@article{
 title = {Multiplexed electrospray emission on a porous wedge},
 type = {article},
 year = {2021},
 volume = {33},
 id = {1f7b15ef-e3c0-3952-97b9-919f075ab49b},
 created = {2021-07-30T18:06:21.528Z},
 file_attached = {true},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-10-15T05:36:12.385Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Linear porous wedge electrospray emitters exhibit a discrete number of emission sites that naturally form during operation. An analytical model is developed to examine the behavior and spacing of these emission sites via the pressure variation in the porous fluid flow associated with the flow focusing on each emission site, which is coupled with the local electric field. The solution for site spacing and current is informed by empirical results with support from electric field modeling and investigation of porous media parameters. Emission site currents of up to 500 nA and site spacings of roughly 50 μm-300 μm are predicted. Results from the model match well with experimental trends and provide further insights into the current and spacing of the discrete emission sites. These insights include the following: (1) for the investigated geometry, the total current can be estimated without taking into account the effects local to each emission site, (2) the wedge hydraulic resistance shows how the emitter output scales with emitter geometry and propellant properties, and (3) the emitted charge to mass ratio increases with the applied electric field. Last, we present a physical description of how specific charge increases with the restorative pressure from the reservoir.},
 bibtype = {article},
 author = {Wright, Peter L. and Wirz, Richard E.},
 doi = {10.1063/5.0030031},
 journal = {Physics of Fluids},
 number = {1}
}

Linear porous wedge electrospray emitters exhibit a discrete number of emission sites that naturally form during operation. An analytical model is developed to examine the behavior and spacing of these emission sites via the pressure variation in the porous fluid flow associated with the flow focusing on each emission site, which is coupled with the local electric field. The solution for site spacing and current is informed by empirical results with support from electric field modeling and investigation of porous media parameters. Emission site currents of up to 500 nA and site spacings of roughly 50 μm-300 μm are predicted. Results from the model match well with experimental trends and provide further insights into the current and spacing of the discrete emission sites. These insights include the following: (1) for the investigated geometry, the total current can be estimated without taking into account the effects local to each emission site, (2) the wedge hydraulic resistance shows how the emitter output scales with emitter geometry and propellant properties, and (3) the emitted charge to mass ratio increases with the applied electric field. Last, we present a physical description of how specific charge increases with the restorative pressure from the reservoir.

@article{
 title = {Tiny cold atmospheric plasma jet for biomedical applications},
 type = {article},
 year = {2021},
 pages = {249},
 volume = {9},
 publisher = {Multidisciplinary Digital Publishing Institute},
 id = {8061b357-87de-3071-8036-c5833cddeaa7},
 created = {2021-08-11T17:14:55.649Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-08-11T17:14:55.649Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 source_type = {JOUR},
 private_publication = {false},
 bibtype = {article},
 author = {Chen, Zhitong and Obenchain, Richard and Wirz, Richard E},
 journal = {Processes},
 number = {2}
}

@article{
 title = {Parametric Analysis of High-Delta-V CubeSat Missions with a Miniature Ion Thruster},
 type = {article},
 year = {2021},
 pages = {754-763},
 volume = {58},
 publisher = {American Institute of Aeronautics and Astronautics},
 id = {3eb852b2-77a3-3554-8c58-a215cd485aba},
 created = {2021-08-11T17:14:56.274Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-08-11T17:14:56.274Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 source_type = {JOUR},
 private_publication = {false},
 bibtype = {article},
 author = {Samples, Stephen and Wirz, Richard},
 journal = {Journal of Spacecraft and Rockets},
 number = {3}
}

@article{
 title = {In situ microscopy for plasma erosion of complex surfaces},
 type = {article},
 year = {2021},
 pages = {073701},
 volume = {92},
 publisher = {AIP Publishing LLC},
 id = {9df08260-799b-304a-938d-e2c93f55f278},
 created = {2021-08-11T17:14:56.906Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-08-11T17:14:56.906Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 source_type = {JOUR},
 private_publication = {false},
 bibtype = {article},
 author = {Ottaviano, Angelica and Thuppul, Anirudh and Hayes, John and Dodson, Chris and Li, Gary Z and Chen, Zhitong and Wirz, Richard E},
 journal = {Review of Scientific Instruments},
 number = {7}
}

@article{
 title = {Atmospheric scattering of energetic electrons from near-Earth space},
 type = {article},
 year = {2021},
 id = {15e71668-d6fc-3d0e-97b8-3ac96c4808b9},
 created = {2021-08-11T17:14:58.262Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-08-11T17:14:58.262Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 source_type = {JOUR},
 private_publication = {false},
 bibtype = {article},
 author = {Angelopoulos, Vassilis and Tsai, Ethan and Wilkins, Colin and Zhang, Xiaojia and Artemyev, Anton and Liu, Jiang and Runov, Andrei and Iglesias, Laura and Turner, Drew and Strangeway, Robert}
}

@article{
 title = {Persistent Sputtering Yield Reduction in Plasma-Infused Foams},
 type = {article},
 year = {2021},
 volume = {126},
 month = {1},
 publisher = {American Physical Society},
 day = {22},
 id = {255104b1-2452-3be0-b27c-ca79658b6efc},
 created = {2021-10-15T05:33:13.532Z},
 file_attached = {true},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-10-15T05:33:14.356Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Aluminum microfoams are found to exhibit persistent sputtering yield reductions of 40%-80% compared to a flat aluminum surface under 100 to 300 eV argon plasma bombardment. An analytical model reveals a strong dependency of the yield on the foam geometry and plasma sheath. For foam pore sizes near or larger than the sheath thickness, the plasma infuses the foam and transitions the plasma-surface interactions from superficial to volumetric phenomena. By defining a plasma infusion parameter, the sputtering behavior of foams is shown to be separated into the plasma-facing and plasma-infused regimes. While plasma infusion leads to a larger effective sputtering area, geometric recapture of ejected particles facilitates an overall reduction in yield. For a given level of plasma infusion, the reductions in normalized yield are more pronounced at lower ion energies since angular sputtering effects enable more effective geometric recapture of sputterants.},
 bibtype = {article},
 author = {Li, Gary Z. and Wirz, Richard E.},
 doi = {10.1103/PhysRevLett.126.035001},
 journal = {Physical Review Letters},
 number = {3}
}

Aluminum microfoams are found to exhibit persistent sputtering yield reductions of 40%-80% compared to a flat aluminum surface under 100 to 300 eV argon plasma bombardment. An analytical model reveals a strong dependency of the yield on the foam geometry and plasma sheath. For foam pore sizes near or larger than the sheath thickness, the plasma infuses the foam and transitions the plasma-surface interactions from superficial to volumetric phenomena. By defining a plasma infusion parameter, the sputtering behavior of foams is shown to be separated into the plasma-facing and plasma-infused regimes. While plasma infusion leads to a larger effective sputtering area, geometric recapture of ejected particles facilitates an overall reduction in yield. For a given level of plasma infusion, the reductions in normalized yield are more pronounced at lower ion energies since angular sputtering effects enable more effective geometric recapture of sputterants.

@article{
 title = {Transdermal cold atmospheric plasma-mediated immune checkpoint blockade therapy},
 type = {article},
 year = {2020},
 volume = {117},
 id = {ecd534ff-47a6-335f-beec-6582a40e4c30},
 created = {2021-07-30T18:18:18.102Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T18:18:18.102Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Despite the promise of immune checkpoint blockade (ICB) therapy against cancer, challenges associated with low objective response rates and severe systemic side effects still remain and limit its clinical applications. Here, we described a cold atmospheric plasma (CAP)mediated ICB therapy integrated with microneedles (MN) for the transdermal delivery of ICB. We found that a hollow-structured MN (hMN) patch facilitates the transportation of CAP through the skin, causing tumor cell death. The release of tumor-associated antigens then promotes the maturation of dendritic cells in the tumor-draining lymph nodes, subsequently initiating T cell-mediated immune response. Anti-programmed death-ligand 1 antibody (aPDL1), an immune checkpoint inhibitor, released from the MN patch further augments the antitumor immunity. Our findings indicate that the proposed transdermal combined CAP and ICB therapy can inhibit the tumor growth of both primary tumors and distant tumors, prolonging the survival of tumor-bearing mice.},
 bibtype = {article},
 author = {Chen, Guojun and Chen, Zhitong and Wen, Di and Wang, Zejun and Li, Hongjun and Zeng, Yi and Dotti, Gianpietro and Wirz, Richard E. and Gu, Zhen},
 doi = {10.1073/pnas.1917891117},
 journal = {Proceedings of the National Academy of Sciences of the United States of America},
 number = {7}
}

Despite the promise of immune checkpoint blockade (ICB) therapy against cancer, challenges associated with low objective response rates and severe systemic side effects still remain and limit its clinical applications. Here, we described a cold atmospheric plasma (CAP)mediated ICB therapy integrated with microneedles (MN) for the transdermal delivery of ICB. We found that a hollow-structured MN (hMN) patch facilitates the transportation of CAP through the skin, causing tumor cell death. The release of tumor-associated antigens then promotes the maturation of dendritic cells in the tumor-draining lymph nodes, subsequently initiating T cell-mediated immune response. Anti-programmed death-ligand 1 antibody (aPDL1), an immune checkpoint inhibitor, released from the MN patch further augments the antitumor immunity. Our findings indicate that the proposed transdermal combined CAP and ICB therapy can inhibit the tumor growth of both primary tumors and distant tumors, prolonging the survival of tumor-bearing mice.

@article{
 title = {Polyatomic ion-induced electron emission (Iiee) in electrospray thrusters},
 type = {article},
 year = {2020},
 volume = {7},
 id = {4247878d-aa36-3454-b695-4be36bf749f7},
 created = {2021-07-30T18:18:19.541Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T18:18:19.541Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {To better characterize the lifetime and performance of electrospray thrusters, electron emission due to electrode impingement by the propellant cation 1-ethyl-3-methylimidazolium (EMI+) has been evaluated with semi-empirical modeling techniques. Results demonstrate that electron emission due to grid impingement by EMI+ cations becomes significant once EMI+ attains a threshold velocity of ∼9 × 105 cm s−1. The mean secondary electron yield, γ, exhibits strong linearity with respect to EMI+ velocity for typical electrospray operating regimes, and we present a simple linear fit equation corresponding to thruster potentials greater than 1 kV. The model chosen for our analysis was shown to be the most appropriate for molecular ion bombardments and is a useful tool in estimating IIEE yields in electrospray devices for molecular ion masses less than ∼1000 u and velocities greater than ∼106 cm s−1. Droplet-induced electron emission (DIEE) in electrospray thrusters was considered by treating a droplet as a macro-ion, with low charge-to-mass ratio, impacting a solid surface. This approach appears to oversimplify back-spray phenomena, meaning a more complex analysis is required. While semi-empirical models of IIEE, and the decades of solid state theory they are based upon, represent an invaluable advance in understanding secondary electron emission in electrospray devices, further progress would be gained by investigating the complex surfaces the electrodes acquire over their lifetimes and considering other possible emission processes.},
 bibtype = {article},
 author = {Magnusson, Jared M. and Collins, Adam L. and Wirz, Richard E.},
 doi = {10.3390/aerospace7110153},
 journal = {Aerospace},
 number = {11}
}

To better characterize the lifetime and performance of electrospray thrusters, electron emission due to electrode impingement by the propellant cation 1-ethyl-3-methylimidazolium (EMI+) has been evaluated with semi-empirical modeling techniques. Results demonstrate that electron emission due to grid impingement by EMI+ cations becomes significant once EMI+ attains a threshold velocity of ∼9 × 105 cm s−1. The mean secondary electron yield, γ, exhibits strong linearity with respect to EMI+ velocity for typical electrospray operating regimes, and we present a simple linear fit equation corresponding to thruster potentials greater than 1 kV. The model chosen for our analysis was shown to be the most appropriate for molecular ion bombardments and is a useful tool in estimating IIEE yields in electrospray devices for molecular ion masses less than ∼1000 u and velocities greater than ∼106 cm s−1. Droplet-induced electron emission (DIEE) in electrospray thrusters was considered by treating a droplet as a macro-ion, with low charge-to-mass ratio, impacting a solid surface. This approach appears to oversimplify back-spray phenomena, meaning a more complex analysis is required. While semi-empirical models of IIEE, and the decades of solid state theory they are based upon, represent an invaluable advance in understanding secondary electron emission in electrospray devices, further progress would be gained by investigating the complex surfaces the electrodes acquire over their lifetimes and considering other possible emission processes.

@misc{
 title = {Comment on “Jet propulsion by microwave air plasma in the atmosphere” [AIP Adv. 10, 055002 (2020)]},
 type = {misc},
 year = {2020},
 source = {AIP Advances},
 volume = {10},
 issue = {9},
 id = {d228d3a3-4d06-3ccf-9846-aa2038b66c81},
 created = {2021-07-30T18:18:21.624Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T18:18:21.624Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {In this Comment, we analyze the performance of a microwave plasma device presented by Ye et al. [AIP Adv. 10, 055002 (2020)]. The efficiency analysis, using conservation of energy, shows that the methods used by the original authors predict up to 8000% device efficiency. Our analytical model is based on a control volume analysis of the original authors’ experimental setup and conditions, indicating that blocking the exit of the device yields stagnation pressure rather than jet pressure. The results from this analysis are consistent with the reported experimental data, demonstrating that the measured pressure using this method is internal chamber pressure and cannot be used to estimate thrust.},
 bibtype = {misc},
 author = {Wright, Peter L. and Samples, Stephen A. and Uchizono, Nolan M. and Wirz, Richard E.},
 doi = {10.1063/5.0013575}
}

In this Comment, we analyze the performance of a microwave plasma device presented by Ye et al. [AIP Adv. 10, 055002 (2020)]. The efficiency analysis, using conservation of energy, shows that the methods used by the original authors predict up to 8000% device efficiency. Our analytical model is based on a control volume analysis of the original authors’ experimental setup and conditions, indicating that blocking the exit of the device yields stagnation pressure rather than jet pressure. The results from this analysis are consistent with the reported experimental data, demonstrating that the measured pressure using this method is internal chamber pressure and cannot be used to estimate thrust.

@article{
 title = {Tunable reflectionless absorption of electromagnetic waves in a plasma-metamaterial composite structure},
 type = {article},
 year = {2020},
 volume = {29},
 id = {03f20df4-b114-3301-b7e4-1d3e3e6d3589},
 created = {2021-07-30T18:18:24.284Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T18:18:24.284Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {We present the first experimental demonstration of a tunable reflectionless absorption resonance in a metamaterial integrated with a plasma discharge. A one-dimensional metamaterial structure excites transverse magnetic slow-wave modes known as 'spoof' surface plasmon polaritons. When interfaced with an argon plasma discharge, the metamaterial-induced 'spoof' plasmon mode is converted to a plasmon polariton mode confined to the plasma/dielectric interface. The reflectionless absorption band that manifests in the metamaterial's spectral response exhibits a dependency on the plasma's electron density that agrees well with theory.},
 bibtype = {article},
 author = {Uchizono, Nolan M. and Samples, Stephen A. and Wirz, Richard E.},
 doi = {10.1088/1361-6595/aba489},
 journal = {Plasma Sources Science and Technology},
 number = {8}
}

We present the first experimental demonstration of a tunable reflectionless absorption resonance in a metamaterial integrated with a plasma discharge. A one-dimensional metamaterial structure excites transverse magnetic slow-wave modes known as 'spoof' surface plasmon polaritons. When interfaced with an argon plasma discharge, the metamaterial-induced 'spoof' plasmon mode is converted to a plasmon polariton mode confined to the plasma/dielectric interface. The reflectionless absorption band that manifests in the metamaterial's spectral response exhibits a dependency on the plasma's electron density that agrees well with theory.

@article{
 title = {Cold atmospheric plasma for SARS-CoV-2 inactivation},
 type = {article},
 year = {2020},
 volume = {32},
 id = {48e6a382-4c56-3a15-97bc-7f7633fb4040},
 created = {2021-07-30T18:29:08.845Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T18:29:08.845Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Syndrome coronavirus 2 (SARS-CoV-2) infectious virions are viable on various surfaces (e.g., plastic, metals, and cardboard) for several hours. This presents a transmission cycle for human infection that can be broken by developing new inactivation approaches. We employed an efficient cold atmospheric plasma (CAP) with argon feed gas to inactivate SARS-CoV-2 on various surfaces including plastic, metal, cardboard, basketball composite leather, football leather, and baseball leather. These results demonstrate the great potential of CAP as a safe and effective means to prevent virus transmission and infections for a wide range of surfaces that experience frequent human contact. Since this is the first-ever demonstration of cold plasma inactivation of SARS-CoV-2, it is a significant milestone in the prevention and treatment of coronavirus disease 2019 (COVID-19) and presents a new opportunity for the scientific, engineering, and medical communities.},
 bibtype = {article},
 author = {Chen, Zhitong and Garcia, Gustavo and Arumugaswami, Vaithilingaraja and Wirz, Richard E.},
 doi = {10.1063/5.0031332},
 journal = {Physics of Fluids},
 number = {11}
}

Syndrome coronavirus 2 (SARS-CoV-2) infectious virions are viable on various surfaces (e.g., plastic, metals, and cardboard) for several hours. This presents a transmission cycle for human infection that can be broken by developing new inactivation approaches. We employed an efficient cold atmospheric plasma (CAP) with argon feed gas to inactivate SARS-CoV-2 on various surfaces including plastic, metal, cardboard, basketball composite leather, football leather, and baseball leather. These results demonstrate the great potential of CAP as a safe and effective means to prevent virus transmission and infections for a wide range of surfaces that experience frequent human contact. Since this is the first-ever demonstration of cold plasma inactivation of SARS-CoV-2, it is a significant milestone in the prevention and treatment of coronavirus disease 2019 (COVID-19) and presents a new opportunity for the scientific, engineering, and medical communities.

@article{
 title = {Emission modes in electrospray thrusters operating with high conductivity ionic liquids},
 type = {article},
 year = {2020},
 volume = {7},
 id = {a8b2f7f8-c725-3f13-87e5-e2f967dbb941},
 created = {2021-07-30T18:29:10.303Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T18:29:10.303Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Electrospray thruster life and mission performance are strongly influenced by grid impingement, the extent of which can be correlated with emission modes that occur at steady-state extraction voltages, and thruster command transients. Most notably, we experimentally observed skewed cone-jet emission during steady-state electrospray thruster operation, which leads to the definition of an additional grid impingement mechanism that we termed “tilted emission”. Long distance microscopy was used in conjunction with high speed videography to observe the emission site of an electrospray thruster operating with an ionic liquid propellant (EMI-Im). During steady-state thruster operation, no unsteady electrohydrodynamic emission modes were observed, though the conical meniscus exhibited steady off-axis tilt of up to 15◦. Cone tilt angle was independent over a wide range of flow rates but proved strongly dependent on extraction voltage. For the geometry and propellant used, the optimal extraction voltage was near 1.6 kV. A second experiment characterized transient emission behavior by observing startup and shutdown of the thruster via flow or voltage. Three of the four possible startup and shutdown procedures transition to quiescence within ∼475 µs, with no observed unsteady modes. However, during voltage-induced thruster startup, unsteady electrohydrodynamic modes were observed.},
 bibtype = {article},
 author = {Uchizono, Nolan M. and Collins, Adam L. and Thuppul, Anirudh and Wright, Peter L. and Eckhardt, Daniel Q. and Ziemer, John and Wirz, Richard E.},
 doi = {10.3390/aerospace7100141},
 journal = {Aerospace},
 number = {10}
}

Electrospray thruster life and mission performance are strongly influenced by grid impingement, the extent of which can be correlated with emission modes that occur at steady-state extraction voltages, and thruster command transients. Most notably, we experimentally observed skewed cone-jet emission during steady-state electrospray thruster operation, which leads to the definition of an additional grid impingement mechanism that we termed “tilted emission”. Long distance microscopy was used in conjunction with high speed videography to observe the emission site of an electrospray thruster operating with an ionic liquid propellant (EMI-Im). During steady-state thruster operation, no unsteady electrohydrodynamic emission modes were observed, though the conical meniscus exhibited steady off-axis tilt of up to 15◦. Cone tilt angle was independent over a wide range of flow rates but proved strongly dependent on extraction voltage. For the geometry and propellant used, the optimal extraction voltage was near 1.6 kV. A second experiment characterized transient emission behavior by observing startup and shutdown of the thruster via flow or voltage. Three of the four possible startup and shutdown procedures transition to quiescence within ∼475 µs, with no observed unsteady modes. However, during voltage-induced thruster startup, unsteady electrohydrodynamic modes were observed.

@article{
 title = {Lifetime Considerations for Electrospray Thrusters},
 type = {article},
 year = {2020},
 publisher = {MDPI},
 id = {40affdc0-9cab-37d1-8bdb-f5f69b890cb1},
 created = {2021-07-30T19:03:04.392Z},
 file_attached = {true},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T19:03:05.478Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {true},
 hidden = {false},
 citation_key = {Thuppul2020},
 source_type = {article},
 private_publication = {false},
 bibtype = {article},
 author = {Thuppul, A and Wright, P L and Collins, A L and Ziemer, J and Wirz, R E},
 journal = {Aerospace}
}

@article{
 title = {Development of the MiXI Thruster with the ARCH Discharge},
 type = {article},
 year = {2020},
 volume = {2},
 id = {9a0c14f6-fc75-3900-9e79-cc72b6afa7b9},
 created = {2021-07-30T20:26:26.025Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:26:26.025Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The Miniature Xenon Ion (MiXI) thruster with the Axial Ring-Cusp Hybrid 'MiXI(ARCH)' discharge was developed and operated with beam extraction at 1 kV. The thruster achieved 59% cathode-free total discharge efficiency at 23.7 mA xenon beam current with filament cathodes and low temperature operation, corresponding to a discharge loss of 226 W/A and propellant utilization of 72%. Thruster efficiency was observed to increase with increasing flow rate and decrease with increasing temperature up to thermal steady state. At thermal steady state, the thruster anode reached ∼320 °C due to the thermal isolation of the thruster head. Reducing the discharge chamber aspect ratio from 0.5 to 0.4 increased thermal steady state efficiency from 46% to 57% but required slow ramping of beam voltage and was limited to stable operation to above 0.5 sccm discharge propellant flow. In contrast to the 3-ring cusp configuration, MiXI(3-Ring), the performance is generally higher but is not able to achieve lower thrust levels and requires more complex start-up for stable operation. An analytical single-cell model was developed and applied to investigate internal processes of the MiXI(ARCH) discharge. The model emulated the effect of increasing flow on performance, indicating that the dominant loss mechanism is plasma electron current to the anode, in contrast to the 3-Ring geometry, which is dominated by primary electron losses. This model also matched trends reported in previous works of strongly increasing electron temperature and primary density with propellant utilization. Through this effort, the MiXI thruster's highest achievable total efficiency has been increased, and several mechanisms for further improved efficiency have been identified.},
 bibtype = {article},
 author = {Samples, Stephen A. and Wirz, Richard E.},
 doi = {10.1088/2516-1067/ab906d},
 journal = {Plasma Research Express},
 number = {2}
}

The Miniature Xenon Ion (MiXI) thruster with the Axial Ring-Cusp Hybrid 'MiXI(ARCH)' discharge was developed and operated with beam extraction at 1 kV. The thruster achieved 59% cathode-free total discharge efficiency at 23.7 mA xenon beam current with filament cathodes and low temperature operation, corresponding to a discharge loss of 226 W/A and propellant utilization of 72%. Thruster efficiency was observed to increase with increasing flow rate and decrease with increasing temperature up to thermal steady state. At thermal steady state, the thruster anode reached ∼320 °C due to the thermal isolation of the thruster head. Reducing the discharge chamber aspect ratio from 0.5 to 0.4 increased thermal steady state efficiency from 46% to 57% but required slow ramping of beam voltage and was limited to stable operation to above 0.5 sccm discharge propellant flow. In contrast to the 3-ring cusp configuration, MiXI(3-Ring), the performance is generally higher but is not able to achieve lower thrust levels and requires more complex start-up for stable operation. An analytical single-cell model was developed and applied to investigate internal processes of the MiXI(ARCH) discharge. The model emulated the effect of increasing flow on performance, indicating that the dominant loss mechanism is plasma electron current to the anode, in contrast to the 3-Ring geometry, which is dominated by primary electron losses. This model also matched trends reported in previous works of strongly increasing electron temperature and primary density with propellant utilization. Through this effort, the MiXI thruster's highest achievable total efficiency has been increased, and several mechanisms for further improved efficiency have been identified.

@article{
 title = {The ELFIN mission},
 type = {article},
 year = {2020},
 pages = {1-45},
 volume = {216},
 publisher = {Springer},
 id = {750bbaa5-9fcd-32e2-ba5c-f1f8cf30587c},
 created = {2021-08-11T17:14:57.696Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-08-11T17:14:57.696Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 source_type = {JOUR},
 private_publication = {false},
 bibtype = {article},
 author = {Angelopoulos, V and Tsai, E and Bingley, L and Shaffer, C and Turner, D L and Runov, A and Li, W and Liu, J and Artemyev, A V and Zhang, X-J},
 journal = {Space science reviews},
 number = {5}
}

@article{
 title = {Measurements of ion velocity and wave propagation in a hollow cathode plume},
 type = {article},
 year = {2019},
 volume = {28},
 id = {4fe68305-7980-3212-addf-3426ce430b26},
 created = {2021-07-30T18:29:11.823Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T18:29:11.823Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The mechanism responsible for the production of energetic ions in the plume of hollow cathodes for electric propulsion is still an open issue. These ions are of concern to cathode and thruster lifetime, particularly for cathodes operating at high (>20 A) discharge currents. Recent theoretical and experimental investigations suggest that there is a correlation between ion energy gain and ion acoustic turbulence. In this paper we present measurements of the evolution of the ion velocity distribution function in the near plume of a 100 A-class hollow cathode, operated in a regime in which the dominant mode is ion acoustic turbulence. Ion flow and thermal properties were related to measurements of the background plasma, fluctuation spectra, and dispersion relations obtained from an array of Langmuir probes. We found ions to flow outward from the cathode and accelerate downstream, to supersonic speeds, approximately aligned with the acoustic wave group velocity vectors. The directions of the ion flow and wave propagation were similar for a range of discharge currents and mass flow rates in the jet region of the plume. One operating condition showed a significant temperature increase, also in the direction of acoustic wave propagation, corresponding to the highest wave energy condition. These results are interpreted in the context of ion acoustic turbulence as a contributing mechanism for ion energy gain.},
 bibtype = {article},
 author = {Dodson, Christopher and Jorns, Benjamin and Wirz, Richard},
 doi = {10.1088/1361-6595/ab1c48},
 journal = {Plasma Sources Science and Technology},
 number = {6}
}

The mechanism responsible for the production of energetic ions in the plume of hollow cathodes for electric propulsion is still an open issue. These ions are of concern to cathode and thruster lifetime, particularly for cathodes operating at high (>20 A) discharge currents. Recent theoretical and experimental investigations suggest that there is a correlation between ion energy gain and ion acoustic turbulence. In this paper we present measurements of the evolution of the ion velocity distribution function in the near plume of a 100 A-class hollow cathode, operated in a regime in which the dominant mode is ion acoustic turbulence. Ion flow and thermal properties were related to measurements of the background plasma, fluctuation spectra, and dispersion relations obtained from an array of Langmuir probes. We found ions to flow outward from the cathode and accelerate downstream, to supersonic speeds, approximately aligned with the acoustic wave group velocity vectors. The directions of the ion flow and wave propagation were similar for a range of discharge currents and mass flow rates in the jet region of the plume. One operating condition showed a significant temperature increase, also in the direction of acoustic wave propagation, corresponding to the highest wave energy condition. These results are interpreted in the context of ion acoustic turbulence as a contributing mechanism for ion energy gain.

@article{
 title = {Numerical Simulation of Electrospray Thruster Extraction},
 type = {article},
 year = {2019},
 pages = {1-11},
 id = {953cdce4-1feb-384b-9c3a-55b78aca5995},
 created = {2021-07-30T19:03:13.891Z},
 file_attached = {true},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T19:03:14.888Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {true},
 hidden = {false},
 private_publication = {false},
 abstract = {The UCLA Plasma & Space Propulsion Laboratory developed an Electrohydrodynamics (EHD) OpenFOAM solver (PSPL-EHD) based on the electrowetting model to understand the physics of cone-jet and droplet formation of electrospray thrusters. The code requires no simplification of charge conservation laws and includes improvements to handle the uniquely challenging properties of highly conductive propellants. Gauss's law and the charge conservation equation are solved alongside the Navier-Stokes equation for flow field and charge distribution. The full charge conservation equation is applied in order to consider the limits of both leaky dielectrics and perfect conductors in this work. The model captures unsteady and time-dependent behavior, which are crucial for investigating the lifetime and performance of electrospray thrusters. To verify and validate the model, simulations were performed for a heptane electrospray at an applied voltage of 5.5 kV between an emitter and extractor plate and a liquid flow rate at 10 cm 3 /s. The droplet diameters are calculated in the range of 20-27 µm, in reasonable agreement with the 10 µm average size reported in experimental results. Also, simulations, where the applied voltage is increased, steepen the shape of the cone-jet and shrink the droplet diameter, consistent with trends from electrospray theory and observations.},
 bibtype = {article},
 author = {Huh, Henry and Wirz, Richard},
 journal = {36th International Electric Propulsion Conference},
 number = {September}
}

The UCLA Plasma & Space Propulsion Laboratory developed an Electrohydrodynamics (EHD) OpenFOAM solver (PSPL-EHD) based on the electrowetting model to understand the physics of cone-jet and droplet formation of electrospray thrusters. The code requires no simplification of charge conservation laws and includes improvements to handle the uniquely challenging properties of highly conductive propellants. Gauss's law and the charge conservation equation are solved alongside the Navier-Stokes equation for flow field and charge distribution. The full charge conservation equation is applied in order to consider the limits of both leaky dielectrics and perfect conductors in this work. The model captures unsteady and time-dependent behavior, which are crucial for investigating the lifetime and performance of electrospray thrusters. To verify and validate the model, simulations were performed for a heptane electrospray at an applied voltage of 5.5 kV between an emitter and extractor plate and a liquid flow rate at 10 cm 3 /s. The droplet diameters are calculated in the range of 20-27 µm, in reasonable agreement with the 10 µm average size reported in experimental results. Also, simulations, where the applied voltage is increased, steepen the shape of the cone-jet and shrink the droplet diameter, consistent with trends from electrospray theory and observations.

@article{
 title = {Assessment of Grid Impingement for Electrospray Thruster Lifetime},
 type = {article},
 year = {2019},
 pages = {1-18},
 id = {89f58ffc-ec2c-3c1a-be7a-24d4eda0e3f2},
 created = {2021-07-30T19:03:22.015Z},
 file_attached = {true},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T19:03:23.106Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {true},
 hidden = {false},
 private_publication = {false},
 bibtype = {article},
 author = {Wright, Peter L and Huh, Henry and Davis, Mckenna J},
 journal = {36th International Electric Propulsion Conference},
 number = {September}
}

@article{
 title = {Spatially-Resolved Mass Flux and Current Measurements of Electrospray Plumes},
 type = {article},
 year = {2019},
 pages = {1-10},
 id = {5ad02d2f-ce58-3804-9900-9a369744cc28},
 created = {2021-07-30T19:03:29.903Z},
 file_attached = {true},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T19:03:31.088Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {true},
 hidden = {false},
 private_publication = {false},
 abstract = {In order to understand the performance and lifetime capabilities of electrospray thrusters, the evolution of their plumes downstream from the emission site must be characterized. This study investigates current density and mass ux measurements of an ionic liquid elec- trospray plume for implications on thruster performance and lifetime. A capillary electro- spray emitter is operated using 1-ethyl-3-methylimidazolium bis(tri ouromethylsulfonyl)imide (EMI-Im) propellant to determine plume pro les at multiple emitter ow rates and emitter bias voltages. A quartz crystal microbalance (QCM) and current probe are swept across the plume to independently measure spatially-resolved mass ux and current density pro- les out to high half-angles of 40. Measured pro les show that mass ux and current density pro les deviate from each other, with the mass ux pro les exhibiting a at-top distribution with a steeper-than-Gaussian decay. Charge-to-mass ratio distributions are es- timated and show that the small half-angle regions of the plume have higher ratios of larger droplets to smaller droplets, while the large half-angle regions are primarily composed of smaller droplets.},
 bibtype = {article},
 author = {Thuppul, Anirudh and Collins, Adam L. and Wright, Peter L. and Uchizono, Nolan, M. and Wirz, Richard E},
 journal = {36th International Electric Propulsion Conference},
 number = {September}
}

In order to understand the performance and lifetime capabilities of electrospray thrusters, the evolution of their plumes downstream from the emission site must be characterized. This study investigates current density and mass ux measurements of an ionic liquid elec- trospray plume for implications on thruster performance and lifetime. A capillary electro- spray emitter is operated using 1-ethyl-3-methylimidazolium bis(tri ouromethylsulfonyl)imide (EMI-Im) propellant to determine plume pro les at multiple emitter ow rates and emitter bias voltages. A quartz crystal microbalance (QCM) and current probe are swept across the plume to independently measure spatially-resolved mass ux and current density pro- les out to high half-angles of 40. Measured pro les show that mass ux and current density pro les deviate from each other, with the mass ux pro les exhibiting a at-top distribution with a steeper-than-Gaussian decay. Charge-to-mass ratio distributions are es- timated and show that the small half-angle regions of the plume have higher ratios of larger droplets to smaller droplets, while the large half-angle regions are primarily composed of smaller droplets.

@inproceedings{
 title = {Electrospray Plume Evolution Via Discrete Simulations},
 type = {inproceedings},
 year = {2019},
 id = {d1e60eed-fd4e-32c2-b3ca-e5ca9d00c015},
 created = {2021-07-30T19:03:36.622Z},
 file_attached = {true},
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 authored = {false},
 confirmed = {true},
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 citation_key = {Davis2019},
 source_type = {inproceedings},
 private_publication = {false},
 bibtype = {inproceedings},
 author = {Davis, McKenna. J and Collins, Adam. L and Wirz, Richard. E},
 booktitle = {Proc. 36th Int. Electr. Propuls. Conf., IEPC-2019-590}
}

@inproceedings{
 title = {A Novel Variable Mode Emitter for Electrospray Thrusters},
 type = {inproceedings},
 year = {2019},
 id = {49875a14-0b0b-3373-987c-218d391b45c3},
 created = {2021-07-30T19:03:42.815Z},
 file_attached = {true},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
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 last_modified = {2021-07-30T19:03:43.765Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {true},
 hidden = {false},
 citation_key = {wright2019anovel},
 source_type = {inproceedings},
 private_publication = {false},
 bibtype = {inproceedings},
 author = {Wright, Peter L and Huh, Henry and Uchizono, Nolan M and Thuppul, Anirudh and Wirz, Richard E},
 booktitle = {Proc. 36th Int. Electr. Propuls. Conf., IEPC-2019-650}
}

@article{
 title = {Electrospray Steady-State and Transient Emission Behavior},
 type = {article},
 year = {2019},
 pages = {1-13},
 id = {ec146e0b-b4c4-3f94-9171-c7bfdaf90ba0},
 created = {2021-07-30T19:03:48.647Z},
 file_attached = {true},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T19:03:49.708Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {true},
 hidden = {false},
 private_publication = {false},
 abstract = {The UCLA Plasma & Space Propulsion Lab is using both high-speed microscopy and emitter current measurements to provide insight into the electrohydrodynamics of electrospray emission processes. Test are being performed via benchtop testing of low-conductivity ethanol and in vacuo testing of high-conductivity ionic propellant (EMI-Im). The physical deformation of the liquid meniscus and angle of the cone-jet with respect to the emitter/extractor axis is observed over a range of flow rates and extractor voltages. The results of this study indicate that EMI-Im electrosprays are stable over a very wide range of setpoints, in comparison to lower-conductivity propellants, but exhibit off-axis asymmetries that must be considered for thruster life and performance. Furthermore, under certain conditions, transient instabilities occur during startup and shutdown that must be consider for thruster operation and lifetime.},
 bibtype = {article},
 author = {Uchizono, Nolan and Collins, Adam L. and Thuppul, Anirudh and Wright, Peter L. and Eckhardt, Daniel Q. and Ziemer, J. K. and Wirz, Richard E.},
 journal = {The 36th International Electric Propulsion Conference},
 number = {September}
}

The UCLA Plasma & Space Propulsion Lab is using both high-speed microscopy and emitter current measurements to provide insight into the electrohydrodynamics of electrospray emission processes. Test are being performed via benchtop testing of low-conductivity ethanol and in vacuo testing of high-conductivity ionic propellant (EMI-Im). The physical deformation of the liquid meniscus and angle of the cone-jet with respect to the emitter/extractor axis is observed over a range of flow rates and extractor voltages. The results of this study indicate that EMI-Im electrosprays are stable over a very wide range of setpoints, in comparison to lower-conductivity propellants, but exhibit off-axis asymmetries that must be considered for thruster life and performance. Furthermore, under certain conditions, transient instabilities occur during startup and shutdown that must be consider for thruster operation and lifetime.

@article{
 title = {Electric Propulsion Activities at the UCLA Plasma & Space Propulsion Laboratory},
 type = {article},
 year = {2019},
 pages = {1-24},
 id = {cf4c0921-0a0b-3293-bb32-e200218574aa},
 created = {2021-07-30T19:03:58.273Z},
 file_attached = {true},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T19:03:59.359Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {true},
 hidden = {false},
 private_publication = {false},
 bibtype = {article},
 author = {Wirz, Richard E and Collins, Adam L and Chen, Zhitong and Huerta, Cesar E and Li, Gary Z and Samples, Stephen A and Thuppul, Anirudh and Wright, Peter L and Uchizono, Nolan M}
}

@article{
 title = {Numerical Simulation of Electrospray Thruster Extraction},
 type = {article},
 year = {2019},
 id = {bc9bd069-40d5-3fe6-9223-4b68f54a3ca7},
 created = {2021-07-30T22:42:47.374Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:47.374Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The UCLA Plasma & Space Propulsion Laboratory developed an Electrohydrodynamics (EHD) OpenFOAM solver (PSPL-EHD) based on the electrowetting model to understand the physics of cone-jet and droplet formation of electrospray thrusters. The code requires no simplification of charge conservation laws and includes improvements to handle the uniquely challenging properties of highly conductive propellants. Gauss's law and the charge conservation equation are solved alongside the Navier-Stokes equation for flow field and charge distribution. The full charge conservation equation is applied in order to consider the limits of both leaky dielectrics and perfect conductors in this work. The model captures unsteady and time-dependent behavior, which are crucial for investigating the lifetime and performance of electrospray thrusters. To verify and validate the model, simulations were performed for a heptane electrospray at an applied voltage of 5.5 kV between an emitter and extractor plate and a liquid flow rate at 10 cm 3 /s. The droplet diameters are calculated in the range of 20-27 µm, in reasonable agreement with the 10 µm average size reported in experimental results. Also, simulations, where the applied voltage is increased, steepen the shape of the cone-jet and shrink the droplet diameter, consistent with trends from electrospray theory and observations.},
 bibtype = {article},
 author = {Huh, Henry and Wirz, Richard},
 journal = {36th International Electric Propulsion Conference},
 number = {September}
}

The UCLA Plasma & Space Propulsion Laboratory developed an Electrohydrodynamics (EHD) OpenFOAM solver (PSPL-EHD) based on the electrowetting model to understand the physics of cone-jet and droplet formation of electrospray thrusters. The code requires no simplification of charge conservation laws and includes improvements to handle the uniquely challenging properties of highly conductive propellants. Gauss's law and the charge conservation equation are solved alongside the Navier-Stokes equation for flow field and charge distribution. The full charge conservation equation is applied in order to consider the limits of both leaky dielectrics and perfect conductors in this work. The model captures unsteady and time-dependent behavior, which are crucial for investigating the lifetime and performance of electrospray thrusters. To verify and validate the model, simulations were performed for a heptane electrospray at an applied voltage of 5.5 kV between an emitter and extractor plate and a liquid flow rate at 10 cm 3 /s. The droplet diameters are calculated in the range of 20-27 µm, in reasonable agreement with the 10 µm average size reported in experimental results. Also, simulations, where the applied voltage is increased, steepen the shape of the cone-jet and shrink the droplet diameter, consistent with trends from electrospray theory and observations.

@inproceedings{
 title = {Lifetime considerations and estimation for electrospray thrusters},
 type = {inproceedings},
 year = {2018},
 id = {97af1442-e7fc-3b14-b580-c361a13cd3c7},
 created = {2021-07-30T18:18:23.103Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T18:18:23.103Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Ionic liquid electrospray thrusters are capable of producing micronewton precision thrust with high thrust-power ratio but have yet to demonstrate lifetimes that are suitable for most missions. Accumulation of propellant on the extractor and accelerator grids is the most significant life-limiting mechanism. In this study, we develop a life model to examine the effects of design features, operating conditions, and emission properties on the porous accelerator grid saturation time of a thruster operating in droplet emission mode. Characterizing a range of geometries and operating conditions reveals that modifying grid aperture radius and grid spacing can significantly improve thruster lifetime. Misalignment and tolerance analysis shows the potential for up to 50% lifetime reduction. In addition, examining the impact of electron backstreaming shows that increasing aperture radius produces a significant increase in backstreaming current compared to changing grid spacing. A study of accelerator grid bias voltages reveals that applying reasonably strong accelerator grid potential can minimize backstreaming current to negligible amounts for a range of geometries.},
 bibtype = {inproceedings},
 author = {Thuppul, Anirudh and Wright, Peter L. and Wirz, Richard E.},
 doi = {10.2514/6.2018-4652},
 booktitle = {2018 Joint Propulsion Conference}
}

Ionic liquid electrospray thrusters are capable of producing micronewton precision thrust with high thrust-power ratio but have yet to demonstrate lifetimes that are suitable for most missions. Accumulation of propellant on the extractor and accelerator grids is the most significant life-limiting mechanism. In this study, we develop a life model to examine the effects of design features, operating conditions, and emission properties on the porous accelerator grid saturation time of a thruster operating in droplet emission mode. Characterizing a range of geometries and operating conditions reveals that modifying grid aperture radius and grid spacing can significantly improve thruster lifetime. Misalignment and tolerance analysis shows the potential for up to 50% lifetime reduction. In addition, examining the impact of electron backstreaming shows that increasing aperture radius produces a significant increase in backstreaming current compared to changing grid spacing. A study of accelerator grid bias voltages reveals that applying reasonably strong accelerator grid potential can minimize backstreaming current to negligible amounts for a range of geometries.

@inproceedings{
 title = {Life-Limiting Emission Modes for Electrospray Thrusters},
 type = {inproceedings},
 year = {2018},
 id = {10fb4ee4-8a46-3e64-a654-e4cd2e96eec4},
 created = {2021-07-30T19:04:14.047Z},
 file_attached = {true},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T19:04:15.110Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {true},
 hidden = {false},
 citation_key = {wright2018life},
 source_type = {inproceedings},
 private_publication = {false},
 bibtype = {inproceedings},
 author = {Wright, Peter and Thuppul, Anirudh and Wirz, Richard E},
 booktitle = {2018 Joint Propulsion Conference, AIAA 2018-4726}
}

@article{
 title = {Demonstration of a low cost, high temperature elemental sulfur thermal battery},
 type = {article},
 year = {2018},
 volume = {137},
 id = {dd7c2041-f575-3635-9158-2b14d4cbdcd3},
 created = {2021-07-30T19:22:23.973Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T19:22:23.973Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Elemental sulfur is a low-cost energy storage media suitable for many medium to high temperature applications, including trough and tower concentrated solar power (CSP) and combined heat and power (CHP) systems. In this study, we have demonstrated the viability of an elemental sulfur thermal energy storage (SulfurTES) system using a laboratory-scale thermal battery. The SulfurTES battery design uses a shell-and-tube thermal battery configuration, wherein stationary elemental sulfur is isochorically stored in multiple stainless steel tubes and a heat transfer fluid (air) is passed over them through the surrounding shell. The safe and reliable operation was demonstrated for twelve thermal charge–discharge cycles in the temperature range of 200–600 °C, during which the SulfurTES battery stored up to 7.6 kW h of thermal energy with volumetric energy density range up to 255 kW h/m3. Furthermore, the SulfurTES battery is operated in a hybrid thermal charging mode to demonstrate its ability to store surplus electrical energy. The present study establishes the feasibility of SulfurTES as a concept that could provide attractive system cost and volumetric energy density for a wide range of thermal energy storage applications.},
 bibtype = {article},
 author = {Barde, Amey and Jin, Kaiyuan and Shinn, Mitchell and Nithyanandam, Karthik and Wirz, Richard E.},
 doi = {10.1016/j.applthermaleng.2018.02.094},
 journal = {Applied Thermal Engineering}
}

Elemental sulfur is a low-cost energy storage media suitable for many medium to high temperature applications, including trough and tower concentrated solar power (CSP) and combined heat and power (CHP) systems. In this study, we have demonstrated the viability of an elemental sulfur thermal energy storage (SulfurTES) system using a laboratory-scale thermal battery. The SulfurTES battery design uses a shell-and-tube thermal battery configuration, wherein stationary elemental sulfur is isochorically stored in multiple stainless steel tubes and a heat transfer fluid (air) is passed over them through the surrounding shell. The safe and reliable operation was demonstrated for twelve thermal charge–discharge cycles in the temperature range of 200–600 °C, during which the SulfurTES battery stored up to 7.6 kW h of thermal energy with volumetric energy density range up to 255 kW h/m3. Furthermore, the SulfurTES battery is operated in a hybrid thermal charging mode to demonstrate its ability to store surplus electrical energy. The present study establishes the feasibility of SulfurTES as a concept that could provide attractive system cost and volumetric energy density for a wide range of thermal energy storage applications.

@inproceedings{
 title = {Ion heating measurements on the centerline of a high-current hollow cathode plume},
 type = {inproceedings},
 year = {2018},
 volume = {34},
 issue = {5},
 id = {cbe4aca6-406e-3845-9ca5-e42c965fc1e5},
 created = {2021-07-30T19:23:19.693Z},
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 hidden = {false},
 private_publication = {false},
 abstract = {An experimental investigation into the correlation between ion acoustic turbulence (IAT) and anomalous ion heating in the plume of a 100 A-class LaB6 hollow cathode is presented. Laser-induced fluorescence is employed to measure the ion velocity distribution function, and a translating ion saturation probe is used to quantify the spatial dependence of the IAT wave energy. It is found that over a range of flow rates and operating currents both the ion temperature and IAT energy increase downstream of the cathode in qualitatively similar ways. Both parameters also are shown to be impacted by operating conditions: the IAT energy and ion temperature decrease at higher flow rates and lower discharge currents. It is shown that the ratio between ion temperature and wave energy is related by a scaling parameter that depends on the background plasma parameters, and this relation is examined in the context of previous analytical work on IAT-induced ion heating.},
 bibtype = {inproceedings},
 author = {Dodson, Christopher A. and Perez-Grande, Daniel and Jorns, Benjamin A. and Goebel, Dan M. and Wirz, Richard E.},
 doi = {10.2514/1.B36788},
 booktitle = {Journal of Propulsion and Power}
}

An experimental investigation into the correlation between ion acoustic turbulence (IAT) and anomalous ion heating in the plume of a 100 A-class LaB6 hollow cathode is presented. Laser-induced fluorescence is employed to measure the ion velocity distribution function, and a translating ion saturation probe is used to quantify the spatial dependence of the IAT wave energy. It is found that over a range of flow rates and operating currents both the ion temperature and IAT energy increase downstream of the cathode in qualitatively similar ways. Both parameters also are shown to be impacted by operating conditions: the IAT energy and ion temperature decrease at higher flow rates and lower discharge currents. It is shown that the ratio between ion temperature and wave energy is related by a scaling parameter that depends on the background plasma parameters, and this relation is examined in the context of previous analytical work on IAT-induced ion heating.

@article{
 title = {Characterization of xenon ion and neutral interactions in a well-characterized experiment},
 type = {article},
 year = {2018},
 volume = {25},
 id = {2fbc2659-4385-3f17-a530-9fbab4d95167},
 created = {2021-07-30T19:24:54.627Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T19:24:54.627Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Interactions between fast ions and slow neutral atoms are commonly dominated by charge-exchange and momentum-exchange collisions, which are important to understanding and simulating the performance and behavior of many plasma devices. To investigate these interactions, this work developed a simple, well-characterized experiment that accurately measures the behavior of high energy xenon ions incident on a background of xenon neutral atoms. By using well-defined operating conditions and a simple geometry, these results serve as canonical data for the development and validation of plasma models and models of neutral beam sources that need to ensure accurate treatment of angular scattering distributions of charge-exchange and momentum-exchange ions and neutrals. The energies used in this study are relevant for electric propulsion devices ∼1.5 keV and can be used to improve models of ion-neutral interactions in the plume. By comparing these results to both analytical and computational models of ion-neutral interactions, we discovered the importance of (1) accurately treating the differential cross-sections for momentum-exchange and charge-exchange collisions over a large range of neutral background pressures and (2) properly considering commonly overlooked interactions, such as ion-induced electron emission from nearby surfaces and neutral-neutral ionization collisions.},
 bibtype = {article},
 author = {Patino, Marlene I. and Wirz, Richard E.},
 doi = {10.1063/1.5030464},
 journal = {Physics of Plasmas},
 number = {6}
}

Interactions between fast ions and slow neutral atoms are commonly dominated by charge-exchange and momentum-exchange collisions, which are important to understanding and simulating the performance and behavior of many plasma devices. To investigate these interactions, this work developed a simple, well-characterized experiment that accurately measures the behavior of high energy xenon ions incident on a background of xenon neutral atoms. By using well-defined operating conditions and a simple geometry, these results serve as canonical data for the development and validation of plasma models and models of neutral beam sources that need to ensure accurate treatment of angular scattering distributions of charge-exchange and momentum-exchange ions and neutrals. The energies used in this study are relevant for electric propulsion devices ∼1.5 keV and can be used to improve models of ion-neutral interactions in the plume. By comparing these results to both analytical and computational models of ion-neutral interactions, we discovered the importance of (1) accurately treating the differential cross-sections for momentum-exchange and charge-exchange collisions over a large range of neutral background pressures and (2) properly considering commonly overlooked interactions, such as ion-induced electron emission from nearby surfaces and neutral-neutral ionization collisions.

@article{
 title = {Angular, temperature, and impurity effects on secondary electron emission from Ni(110)},
 type = {article},
 year = {2018},
 volume = {124},
 id = {22854270-680f-3039-988a-964e38284e8a},
 created = {2021-07-30T19:25:42.643Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T19:25:42.643Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The secondary electron emission from a temperature-controlled Ni(110) sample was examined for 50-1500 eV electrons impacting at 0°-35°, 50°, and 78°. Measurements showed a non-cosine dependence on an electron incidence angle: the yield has a maximum at 0°, minima at ±12°, and increases at larger angles up to 35°. This trend in angular dependence is characteristic of single crystal materials and is due to increased secondary electron generation when primary electrons are directed along a close-packed direction. For example, compared to polycrystalline nickel, the yield for Ni(110) from primary electrons at 0° (i.e., along the [110] direction) is up to 36% larger. Additionally, secondary electron yields are highly sensitive to incident electron energy (most notably between 0 and 500 eV) and to the presence of adsorbed carbon monoxide [with an up to 25% decrease compared to clean Ni(110)]. However, yields are independent of sample temperature between 300 and 600 K and of exposure to deuterium ions leading to formation of subsurface hydrogen. These results reaffirm the unique secondary electron emission properties of single crystals materials and highlight the importance of crystal orientation. Results are important for plasma-enhanced chemistry applications that utilize Ni(110) catalysts, since larger secondary electron emission may facilitate reactions of adsorbed species.},
 bibtype = {article},
 author = {Patino, M. I. and Wirz, R. E. and Raitses, Y. and Koel, B. E.},
 doi = {10.1063/1.5025344},
 journal = {Journal of Applied Physics},
 number = {9}
}

The secondary electron emission from a temperature-controlled Ni(110) sample was examined for 50-1500 eV electrons impacting at 0°-35°, 50°, and 78°. Measurements showed a non-cosine dependence on an electron incidence angle: the yield has a maximum at 0°, minima at ±12°, and increases at larger angles up to 35°. This trend in angular dependence is characteristic of single crystal materials and is due to increased secondary electron generation when primary electrons are directed along a close-packed direction. For example, compared to polycrystalline nickel, the yield for Ni(110) from primary electrons at 0° (i.e., along the [110] direction) is up to 36% larger. Additionally, secondary electron yields are highly sensitive to incident electron energy (most notably between 0 and 500 eV) and to the presence of adsorbed carbon monoxide [with an up to 25% decrease compared to clean Ni(110)]. However, yields are independent of sample temperature between 300 and 600 K and of exposure to deuterium ions leading to formation of subsurface hydrogen. These results reaffirm the unique secondary electron emission properties of single crystals materials and highlight the importance of crystal orientation. Results are important for plasma-enhanced chemistry applications that utilize Ni(110) catalysts, since larger secondary electron emission may facilitate reactions of adsorbed species.

@article{
 title = {Heat transfer behavior of elemental sulfur for low temperature thermal energy storage applications},
 type = {article},
 year = {2018},
 volume = {127},
 id = {db4366de-1a61-3252-9d06-118b7383ebe8},
 created = {2021-07-30T20:24:47.060Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:24:47.060Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Elemental sulfur provides a low-cost, high-performance thermal storage option for a wide range of applications and over an exceptionally wide range of temperatures (50 °C to over 600 °C). In previous efforts we have shown impressive performance for 200–600 °C, while in this study we examine the low-temperature (50–200 °C) thermal charge and discharge behavior of isochoric sulfur-based storage using a detailed computational model solving for the conjugate heat transfer and solid-liquid phase change dynamics. The model provides excellent agreement with experimental results. We show that sulfur exhibits lower viscosity because of reduction in the chain-length of polymeric sulfur caused by trace amounts of organic substances resulting in attractive charge and discharge performance. The results from the parametric analysis of pipe diameter on the charge and discharge heat transfer characteristics are used to develop a simple, generalized correlation that relates the transient sulfur temperature and liquid fraction evolution as a function of dynamically evolving buoyancy-Fourier number due to the solid-liquid phase change. This solid-liquid buoyancy-Fourier, BFs-l, correlation can be used for effectively designing sulfur-based thermal energy storage systems for transient operation in low temperature applications.},
 bibtype = {article},
 author = {Nithyanandam, K. and Barde, A. and Wirz, R. E.},
 doi = {10.1016/j.ijheatmasstransfer.2018.07.089},
 journal = {International Journal of Heat and Mass Transfer}
}

Elemental sulfur provides a low-cost, high-performance thermal storage option for a wide range of applications and over an exceptionally wide range of temperatures (50 °C to over 600 °C). In previous efforts we have shown impressive performance for 200–600 °C, while in this study we examine the low-temperature (50–200 °C) thermal charge and discharge behavior of isochoric sulfur-based storage using a detailed computational model solving for the conjugate heat transfer and solid-liquid phase change dynamics. The model provides excellent agreement with experimental results. We show that sulfur exhibits lower viscosity because of reduction in the chain-length of polymeric sulfur caused by trace amounts of organic substances resulting in attractive charge and discharge performance. The results from the parametric analysis of pipe diameter on the charge and discharge heat transfer characteristics are used to develop a simple, generalized correlation that relates the transient sulfur temperature and liquid fraction evolution as a function of dynamically evolving buoyancy-Fourier number due to the solid-liquid phase change. This solid-liquid buoyancy-Fourier, BFs-l, correlation can be used for effectively designing sulfur-based thermal energy storage systems for transient operation in low temperature applications.

@article{
 title = {Electron emission from carbon velvet due to incident xenon ions},
 type = {article},
 year = {2018},
 volume = {113},
 id = {7ad68a41-d088-3ab3-8805-c8c4a9fd791a},
 created = {2021-07-30T20:24:49.065Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:24:49.065Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {We present measurements of the ion-induced electron emission from carbon velvet. The results from carbon velvet with high aspect ratio vertical fibers (6.8 μm diameter and 2.6 mm length) show a more than 60% reduction in ion-induced electron emission for normal incident xenon ions over the entire ion incident energy investigated (i.e., 500-2000 eV) when compared to graphite. This is important for plasma-facing surfaces that are exposed to large fluxes of energetic ions, such as beam dumps and chamber walls used to control facility effects in plasma-thruster ground tests.},
 bibtype = {article},
 author = {Patino, M. I. and Wirz, R. E.},
 doi = {10.1063/1.5037200},
 journal = {Applied Physics Letters},
 number = {4}
}

We present measurements of the ion-induced electron emission from carbon velvet. The results from carbon velvet with high aspect ratio vertical fibers (6.8 μm diameter and 2.6 mm length) show a more than 60% reduction in ion-induced electron emission for normal incident xenon ions over the entire ion incident energy investigated (i.e., 500-2000 eV) when compared to graphite. This is important for plasma-facing surfaces that are exposed to large fluxes of energetic ions, such as beam dumps and chamber walls used to control facility effects in plasma-thruster ground tests.

@article{
 title = {A dimensionless model for transient turbulent natural convection in isochoric vertical thermal energy storage tubes},
 type = {article},
 year = {2018},
 volume = {10},
 id = {98552d0c-8b48-3ab0-bfae-037a6973802f},
 created = {2021-07-30T20:24:51.769Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:24:51.769Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {In this study, turbulent natural convection heat transfer during the charge cycle of an isochoric vertically oriented thermal energy storage (TES) tube is studied computationally and analytically. The storage fluids considered in this study (supercritical CO2 and liquid toluene) cover a wide range of Rayleigh numbers. The volume of the storage tube is constant and the thermal storage happens in an isochoric process. A computational model was utilized to study turbulent natural convection during the charge cycle. The computational results were further utilized to develop a conceptual and dimensionless model that views the thermal storage process as a hot boundary layer that rises along the tube wall and falls in the center to replace the cold fluid in the core. The dimensionless model predicts that the dimensionless mean temperature of the storage fluid and average Nusselt number of natural convection are functions of L/D ratio, Rayleigh number, and Fourier number that are combined to form a buoyancy-Fourier number.},
 bibtype = {article},
 author = {Lakeh, Reza Baghaei and Wirz, Richard E. and Kavehpour, Pirouz and Lavine, Adrienne S.},
 doi = {10.1115/1.4038587},
 journal = {Journal of Thermal Science and Engineering Applications},
 number = {3}
}

In this study, turbulent natural convection heat transfer during the charge cycle of an isochoric vertically oriented thermal energy storage (TES) tube is studied computationally and analytically. The storage fluids considered in this study (supercritical CO2 and liquid toluene) cover a wide range of Rayleigh numbers. The volume of the storage tube is constant and the thermal storage happens in an isochoric process. A computational model was utilized to study turbulent natural convection during the charge cycle. The computational results were further utilized to develop a conceptual and dimensionless model that views the thermal storage process as a hot boundary layer that rises along the tube wall and falls in the center to replace the cold fluid in the core. The dimensionless model predicts that the dimensionless mean temperature of the storage fluid and average Nusselt number of natural convection are functions of L/D ratio, Rayleigh number, and Fourier number that are combined to form a buoyancy-Fourier number.

@article{
 title = {Charge and discharge behavior of elemental sulfur in isochoric high temperature thermal energy storage systems},
 type = {article},
 year = {2018},
 volume = {214},
 id = {ecf36487-fd0c-35cd-848b-ca03a08cee33},
 created = {2021-07-30T20:24:53.604Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:24:53.604Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Thermal energy storage with elemental sulfur is a low-cost alternative to molten salts for many medium to high-temperature energy applications (200–600 °C). In this effort, by examining elemental sulfur stored isochorically inside isolated pipes, we find that sulfur provides attractive charge/discharge performance since it operates in the liquid-vapor regime at the temperatures relevant to many important applications, such as combined heat and power (CHP) plants and concentrating solar power (CSP) plants with advanced power cycle systems. The isolated pipe configuration is relevant to shell-and-tube thermal battery applications where the heat transfer fluid flows over the storage pipes through the shell. We analyze the transient charge and discharge behavior of sulfur inside the pipes using detailed computational modeling of the complex conjugate heat transfer and fluid flow phenomena. The computational model is validated against experiments of a single tube with well-defined temperature boundary conditions and internal temperature measurements. The model results evaluate the influence of pipe diameter on charge and discharge times, heat transfer rate, and Nusselt number due to buoyancy driven convection currents. Depending on the Rayleigh number (pipe diameter), the average Nusselt number obtained for discharge is 3–14 times higher than proposed solid-liquid phase change technologies based on molten salt, which are limited in their performance due to conduction based solidification and low thermal conductivity. The results show competing trade-offs between increase in heat transfer coefficient, thermal energy stored in sulfur, and increase in charge and discharge time with increase in pipe diameter. A preferred pipe diameter can be determined for target applications based on their requirements and these competing trade-offs. A validated fundamental correlation for Nusselt number as a function of Rayleigh number for charge and discharge is developed that can be used to design the sulfur-based thermal storage system for transient operation.},
 bibtype = {article},
 author = {Nithyanandam, K. and Barde, A. and Lakeh, R. Baghaei and Wirz, R. E.},
 doi = {10.1016/j.apenergy.2017.12.121},
 journal = {Applied Energy}
}

Thermal energy storage with elemental sulfur is a low-cost alternative to molten salts for many medium to high-temperature energy applications (200–600 °C). In this effort, by examining elemental sulfur stored isochorically inside isolated pipes, we find that sulfur provides attractive charge/discharge performance since it operates in the liquid-vapor regime at the temperatures relevant to many important applications, such as combined heat and power (CHP) plants and concentrating solar power (CSP) plants with advanced power cycle systems. The isolated pipe configuration is relevant to shell-and-tube thermal battery applications where the heat transfer fluid flows over the storage pipes through the shell. We analyze the transient charge and discharge behavior of sulfur inside the pipes using detailed computational modeling of the complex conjugate heat transfer and fluid flow phenomena. The computational model is validated against experiments of a single tube with well-defined temperature boundary conditions and internal temperature measurements. The model results evaluate the influence of pipe diameter on charge and discharge times, heat transfer rate, and Nusselt number due to buoyancy driven convection currents. Depending on the Rayleigh number (pipe diameter), the average Nusselt number obtained for discharge is 3–14 times higher than proposed solid-liquid phase change technologies based on molten salt, which are limited in their performance due to conduction based solidification and low thermal conductivity. The results show competing trade-offs between increase in heat transfer coefficient, thermal energy stored in sulfur, and increase in charge and discharge time with increase in pipe diameter. A preferred pipe diameter can be determined for target applications based on their requirements and these competing trade-offs. A validated fundamental correlation for Nusselt number as a function of Rayleigh number for charge and discharge is developed that can be used to design the sulfur-based thermal storage system for transient operation.

@article{
 title = {Secondary electron emission from textured surfaces},
 type = {article},
 year = {2018},
 volume = {51},
 id = {ef0e33f5-9378-3e0b-85a4-10231c85afd1},
 created = {2021-07-30T20:24:55.598Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:24:55.598Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {In this work, a Monte Carlo model is used to investigate electron induced secondary electron emission for varying effects of complex surfaces by using simple geometric constructs. Geometries used in the model include: vertical fibers for velvet-like surfaces, tapered pillars for carpet-like surfaces, and a cage-like configuration of interlaced horizontal and vertical fibers for nano-structured fuzz. The model accurately captures the secondary electron emission yield dependence on incidence angle. The model shows that unlike other structured surfaces previously studied, tungsten fuzz exhibits secondary electron emission yield that is independent of primary electron incidence angle, due to the prevalence of horizontally-oriented fibers in the fuzz geometry. This is confirmed with new data presented herein of the secondary electron emission yield of tungsten fuzz at incidence angles from 0-60°.},
 bibtype = {article},
 author = {Huerta, C. E. and Patino, M. I. and Wirz, R. E.},
 doi = {10.1088/1361-6463/aab1ac},
 journal = {Journal of Physics D: Applied Physics},
 number = {14}
}

In this work, a Monte Carlo model is used to investigate electron induced secondary electron emission for varying effects of complex surfaces by using simple geometric constructs. Geometries used in the model include: vertical fibers for velvet-like surfaces, tapered pillars for carpet-like surfaces, and a cage-like configuration of interlaced horizontal and vertical fibers for nano-structured fuzz. The model accurately captures the secondary electron emission yield dependence on incidence angle. The model shows that unlike other structured surfaces previously studied, tungsten fuzz exhibits secondary electron emission yield that is independent of primary electron incidence angle, due to the prevalence of horizontally-oriented fibers in the fuzz geometry. This is confirmed with new data presented herein of the secondary electron emission yield of tungsten fuzz at incidence angles from 0-60°.

@article{
 title = {Sulfur-based thermal energy storage system using intermodal containment: Design and performance analysis},
 type = {article},
 year = {2018},
 volume = {128},
 id = {978ce3ca-32a5-3ae5-b042-ca512dc8c327},
 created = {2021-07-30T20:25:00.907Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:00.907Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Thermal energy storage (TES) is an important energy storage technology that can be coupled to intermittent energy sources to improve system dispatchability. Elemental sulfur is a promising candidate storage fluid for high temperature TES systems due to its high energy density, moderate vapor pressure, high thermal stability, and low cost. This study uses a transient, two-dimensional numerical model to investigate the design and performance of a thermal energy storage (TES) system that uses sulfur stored isochorically in an intermodal shell and tube thermal battery configuration. Parametric analyses of key design and operating parameters show that there is a preferred tube diameter based on the competing influence of system-level energy storage utilization, exergetic efficiency, and cost. The results show that designs with smaller tube dimensions in the range of 2″ NPS to 4″ NPS provide exergetic efficiencies close to 95% while tube dimensions in the range of 4″ NPS to 8″ NPS meet the Department of Energy cost target of $15/kWh with costs being as low as $8.41/kWh. Finally, a table of preferred designs that meet the DOE cost goals is presented to help guide future design and experimentation efforts.},
 bibtype = {article},
 author = {Shinn, M. and Nithyanandam, K. and Barde, A. and Wirz, R. E.},
 doi = {10.1016/j.applthermaleng.2017.08.167},
 journal = {Applied Thermal Engineering}
}

Thermal energy storage (TES) is an important energy storage technology that can be coupled to intermittent energy sources to improve system dispatchability. Elemental sulfur is a promising candidate storage fluid for high temperature TES systems due to its high energy density, moderate vapor pressure, high thermal stability, and low cost. This study uses a transient, two-dimensional numerical model to investigate the design and performance of a thermal energy storage (TES) system that uses sulfur stored isochorically in an intermodal shell and tube thermal battery configuration. Parametric analyses of key design and operating parameters show that there is a preferred tube diameter based on the competing influence of system-level energy storage utilization, exergetic efficiency, and cost. The results show that designs with smaller tube dimensions in the range of 2″ NPS to 4″ NPS provide exergetic efficiencies close to 95% while tube dimensions in the range of 4″ NPS to 8″ NPS meet the Department of Energy cost target of $15/kWh with costs being as low as $8.41/kWh. Finally, a table of preferred designs that meet the DOE cost goals is presented to help guide future design and experimentation efforts.

@article{
 title = {Axial Ring-Cusp Hybrid (ARCH) plasma discharge: an approach to highly efficient miniature-scale ion sources},
 type = {article},
 year = {2018},
 volume = {27},
 id = {4334f9d8-454d-3288-aaf3-bd3d49421caf},
 created = {2021-07-30T20:25:02.169Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:02.169Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The miniaturization of conventional direct-current ion sources is predominantly restricted by efficiency limitations associated with the increased surface area-to-volume ratio of smaller-scale discharge chambers - reducing the effective confinement length of the high-energy 'primary' electrons that is necessary for efficient plasma generation. The Axial Ring-Cusp Hybrid (ARCH) plasma discharge addresses this scaling limitation by using a new approach that combines magnetic and electrostatic confinement to decouple the primary and plasma electrons loss mechanisms. Simulated ion thruster performance measurements show that the ARCH discharge may be capable of achieving a discharge loss and a propellant mass utilization of 175 eV/ion and 0.87, respectively. These estimates are supported by full internal maps of the plasma properties, including the electron energy distribution function, inside the discharge chamber. The measurements show highly effective confinement of the primary electrons, high average plasma electron temperatures of ∼5 eV, and low plasma sheath potential relative to the anode - attributes generally found only in efficient conventional-scale discharges with good overall plasma confinement. As such, the new ARCH discharge design approach may allow miniature ion thrusters to achieve the performance and efficiency levels similar to those of highly efficient conventional ion thrusters.},
 bibtype = {article},
 author = {Dankongkakul, Ben and Wirz, Richard E.},
 doi = {10.1088/1361-6595/aae63c},
 journal = {Plasma Sources Science and Technology},
 number = {12}
}

The miniaturization of conventional direct-current ion sources is predominantly restricted by efficiency limitations associated with the increased surface area-to-volume ratio of smaller-scale discharge chambers - reducing the effective confinement length of the high-energy 'primary' electrons that is necessary for efficient plasma generation. The Axial Ring-Cusp Hybrid (ARCH) plasma discharge addresses this scaling limitation by using a new approach that combines magnetic and electrostatic confinement to decouple the primary and plasma electrons loss mechanisms. Simulated ion thruster performance measurements show that the ARCH discharge may be capable of achieving a discharge loss and a propellant mass utilization of 175 eV/ion and 0.87, respectively. These estimates are supported by full internal maps of the plasma properties, including the electron energy distribution function, inside the discharge chamber. The measurements show highly effective confinement of the primary electrons, high average plasma electron temperatures of ∼5 eV, and low plasma sheath potential relative to the anode - attributes generally found only in efficient conventional-scale discharges with good overall plasma confinement. As such, the new ARCH discharge design approach may allow miniature ion thrusters to achieve the performance and efficiency levels similar to those of highly efficient conventional ion thrusters.

@inproceedings{
 title = {Computational modeling of an axial ring cusp hybrid discharge for the MiXI thruster},
 type = {inproceedings},
 year = {2018},
 id = {fd1e8446-b439-311f-877e-cf1b8394520c},
 created = {2021-07-30T22:42:48.002Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:48.002Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The ion thruster computational model DC-ION is applied to the Axial Ring Cusp Hybrid (ARCH) discharge in order to study the primary electron and plasma confinement behavior. The DC-ION results are compared to internally mapped Langmuir probe data, showing qualitatively good agreement in predicting the plasma structure. We present several thruster sensitivity studies including backplate bias, neutral injection variation, outer magnet displacement, and filament vs. hollow cathode emission. Results show that electrostatic confinement at the backplate significantly improves overall discharge performance. Changing the neutral injection point from the backplate to 45◦ off-normal near the grids results in similar plasma properties and performance. The outer magnet sensitivity study reveals that a radial displacement of 1 to 2 mm highly improves the beam profile with a flatness parameter of up to 0.76. The hollow cathode simulations show that centerline placement results in highly peaked beam profiles, and that a radially offset hollow cathode flush with the backplate produces desirable plasma properties and performance. These studies provide actionable recommendations for further improvements to the MiXI(ARCH) thruster discharge.},
 bibtype = {inproceedings},
 author = {Li, Gary Z. and Huerta, Cesar E. and Wirz, Richard E.},
 doi = {10.2514/6.2018-4814},
 booktitle = {2018 Joint Propulsion Conference}
}

The ion thruster computational model DC-ION is applied to the Axial Ring Cusp Hybrid (ARCH) discharge in order to study the primary electron and plasma confinement behavior. The DC-ION results are compared to internally mapped Langmuir probe data, showing qualitatively good agreement in predicting the plasma structure. We present several thruster sensitivity studies including backplate bias, neutral injection variation, outer magnet displacement, and filament vs. hollow cathode emission. Results show that electrostatic confinement at the backplate significantly improves overall discharge performance. Changing the neutral injection point from the backplate to 45◦ off-normal near the grids results in similar plasma properties and performance. The outer magnet sensitivity study reveals that a radial displacement of 1 to 2 mm highly improves the beam profile with a flatness parameter of up to 0.76. The hollow cathode simulations show that centerline placement results in highly peaked beam profiles, and that a radially offset hollow cathode flush with the backplate produces desirable plasma properties and performance. These studies provide actionable recommendations for further improvements to the MiXI(ARCH) thruster discharge.

@inproceedings{
 title = {Performance testing and development of the MiXI thruster with the ARCH discharge},
 type = {inproceedings},
 year = {2018},
 id = {9c4a4e38-0318-3236-8f12-e8cddb1b6c06},
 created = {2021-07-30T22:42:48.610Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:48.610Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The Miniature Xenon Ion thruster with the Axial Ring-Cusp Hybrid discharge was operated with beam extraction for the first time at 1 kV. When cold, the thruster achieved 59% total efficiency at 23.7 mA beam current, corresponding to a discharge loss of 226 W/A and propellant utilization of 0.72. This is an improvement on previous results with an increase in total efficiency of 3% and a decrease in minimum efficient power level of 30%. Thruster efficiency was observed to increase with increasing flow rate and decrease with increasing temperature up to thermal steady state. At thermal steady state, the thruster anode reached ~320 °C due to thermal isolation of the setup. Reducing the anode aspect ratio from 0.5 to 0.4 increased thermal steady state efficiency from 46% to 57%, but required slow ramping of beam voltage and was limited to above 0.5 sccm. Through this effort, the MiXi thruster’s highest achievable total efficiency has been increased, and several mechanisms for further improved efficiency have been identified.},
 bibtype = {inproceedings},
 author = {Samples, Stephen A. and Li, Gary Z. and Wirz, Richard E.},
 doi = {10.2514/6.2018-4648},
 booktitle = {2018 Joint Propulsion Conference}
}

The Miniature Xenon Ion thruster with the Axial Ring-Cusp Hybrid discharge was operated with beam extraction for the first time at 1 kV. When cold, the thruster achieved 59% total efficiency at 23.7 mA beam current, corresponding to a discharge loss of 226 W/A and propellant utilization of 0.72. This is an improvement on previous results with an increase in total efficiency of 3% and a decrease in minimum efficient power level of 30%. Thruster efficiency was observed to increase with increasing flow rate and decrease with increasing temperature up to thermal steady state. At thermal steady state, the thruster anode reached ~320 °C due to thermal isolation of the setup. Reducing the anode aspect ratio from 0.5 to 0.4 increased thermal steady state efficiency from 46% to 57%, but required slow ramping of beam voltage and was limited to above 0.5 sccm. Through this effort, the MiXi thruster’s highest achievable total efficiency has been increased, and several mechanisms for further improved efficiency have been identified.

@inproceedings{
 title = {DC-ION model validation and convergence studies},
 type = {inproceedings},
 year = {2018},
 id = {ffcd44a6-ccb3-3ce1-b0b8-587429edfd8d},
 created = {2021-07-30T22:42:49.349Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:49.349Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The DC-ION model is used to design and predict the performance of ion thrusters. For this effort, improvements to DC-ION’s memory management, magnetic field sub-model, and linear algebra routines are investigated. These modifications significantly increase the allowable resolution of primary electron and plasma density results. For particle tracking of primary electrons, the Wirz method is shown to provide significant accuracy and computational efficiency benefits over the commonly-used Boris method, most notably for the high magnetic field gradients used in modern ion thrusters. The model is validated with experimental plasma maps of the MiXI thruster, showing good overall agreement. Convergence studies are also undertaken to ensure modeling precision, specifically with regards to the mesh resolution and number of primary electrons tracked.},
 bibtype = {inproceedings},
 author = {Huerta, Cesar E. and Wirz, Richard E.},
 doi = {10.2514/6.2018-4813},
 booktitle = {2018 Joint Propulsion Conference}
}

The DC-ION model is used to design and predict the performance of ion thrusters. For this effort, improvements to DC-ION’s memory management, magnetic field sub-model, and linear algebra routines are investigated. These modifications significantly increase the allowable resolution of primary electron and plasma density results. For particle tracking of primary electrons, the Wirz method is shown to provide significant accuracy and computational efficiency benefits over the commonly-used Boris method, most notably for the high magnetic field gradients used in modern ion thrusters. The model is validated with experimental plasma maps of the MiXI thruster, showing good overall agreement. Convergence studies are also undertaken to ensure modeling precision, specifically with regards to the mesh resolution and number of primary electrons tracked.

@article{
 title = {Fluence-dependent sputtering yield of micro-architectured materials},
 type = {article},
 year = {2017},
 volume = {407},
 id = {567a7fd9-c8ac-3750-a9d1-132dde3ed65d},
 created = {2021-07-30T18:04:26.664Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T18:04:26.664Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {We present an experimental examination of the relationship between the surface morphology of Mo and its instantaneous sputtering rate as function of low-energy plasma ion fluence. We quantify the dynamic evolution of nano/micro features of surfaces with built-in architecture, and the corresponding variation in the sputtering yield. Ballistic deposition of sputtered atoms as a result of geometric re-trapping is observed, and re-growth of surface layers is confirmed. This provides a self-healing mechanism of micro-architectured surfaces during plasma exposure. A variety of material characterization techniques are used to show that the sputtering yield is not a fundamental property, but that it is quantitatively related to the initial surface architecture and to its subsequent evolution. The sputtering yield of textured molybdenum samples exposed to 300 eV Ar plasma is roughly 1/2 of the corresponding value for flat samples, and increases with ion fluence. Mo samples exhibited a sputtering yield initially as low as 0.22 ± 5%, converging to 0.4 ± 5% at high fluence. The sputtering yield exhibits a transient behavior as function of the integrated ion fluence, reaching a steady-state value that is independent of initial surface conditions. A phenomenological model is proposed to explain the observed transient sputtering phenomenon, and to show that the saturation fluence is solely determined by the initial surface roughness.},
 bibtype = {article},
 author = {Matthes, Christopher S.R. and Ghoniem, Nasr M. and Li, Gary Z. and Matlock, Taylor S. and Goebel, Dan M. and Dodson, Chris A. and Wirz, Richard E.},
 doi = {10.1016/j.apsusc.2017.02.140},
 journal = {Applied Surface Science}
}

We present an experimental examination of the relationship between the surface morphology of Mo and its instantaneous sputtering rate as function of low-energy plasma ion fluence. We quantify the dynamic evolution of nano/micro features of surfaces with built-in architecture, and the corresponding variation in the sputtering yield. Ballistic deposition of sputtered atoms as a result of geometric re-trapping is observed, and re-growth of surface layers is confirmed. This provides a self-healing mechanism of micro-architectured surfaces during plasma exposure. A variety of material characterization techniques are used to show that the sputtering yield is not a fundamental property, but that it is quantitatively related to the initial surface architecture and to its subsequent evolution. The sputtering yield of textured molybdenum samples exposed to 300 eV Ar plasma is roughly 1/2 of the corresponding value for flat samples, and increases with ion fluence. Mo samples exhibited a sputtering yield initially as low as 0.22 ± 5%, converging to 0.4 ± 5% at high fluence. The sputtering yield exhibits a transient behavior as function of the integrated ion fluence, reaching a steady-state value that is independent of initial surface conditions. A phenomenological model is proposed to explain the observed transient sputtering phenomenon, and to show that the saturation fluence is solely determined by the initial surface roughness.

@article{
 title = {Propagation of ion acoustic wave energy in the plume of a high-current LaB6 hollow cathode},
 type = {article},
 year = {2017},
 volume = {96},
 id = {05356c1c-5f60-3b2d-a074-bca53d9c0dd8},
 created = {2021-07-30T20:25:03.854Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:03.854Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A frequency-averaged quasilinear model is derived and experimentally validated for the evolution of ion acoustic turbulence (IAT) along the centerline of a 100-A class, LaB6 hollow cathode. Probe-based diagnostics and a laser induced fluorescence system are employed to measure the properties of both the turbulence and the background plasma parameters as they vary spatially in the cathode plume. It is shown that for the three discharge currents investigated, 100 A, 130 A, and 160 A, the spatial growth of the total energy density of the IAT in the near field of the cathode plume is exponential and agrees quantitatively with the predicted growth rates from the quasilinear formulation. However, in the downstream region of the cathode plume, the growth of IAT energy saturates at a level that is commensurate with the Sagdeev limit. The experimental validation of the quasilinear model for IAT growth and its limitations are discussed in the context of numerical efforts to describe self-consistently the plasma processes in the hollow cathode plume.},
 bibtype = {article},
 author = {Jorns, Benjamin A. and Dodson, Christoper and Goebel, Dan M. and Wirz, Richard},
 doi = {10.1103/PhysRevE.96.023208},
 journal = {Physical Review E},
 number = {2}
}

A frequency-averaged quasilinear model is derived and experimentally validated for the evolution of ion acoustic turbulence (IAT) along the centerline of a 100-A class, LaB6 hollow cathode. Probe-based diagnostics and a laser induced fluorescence system are employed to measure the properties of both the turbulence and the background plasma parameters as they vary spatially in the cathode plume. It is shown that for the three discharge currents investigated, 100 A, 130 A, and 160 A, the spatial growth of the total energy density of the IAT in the near field of the cathode plume is exponential and agrees quantitatively with the predicted growth rates from the quasilinear formulation. However, in the downstream region of the cathode plume, the growth of IAT energy saturates at a level that is commensurate with the Sagdeev limit. The experimental validation of the quasilinear model for IAT growth and its limitations are discussed in the context of numerical efforts to describe self-consistently the plasma processes in the hollow cathode plume.

@article{
 title = {Miniature ion thruster ring-cusp discharge performance and behavior},
 type = {article},
 year = {2017},
 volume = {122},
 id = {15df995a-018d-3d91-a581-8444841e5cdf},
 created = {2021-07-30T20:25:05.593Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:05.593Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Miniature ion thrusters are an attractive option for a wide range of space missions due to their low power levels and high specific impulse. Thrusters using ring-cusp plasma discharges promise the highest performance, but are still limited by the challenges of efficiently maintaining a plasma discharge at such small scales (typically 1-3 cm diameter). This effort significantly advances the understanding of miniature-scale plasma discharges by comparing the performance and xenon plasma confinement behavior for 3-ring, 4-ring, and 5-ring cusp by using the 3 cm Miniature Xenon Ion thruster as a modifiable platform. By measuring and comparing the plasma and electron energy distribution maps throughout the discharge, we find that miniature ring-cusp plasma behavior is dominated by the high magnetic fields from the cusps; this can lead to high loss rates of high-energy primary electrons to the anode walls. However, the primary electron confinement was shown to considerably improve by imposing an axial magnetic field or by using cathode terminating cusps, which led to increases in the discharge efficiency of up to 50%. Even though these design modifications still present some challenges, they show promise to bypassing what were previously seen as inherent limitations to ring-cusp discharge efficiency at miniature scales.},
 bibtype = {article},
 author = {Dankongkakul, Ben and Wirz, Richard E.},
 doi = {10.1063/1.4995638},
 journal = {Journal of Applied Physics},
 number = {24}
}

Miniature ion thrusters are an attractive option for a wide range of space missions due to their low power levels and high specific impulse. Thrusters using ring-cusp plasma discharges promise the highest performance, but are still limited by the challenges of efficiently maintaining a plasma discharge at such small scales (typically 1-3 cm diameter). This effort significantly advances the understanding of miniature-scale plasma discharges by comparing the performance and xenon plasma confinement behavior for 3-ring, 4-ring, and 5-ring cusp by using the 3 cm Miniature Xenon Ion thruster as a modifiable platform. By measuring and comparing the plasma and electron energy distribution maps throughout the discharge, we find that miniature ring-cusp plasma behavior is dominated by the high magnetic fields from the cusps; this can lead to high loss rates of high-energy primary electrons to the anode walls. However, the primary electron confinement was shown to considerably improve by imposing an axial magnetic field or by using cathode terminating cusps, which led to increases in the discharge efficiency of up to 50%. Even though these design modifications still present some challenges, they show promise to bypassing what were previously seen as inherent limitations to ring-cusp discharge efficiency at miniature scales.

@article{
 title = {Performance analysis of a low-power magnetically shielded hall thruster: Experiments},
 type = {article},
 year = {2017},
 volume = {33},
 id = {5b32ae65-d0b4-338c-9a11-2249bd75241d},
 created = {2021-07-30T20:25:07.989Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:07.989Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The successful application of a fully shielding magnetic field topology in a low-power Hall thruster is demonstrated through the testing of the MaSMi-60 Hall thruster (an improved variant of the original Magnetically Shielded Miniature Hall thruster). The device was operated at discharge powers from 160 to 750 W at discharge voltages ranging from 200 to 400 V. Several techniques were used to determine the effectiveness of magnetic shielding achieved by the MaSMi-60 and to estimate the reduction in discharge channel erosion rate enabled by the shielding field topology. This ultimately suggested an improvement in discharge channel life by a factor of at least 10 times, and likely greater than 100 times, when compared to unshielded devices. Thruster performance, measured both directly by a thrust stand and indirectly by plume diagnostics, was lower than expected when compared to results from high-power magnetically shielded Hall thrusters. However, the plume diagnostic measurements enabled the identification of the primary causes for the MaSMi-60's moderate performance.},
 bibtype = {article},
 author = {Conversano, Ryan W. and Goebel, Dan M. and Hofer, Richard R. and Mikellides, Ioannis G. and Wirz, Richard E.},
 doi = {10.2514/1.B36230},
 journal = {Journal of Propulsion and Power},
 number = {4}
}

The successful application of a fully shielding magnetic field topology in a low-power Hall thruster is demonstrated through the testing of the MaSMi-60 Hall thruster (an improved variant of the original Magnetically Shielded Miniature Hall thruster). The device was operated at discharge powers from 160 to 750 W at discharge voltages ranging from 200 to 400 V. Several techniques were used to determine the effectiveness of magnetic shielding achieved by the MaSMi-60 and to estimate the reduction in discharge channel erosion rate enabled by the shielding field topology. This ultimately suggested an improvement in discharge channel life by a factor of at least 10 times, and likely greater than 100 times, when compared to unshielded devices. Thruster performance, measured both directly by a thrust stand and indirectly by plume diagnostics, was lower than expected when compared to results from high-power magnetically shielded Hall thrusters. However, the plume diagnostic measurements enabled the identification of the primary causes for the MaSMi-60's moderate performance.

@article{
 title = {Experimental measurements of surface damage and residual stresses in micro-engineered plasma facing materials},
 type = {article},
 year = {2017},
 volume = {486},
 id = {4b2dd4a9-8047-3b99-b87d-5edc49bb5daa},
 created = {2021-07-30T20:25:09.714Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:09.714Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The thermomechanical damage and residual stresses in plasma-facing materials operating at high heat flux are experimentally investigated. Materials with micro-surfaces are found to be more resilient, when exposed to cyclic high heat flux generated by an arc-jet plasma. An experimental facility, dedicated to High Energy Flux Testing (HEFTY), is developed for testing cyclic heat flux in excess of 10 MW/m2. We show that plastic deformation and subsequent fracture of the surface can be controlled by sample cooling. We demonstrate that W surfaces with micro-pillar type surface architecture have significantly reduced residual thermal stresses after plasma exposure, as compared to those with flat surfaces. X-ray diffraction (XRD) spectra of the W-(110) peak reveal that broadening of the Full Width at Half Maximum (FWHM) for micro-engineered samples is substantially smaller than corresponding flat surfaces. Spectral shifts of XRD signals indicate that residual stresses due to plasma exposure of micro-engineered surfaces build up in the first few cycles of exposure. Subsequent cyclic plasma heat loading is shown to anneal out most of the built-up residual stresses in micro-engineered surfaces. These findings are consistent with relaxation of residual thermal stresses in surfaces with micro-engineered features. The initial residual stress state of highly polished flat W samples is compressive (≈ -1.3 GPa). After exposure to 50 plasma cycles, the surface stress relaxes to −1.0 GPa. Micro-engineered samples exposed to the same thermal cycling show that the initial residual stress state is compressive at (- 250 MPa), and remains largely unchanged after plasma exposure.},
 bibtype = {article},
 author = {Rivera, David and Wirz, Richard E. and Ghoniem, Nasr M.},
 doi = {10.1016/j.jnucmat.2016.12.035},
 journal = {Journal of Nuclear Materials}
}

The thermomechanical damage and residual stresses in plasma-facing materials operating at high heat flux are experimentally investigated. Materials with micro-surfaces are found to be more resilient, when exposed to cyclic high heat flux generated by an arc-jet plasma. An experimental facility, dedicated to High Energy Flux Testing (HEFTY), is developed for testing cyclic heat flux in excess of 10 MW/m2. We show that plastic deformation and subsequent fracture of the surface can be controlled by sample cooling. We demonstrate that W surfaces with micro-pillar type surface architecture have significantly reduced residual thermal stresses after plasma exposure, as compared to those with flat surfaces. X-ray diffraction (XRD) spectra of the W-(110) peak reveal that broadening of the Full Width at Half Maximum (FWHM) for micro-engineered samples is substantially smaller than corresponding flat surfaces. Spectral shifts of XRD signals indicate that residual stresses due to plasma exposure of micro-engineered surfaces build up in the first few cycles of exposure. Subsequent cyclic plasma heat loading is shown to anneal out most of the built-up residual stresses in micro-engineered surfaces. These findings are consistent with relaxation of residual thermal stresses in surfaces with micro-engineered features. The initial residual stress state of highly polished flat W samples is compressive (≈ -1.3 GPa). After exposure to 50 plasma cycles, the surface stress relaxes to −1.0 GPa. Micro-engineered samples exposed to the same thermal cycling show that the initial residual stress state is compressive at (- 250 MPa), and remains largely unchanged after plasma exposure.

@article{
 title = {In situ plasma sputtering and angular distribution measurements for structured molybdenum surfaces},
 type = {article},
 year = {2017},
 volume = {26},
 id = {b6dd76ae-067a-3300-8cb9-6474a7bf11d4},
 created = {2021-07-30T20:25:11.547Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:11.547Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {We present in situ sputtering yield measurements of the time-dependent erosion of flat and micro-architectured molybdenum samples in a plasma environment. The measurements are performed using the plasma interactions (Pi) Facility at UCLA, which focuses a magnetized hollow cathode plasma to a material target with an exposure diameter of approximately 1.5 cm. During plasma exposure, a scanning quartz crystal microbalance (QCM) provides angular sputtering profiles that are integrated to estimate the total sputtering yield. This technique is validated to within the scatter of previous experimental data for a planar molybdenum target exposed to argon ion energies from 100 to 300 eV. The QCM is then used to obtain in situ measurements during a 17 h exposure of a micro-architectured-surface molybdenum sample to 300 eV incident argon ions. The time-dependent angular sputtering profile is shown to deviate from classical planar profiles, demonstrating the unique temporal and spatial sputtering effects of micro-architectured materials. Notably, the sputtering yield for the micro-architectured sample is initially much less than that for planar molybdenum, but then gradually asymptotes to the value for planar molybdenum after approximately 10 h as the surface features are eroded away.},
 bibtype = {article},
 author = {Li, Gary Z. and Matlock, Taylor S. and Goebel, Dan M. and Dodson, Christopher A. and Matthes, Christopher S.R. and Ghoniem, Nasr M. and Wirz, Richard E.},
 doi = {10.1088/1361-6595/aa6a7d},
 journal = {Plasma Sources Science and Technology},
 number = {6}
}

We present in situ sputtering yield measurements of the time-dependent erosion of flat and micro-architectured molybdenum samples in a plasma environment. The measurements are performed using the plasma interactions (Pi) Facility at UCLA, which focuses a magnetized hollow cathode plasma to a material target with an exposure diameter of approximately 1.5 cm. During plasma exposure, a scanning quartz crystal microbalance (QCM) provides angular sputtering profiles that are integrated to estimate the total sputtering yield. This technique is validated to within the scatter of previous experimental data for a planar molybdenum target exposed to argon ion energies from 100 to 300 eV. The QCM is then used to obtain in situ measurements during a 17 h exposure of a micro-architectured-surface molybdenum sample to 300 eV incident argon ions. The time-dependent angular sputtering profile is shown to deviate from classical planar profiles, demonstrating the unique temporal and spatial sputtering effects of micro-architectured materials. Notably, the sputtering yield for the micro-architectured sample is initially much less than that for planar molybdenum, but then gradually asymptotes to the value for planar molybdenum after approximately 10 h as the surface features are eroded away.

@article{
 title = {Ion Acoustic Wave Propagation and Heating in a High-Current Hollow Cathode Plume},
 type = {article},
 year = {2017},
 id = {1f06e4ee-77e0-3ef4-a66f-296af219ef19},
 created = {2021-07-30T22:42:49.913Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:49.913Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {High current hollow cathodes used for electric propulsion have been found to have en-ergetic ions in the plume caused by a mechanism still not fully understood. These anoma-lously energetic ions have been shown to erode cathode keepers and pose a risk to cathodes used for long-duration missions. This work builds on previous work investigating the effects of ion acoustic turbulence (IAT) in the plume and its effects on the ion population. Laser-induced fluorescence (LIF) and plasma probes were used to measure several steady-state and fluctuating plasma properties related to ion acoustic waves. A preliminary analysis of these data show the ion distribution function along the the centerline to be heated primar-ily in the axial direction resulting in ion temperature anisotropy. A bimodal ion velocity distribution function was observed with the axial LIF beam near the anode, consisting of slow and fast populations. It was not possible to detect two distinct populations using the radial LIF measurements. The fast population was found to accelerate with distance from the keeper orifice whereas the slow ion population remained nearly stationary, and is be-lieved to be caused by ion generation in the plume through ionization or charge-exchange collisions. Wave energy and the ion acoustic wave direction were measured and show an increase in wave energy downstream, highly concentrated along the centerline. The ion acoustic waves were found to propagate outward from the orifice and are approximately collinear with the drift direction of the fast ion population.},
 bibtype = {article},
 author = {Dodson, Christopher A and Jorns, Benjamin A and Wirz, Richard E},
 journal = {International Electric Propulsion Conference}
}

High current hollow cathodes used for electric propulsion have been found to have en-ergetic ions in the plume caused by a mechanism still not fully understood. These anoma-lously energetic ions have been shown to erode cathode keepers and pose a risk to cathodes used for long-duration missions. This work builds on previous work investigating the effects of ion acoustic turbulence (IAT) in the plume and its effects on the ion population. Laser-induced fluorescence (LIF) and plasma probes were used to measure several steady-state and fluctuating plasma properties related to ion acoustic waves. A preliminary analysis of these data show the ion distribution function along the the centerline to be heated primar-ily in the axial direction resulting in ion temperature anisotropy. A bimodal ion velocity distribution function was observed with the axial LIF beam near the anode, consisting of slow and fast populations. It was not possible to detect two distinct populations using the radial LIF measurements. The fast population was found to accelerate with distance from the keeper orifice whereas the slow ion population remained nearly stationary, and is be-lieved to be caused by ion generation in the plume through ionization or charge-exchange collisions. Wave energy and the ion acoustic wave direction were measured and show an increase in wave energy downstream, highly concentrated along the centerline. The ion acoustic waves were found to propagate outward from the orifice and are approximately collinear with the drift direction of the fast ion population.

@article{
 title = {Design and Performance of the Axial Ring-Cusp Hybrid (ARCH) Discharge for Miniature Ion Thrusters},
 type = {article},
 year = {2017},
 id = {35380f00-4a73-3652-b6c0-f2fea3026d40},
 created = {2021-07-30T22:42:50.456Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:50.456Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The miniaturization of traditional ring-cusp discharges is restricted by the increased surface area-to-volume ratio that reduces the primary electron confinement time necessary for efficient plasma generation. The ARCH discharge design decouples primary and plasma electron losses, in a manner similar to a Kaufmann and Magneto-Electric discharges, while maintaining the advantages of using ring-cusp designs. Discharge mapping and performance testing of the Axial Ring-Cusp Hybrid (ARCH) discharge demonstrate favorable plasma conditions within the discharge and near the extraction plane. These results suggest that the current 3 cm implementaion of the ARCH discharge can potentially achieve discharge losses and mass utilization efficiencies of 175 W/A and 0.87, respectively. As such, the new ARCH design approach may allow miniature ion thrusters to achieve the performance and efficiency levels similar those of highly efficient conventional ion thrusters.},
 bibtype = {article},
 author = {Dankongkakul, Ben and Wirz, Richard E.},
 journal = {35th International Electric Propulsion Conference}
}

The miniaturization of traditional ring-cusp discharges is restricted by the increased surface area-to-volume ratio that reduces the primary electron confinement time necessary for efficient plasma generation. The ARCH discharge design decouples primary and plasma electron losses, in a manner similar to a Kaufmann and Magneto-Electric discharges, while maintaining the advantages of using ring-cusp designs. Discharge mapping and performance testing of the Axial Ring-Cusp Hybrid (ARCH) discharge demonstrate favorable plasma conditions within the discharge and near the extraction plane. These results suggest that the current 3 cm implementaion of the ARCH discharge can potentially achieve discharge losses and mass utilization efficiencies of 175 W/A and 0.87, respectively. As such, the new ARCH design approach may allow miniature ion thrusters to achieve the performance and efficiency levels similar those of highly efficient conventional ion thrusters.

@inproceedings{
 title = {Development and initial performance testing of a low-power magnetically shielded Hall thruster with an internally-mounted hollow cathode},
 type = {inproceedings},
 year = {2017},
 pages = {3},
 id = {4e536dad-a028-330c-9876-fa2625c00199},
 created = {2021-07-30T22:59:30.312Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:59:30.312Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 source_type = {CONF},
 private_publication = {false},
 bibtype = {inproceedings},
 author = {Conversano, Ryan W and Lobbia, Robert B and Tilley, Kimberlyn C and Goebel, Dan M and Reilly, Sean W and Mikellides, Ioannis G and Hofer, Richard R},
 booktitle = {35th International Electric Propulsion Conference}
}

@article{
 title = {View factor modeling of sputter-deposition on micron-scale-architectured surfaces exposed to plasma},
 type = {article},
 year = {2016},
 volume = {119},
 id = {5795d4d4-f387-3d81-83e0-662355407365},
 created = {2021-07-30T20:25:13.173Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:13.173Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The sputter-deposition on surfaces exposed to plasma plays an important role in the erosion behavior and overall performance of a wide range of plasma devices. Plasma models in the low density, low energy plasma regime typically neglect micron-scale surface feature effects on the net sputter yield and erosion rate. The model discussed in this paper captures such surface architecture effects via a computationally efficient view factor model. The model compares well with experimental measurements of argon ion sputter yield from a nickel surface with a triangle wave geometry with peak heights in the hundreds of microns range. Further analysis with the model shows that increasing the surface pitch angle beyond about 45° can lead to significant decreases in the normalized net sputter yield for all simulated ion incident energies (i.e., 75, 100, 200, and 400 eV) for both smooth and roughened surfaces. At higher incident energies, smooth triangular surfaces exhibit a nonmonotonic trend in the normalized net sputter yield with surface pitch angle with a maximum yield above unity over a range of intermediate angles. The resulting increased erosion rate occurs because increased sputter yield due to the local ion incidence angle outweighs increased deposition due to the sputterant angular distribution. The model also compares well with experimentally observed radial expansion of protuberances (measuring tens of microns) in a nano-rod field exposed to an argon beam. The model captures the coalescence of sputterants at the protuberance sites and accurately illustrates the structure's expansion due to deposition from surrounding sputtering surfaces; these capabilities will be used for future studies into more complex surface architectures.},
 bibtype = {article},
 author = {Huerta, C. E. and Matlock, T. S. and Wirz, R. E.},
 doi = {10.1063/1.4944035},
 journal = {Journal of Applied Physics},
 number = {11}
}

The sputter-deposition on surfaces exposed to plasma plays an important role in the erosion behavior and overall performance of a wide range of plasma devices. Plasma models in the low density, low energy plasma regime typically neglect micron-scale surface feature effects on the net sputter yield and erosion rate. The model discussed in this paper captures such surface architecture effects via a computationally efficient view factor model. The model compares well with experimental measurements of argon ion sputter yield from a nickel surface with a triangle wave geometry with peak heights in the hundreds of microns range. Further analysis with the model shows that increasing the surface pitch angle beyond about 45° can lead to significant decreases in the normalized net sputter yield for all simulated ion incident energies (i.e., 75, 100, 200, and 400 eV) for both smooth and roughened surfaces. At higher incident energies, smooth triangular surfaces exhibit a nonmonotonic trend in the normalized net sputter yield with surface pitch angle with a maximum yield above unity over a range of intermediate angles. The resulting increased erosion rate occurs because increased sputter yield due to the local ion incidence angle outweighs increased deposition due to the sputterant angular distribution. The model also compares well with experimentally observed radial expansion of protuberances (measuring tens of microns) in a nano-rod field exposed to an argon beam. The model captures the coalescence of sputterants at the protuberance sites and accurately illustrates the structure's expansion due to deposition from surrounding sputtering surfaces; these capabilities will be used for future studies into more complex surface architectures.

@article{
 title = {Secondary electron emission from plasma-generated nanostructured tungsten fuzz},
 type = {article},
 year = {2016},
 volume = {109},
 id = {49829207-7c2d-3a8b-905a-a7abe46f3279},
 created = {2021-07-30T20:25:15.222Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:15.222Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Recently, several researchers [e.g., Yang et al., Sci. Rep. 5, 10959 (2015)] have shown that tungsten fuzz can grow on a hot tungsten surface under bombardment by energetic helium ions in different plasma discharges and applications, including magnetic fusion devices with plasma facing tungsten components. This work reports the direct measurements of the total effective secondary electron emission (SEE) from tungsten fuzz. Using dedicated material surface diagnostics and in-situ characterization, we find two important results: (1) SEE values for tungsten fuzz are 40%-63% lower than for smooth tungsten and (2) the SEE values for tungsten fuzz are independent of the angle of the incident electron. The reduction in SEE from tungsten fuzz is most pronounced at high incident angles, which has important implications for many plasma devices since in a negative-going sheath the potential structure leads to relatively high incident angles for the electrons at the plasma confining walls. Overall, low SEE will create a relatively higher sheath potential difference that reduces plasma electron energy loss to the confining wall. Thus, the presence or self-generation in a plasma of a low SEE surface such as tungsten fuzz can be desirable for improved performance of many plasma devices.},
 bibtype = {article},
 author = {Patino, M. and Raitses, Y. and Wirz, R.},
 doi = {10.1063/1.4967830},
 journal = {Applied Physics Letters},
 number = {20}
}

Recently, several researchers [e.g., Yang et al., Sci. Rep. 5, 10959 (2015)] have shown that tungsten fuzz can grow on a hot tungsten surface under bombardment by energetic helium ions in different plasma discharges and applications, including magnetic fusion devices with plasma facing tungsten components. This work reports the direct measurements of the total effective secondary electron emission (SEE) from tungsten fuzz. Using dedicated material surface diagnostics and in-situ characterization, we find two important results: (1) SEE values for tungsten fuzz are 40%-63% lower than for smooth tungsten and (2) the SEE values for tungsten fuzz are independent of the angle of the incident electron. The reduction in SEE from tungsten fuzz is most pronounced at high incident angles, which has important implications for many plasma devices since in a negative-going sheath the potential structure leads to relatively high incident angles for the electrons at the plasma confining walls. Overall, low SEE will create a relatively higher sheath potential difference that reduces plasma electron energy loss to the confining wall. Thus, the presence or self-generation in a plasma of a low SEE surface such as tungsten fuzz can be desirable for improved performance of many plasma devices.

@inproceedings{
 title = {Implementation of in situ diagnostics for sputter yield measurements in a focused plasma},
 type = {inproceedings},
 year = {2016},
 id = {ed645c88-48af-3e0c-ac33-f61ab3082e79},
 created = {2021-07-30T22:42:51.008Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:51.008Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The Plasma-interactions test facility at UCLA features a high current hollow cathode plasma source that delivers a cylindrical plasma through solenoids to a target downstream. The near-target plasma is focused to a spot size of 1.5 cm for localized sputtering. The target is then biased negative with respect to the plasma potential to provide energetic ion bombardment. To perform in situ sputter yield measurements, a quartz crystal niicrobalance and optical emission spectroscopy have been implemented. Preliminary results for an argon plasma on molybdenum sputtering experiment are presented to validate these diagnostics. The quartz crystal niicrobalance method along with the traditional weight loss method produce sputter yields that agree well with published results. Angular distributions and emission spectra for these test cases are also provided.},
 bibtype = {inproceedings},
 author = {Li, Gary Z. and Dodson, Christopher A. and Wirz, Richard E. and Matlock, Taylor S. and Goebel, Dan M.},
 doi = {10.2514/6.2016-4841},
 booktitle = {52nd AIAA/SAE/ASEE Joint Propulsion Conference, 2016}
}

The Plasma-interactions test facility at UCLA features a high current hollow cathode plasma source that delivers a cylindrical plasma through solenoids to a target downstream. The near-target plasma is focused to a spot size of 1.5 cm for localized sputtering. The target is then biased negative with respect to the plasma potential to provide energetic ion bombardment. To perform in situ sputter yield measurements, a quartz crystal niicrobalance and optical emission spectroscopy have been implemented. Preliminary results for an argon plasma on molybdenum sputtering experiment are presented to validate these diagnostics. The quartz crystal niicrobalance method along with the traditional weight loss method produce sputter yields that agree well with published results. Angular distributions and emission spectra for these test cases are also provided.

@inproceedings{
 title = {Laser-induced fluorescence measurements of energetic ions in a 100-A LaB6 hollow cathode plume},
 type = {inproceedings},
 year = {2016},
 id = {5e678e83-f057-3987-9759-92fe56b5f105},
 created = {2021-07-30T22:42:51.616Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:51.616Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Ion acoustic turbulence (IAT) has been found to exist in the plume of hollow cathodes and is suspected to be one of the contributing mechanisms to the production of energetic ions. In this work the ion velocity distribution has been measured using laser-induced fluorescence (LIF) at several high-current operating conditions on the centerline of a 100 A class hollow cathode. The wave energy associated with IAT was also measured and shown to correlate with the ion temperature deduced from the LIF measurements. Both parameters are found to increase with distance downstream of the cathode keeper face. A parametric study of the ion heating as a function of discharge current and cathode flow rate was also performed. It was found that both increasing current and decreasing flow rate increased the amount of ion heating but the change in heating was more sensitive to flow rate. These results strongly support the theory that IAT is a significant contributor to energetic ion production in hollow cathodes.},
 bibtype = {inproceedings},
 author = {Dodson, Christopher A. and Perez-Grande, Daniel and Jorns, Benjamin A. and Goebel, Dan M. and Wirz, Richard E.},
 doi = {10.2514/6.2016-4838},
 booktitle = {52nd AIAA/SAE/ASEE Joint Propulsion Conference, 2016}
}

Ion acoustic turbulence (IAT) has been found to exist in the plume of hollow cathodes and is suspected to be one of the contributing mechanisms to the production of energetic ions. In this work the ion velocity distribution has been measured using laser-induced fluorescence (LIF) at several high-current operating conditions on the centerline of a 100 A class hollow cathode. The wave energy associated with IAT was also measured and shown to correlate with the ion temperature deduced from the LIF measurements. Both parameters are found to increase with distance downstream of the cathode keeper face. A parametric study of the ion heating as a function of discharge current and cathode flow rate was also performed. It was found that both increasing current and decreasing flow rate increased the amount of ion heating but the change in heating was more sensitive to flow rate. These results strongly support the theory that IAT is a significant contributor to energetic ion production in hollow cathodes.

@inproceedings{
 title = {Surface geometry effects on secondary electron emission via monte carlo modeling},
 type = {inproceedings},
 year = {2016},
 id = {eefef882-45d1-39c3-ada3-1de4177685f4},
 created = {2021-07-30T22:42:52.334Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:52.334Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A secondary electron emission model have been developed to simulate electron induced SEE from complex, textured surfaces for any material. In this paper, the model employs two simple geometries to simulate ion-textured copper surfaces. An exploration of nondimensional geometric parameters shows the effects that fiber characteristic lengths have in the reduction of true yield by capturing secondaries emitted from the underlying substrate. Sensitivity studies of fiber radius and surface properties show significant correlations between the true yield and these parameters.},
 bibtype = {inproceedings},
 author = {Huerta, Cesar E. and Wirz, Richard E.},
 doi = {10.2514/6.2016-4840},
 booktitle = {52nd AIAA/SAE/ASEE Joint Propulsion Conference, 2016}
}

A secondary electron emission model have been developed to simulate electron induced SEE from complex, textured surfaces for any material. In this paper, the model employs two simple geometries to simulate ion-textured copper surfaces. An exploration of nondimensional geometric parameters shows the effects that fiber characteristic lengths have in the reduction of true yield by capturing secondaries emitted from the underlying substrate. Sensitivity studies of fiber radius and surface properties show significant correlations between the true yield and these parameters.

@article{
 title = {Development and initial testing of a magnetically shielded miniature hall thruster},
 type = {article},
 year = {2015},
 volume = {43},
 id = {dab7dfcc-16a5-35ca-bf5e-bcebbc72ad93},
 created = {2021-07-30T20:25:19.938Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:19.938Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The scaling of magnetically shielded Hall thrusters to low power is investigated through the development and fabrication of a 4-cm Hall thruster. During initial testing, the magnetically shielded miniature Hall thruster was operated at 275 V discharge voltage and 325-W discharge power. Inspection of the channel walls after testing suggests that the outer discharge channel wall was successfully shielded from high-energy ion erosion while the inner channel wall showed evidence of weaker shielding, likely due to magnetic circuit saturation. Scanning planar probe measurements taken at two locations downstream of the thruster face provided ion current density profiles. The ion current calculated by integrating these data was 1.04 A with a plume divergence half-angle of 30°. Swept retarding potential analyzer measurements taken 80-cm axially downstream of the thruster measured the most probable ion voltage to be 252 V. The total thruster efficiency was calculated from probe measurements to be 43% (anode efficiency of 59%) corresponding to a thrust of 19 mN at a specific impulse of 1870 s. Discharge channel erosion rates were found to be approximately three orders of magnitude less than unshielded Hall thrusters, suggesting the potential for a significant increase in operational life.},
 bibtype = {article},
 author = {Conversano, Ryan W. and Goebel, Dan M. and Hofer, Richard R. and Matlock, Taylor S. and Wirz, Richard E.},
 doi = {10.1109/TPS.2014.2321107},
 journal = {IEEE Transactions on Plasma Science},
 number = {1}
}

The scaling of magnetically shielded Hall thrusters to low power is investigated through the development and fabrication of a 4-cm Hall thruster. During initial testing, the magnetically shielded miniature Hall thruster was operated at 275 V discharge voltage and 325-W discharge power. Inspection of the channel walls after testing suggests that the outer discharge channel wall was successfully shielded from high-energy ion erosion while the inner channel wall showed evidence of weaker shielding, likely due to magnetic circuit saturation. Scanning planar probe measurements taken at two locations downstream of the thruster face provided ion current density profiles. The ion current calculated by integrating these data was 1.04 A with a plume divergence half-angle of 30°. Swept retarding potential analyzer measurements taken 80-cm axially downstream of the thruster measured the most probable ion voltage to be 252 V. The total thruster efficiency was calculated from probe measurements to be 43% (anode efficiency of 59%) corresponding to a thrust of 19 mN at a specific impulse of 1870 s. Discharge channel erosion rates were found to be approximately three orders of magnitude less than unshielded Hall thrusters, suggesting the potential for a significant increase in operational life.

@article{
 title = {Analysis of secondary electron emission for conducting materials using 4-grid LEED/AES optics},
 type = {article},
 year = {2015},
 volume = {48},
 id = {507bb701-c96f-3d34-818a-9b6b07555bcc},
 created = {2021-07-30T20:25:22.378Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:22.378Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A facility utilizing 4-grid optics for LEED/AES (low energy electron diffraction/Auger electron spectroscopy) was developed to measure the total secondary electron yield and secondary electron energy distribution function for conducting materials. The facility and experimental procedure were validated with measurements of 50-500 eV primary electrons impacting graphite. The total yield was calculated from measurements of the secondary electron current (i) from the sample and (ii) from the collection assembly, by biasing each surface. Secondary electron yield results from both methods agreed well with each other and were within the spread of previous results for the total yield from graphite. Additionally, measurements of the energy distribution function of secondary electrons from graphite are provided for a wider range of incident electron energies. These results can be used in modeling plasma-wall interactions in plasmas bounded by graphite walls, such as are found in plasma thrusters, and divertors and limiters of magnetic fusion devices.},
 bibtype = {article},
 author = {Patino, M. I. and Raitses, Y. and Koel, B. E. and Wirz, R. E.},
 doi = {10.1088/0022-3727/48/19/195204},
 journal = {Journal of Physics D: Applied Physics},
 number = {19}
}

A facility utilizing 4-grid optics for LEED/AES (low energy electron diffraction/Auger electron spectroscopy) was developed to measure the total secondary electron yield and secondary electron energy distribution function for conducting materials. The facility and experimental procedure were validated with measurements of 50-500 eV primary electrons impacting graphite. The total yield was calculated from measurements of the secondary electron current (i) from the sample and (ii) from the collection assembly, by biasing each surface. Secondary electron yield results from both methods agreed well with each other and were within the spread of previous results for the total yield from graphite. Additionally, measurements of the energy distribution function of secondary electrons from graphite are provided for a wider range of incident electron energies. These results can be used in modeling plasma-wall interactions in plasmas bounded by graphite walls, such as are found in plasma thrusters, and divertors and limiters of magnetic fusion devices.

@article{
 title = {Exergetic optimization and performance evaluation of multi-phase thermal energy storage systems},
 type = {article},
 year = {2015},
 volume = {122},
 id = {e53f5e06-1b0d-349a-baf9-58d4e4317a73},
 created = {2021-07-30T20:25:24.067Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:24.067Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {This study outlines a methodology for modeling and optimizing multi-phase thermal energy storage systems for solar thermal power plant (STPP) operation by incorporating energy and exergy analyses to a TES system employing a storage medium that can undergo multi-phase operation during the charging and discharging period. First, a numerical model is developed to investigate the transient thermodynamic and heat transfer characteristics of the storage system by coupling conservation of energy with an equation of state to model the spatial and temporal variations in fluid properties during the entire working cycle of the TES tank. Second, parametric studies are conducted to determine the impact of key design parameters on both energy and exergy efficiencies. The optimal values must balance exergy destroyed due to heat transfer and exergy destroyed due to pressure losses, which have competing effects. Optimization is utilized to determine parameter values within a feasible design window, which leads to a maximum exergetic efficiency of 87%.},
 bibtype = {article},
 author = {Tse, Louis A. and Lavine, Adrienne S. and Lakeh, Reza Baghaei and Wirz, Richard E.},
 doi = {10.1016/j.solener.2015.08.026},
 journal = {Solar Energy}
}

This study outlines a methodology for modeling and optimizing multi-phase thermal energy storage systems for solar thermal power plant (STPP) operation by incorporating energy and exergy analyses to a TES system employing a storage medium that can undergo multi-phase operation during the charging and discharging period. First, a numerical model is developed to investigate the transient thermodynamic and heat transfer characteristics of the storage system by coupling conservation of energy with an equation of state to model the spatial and temporal variations in fluid properties during the entire working cycle of the TES tank. Second, parametric studies are conducted to determine the impact of key design parameters on both energy and exergy efficiencies. The optimal values must balance exergy destroyed due to heat transfer and exergy destroyed due to pressure losses, which have competing effects. Optimization is utilized to determine parameter values within a feasible design window, which leads to a maximum exergetic efficiency of 87%.

@inproceedings{
 title = {Optimal operation strategies for thermal energy storage systems in solar thermal power plants},
 type = {inproceedings},
 year = {2015},
 volume = {2},
 id = {e52141c8-96ba-3953-b9bb-dfbd1bb82680},
 created = {2021-07-30T20:25:25.559Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:25.559Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {This paper examines the economic benefits of various operation strategies for a thermal energy storage (TES) system in a solar thermal power plant. A thermodynamic model developed to evaluate different design options has been utilized to calculate system performance and assess the impact of operation strategies, storage capacity, and market prices on the value of TES. The overall performance is also investigated through several parametric studies, such as solar multiple, geographic location, and choice of HTF. The influence of these parameters has been evaluated in consideration of exergy destruction due to heat transfer and pressure drop. By incorporating exergy-based optimization alongside traditional energy analyses, the results of this study evaluate the optimal values for key parameters in the design and operation of TES systems, as well as highlight opportunities to minimize thermodynamic losses. Annual performance for each case is characterized both by nominal and part-load efficiency. Levelized cost of electricity (LCOE) is calculated for all cases, illustrating a set of optimal parameters that yield a minimum LCOE value.},
 bibtype = {inproceedings},
 author = {Tse, Louis A. and Wirz, Richard E. and Lavine, Adrienne S.},
 doi = {10.1115/ES2015-49053},
 booktitle = {ASME 2015 9th International Conference on Energy Sustainability, ES 2015, collocated with the ASME 2015 Power Conference, the ASME 2015 13th International Conference on Fuel Cell Science, Engineering and Technology, and the ASME 2015 Nuclear Forum}
}

This paper examines the economic benefits of various operation strategies for a thermal energy storage (TES) system in a solar thermal power plant. A thermodynamic model developed to evaluate different design options has been utilized to calculate system performance and assess the impact of operation strategies, storage capacity, and market prices on the value of TES. The overall performance is also investigated through several parametric studies, such as solar multiple, geographic location, and choice of HTF. The influence of these parameters has been evaluated in consideration of exergy destruction due to heat transfer and pressure drop. By incorporating exergy-based optimization alongside traditional energy analyses, the results of this study evaluate the optimal values for key parameters in the design and operation of TES systems, as well as highlight opportunities to minimize thermodynamic losses. Annual performance for each case is characterized both by nominal and part-load efficiency. Levelized cost of electricity (LCOE) is calculated for all cases, illustrating a set of optimal parameters that yield a minimum LCOE value.

@article{
 title = {Study of turbulent natural convection in vertical storage tubes for supercritical thermal energy storage},
 type = {article},
 year = {2015},
 volume = {67},
 id = {19096fb2-7bd6-3508-a9a2-5dd0f5ad799a},
 created = {2021-07-30T20:25:31.156Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:31.156Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Heat transfer to storage fluid is a critical subject for any thermal energy storage system. The poor thermal conductivity of the storage medium may lead to insufficient heat transfer and may impair the functionality of the system. Supercritical thermal energy storage systems benefit from turbulent natural convection that dominates the heat transfer mechanism and compensates for the low thermal conductivity of the storage fluids. A computational model is validated and adopted to study the buoyancy-driven flow in vertical storage tubes and the effect of the aspect ratio of the vertical storage tubes on the charge time is investigated.},
 bibtype = {article},
 author = {Lakeh, Reza Baghaei and Lavine, Adrienne S. and Kavehpour, H. Pirouz and Wirz, Richard E.},
 doi = {10.1080/10407782.2014.923224},
 journal = {Numerical Heat Transfer; Part A: Applications},
 number = {2}
}

Heat transfer to storage fluid is a critical subject for any thermal energy storage system. The poor thermal conductivity of the storage medium may lead to insufficient heat transfer and may impair the functionality of the system. Supercritical thermal energy storage systems benefit from turbulent natural convection that dominates the heat transfer mechanism and compensates for the low thermal conductivity of the storage fluids. A computational model is validated and adopted to study the buoyancy-driven flow in vertical storage tubes and the effect of the aspect ratio of the vertical storage tubes on the charge time is investigated.

@inproceedings{
 title = {Validation of a plasma-facing surface sputtering and deposition view factor model},
 type = {inproceedings},
 year = {2015},
 id = {dbd4dbf1-0f88-3cd7-af97-85677a38f649},
 created = {2021-07-30T22:42:53.042Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:53.042Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A computational model that will be used to study sputtering, deposition, and erosion of surfaces exposed to low energy, low density plasma environments has been developed and validated. Comparisons with sputter yield curve fits and experimental data show favorable agreement and indicate that the model can accurately capture the effects of incidence angle and energy on sputtering behavior. COMSOL simulations were also used to validate view factor and shadowing calculations. This computational model will be used in the future to test the effects of surface architecturing on gross sputter yield and surface erosion rate, and will serve to drive experimental efforts at UCLA.},
 bibtype = {inproceedings},
 author = {Huerta, Cesar E. and Matlock, Taylor S. and Wirz, Richard E.},
 doi = {10.2514/6.2015-4503},
 booktitle = {AIAA SPACE 2015 Conference and Exposition}
}

A computational model that will be used to study sputtering, deposition, and erosion of surfaces exposed to low energy, low density plasma environments has been developed and validated. Comparisons with sputter yield curve fits and experimental data show favorable agreement and indicate that the model can accurately capture the effects of incidence angle and energy on sputtering behavior. COMSOL simulations were also used to validate view factor and shadowing calculations. This computational model will be used in the future to test the effects of surface architecturing on gross sputter yield and surface erosion rate, and will serve to drive experimental efforts at UCLA.

@article{
 title = {Miniature Ion Thrusters: A Review of Modern Technologies and Mission Capabilities},
 type = {article},
 year = {2015},
 id = {55866832-074c-3660-964f-a9c22fb05fa2},
 created = {2021-07-30T22:42:53.682Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:53.682Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {There are inherent challenges to ion thruster miniaturization, but many current thrusters exhibit desirable performance. Depending on mission requirements, modern ion thrusters provide thruster capabilities that are attractive for missions for both large and small spacecraft. In addition to providing (1) high specific impulse, Isp, (~1000 – 3000 s), these thrusters can provide (2) very precise thrust and impulse bits, (3) low disturbance thrust, and (4) low contamination and spacecraft interaction potential. These capabilities are attractive for high V exploration and orbit/inclination change missions, precision orbit maintenance, formation flying, or precision pointing/control. Many thruster options exist that employ RF, microwave, and DC ring cusp discharges. These thrusters use low contamination propellants such as xenon, but may also use highly-storable propellants such as iodine.},
 bibtype = {article},
 author = {Wirz, Richard E},
 journal = {34th International Electric Propulsion Conference}
}

There are inherent challenges to ion thruster miniaturization, but many current thrusters exhibit desirable performance. Depending on mission requirements, modern ion thrusters provide thruster capabilities that are attractive for missions for both large and small spacecraft. In addition to providing (1) high specific impulse, Isp, (~1000 – 3000 s), these thrusters can provide (2) very precise thrust and impulse bits, (3) low disturbance thrust, and (4) low contamination and spacecraft interaction potential. These capabilities are attractive for high V exploration and orbit/inclination change missions, precision orbit maintenance, formation flying, or precision pointing/control. Many thruster options exist that employ RF, microwave, and DC ring cusp discharges. These thrusters use low contamination propellants such as xenon, but may also use highly-storable propellants such as iodine.

@inproceedings{
 title = {Transient turbulent natural convection in vertical tubes for indirect Thermal Energy Storage},
 type = {inproceedings},
 year = {2015},
 volume = {2},
 id = {06521ac8-3854-3b0c-90d9-c166f9c7f7ea},
 created = {2021-07-30T22:42:59.629Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:59.629Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {In this study, turbulent natural convection heat transfer during the charge cycle of a Thermal Energy Storage system was studied computationally and analytically. The storage fluids were supercritical CO2 and liquid toluene which are stored in vertical and sealed storage tubes. A computational model was developed and validated to study turbulent natural convection during the charge cycle. The results of this study show that the aspect ratio of the storage tube (L/D) has an important effect on the heat transfer characteristics. A conceptual model was developed that views the thermal storage process as a hot boundary layer that rises along the tube wall and falls in the center to replace the cold fluid in the core. This model shows that dimensionless mean temperature of the storage fluid and average Nusselt number are functions of a Buoyancy-Fourier number.},
 bibtype = {inproceedings},
 author = {Lakeh, Reza Baghaei and Wirz, Richard E. and Kavehpour, H. Pirouz and Lavine, Adrienne S.},
 doi = {10.1115/ES2015-49092},
 booktitle = {ASME 2015 9th International Conference on Energy Sustainability, ES 2015, collocated with the ASME 2015 Power Conference, the ASME 2015 13th International Conference on Fuel Cell Science, Engineering and Technology, and the ASME 2015 Nuclear Forum}
}

In this study, turbulent natural convection heat transfer during the charge cycle of a Thermal Energy Storage system was studied computationally and analytically. The storage fluids were supercritical CO2 and liquid toluene which are stored in vertical and sealed storage tubes. A computational model was developed and validated to study turbulent natural convection during the charge cycle. The results of this study show that the aspect ratio of the storage tube (L/D) has an important effect on the heat transfer characteristics. A conceptual model was developed that views the thermal storage process as a hot boundary layer that rises along the tube wall and falls in the center to replace the cold fluid in the core. This model shows that dimensionless mean temperature of the storage fluid and average Nusselt number are functions of a Buoyancy-Fourier number.

@article{
 title = {Spatial and temporal modeling of sub- and supercritical thermal energy storage},
 type = {article},
 year = {2014},
 volume = {103},
 id = {af27892b-5abd-3167-9517-b2cdfb312985},
 created = {2021-07-30T20:25:27.424Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:27.424Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {This paper describes a thermodynamic model that simulates the discharge cycle of a single-tank thermal energy storage (TES) system that can operate from the two-phase (liquid-vapor) to supercritical regimes for storage fluid temperatures typical of concentrating solar power plants. State-of-the-art TES design utilizes a two-tank system with molten nitrate salts; one major problem is the high capital cost of the salts (International Renewable Energy Agency, 2012). The alternate approach explored here opens up the use of low-cost fluids by considering operation at higher pressures associated with the two-phase and supercritical regimes.The main challenge to such a system is its high pressures and temperatures which necessitate a relatively high-cost containment vessel that represents a large fraction of the system capital cost. To mitigate this cost, the proposed design utilizes a single-tank TES system, effectively halving the required wall material. A single-tank approach also significantly reduces the complexity of the system in comparison to the two-tank systems, which require expensive pumps and external heat exchangers.A thermodynamic model is used to evaluate system performance; in particular it predicts the volume of tank wall material needed to encapsulate the storage fluid. The transient temperature of the tank is observed to remain hottest at the storage tank exit, which is beneficial to system operation. It is also shown that there is an optimum storage fluid loading that generates a given turbine energy output while minimizing the required tank wall material.Overall, this study explores opportunities to further improve current solar thermal technologies. The proposed single-tank system shows promise for decreasing the cost of thermal energy storage. © 2014 Elsevier Ltd.},
 bibtype = {article},
 author = {Tse, Louis A. and Ganapathi, Gani B. and Wirz, Richard E. and Lavine, Adrienne S.},
 doi = {10.1016/j.solener.2014.02.040},
 journal = {Solar Energy}
}

This paper describes a thermodynamic model that simulates the discharge cycle of a single-tank thermal energy storage (TES) system that can operate from the two-phase (liquid-vapor) to supercritical regimes for storage fluid temperatures typical of concentrating solar power plants. State-of-the-art TES design utilizes a two-tank system with molten nitrate salts; one major problem is the high capital cost of the salts (International Renewable Energy Agency, 2012). The alternate approach explored here opens up the use of low-cost fluids by considering operation at higher pressures associated with the two-phase and supercritical regimes.The main challenge to such a system is its high pressures and temperatures which necessitate a relatively high-cost containment vessel that represents a large fraction of the system capital cost. To mitigate this cost, the proposed design utilizes a single-tank TES system, effectively halving the required wall material. A single-tank approach also significantly reduces the complexity of the system in comparison to the two-tank systems, which require expensive pumps and external heat exchangers.A thermodynamic model is used to evaluate system performance; in particular it predicts the volume of tank wall material needed to encapsulate the storage fluid. The transient temperature of the tank is observed to remain hottest at the storage tank exit, which is beneficial to system operation. It is also shown that there is an optimum storage fluid loading that generates a given turbine energy output while minimizing the required tank wall material.Overall, this study explores opportunities to further improve current solar thermal technologies. The proposed single-tank system shows promise for decreasing the cost of thermal energy storage. © 2014 Elsevier Ltd.

@article{
 title = {A dc plasma source for plasma-material interaction experiments},
 type = {article},
 year = {2014},
 volume = {23},
 id = {1097811b-3d6e-3eca-abec-51ce8e819ff0},
 created = {2021-07-30T20:25:32.975Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:32.975Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A new device has been constructed for the investigation of interactions between engineered materials and a plasma in regimes relevant to electric propulsion and pulsed power devices. A linear plasma source, consisting of a hollow cathode, cylindrical anode, and axial magnetic field, delivers a 3 cm diameter beam to a biased target 70 cm away. The ion energy impacting the surface is controlled by biasing the sample from 0 to 500 V below the local plasma potential. This paper discusses the major aspects of the plasma source design and presents measurements of the plasma parameters achieved to date on argon and xenon. Experiments show that splitting the gas injection between the hollow cathode and the anode region provides control of the discharge voltage to minimize cathode sputtering while providing ion fluxes to the target in excess of 1021 m-2 s-1. Sputtering rate measurements on a non-textured molybdenum sample show close agreement with those established in the literature. © 2014 IOP Publishing Ltd.},
 bibtype = {article},
 author = {Matlock, T. S. and Goebel, D. M. and Conversano, R. and Wirz, R. E.},
 doi = {10.1088/0963-0252/23/2/025014},
 journal = {Plasma Sources Science and Technology},
 number = {2}
}

A new device has been constructed for the investigation of interactions between engineered materials and a plasma in regimes relevant to electric propulsion and pulsed power devices. A linear plasma source, consisting of a hollow cathode, cylindrical anode, and axial magnetic field, delivers a 3 cm diameter beam to a biased target 70 cm away. The ion energy impacting the surface is controlled by biasing the sample from 0 to 500 V below the local plasma potential. This paper discusses the major aspects of the plasma source design and presents measurements of the plasma parameters achieved to date on argon and xenon. Experiments show that splitting the gas injection between the hollow cathode and the anode region provides control of the discharge voltage to minimize cathode sputtering while providing ion fluxes to the target in excess of 1021 m-2 s-1. Sputtering rate measurements on a non-textured molybdenum sample show close agreement with those established in the literature. © 2014 IOP Publishing Ltd.

@article{
 title = {Structural design of spars for 100-m biplane wind turbine blades},
 type = {article},
 year = {2014},
 volume = {71},
 id = {5eb710ae-b141-31f6-917e-e74d9404a3ae},
 created = {2021-07-30T20:25:34.904Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:34.904Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Large wind turbine blades are being developed at lengths of 75-100m, in order to improve energy capture and reduce the cost of wind energy. Bending loads in the inboard region of the blade make large blade development challenging. The "biplane blade" design was proposed to use a biplane inboard region to improve the design of the inboard region and improve overall performance of large blades. This paper focuses on the design of the internal "biplane spar" structure for 100-m biplane blades. Several spars were designed to approximate the Sandia SNL100-00 blade ("monoplane spar") and the biplane blade ("biplane spar"). Analytical and computational models are developed to analyze these spars. The analytical model used the method of minimum total potential energy; the computational model used beam finite elements with cross-sectional analysis. Simple load cases were applied to each spar and their deflections, bending moments, axial forces, and stresses were compared. Similar performance trends are identified with both the analytical and computational models. An approximate buckling analysis shows that compressive loads in the inboard biplane region do not exceed buckling loads. A parametric analysis shows biplane spar configurations have 25-35% smaller tip deflections and 75% smaller maximum root bending moments than monoplane spars of the same length and mass per unit span. Root bending moments in the biplane spar are largely relieved by axial forces in the biplane region, which are not significant in the monoplane spar. The benefits for the inboard region could lead to weight reductions in wind turbine blades. Innovations that create lighter blades can make large blades a reality, suggesting that the biplane blade may be an attractive design for large (100-m) blades. © 2014 Elsevier Ltd.},
 bibtype = {article},
 author = {Roth-Johnson, Perry and Wirz, Richard E. and Lin, Edward},
 doi = {10.1016/j.renene.2014.05.030},
 journal = {Renewable Energy}
}

Large wind turbine blades are being developed at lengths of 75-100m, in order to improve energy capture and reduce the cost of wind energy. Bending loads in the inboard region of the blade make large blade development challenging. The "biplane blade" design was proposed to use a biplane inboard region to improve the design of the inboard region and improve overall performance of large blades. This paper focuses on the design of the internal "biplane spar" structure for 100-m biplane blades. Several spars were designed to approximate the Sandia SNL100-00 blade ("monoplane spar") and the biplane blade ("biplane spar"). Analytical and computational models are developed to analyze these spars. The analytical model used the method of minimum total potential energy; the computational model used beam finite elements with cross-sectional analysis. Simple load cases were applied to each spar and their deflections, bending moments, axial forces, and stresses were compared. Similar performance trends are identified with both the analytical and computational models. An approximate buckling analysis shows that compressive loads in the inboard biplane region do not exceed buckling loads. A parametric analysis shows biplane spar configurations have 25-35% smaller tip deflections and 75% smaller maximum root bending moments than monoplane spars of the same length and mass per unit span. Root bending moments in the biplane spar are largely relieved by axial forces in the biplane region, which are not significant in the monoplane spar. The benefits for the inboard region could lead to weight reductions in wind turbine blades. Innovations that create lighter blades can make large blades a reality, suggesting that the biplane blade may be an attractive design for large (100-m) blades. © 2014 Elsevier Ltd.

@article{
 title = {Aero-structural investigation of biplane wind turbine blades},
 type = {article},
 year = {2014},
 volume = {17},
 id = {13d86895-cf46-344f-a6b7-0469402a9f2b},
 created = {2021-07-30T20:25:40.262Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:40.262Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {As wind turbines grow larger, loads increase dramatically, particularly in the inboard region of the blade. A key problem is to design a strong inboard region that supports these loads without sacrificing too much aerodynamic performance. A new design is proposed: a biplane inboard region that transitions into a joint, which connects to a monoplane outboard region. The objective is to develop biplane inboard configurations that improve the aero-structural performance of blades. To approximately compare a conventional inboard region with a biplane inboard region, cross-sectional properties of a thick monoplane and a biplane were measured. Numerical simulations were used to explicitly compare the aerodynamic performance of a thick monoplane with a biplane. Then, several model beams were designed to be simple approximations of a conventional blade ('monoplane beam') and the biplane blade ('biplane beam'). Canonical bending loads were applied to each model beam, and their deflections were compared. Numerical simulations show that the lift-to-drag ratio is significantly greater for the biplane than the thick monoplane for 0° < α < 15.5°. A parametric analysis of biplane beam configurations shows that their tip deflections are smaller than monoplane beams of the same length. These benefits for the inboard region of (i) improved aerodynamics and (ii) improved strength could lead to weight reductions in wind turbine blades. Innovations that create lighter blades can make large blades a reality. These results suggest that the biplane blade is an attractive design for large blades. Copyright © 2012 John Wiley & Sons, Ltd.},
 bibtype = {article},
 author = {Roth-Johnson, Perry and Wirz, Richard E.},
 doi = {10.1002/we.1583},
 journal = {Wind Energy},
 number = {3}
}

As wind turbines grow larger, loads increase dramatically, particularly in the inboard region of the blade. A key problem is to design a strong inboard region that supports these loads without sacrificing too much aerodynamic performance. A new design is proposed: a biplane inboard region that transitions into a joint, which connects to a monoplane outboard region. The objective is to develop biplane inboard configurations that improve the aero-structural performance of blades. To approximately compare a conventional inboard region with a biplane inboard region, cross-sectional properties of a thick monoplane and a biplane were measured. Numerical simulations were used to explicitly compare the aerodynamic performance of a thick monoplane with a biplane. Then, several model beams were designed to be simple approximations of a conventional blade ('monoplane beam') and the biplane blade ('biplane beam'). Canonical bending loads were applied to each model beam, and their deflections were compared. Numerical simulations show that the lift-to-drag ratio is significantly greater for the biplane than the thick monoplane for 0° < α < 15.5°. A parametric analysis of biplane beam configurations shows that their tip deflections are smaller than monoplane beams of the same length. These benefits for the inboard region of (i) improved aerodynamics and (ii) improved strength could lead to weight reductions in wind turbine blades. Innovations that create lighter blades can make large blades a reality. These results suggest that the biplane blade is an attractive design for large blades. Copyright © 2012 John Wiley & Sons, Ltd.

@article{
 title = {Magnetic field structure influence on primary electron cusp losses for micro-scale discharges},
 type = {article},
 year = {2014},
 volume = {21},
 id = {5e0fc2d6-bd6c-3a4a-90e1-ea40b421448d},
 created = {2021-07-30T20:25:42.247Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:42.247Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {An experimental effort was used to examine the primary electron loss behavior for micro-scale (<3cm diameter) discharges. The experiment uses an electron flood gun source and an axially aligned arrangement of ring-cusps to guide the electrons to a downstream point cusp. Measurements of the electron current collected at the point cusp show an unexpectedly complex loss pattern with azimuthally periodic structures. Additionally, in contrast to conventional theory for cusp losses, the overall radii of the measured collection areas are over an order of magnitude larger than the electron gyroradius. Comparing these results to Monte Carlo particle tracking simulations and a simplified analytical analysis shows that azimuthal asymmetries of the magnetic field far upstream of the collection surface can substantially affect the electron loss structure and overall loss area. © 2014 AIP Publishing LLC.},
 bibtype = {article},
 author = {Dankongkakul, Ben and Araki, Samuel J. and Wirz, Richard E.},
 doi = {10.1063/1.4871724},
 journal = {Physics of Plasmas},
 number = {4}
}

An experimental effort was used to examine the primary electron loss behavior for micro-scale (<3cm diameter) discharges. The experiment uses an electron flood gun source and an axially aligned arrangement of ring-cusps to guide the electrons to a downstream point cusp. Measurements of the electron current collected at the point cusp show an unexpectedly complex loss pattern with azimuthally periodic structures. Additionally, in contrast to conventional theory for cusp losses, the overall radii of the measured collection areas are over an order of magnitude larger than the electron gyroradius. Comparing these results to Monte Carlo particle tracking simulations and a simplified analytical analysis shows that azimuthal asymmetries of the magnetic field far upstream of the collection surface can substantially affect the electron loss structure and overall loss area. © 2014 AIP Publishing LLC.

@article{
 title = {Plasma structure of miniature ring-cusp ion thruster discharges},
 type = {article},
 year = {2014},
 volume = {30},
 id = {50f76cc5-5b29-3914-8413-2675274c46fe},
 created = {2021-07-30T20:25:45.431Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:45.431Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Previous miniature ion thruster studies have demonstrated impressive performance using ring-cusp discharges. These studies suggest that the magnetic field must be sufficiently strong to increase primary electron confinement times for ionization, but weak enough to allow plasma electrons to escape and maintain the plasma potential necessary for ionization. To investigate these phenomena, an experiment was developed to allow detailed measurements of the internal structure and characteristics of a miniature ring-cusp discharge. These measurements provide spatially resolved values for plasma density, electron temperature, and plasma potential along a meridian plane. The magnetic field configuration is arranged as a quasi-periodic domain in order to generalize the findings to all multipole discharges. The results show that the magnetic field strength drives the plasma structure, and the dependence on discharge power can be removed with proper scaling of the plasma parameters. The stronger magnetic field results in a higher peak plasma density, but relatively low discharge utilization efficiency. In addition, the potential measurements indicate the likely onset of discharge instability. In contrast, the weaker magnetic field, or baseline configuration, better uses the volume of the chamber. This leads to a higher and more uniform density near the downstream end of the discharge where ion extraction would occur, implying superior discharge utilization. Copyright © 2013 by the American Institute of Aeronautics and Astronautics, Inc.},
 bibtype = {article},
 author = {Mao, Hann Shin and Wirz, Richard E. and Goebel, Dan M.},
 doi = {10.2514/1.B34759},
 journal = {Journal of Propulsion and Power},
 number = {3}
}

Previous miniature ion thruster studies have demonstrated impressive performance using ring-cusp discharges. These studies suggest that the magnetic field must be sufficiently strong to increase primary electron confinement times for ionization, but weak enough to allow plasma electrons to escape and maintain the plasma potential necessary for ionization. To investigate these phenomena, an experiment was developed to allow detailed measurements of the internal structure and characteristics of a miniature ring-cusp discharge. These measurements provide spatially resolved values for plasma density, electron temperature, and plasma potential along a meridian plane. The magnetic field configuration is arranged as a quasi-periodic domain in order to generalize the findings to all multipole discharges. The results show that the magnetic field strength drives the plasma structure, and the dependence on discharge power can be removed with proper scaling of the plasma parameters. The stronger magnetic field results in a higher peak plasma density, but relatively low discharge utilization efficiency. In addition, the potential measurements indicate the likely onset of discharge instability. In contrast, the weaker magnetic field, or baseline configuration, better uses the volume of the chamber. This leads to a higher and more uniform density near the downstream end of the discharge where ion extraction would occur, implying superior discharge utilization. Copyright © 2013 by the American Institute of Aeronautics and Astronautics, Inc.

@article{
 title = {Cell-centered particle weighting algorithm for PIC simulations in a non-uniform 2D axisymmetric mesh},
 type = {article},
 year = {2014},
 volume = {272},
 id = {7314ab5f-c21c-3d96-938b-32fc49a9efa3},
 created = {2021-07-30T20:25:50.411Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:50.411Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Standard area weighting methods for particle-in-cell simulations result in systematic errors on particle densities for a non-uniform mesh in cylindrical coordinates. These errors can be significantly reduced by using weighted cell volumes for density calculations. A detailed description on the corrected volume calculations and cell-centered weighting algorithm in a non-uniform mesh is provided. The simple formulas for the corrected volume can be used for any type of quadrilateral and/or triangular mesh in cylindrical coordinates. Density errors arising from the cell-centered weighting algorithm are computed for radial density profiles of uniform, linearly decreasing, and Bessel function in an adaptive Cartesian mesh and an unstructured mesh. For all the density profiles, it is shown that the weighting algorithm provides a significant improvement for density calculations. However, relatively large density errors may persist at outermost cells for monotonically decreasing density profiles. A further analysis has been performed to investigate the effect of the density errors in potential calculations, and it is shown that the error at the outermost cell does not propagate into the potential solution for the density profiles investigated. © 2014 Elsevier Inc.},
 bibtype = {article},
 author = {Araki, Samuel J. and Wirz, Richard E.},
 doi = {10.1016/j.jcp.2014.04.037},
 journal = {Journal of Computational Physics}
}

Standard area weighting methods for particle-in-cell simulations result in systematic errors on particle densities for a non-uniform mesh in cylindrical coordinates. These errors can be significantly reduced by using weighted cell volumes for density calculations. A detailed description on the corrected volume calculations and cell-centered weighting algorithm in a non-uniform mesh is provided. The simple formulas for the corrected volume can be used for any type of quadrilateral and/or triangular mesh in cylindrical coordinates. Density errors arising from the cell-centered weighting algorithm are computed for radial density profiles of uniform, linearly decreasing, and Bessel function in an adaptive Cartesian mesh and an unstructured mesh. For all the density profiles, it is shown that the weighting algorithm provides a significant improvement for density calculations. However, relatively large density errors may persist at outermost cells for monotonically decreasing density profiles. A further analysis has been performed to investigate the effect of the density errors in potential calculations, and it is shown that the error at the outermost cell does not propagate into the potential solution for the density profiles investigated. © 2014 Elsevier Inc.

@article{
 title = {Algorithm to optimize transient hot-wire thermal property measurement},
 type = {article},
 year = {2014},
 volume = {85},
 id = {7520f134-6d3b-3cf6-95b0-5b2ba2ff79ce},
 created = {2021-07-30T20:25:52.137Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:52.137Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The transient hot-wire method has been widely used to measure the thermal conductivity of fluids. The ideal working equation is based on the solution of the transient heat conduction equation for an infinite linear heat source assuming no natural convection or thermal end effects. In practice, the assumptions inherent in the model are only valid for a portion of the measurement time. In this study, an algorithm was developed to automatically select the proper data range from a transient hot-wire experiment. Numerical simulations of the experiment were used in order to validate the algorithm. The experimental results show that the developed algorithm can be used to improve the accuracy of thermal conductivity measurements. © 2014 AIP Publishing LLC.},
 bibtype = {article},
 author = {Bran-Anleu, Gabriela and Lavine, Adrienne S. and Wirz, Richard E. and Kavehpour, H. Pirouz},
 doi = {10.1063/1.4870275},
 journal = {Review of Scientific Instruments},
 number = {4}
}

The transient hot-wire method has been widely used to measure the thermal conductivity of fluids. The ideal working equation is based on the solution of the transient heat conduction equation for an infinite linear heat source assuming no natural convection or thermal end effects. In practice, the assumptions inherent in the model are only valid for a portion of the measurement time. In this study, an algorithm was developed to automatically select the proper data range from a transient hot-wire experiment. Numerical simulations of the experiment were used in order to validate the algorithm. The experimental results show that the developed algorithm can be used to improve the accuracy of thermal conductivity measurements. © 2014 AIP Publishing LLC.

@inproceedings{
 title = {Magnetically shielded miniature hall thruster: Performance assessment and status update},
 type = {inproceedings},
 year = {2014},
 id = {4d68d241-757a-31b1-9800-56bf26c10317},
 created = {2021-07-30T22:42:54.228Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:54.228Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The magnetically shielded miniature Hall thruster, originally tested at the University of California, Los Angeles, underwent performance validation experiments at the Jet Propulsion Laboratory. The thruster was operated over a range of discharge voltages, from 150 V – 300V, and currents, from 1 A – 2.3 A. It was discovered that the thruster operated in two distinct modes which were dependent on the thruster’s temperature: a “jet” mode and a “diffuse” mode. At the nominal condition of 275 V and 1.2 A in the jet mode, a thrust of approximately 12 – 13 mN was measured by a thrust stand with an anode efficiency of approximately24%. At the same nominal conditions, the diffuse mode showed a thrust of 11 – 12 mN and an anode efficiency of approximately 21%. Characterization of the plume in both operating modes was accomplished using a shielded Faraday probe, a retarding potential analyzer, and an ExB probe. Discharge current oscillations on the order of 2.5 – 4 times the mean current were observed during jet mode operation, while the oscillations in the diffuse mode were on the order of 20% of the mean current. Results from the plume characterization, post-operation discharge channel inspection, and discharge oscillations, combined with the temperature-dependent mode shift, suggest that changes to the magnetic field strength and topology caused by saturation of the thruster’s magnetic circuit may be occurring at elevated operating temperatures.},
 bibtype = {inproceedings},
 author = {Conversano, Ryan W. and Goebel, Dan M. and Mikellides, Ioannis G. and Hofer, Richard R. and Matlock, Taylor S. and Wirz, Richard E.},
 doi = {10.2514/6.2014-3896},
 booktitle = {50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference 2014}
}

The magnetically shielded miniature Hall thruster, originally tested at the University of California, Los Angeles, underwent performance validation experiments at the Jet Propulsion Laboratory. The thruster was operated over a range of discharge voltages, from 150 V – 300V, and currents, from 1 A – 2.3 A. It was discovered that the thruster operated in two distinct modes which were dependent on the thruster’s temperature: a “jet” mode and a “diffuse” mode. At the nominal condition of 275 V and 1.2 A in the jet mode, a thrust of approximately 12 – 13 mN was measured by a thrust stand with an anode efficiency of approximately24%. At the same nominal conditions, the diffuse mode showed a thrust of 11 – 12 mN and an anode efficiency of approximately 21%. Characterization of the plume in both operating modes was accomplished using a shielded Faraday probe, a retarding potential analyzer, and an ExB probe. Discharge current oscillations on the order of 2.5 – 4 times the mean current were observed during jet mode operation, while the oscillations in the diffuse mode were on the order of 20% of the mean current. Results from the plume characterization, post-operation discharge channel inspection, and discharge oscillations, combined with the temperature-dependent mode shift, suggest that changes to the magnetic field strength and topology caused by saturation of the thruster’s magnetic circuit may be occurring at elevated operating temperatures.

@inproceedings{
 title = {Magnetic field aligned mesh for ring-cusp discharge chambers},
 type = {inproceedings},
 year = {2014},
 id = {b500ba47-a12e-3a89-91d4-4053cf754596},
 created = {2021-07-30T22:42:54.765Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:54.765Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Ring-cusp discharges use multiple permanent magnet rings to facilitate confinement of the high energy electrons and other plasma species. The magnetic field configuration con- sists of converging fields toward the magnetic cusps with a large ratio of the maximum to the minimum magnetic field strength. In such configurations, a plasma fluid simula- tion with a regular mesh can lead to undesirable numerical diffusion errors caused by the anisotropicity of plasma and the misalignment of the grid with the field. In this paper, details of a custom technique for magnetic field aligned (MFA) mesh generation are pro- vided. The MFA meshes may consist of irregular nodes that are attached to more than four mesh elements. The numerical method developed here can be applied to such meshes to solve the plasma diffusion equation. The combination of the MFA mesh generation and the numerical method is applied to two test cases: perfectly anisotropic plasma in a single cusp configuration and moderately anisotropic plasma in a ring-cusp discharge-like configuration. It is shown that a simulation with the MFA mesh can provide a more ac- curate solution compared with the regular mesh with a similar number of mesh elements. However, realizing the inherent benefits of the MFA mesh often requires improvements on the mesh quality near the boundary, in particular, the smoothness of the near-boundary elements.},
 bibtype = {inproceedings},
 author = {Araki, Samuel J. and Wirz, Richard E.},
 doi = {10.2514/6.2014-3830},
 booktitle = {50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference 2014}
}

Ring-cusp discharges use multiple permanent magnet rings to facilitate confinement of the high energy electrons and other plasma species. The magnetic field configuration con- sists of converging fields toward the magnetic cusps with a large ratio of the maximum to the minimum magnetic field strength. In such configurations, a plasma fluid simula- tion with a regular mesh can lead to undesirable numerical diffusion errors caused by the anisotropicity of plasma and the misalignment of the grid with the field. In this paper, details of a custom technique for magnetic field aligned (MFA) mesh generation are pro- vided. The MFA meshes may consist of irregular nodes that are attached to more than four mesh elements. The numerical method developed here can be applied to such meshes to solve the plasma diffusion equation. The combination of the MFA mesh generation and the numerical method is applied to two test cases: perfectly anisotropic plasma in a single cusp configuration and moderately anisotropic plasma in a ring-cusp discharge-like configuration. It is shown that a simulation with the MFA mesh can provide a more ac- curate solution compared with the regular mesh with a similar number of mesh elements. However, realizing the inherent benefits of the MFA mesh often requires improvements on the mesh quality near the boundary, in particular, the smoothness of the near-boundary elements.

@inproceedings{
 title = {Exergy-based optimization of sub- and supercritical thermal energy storage systems},
 type = {inproceedings},
 year = {2014},
 volume = {2},
 id = {41887687-ca03-3b84-a1c7-b0856e148320},
 created = {2021-07-30T22:42:55.312Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:55.312Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {In this work, energy and exergy analyses are applied to a thermal energy storage system employing a storage medium in the two-phase or supercritical regime. First, a numerical model is developed to investigate the transient thermodynamic and heat transfer characteristics of the storage system by coupling conservation of energy with an equation of state to model the spatial and temporal variations in fluid properties during the entire working cycle of the TES tank. Second, parametric studies are conducted to determine the impact of key variables (such as heat transfer fluid mass flow rate and maximum storage temperature) on both energy and exergy efficiencies. The optimum heat transfer fluid mass flow rate during charging must balance exergy destroyed due to heat transfer and exergy destroyed due to pressure losses, which have competing effects. Similarly, the optimum maximum storage fluid temperature is evaluated to optimize exergetic efficiency. By incorporating exergy-based optimization alongside traditional energy analyses, the results of this study evaluate the optimal values for key parameters in the design and operation of TES systems, as well as highlight opportunities to minimize thermodynamic losses.},
 bibtype = {inproceedings},
 author = {Tse, Louis A. and Wirz, Richard E. and Lakeh, Reza Baghaei and Lavine, Adrienne S.},
 doi = {10.1115/ES2014-6302},
 booktitle = {ASME 2014 8th International Conference on Energy Sustainability, ES 2014 Collocated with the ASME 2014 12th International Conference on Fuel Cell Science, Engineering and Technology}
}

In this work, energy and exergy analyses are applied to a thermal energy storage system employing a storage medium in the two-phase or supercritical regime. First, a numerical model is developed to investigate the transient thermodynamic and heat transfer characteristics of the storage system by coupling conservation of energy with an equation of state to model the spatial and temporal variations in fluid properties during the entire working cycle of the TES tank. Second, parametric studies are conducted to determine the impact of key variables (such as heat transfer fluid mass flow rate and maximum storage temperature) on both energy and exergy efficiencies. The optimum heat transfer fluid mass flow rate during charging must balance exergy destroyed due to heat transfer and exergy destroyed due to pressure losses, which have competing effects. Similarly, the optimum maximum storage fluid temperature is evaluated to optimize exergetic efficiency. By incorporating exergy-based optimization alongside traditional energy analyses, the results of this study evaluate the optimal values for key parameters in the design and operation of TES systems, as well as highlight opportunities to minimize thermodynamic losses.

@inproceedings{
 title = {An algorithm to determine the optimized data range in transient hotwire thermal property measurement},
 type = {inproceedings},
 year = {2013},
 volume = {8 C},
 id = {d5ac8159-bb68-30e1-b4e1-62e6ad041bf1},
 created = {2021-07-30T20:25:29.202Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:29.202Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {There exists a limited amount of data for thermal properties of supercritical fluids. Accurate measurement of these properties is needed to supply benchmark data to the scientific community and to establish promising candidates for supercritical fluids applications such as energy storage systems [1]. The transient hotwire method has been widely used to measure the thermal conductivity of fluids. The ideal working equation is based on the transient solution of Fourier's law for an infinite linear heat source assuming no natural convection or thermal end effects. In practice, the assumptions inherent in the model are only valid for a portion of the measurement time. In this study, an algorithm was developed to automatically select the proper data range from a transient hotwire experiment. Numerical simulations of the experiment were used in order to validate the algorithm. The experimental results show that the developed algorithm can be used to improve the accuracy of thermal conductivity measurements. Copyright © 2013 by ASME.},
 bibtype = {inproceedings},
 author = {Bran-Anleu, Gabriela and Lavine, Adrienne S. and Wirz, Richard E. and Lakeh, Reza Baghaei and Pirouz Kavehpour, H.},
 doi = {10.1115/IMECE2013-64940},
 booktitle = {ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}
}

There exists a limited amount of data for thermal properties of supercritical fluids. Accurate measurement of these properties is needed to supply benchmark data to the scientific community and to establish promising candidates for supercritical fluids applications such as energy storage systems [1]. The transient hotwire method has been widely used to measure the thermal conductivity of fluids. The ideal working equation is based on the transient solution of Fourier's law for an infinite linear heat source assuming no natural convection or thermal end effects. In practice, the assumptions inherent in the model are only valid for a portion of the measurement time. In this study, an algorithm was developed to automatically select the proper data range from a transient hotwire experiment. Numerical simulations of the experiment were used in order to validate the algorithm. The experimental results show that the developed algorithm can be used to improve the accuracy of thermal conductivity measurements. Copyright © 2013 by ASME.

@inproceedings{
 title = {Influence of upstream field structure on primary electron loss for a permanent magnet cusp},
 type = {inproceedings},
 year = {2013},
 volume = {1 PartF},
 id = {b3408706-10e2-3eb7-952b-a23cd13c2d10},
 created = {2021-07-30T20:25:43.789Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:43.789Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {An improved understanding of the electron loss behavior for permanent magnet cusps is needed to enable the design of efficient micro-scale plasma devices. Such devices can be used for a variety of applications, including high performance micro-thrusters that are attractive for primary propulsion for microsatellites, secondary propulsion for larger spacecraft, and formation flying. Conventional plasma loss theory for magnetic cusps generally relates the loss area to an analytical expression related to the magnetic field strength at the loss surface. In contrast, this study examines the importance of the upstream magnetic field structure on the loss behavior of high energy electrons. For the experimental effort, an electron gun is used to inject monoenergetic electrons towards a cusp confined discharge, while precision electron loss measurements are made at the face of a single permanent magnet point cusp. Measurements are taken at facility base pressure and with xenon background gas. A Monte-Carlo model is used to provide detailed examination of the electron confinement and loss behavior. Comparison of the experimental and computational results shows that the primary electron loss behavior is strongly influenced by the upstream magnetic field structure and is not simply dictated by the field strength very near the cusp collection surface.},
 bibtype = {inproceedings},
 author = {Dankongkakul, Ben and Araki, Samuel J. and Wirz, Richard E.},
 doi = {10.2514/6.2013-3963},
 booktitle = {49th AIAA/ASME/SAE/ASEE Joint Propulsion Conference}
}

An improved understanding of the electron loss behavior for permanent magnet cusps is needed to enable the design of efficient micro-scale plasma devices. Such devices can be used for a variety of applications, including high performance micro-thrusters that are attractive for primary propulsion for microsatellites, secondary propulsion for larger spacecraft, and formation flying. Conventional plasma loss theory for magnetic cusps generally relates the loss area to an analytical expression related to the magnetic field strength at the loss surface. In contrast, this study examines the importance of the upstream magnetic field structure on the loss behavior of high energy electrons. For the experimental effort, an electron gun is used to inject monoenergetic electrons towards a cusp confined discharge, while precision electron loss measurements are made at the face of a single permanent magnet point cusp. Measurements are taken at facility base pressure and with xenon background gas. A Monte-Carlo model is used to provide detailed examination of the electron confinement and loss behavior. Comparison of the experimental and computational results shows that the primary electron loss behavior is strongly influenced by the upstream magnetic field structure and is not simply dictated by the field strength very near the cusp collection surface.

@article{
 title = {Effect of laminar and turbulent buoyancydriven flows on thermal energy storage using supercritical fluids},
 type = {article},
 year = {2013},
 volume = {64},
 id = {08c2197d-f0c4-3c50-8996-11d8823eb562},
 created = {2021-07-30T20:25:53.939Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:53.939Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Efficient heat transfer to storage fluid is required for the desirable operation of thermal energy storage systems. Most of the fluid candidates for supercritical thermal storage have poor thermal conductivity; therefore, conduction does not provide sufficient heat transfer. The current study concerns a supercritical thermal energy storage system consisting of horizontal tubes filled with a storage fluid in its supercritical state. The results of this study show that the heat transfer to the supercritical fluid is dominated by laminar and turbulent natural convection. The buoyancy-driven flow inside the storage tubes enhances the heat transfer and dramatically reduces the charge time. Copyright © Taylor & Francis Group, LLC.},
 bibtype = {article},
 author = {Lakeh, Reza Baghaei and Lavine, Adrienne S. and Kavehpour, H. Pirouz and Ganapathi, Gani B. and Wirz, Richard E.},
 doi = {10.1080/10407782.2013.811349},
 journal = {Numerical Heat Transfer; Part A: Applications},
 number = {12}
}

Efficient heat transfer to storage fluid is required for the desirable operation of thermal energy storage systems. Most of the fluid candidates for supercritical thermal storage have poor thermal conductivity; therefore, conduction does not provide sufficient heat transfer. The current study concerns a supercritical thermal energy storage system consisting of horizontal tubes filled with a storage fluid in its supercritical state. The results of this study show that the heat transfer to the supercritical fluid is dominated by laminar and turbulent natural convection. The buoyancy-driven flow inside the storage tubes enhances the heat transfer and dramatically reduces the charge time. Copyright © Taylor & Francis Group, LLC.

@article{
 title = {Reduction of energetic ion production in hollow cathodes by external gas injection},
 type = {article},
 year = {2013},
 volume = {29},
 id = {9542b05b-d566-327d-8219-8451ec454806},
 created = {2021-07-30T20:25:55.700Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:55.700Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Studies of the hollow-cathode discharge have shown the existence of energetic ions at high-discharge currents that are likely responsible for the high erosion rates observed on the cathode keeper electrode. This work examines the effect of neutral gas injection in the discharge plume of a 250 A lanthanum hexaboride hollow cathode on the production of energetic ions to determine the conditions that yield cathode operation and life. Two different gas injector types are used to deliver neutral gas into the discharge plume and a retarding-potential analyzer is used for ion energy measurements. The flow splits between the cathode internal and external flows, and the number and locations of the external gas injection sites are examined as a function of the discharge current. It is found that increasing discharge current increases the energetic ion production at any given flow rate or injection location. External gas injection reduces energetic ion production for constant cathode flow, with collimated gas-jet injection performing better than distributed injection. Lifetime estimates of the keeper electrode surface due to sputter erosion by ion bombardment reveal that high-discharge current operation at low cathode gas flow produced very energetic ions and limited keeper lifetimes to less than 5000 h. Applying sufficient internal cathode gas flow and external gas injection can extend the keeper life to over 10,000 h at discharge currents of up to 200 A.},
 bibtype = {article},
 author = {Chu, Emily and Goebel, Dan M. and Wirz, Richard E.},
 doi = {10.2514/1.B34799},
 journal = {Journal of Propulsion and Power},
 number = {5}
}

Studies of the hollow-cathode discharge have shown the existence of energetic ions at high-discharge currents that are likely responsible for the high erosion rates observed on the cathode keeper electrode. This work examines the effect of neutral gas injection in the discharge plume of a 250 A lanthanum hexaboride hollow cathode on the production of energetic ions to determine the conditions that yield cathode operation and life. Two different gas injector types are used to deliver neutral gas into the discharge plume and a retarding-potential analyzer is used for ion energy measurements. The flow splits between the cathode internal and external flows, and the number and locations of the external gas injection sites are examined as a function of the discharge current. It is found that increasing discharge current increases the energetic ion production at any given flow rate or injection location. External gas injection reduces energetic ion production for constant cathode flow, with collimated gas-jet injection performing better than distributed injection. Lifetime estimates of the keeper electrode surface due to sputter erosion by ion bombardment reveal that high-discharge current operation at low cathode gas flow produced very energetic ions and limited keeper lifetimes to less than 5000 h. Applying sufficient internal cathode gas flow and external gas injection can extend the keeper life to over 10,000 h at discharge currents of up to 200 A.

@article{
 title = {Mission capability assessment of CubeSats using a miniature ion thruster},
 type = {article},
 year = {2013},
 volume = {50},
 id = {d6d1ae8e-47d8-3cf7-a014-df56c1daa61b},
 created = {2021-07-30T20:25:57.358Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:57.358Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The successful miniaturization of many spacecraft subsystems make CubeSats attractive candidates for evermoredemanding scientific missions. A three-cell CubeSat employing the miniature xenon ion thruster, which features high efficiency and impulse capability, yields a unique spacecraft that can be optimized for a variety of missions ranging from significant inclination changes in a low Earth orbit to lunar transfers. A nominal configuration of a high-ΔV CubeSat has a dry mass of approximately 6.3 kg, including a 0.75 kg payload, margins, and contingencies. Depending on the thruster and propellant tank configuration, this CubeSat is capable of delivering mission ΔV values from 1000 to over 7000 m/s, enabling low-Earth-orbit inclination change missions and lunar missions. Aparametric analysis on a three-cell high-ΔV CubeSat bus revealed that a range of payload volumes (up to nearly 1.4 units) and masses (up to nearly 6 kg) can be accommodated depending on the ΔV requirements and mission type. Additionally, this analysis showed that a high-ΔV three-cell CubeSat in a 600 km low Earth orbit can be designed to provide an inclination change of over 80 deg. © 2012 AIAA.},
 bibtype = {article},
 author = {Conversano, Ryan W. and Wirz, Richard E.},
 doi = {10.2514/1.A32435},
 journal = {Journal of Spacecraft and Rockets},
 number = {5}
}

The successful miniaturization of many spacecraft subsystems make CubeSats attractive candidates for evermoredemanding scientific missions. A three-cell CubeSat employing the miniature xenon ion thruster, which features high efficiency and impulse capability, yields a unique spacecraft that can be optimized for a variety of missions ranging from significant inclination changes in a low Earth orbit to lunar transfers. A nominal configuration of a high-ΔV CubeSat has a dry mass of approximately 6.3 kg, including a 0.75 kg payload, margins, and contingencies. Depending on the thruster and propellant tank configuration, this CubeSat is capable of delivering mission ΔV values from 1000 to over 7000 m/s, enabling low-Earth-orbit inclination change missions and lunar missions. Aparametric analysis on a three-cell high-ΔV CubeSat bus revealed that a range of payload volumes (up to nearly 1.4 units) and masses (up to nearly 6 kg) can be accommodated depending on the ΔV requirements and mission type. Additionally, this analysis showed that a high-ΔV three-cell CubeSat in a 600 km low Earth orbit can be designed to provide an inclination change of over 80 deg. © 2012 AIAA.

@article{
 title = {Ion-neutral collision modeling using classical scattering with spin-orbit free interaction potential},
 type = {article},
 year = {2013},
 volume = {41},
 id = {42685627-0b04-32f4-b35f-cce024c5bb76},
 created = {2021-07-30T20:25:59.381Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:59.381Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A particle-in-cell Monte Carlo collision model is developed to explore dominant collisional effects on high-velocity xenon ions incident to a quiescent xenon gas at low neutral pressures. The range of neutral pressure and collisionality examined are applicable for electric propulsion as well as plasma processing devices; therefore, the computational technique described herein can be applied to more complex simulations of those devices. Momentum and resonant charge-exchange collisions between ions and background neutrals are implemented using two different models, classical scattering with spin-orbit free potential and variable-hard-sphere model, depending on the incident particle energy. The primary and charge-exchange ions are tracked separately, and their trajectories within a well-defined 'Test Cell' domain are determined. Predicted electrode currents as a function of the Test Cell pressure are compared with electrode currents measured in an ion gun experiment. The simulation results agree well with the experiment up to a Test Cell pressure corresponding to a mean free path of the Test Cell length and then start to deviate with increasing collisionality at higher pressures. This discrepancy at higher pressures is likely due to the increasing influence of secondary electrons emitted from electrodes due to the high-velocity impacts of heavy species (i.e., beam ions and fast neutrals created by charge-exchange interaction) at the electrode surfaces. © 1973-2012 IEEE.},
 bibtype = {article},
 author = {Araki, Samuel J. and Wirz, Richard E.},
 doi = {10.1109/TPS.2013.2241457},
 journal = {IEEE Transactions on Plasma Science},
 number = {3}
}

A particle-in-cell Monte Carlo collision model is developed to explore dominant collisional effects on high-velocity xenon ions incident to a quiescent xenon gas at low neutral pressures. The range of neutral pressure and collisionality examined are applicable for electric propulsion as well as plasma processing devices; therefore, the computational technique described herein can be applied to more complex simulations of those devices. Momentum and resonant charge-exchange collisions between ions and background neutrals are implemented using two different models, classical scattering with spin-orbit free potential and variable-hard-sphere model, depending on the incident particle energy. The primary and charge-exchange ions are tracked separately, and their trajectories within a well-defined 'Test Cell' domain are determined. Predicted electrode currents as a function of the Test Cell pressure are compared with electrode currents measured in an ion gun experiment. The simulation results agree well with the experiment up to a Test Cell pressure corresponding to a mean free path of the Test Cell length and then start to deviate with increasing collisionality at higher pressures. This discrepancy at higher pressures is likely due to the increasing influence of secondary electrons emitted from electrodes due to the high-velocity impacts of heavy species (i.e., beam ions and fast neutrals created by charge-exchange interaction) at the electrode surfaces. © 1973-2012 IEEE.

@article{
 title = {Fuel optimum low-thrust elliptic transfer using numerical averaging},
 type = {article},
 year = {2013},
 volume = {86},
 id = {16e426e7-4100-33a8-aa7e-a43b492f2f32},
 created = {2021-07-30T20:26:04.816Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:26:04.816Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Low-thrust electric propulsion is increasingly being used for spacecraft missions primarily due to its high propellant efficiency. As a result, a simple and fast method for low-thrust trajectory optimization is of great value for preliminary mission planning. However, few low-thrust trajectory tools are appropriate for preliminary mission design studies. The method presented in this paper provides quick and accurate solutions for a wide range of transfers by using numerical orbital averaging to improve solution convergence and include orbital perturbations. Thus, preliminary trajectories can be obtained for transfers which involve many revolutions about the primary body. This method considers minimum fuel transfers using first-order averaging to obtain the fuel optimum rates of change of the equinoctial orbital elements in terms of each other and the Lagrange multipliers. Constraints on thrust and power, as well as minimum periapsis, are implemented and the equations are averaged numerically using a Gausian quadrature. The use of numerical averaging allows for more complex orbital perturbations to be added in the future without great difficulty. The effects of zonal gravity harmonics, solar radiation pressure, and thrust limitations due to shadowing are included in this study. The solution to a transfer which minimizes the square of the thrust magnitude is used as a preliminary guess for the minimum fuel problem, thus allowing for faster convergence to a wider range of problems. Results from this model are shown to provide a reduction in propellant mass required over previous minimum fuel solutions. © 2013 Elsevier Ltd. All rights reserved.},
 bibtype = {article},
 author = {Tarzi, Zahi and Speyer, Jason and Wirz, Richard},
 doi = {10.1016/j.actaastro.2013.01.003},
 journal = {Acta Astronautica}
}

Low-thrust electric propulsion is increasingly being used for spacecraft missions primarily due to its high propellant efficiency. As a result, a simple and fast method for low-thrust trajectory optimization is of great value for preliminary mission planning. However, few low-thrust trajectory tools are appropriate for preliminary mission design studies. The method presented in this paper provides quick and accurate solutions for a wide range of transfers by using numerical orbital averaging to improve solution convergence and include orbital perturbations. Thus, preliminary trajectories can be obtained for transfers which involve many revolutions about the primary body. This method considers minimum fuel transfers using first-order averaging to obtain the fuel optimum rates of change of the equinoctial orbital elements in terms of each other and the Lagrange multipliers. Constraints on thrust and power, as well as minimum periapsis, are implemented and the equations are averaged numerically using a Gausian quadrature. The use of numerical averaging allows for more complex orbital perturbations to be added in the future without great difficulty. The effects of zonal gravity harmonics, solar radiation pressure, and thrust limitations due to shadowing are included in this study. The solution to a transfer which minimizes the square of the thrust magnitude is used as a preliminary guess for the minimum fuel problem, thus allowing for faster convergence to a wider range of problems. Results from this model are shown to provide a reduction in propellant mass required over previous minimum fuel solutions. © 2013 Elsevier Ltd. All rights reserved.

@inproceedings{
 title = {The plasma interactions facility at ucla},
 type = {inproceedings},
 year = {2013},
 volume = {1 PartF},
 id = {2a539f12-7d8d-3098-9141-ab41e442f557},
 created = {2021-07-30T22:42:55.999Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:55.999Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A new device has been constructed for the investigation of interactions between engineered materials and a plasma in the regime relevant to electric propulsion. The facility is comprised of a cylindrical vacuum chamber with a flange-mounted, hollow cathode plasma source running perpendicular to the chamber axis. The plasma source delivers unmagnetized ions and thermal, magnetized electrons to a cooled target mounted on the opposing flange. The ion energy impacting the surface is controlled by the sample bias relative to the plasma potential, which is maintained near the chamber potential. This paper discusses the major aspects of the plasma source design and presents preliminary measurements of the plasma parameters achieved to date.},
 bibtype = {inproceedings},
 author = {Matlock, Taylor and Goebel, Dan and Conversano, Ryan and Wirz, Richard},
 doi = {10.2514/6.2013-4132},
 booktitle = {49th AIAA/ASME/SAE/ASEE Joint Propulsion Conference}
}

A new device has been constructed for the investigation of interactions between engineered materials and a plasma in the regime relevant to electric propulsion. The facility is comprised of a cylindrical vacuum chamber with a flange-mounted, hollow cathode plasma source running perpendicular to the chamber axis. The plasma source delivers unmagnetized ions and thermal, magnetized electrons to a cooled target mounted on the opposing flange. The ion energy impacting the surface is controlled by the sample bias relative to the plasma potential, which is maintained near the chamber potential. This paper discusses the major aspects of the plasma source design and presents preliminary measurements of the plasma parameters achieved to date.

@inproceedings{
 title = {Magnetically Shielded Miniature Hall Thruster : Development and Initial Testing},
 type = {inproceedings},
 year = {2013},
 id = {417fffaf-9e41-30a7-845a-3c5dfe640471},
 created = {2021-07-30T22:42:56.644Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:56.644Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The scaling of magnetically shielded Hall thrusters to low-power is investigated through the development and fabrication of a 4 cm Hall thruster. During initial testing, the magnetically shielded miniature Hall thruster was operated at 275 V discharge voltage and 325 W discharge power. Inspection of the channel walls after testing suggests that the outer discharge channel wall was successfully shielded from high-energy ion erosion while the inner channel wall showed evidence of weaker shielding, likely due to magnetic circuit saturation. Scanning planar probe measurements taken at 4.4 cm and 10.8 cm downstream of the thruster face provided ion current density profiles. The ion current calculated by integrating these data was 1.04 A with a plume divergence half-angle of 30°. Swept retarding potential analyzer measurements taken at 80 cm axially downstream of the thruster measured the most probable ion voltage to be 252 V. The total thruster efficiency was calculated from probe measurements to be 43% (anode efficiency of 59%) corresponding to a thrust of 19 mN at a specific impulse of 1870 s. Discharge channel erosion rates were found to be approximately three orders of magnitude less than unshielded Hall thrusters, suggesting a significant increase in projected operational life. 1},
 bibtype = {inproceedings},
 author = {Conversano, Ryan W and Goebel, Dan M and Hofer, Richard R},
 booktitle = {IEPC-2013-201 Presented}
}

The scaling of magnetically shielded Hall thrusters to low-power is investigated through the development and fabrication of a 4 cm Hall thruster. During initial testing, the magnetically shielded miniature Hall thruster was operated at 275 V discharge voltage and 325 W discharge power. Inspection of the channel walls after testing suggests that the outer discharge channel wall was successfully shielded from high-energy ion erosion while the inner channel wall showed evidence of weaker shielding, likely due to magnetic circuit saturation. Scanning planar probe measurements taken at 4.4 cm and 10.8 cm downstream of the thruster face provided ion current density profiles. The ion current calculated by integrating these data was 1.04 A with a plume divergence half-angle of 30°. Swept retarding potential analyzer measurements taken at 80 cm axially downstream of the thruster measured the most probable ion voltage to be 252 V. The total thruster efficiency was calculated from probe measurements to be 43% (anode efficiency of 59%) corresponding to a thrust of 19 mN at a specific impulse of 1870 s. Discharge channel erosion rates were found to be approximately three orders of magnitude less than unshielded Hall thrusters, suggesting a significant increase in projected operational life. 1

@inproceedings{
 title = {Isobaric, isochoric and supercritical thermal energy storage in R134a},
 type = {inproceedings},
 year = {2013},
 volume = {6 B},
 id = {a981f4c0-23e6-3329-b0ee-d7a7f99c85e4},
 created = {2021-07-30T22:42:57.338Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:57.338Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The effective thermal energy density of R134a subjected to an isobaric or isochoric process is determined and evaluated in the two-phase and supercritical regimes. The results are qualitatively extended to other fluids via the principle of corresponding states. It is shown that substantial increases in volumetric energy density can be realized in the critical region for isobaric processes. Also, for isobaric processes which utilize the full enthalpy of vaporization at a given pressure, there exists a pressure at which the volumetric energy density is a maximum. For isochoric processes (supercritical and twophase), it is found that there is no appreciable increase in volumetric energy density over sensible liquid heat storage; the effective specific heat can be enhanced in the two-phase, isochoric regime, but only with a significant reduction in volumetric energy density. Copyright © 2013 by ASME.},
 bibtype = {inproceedings},
 author = {Furst, Benjamin I. and Lavine, Adrienne S. and Lakeh, Reza Baghaei and Wirz, Richard E.},
 doi = {10.1115/IMECE2013-64947},
 booktitle = {ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}
}

The effective thermal energy density of R134a subjected to an isobaric or isochoric process is determined and evaluated in the two-phase and supercritical regimes. The results are qualitatively extended to other fluids via the principle of corresponding states. It is shown that substantial increases in volumetric energy density can be realized in the critical region for isobaric processes. Also, for isobaric processes which utilize the full enthalpy of vaporization at a given pressure, there exists a pressure at which the volumetric energy density is a maximum. For isochoric processes (supercritical and twophase), it is found that there is no appreciable increase in volumetric energy density over sensible liquid heat storage; the effective specific heat can be enhanced in the two-phase, isochoric regime, but only with a significant reduction in volumetric energy density. Copyright © 2013 by ASME.

@inproceedings{
 title = {Thermal testing of organic fluids for supercriticalthermal energy storage systems},
 type = {inproceedings},
 year = {2013},
 id = {1793b8dc-f145-3deb-97ea-0d2d4646d8e3},
 created = {2021-07-30T22:42:57.899Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:57.899Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Concentrating solar power (CSP) continues to advance as worldwide interest in renewable energy continues to grow. CSP technologies, including parabolic troughs, power towers, and dish/engines, provide the unique potential for low-cost thermal energy storage that will ensure that renewable energy can become cost-competitive with traditional fossil fuel sources on a large scale and comprise a significant portion of the global energy portfolio. The challenge is to develop cost-effective thermal energy storage to ensure that renewable energy can become a major part of the national and global energy supply. Storage fluid selection is a critical decision that must fulfill a number of criteria to not only provide long-term reliability, but also to remain cost-competitive in the power generation arena. The state-of-the-art thermal storage design uses a 2-tank molten salt configuration. However, most molten salt mixtures have a relatively high freezing temperature, which poses some system design issues. Additionally, the price of molten salt mixtures is steadily increasing. Current laboratory and industry research efforts have shifted focus to exploration of alternative storage fluids to significantly reduce costs. In this study, several storage fluid candidates have been selected based on an attractive combination of thermodynamic properties, cost, and availability. In this paper, rapid screening of fluid candidates is reported, and an expanded series of thermal cycling and supercritical characterization experiments have been planned and are being implemented to determine the long-term durability of the fluid candidates over a range of operating temperatures for extended periods of time. Commercial-grade materials were used, and in the case of naphthalene and biphenyl, the testing procedure was carefully controlled to prevent sublimation of the sample. This paper presents the results of a study investigating the thermal stability of several organic fluids. Samples were extracted and chemical analyses such as nuclear magnetic resonance (NMR) and gas chromatography (GC) were conducted to observe degradation behavior and decomposition pathways. The rapid screening phase provided a timely and effective filter of the best-performing fluid candidates for supercritical thermal energy storage. Copyright © 2013 by ASME.},
 bibtype = {inproceedings},
 author = {Tse, Louis A. and Stopin, Antoine and Ganapathi, Gani B. and Garcia-Garibay, Miguel A. and Wirz, Richard E.},
 doi = {10.1115/ES2013-18195},
 booktitle = {ASME 2013 7th Int. Conf. on Energy Sustainability Collocated with the ASME 2013 Heat Transfer Summer Conf. and the ASME 2013 11th Int. Conf. on Fuel Cell Science, Engineering and Technology, ES 2013}
}

Concentrating solar power (CSP) continues to advance as worldwide interest in renewable energy continues to grow. CSP technologies, including parabolic troughs, power towers, and dish/engines, provide the unique potential for low-cost thermal energy storage that will ensure that renewable energy can become cost-competitive with traditional fossil fuel sources on a large scale and comprise a significant portion of the global energy portfolio. The challenge is to develop cost-effective thermal energy storage to ensure that renewable energy can become a major part of the national and global energy supply. Storage fluid selection is a critical decision that must fulfill a number of criteria to not only provide long-term reliability, but also to remain cost-competitive in the power generation arena. The state-of-the-art thermal storage design uses a 2-tank molten salt configuration. However, most molten salt mixtures have a relatively high freezing temperature, which poses some system design issues. Additionally, the price of molten salt mixtures is steadily increasing. Current laboratory and industry research efforts have shifted focus to exploration of alternative storage fluids to significantly reduce costs. In this study, several storage fluid candidates have been selected based on an attractive combination of thermodynamic properties, cost, and availability. In this paper, rapid screening of fluid candidates is reported, and an expanded series of thermal cycling and supercritical characterization experiments have been planned and are being implemented to determine the long-term durability of the fluid candidates over a range of operating temperatures for extended periods of time. Commercial-grade materials were used, and in the case of naphthalene and biphenyl, the testing procedure was carefully controlled to prevent sublimation of the sample. This paper presents the results of a study investigating the thermal stability of several organic fluids. Samples were extracted and chemical analyses such as nuclear magnetic resonance (NMR) and gas chromatography (GC) were conducted to observe degradation behavior and decomposition pathways. The rapid screening phase provided a timely and effective filter of the best-performing fluid candidates for supercritical thermal energy storage. Copyright © 2013 by ASME.

@article{
 title = {Near anode plasma in the plasma interaction facility at},
 type = {article},
 year = {2013},
 id = {aea4ffa8-e909-33de-ae78-8aaf05d66970},
 created = {2021-07-30T22:42:58.469Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:58.469Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A DC cylindrical discharge plasma column is examined in the region between an upstream anode and an axial, magnetized guide section that delivers the plasma created in the anode region to a target downstream. Radial profiles of the plasma potential, density and temperature are obtained with floating emissive and double probes. An alternative method for double probe implementation is discussed and shown to result in plasma potential profiles closely matching those obtained using a floating emissive probe technique. Plasma profiles measured for a range of magnetic field configurations demonstrate the existence of distinct modes of operation of the plasma source. Time resolved measurements of floating potential with an emissive probe reveal several low frequency oscillations which become significant past a critical magnetic field near 100 Gauss. These measurements are performed to inform analytical and numerical models of the device which is being developed to deliver a stable, well defined plasma to a target surface for plasma-material interaction studies.},
 bibtype = {article},
 author = {Matlock, Taylor and Goebel, Dan and Wirz, Richard and Ucla, undefined},
 journal = {33rd International Electric Propulsion Conference}
}

A DC cylindrical discharge plasma column is examined in the region between an upstream anode and an axial, magnetized guide section that delivers the plasma created in the anode region to a target downstream. Radial profiles of the plasma potential, density and temperature are obtained with floating emissive and double probes. An alternative method for double probe implementation is discussed and shown to result in plasma potential profiles closely matching those obtained using a floating emissive probe technique. Plasma profiles measured for a range of magnetic field configurations demonstrate the existence of distinct modes of operation of the plasma source. Time resolved measurements of floating potential with an emissive probe reveal several low frequency oscillations which become significant past a critical magnetic field near 100 Gauss. These measurements are performed to inform analytical and numerical models of the device which is being developed to deliver a stable, well defined plasma to a target surface for plasma-material interaction studies.

@inproceedings{
 title = {Study of turbulent natural convection in vertical storage tubes for supercritical thermal storage system},
 type = {inproceedings},
 year = {2013},
 volume = {6 B},
 id = {09c44a27-79b0-3e24-ad70-481a1c690b64},
 created = {2021-07-30T22:42:59.034Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:42:59.034Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The effect of turbulent natural convection in vertical storage tubes containing a supercritical fluid is investigated computationally. In a supercritical thermal storage system, thermal energy is transferred to the storage fluid and is stored as the internal energy of the fluid in supercritical state. The heat is conducted from the heat transfer fluid to the storage fluid through the storage tube wall. Unlike phase-change systems, the heat transfer mechanism within the storage tubes of supercritical thermal storage system is dominantly affected by rigorous turbulent natural convection. The natural convection enhances the heat transfer and compensates for the low thermal conductivity of the storage fluid. The turbulent buoyancy-driven flow field in vertical storage tubes with different aspect ratios is investigated in this paper and the effect of vertical orientation of storage tubes on the characteristics of the flow field is explored. A standard k-epsilon method is utilized to model the Reynolds stresses in turbulent natural convection flow. The results of this study show that the turbulent buoyancy-driven flow and natural convection play an important role in charge and discharge of the supercritical thermal storage system. The charge time of the system is a function of Rayleigh number and aspect ratio of the storage tube. Copyright © 2013 by ASME.},
 bibtype = {inproceedings},
 author = {Lakeh, Reza Baghaei and Pirouz Kavehpour, H. and Lavine, Adrienne S. and Ganapathi, Gani B. and Wirz, Richard E.},
 doi = {10.1115/IMECE2013-62716},
 booktitle = {ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)}
}

The effect of turbulent natural convection in vertical storage tubes containing a supercritical fluid is investigated computationally. In a supercritical thermal storage system, thermal energy is transferred to the storage fluid and is stored as the internal energy of the fluid in supercritical state. The heat is conducted from the heat transfer fluid to the storage fluid through the storage tube wall. Unlike phase-change systems, the heat transfer mechanism within the storage tubes of supercritical thermal storage system is dominantly affected by rigorous turbulent natural convection. The natural convection enhances the heat transfer and compensates for the low thermal conductivity of the storage fluid. The turbulent buoyancy-driven flow field in vertical storage tubes with different aspect ratios is investigated in this paper and the effect of vertical orientation of storage tubes on the characteristics of the flow field is explored. A standard k-epsilon method is utilized to model the Reynolds stresses in turbulent natural convection flow. The results of this study show that the turbulent buoyancy-driven flow and natural convection play an important role in charge and discharge of the supercritical thermal storage system. The charge time of the system is a function of Rayleigh number and aspect ratio of the storage tube. Copyright © 2013 by ASME.

@inproceedings{
 title = {Application of Auger spectroscopy for measurement of secondary electron emission from conducting material for electric propulsion devices},
 type = {inproceedings},
 year = {2013},
 pages = {1-8},
 id = {11987dc5-45a4-394a-9e09-5b080fcebf04},
 created = {2021-07-30T22:59:30.934Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:59:30.934Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 source_type = {CONF},
 private_publication = {false},
 bibtype = {inproceedings},
 author = {Patino, Marlene I and Raitses, Yevgeny and Koel, Bruce E and Wirz, Richard E},
 booktitle = {Proc. 33rd Int. Electric Propulsion Conf.(Washington, DC, USA, 6–10 October 2013)}
}

@inproceedings{
 title = {A 5 kWht Lab-Scale Demonstration of a Novel Thermal Energy Storage Concept With Supercritical Fluids},
 type = {inproceedings},
 year = {2013},
 pages = {V001T15A005},
 volume = {55515},
 publisher = {American Society of Mechanical Engineers},
 id = {094eaa5e-b73a-33f0-9898-2743c1109d6c},
 created = {2021-07-30T22:59:31.553Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:59:31.553Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 source_type = {CONF},
 private_publication = {false},
 bibtype = {inproceedings},
 author = {Ganapathi, Gani B and Berisford, Daniel and Furst, Benjamin and Bame, David and Pauken, Michael and Wirz, Richard},
 booktitle = {Energy Sustainability}
}

@article{
 title = {Electron Loss at a Magnetic Cusp for a Range of Upstream Magnetic Field Conditions},
 type = {article},
 year = {2013},
 id = {3b85f48c-b30a-3b3a-921c-abacc9711754},
 created = {2021-07-30T22:59:32.364Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:59:32.364Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 source_type = {JOUR},
 private_publication = {false},
 bibtype = {article},
 author = {Dankongkakul, B and Wirz, R E}
}

@inproceedings{
 title = {Plasma structure of miniature ring-cusp ion thruster discharges},
 type = {inproceedings},
 year = {2012},
 id = {2ee9feab-1132-313d-b9bb-60651a32f67b},
 created = {2021-07-30T20:25:49.018Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:25:49.018Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Previous miniature ion thruster studies have demonstrated impressive performance using ring-cusp discharges. These studies have suggested that optimization is achieved by balancing considerations for magnetic confinement and discharge stability. Essentially, the field must be strong enough for sufficient electron confinement times for ionization, but weak enough to allow electrons to escape and maintain the plasma potential necessary for ionization. To investigate these phenomena, an experiment was developed to allow detailed measurements of the internal structure and characteristics of a miniature ring- cusp discharge. These measurements provide spatially resolved values for plasma density, electron temperature, and plasma potential along a meridian plane. The magnetic field configuration is arranged as a quasi-periodic domain in order to generalize the findings to all multi-pole discharges. The results show that the magnetic field strength drives the plasma structure, and the dependence on discharge power can be removed with proper scaling of the plasma parameters. The stronger magnetic field results in a higher peak plasma density, but relatively low discharge utilization efficiency. Additionally, the poten- tial measurements indicate the likely onset of discharge instability. In contrast, the weaker magnetic field better utilizes the volume of the chamber. This leads to a higher and more uniform density near the downstream end of the discharge where ion extraction would occur, implying superior discharge utilization. © 2012 by Hann-Shin Mao.},
 bibtype = {inproceedings},
 author = {Mao, Hann Shin and Wirz, Richard E. and Goebel, Dan M.},
 doi = {10.2514/6.2012-4021},
 booktitle = {48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit 2012}
}

Previous miniature ion thruster studies have demonstrated impressive performance using ring-cusp discharges. These studies have suggested that optimization is achieved by balancing considerations for magnetic confinement and discharge stability. Essentially, the field must be strong enough for sufficient electron confinement times for ionization, but weak enough to allow electrons to escape and maintain the plasma potential necessary for ionization. To investigate these phenomena, an experiment was developed to allow detailed measurements of the internal structure and characteristics of a miniature ring- cusp discharge. These measurements provide spatially resolved values for plasma density, electron temperature, and plasma potential along a meridian plane. The magnetic field configuration is arranged as a quasi-periodic domain in order to generalize the findings to all multi-pole discharges. The results show that the magnetic field strength drives the plasma structure, and the dependence on discharge power can be removed with proper scaling of the plasma parameters. The stronger magnetic field results in a higher peak plasma density, but relatively low discharge utilization efficiency. Additionally, the poten- tial measurements indicate the likely onset of discharge instability. In contrast, the weaker magnetic field better utilizes the volume of the chamber. This leads to a higher and more uniform density near the downstream end of the discharge where ion extraction would occur, implying superior discharge utilization. © 2012 by Hann-Shin Mao.

@article{
 title = {Quasi-equilibrium electron density along a magnetic field line},
 type = {article},
 year = {2012},
 volume = {101},
 id = {874ac26d-f7f8-3a01-bc32-f3296990e2cd},
 created = {2021-07-30T20:26:07.836Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:26:07.836Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A methodology is developed to determine the density of high-energy electrons along a magnetic field line for a low- β plasma. This method avoids the expense and statistical noise of traditional particle tracking techniques commonly used for high-energy electrons in bombardment plasma generators. By preserving the magnetic mirror and assuming a mixing timescale, typically the elastic collision frequency with neutrals, a quasi-equilibrium electron distribution can be calculated. Following the transient decay, the analysis shows that both the normalized density and the reduction fraction due to collision converge to a single quasi-equilibrium solution. © 2012 American Institute of Physics.},
 bibtype = {article},
 author = {Mao, Hann Shin and Wirz, Richard},
 doi = {10.1063/1.4768301},
 journal = {Applied Physics Letters},
 number = {22}
}

A methodology is developed to determine the density of high-energy electrons along a magnetic field line for a low- β plasma. This method avoids the expense and statistical noise of traditional particle tracking techniques commonly used for high-energy electrons in bombardment plasma generators. By preserving the magnetic mirror and assuming a mixing timescale, typically the elastic collision frequency with neutrals, a quasi-equilibrium electron distribution can be calculated. Following the transient decay, the analysis shows that both the normalized density and the reduction fraction due to collision converge to a single quasi-equilibrium solution. © 2012 American Institute of Physics.

@inproceedings{
 title = {System modeling for a supercritical thermal energy storage system},
 type = {inproceedings},
 year = {2012},
 issue = {PARTS A AND B},
 id = {36012c56-b150-3aa7-886a-7a011b0729be},
 created = {2021-07-30T22:43:20.589Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:20.589Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {This paper describes a thermodynamic model that simulates the discharge cycle of a single-tank thermal energy storage (TES) system using supercritical fluid in a concentrating solar power plant. Current state-of-the-art TES design utilizes a two-tank system with molten nitrate salts; one major problem is the high cost of the fluid. The alternate design explored here involves the use of less expensive fluids at supercritical temperatures and pressures. By cycling the storage fluid between a relatively low temperature two-phase state and a high temperature supercritical state, a large excursion in internal energy can be accessed which includes both sensible heat and latent heat of vaporization. Supercritical storage allows for the consideration of fluids that are significantly cheaper than molten salts; however, a supercritical TES system requires high pressures and temperatures that necessitate a relatively high cost containment vessel that represents a large fraction of the system capital cost. To mitigate this cost, the proposed design utilizes a single-tank TES system, effectively halving the required wall material. A singletank approach also significantly reduces the complexity of the system in comparison to the two-tank systems, which require expensive pumps and external heat exchangers. However, a single-tank approach also results in a loss of turbine power output as the storage fluid temperature declines over time during the discharge cycle. The thermodynamic model is used to evaluate system performance; in particular it predicts the reduction in energy output of the single-tank system relative to a conventional two-tank storage system. Tank wall material volume is also presented and it is shown that there is an optimum average fluid density that generates a given turbine energy output while minimizing the required tank wall material and associated capital cost. Overall, this study illustrates opportunities to further improve current solar thermal technologies. The singletank supercritical fluid system shows great promise for decreasing the cost of thermal energy storage, and ensuring that renewable energy can become a significant part of the national and global energy portfolio. Copyright © 2012 by ASME.},
 bibtype = {inproceedings},
 author = {Tse, Louis and Ganapathi, Gani and Wirz, Richard and Lavine, Adrienne},
 doi = {10.1115/ES2012-91001},
 booktitle = {ASME 2012 6th International Conference on Energy Sustainability, ES 2012, Collocated with the ASME 2012 10th International Conference on Fuel Cell Science, Engineering and Technology}
}

This paper describes a thermodynamic model that simulates the discharge cycle of a single-tank thermal energy storage (TES) system using supercritical fluid in a concentrating solar power plant. Current state-of-the-art TES design utilizes a two-tank system with molten nitrate salts; one major problem is the high cost of the fluid. The alternate design explored here involves the use of less expensive fluids at supercritical temperatures and pressures. By cycling the storage fluid between a relatively low temperature two-phase state and a high temperature supercritical state, a large excursion in internal energy can be accessed which includes both sensible heat and latent heat of vaporization. Supercritical storage allows for the consideration of fluids that are significantly cheaper than molten salts; however, a supercritical TES system requires high pressures and temperatures that necessitate a relatively high cost containment vessel that represents a large fraction of the system capital cost. To mitigate this cost, the proposed design utilizes a single-tank TES system, effectively halving the required wall material. A singletank approach also significantly reduces the complexity of the system in comparison to the two-tank systems, which require expensive pumps and external heat exchangers. However, a single-tank approach also results in a loss of turbine power output as the storage fluid temperature declines over time during the discharge cycle. The thermodynamic model is used to evaluate system performance; in particular it predicts the reduction in energy output of the single-tank system relative to a conventional two-tank storage system. Tank wall material volume is also presented and it is shown that there is an optimum average fluid density that generates a given turbine energy output while minimizing the required tank wall material and associated capital cost. Overall, this study illustrates opportunities to further improve current solar thermal technologies. The singletank supercritical fluid system shows great promise for decreasing the cost of thermal energy storage, and ensuring that renewable energy can become a significant part of the national and global energy portfolio. Copyright © 2012 by ASME.

@inproceedings{
 title = {High density thermal energy storage with supercritical fluids},
 type = {inproceedings},
 year = {2012},
 issue = {PARTS A AND B},
 id = {00850958-3d1d-3e5c-a6dd-63d84580048f},
 created = {2021-07-30T22:43:21.184Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:21.184Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A novel approach to storing thermal energy with supercritical fluids is being investigated, which if successful, promises to transform the way thermal energy is captured and utilized. The use of supercritical fluids allows cost-affordable high-density storage with a combination of latent heat and sensible heat in the two-phase as well as the supercritical state. This technology will enhance penetration of several thermal power generation applications and high temperature water for commercial use if the overall cost of the technology can be demonstrated to be lower than the current state-of-the-art molten salt using sodium nitrate and potassium nitrate eutectic mixtures. An additional attraction is that the volumetric storage density of a supercritical fluid can be higher than a two-tank molten salt system due to the high compressibilities in the supercritical state. This paper looks at different elements for determining the feasibility of this storage concept - thermodynamics of supercritical state with a specific example, naphthalene, fluid and system cost and a representative storage design. A modular storage vessel design based on a shell and heat exchanger concept allows the cost to be minimized as there is no need for a separate pump for transferring fluid from one tank to another as in the molten salt system. Since the heat exchangers are internal to the tank, other advantages such as lower parasitic heat loss, easy fabrication can be achieved. Results from the study indicate that the fluid cost can be reduced by a factor of ten or even twenty depending on the fluid and thermodynamic optimization of loading factor. Results for naphthalene operating between 290°C and 475°C, indicate that the fluid cost is approximately $3/kWh compared with $25-$50/kWh for molten salt. When the storage container costs are factored in, the overall system cost is still very attractive. Studies for a 12-hr storage indicate that for operating at temperatures between 290-450°C, the cost for a molten salt system can vary between $66/kWh to $184/kWh depending on molten salt cost of $2/kg or a more recent quote of $8/kg. In contrast, the cost for a 12-hr supercritical storage system can be as low as $40/kWh. By using less expensive materials than SS 316L, it is possible to reduce the costs even further. Copyright © 2012 by ASME.},
 bibtype = {inproceedings},
 author = {Ganapathi, Gani B. and Wirz, Richard},
 doi = {10.1115/ES2012-91008},
 booktitle = {ASME 2012 6th International Conference on Energy Sustainability, ES 2012, Collocated with the ASME 2012 10th International Conference on Fuel Cell Science, Engineering and Technology}
}

A novel approach to storing thermal energy with supercritical fluids is being investigated, which if successful, promises to transform the way thermal energy is captured and utilized. The use of supercritical fluids allows cost-affordable high-density storage with a combination of latent heat and sensible heat in the two-phase as well as the supercritical state. This technology will enhance penetration of several thermal power generation applications and high temperature water for commercial use if the overall cost of the technology can be demonstrated to be lower than the current state-of-the-art molten salt using sodium nitrate and potassium nitrate eutectic mixtures. An additional attraction is that the volumetric storage density of a supercritical fluid can be higher than a two-tank molten salt system due to the high compressibilities in the supercritical state. This paper looks at different elements for determining the feasibility of this storage concept - thermodynamics of supercritical state with a specific example, naphthalene, fluid and system cost and a representative storage design. A modular storage vessel design based on a shell and heat exchanger concept allows the cost to be minimized as there is no need for a separate pump for transferring fluid from one tank to another as in the molten salt system. Since the heat exchangers are internal to the tank, other advantages such as lower parasitic heat loss, easy fabrication can be achieved. Results from the study indicate that the fluid cost can be reduced by a factor of ten or even twenty depending on the fluid and thermodynamic optimization of loading factor. Results for naphthalene operating between 290°C and 475°C, indicate that the fluid cost is approximately $3/kWh compared with $25-$50/kWh for molten salt. When the storage container costs are factored in, the overall system cost is still very attractive. Studies for a 12-hr storage indicate that for operating at temperatures between 290-450°C, the cost for a molten salt system can vary between $66/kWh to $184/kWh depending on molten salt cost of $2/kg or a more recent quote of $8/kg. In contrast, the cost for a 12-hr supercritical storage system can be as low as $40/kWh. By using less expensive materials than SS 316L, it is possible to reduce the costs even further. Copyright © 2012 by ASME.

@inproceedings{
 title = {Near-surface cusp confinement for weakly ionized plasma},
 type = {inproceedings},
 year = {2012},
 id = {4e794c6b-b88b-3af1-980e-785fe6d6ba8e},
 created = {2021-07-30T22:43:21.718Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:21.718Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {For this study, we use a combined experimental and computational approach to investigate the near-surface dynamics and structure of plasma confined by a permanent magnet cusp. Improved understanding in this region allows electric propulsion designers to take advantage of cusp confinement for micro-scale discharges (≤ 1 cm), enabling high performance microthrusters that are attractive for both microsatellite and formation flying missions. An electron gun experiment designed specifically for this effort provides detailed data on the plasma for a single permanent magnet cusp for electron-only conditions and multispecies (electron, neutral, and ion) weakly ionized conditions. For these experimental conditions, a multispecies particle-in-cell model is developed that uses an adaptive mesh and analytical solutions for permanent magnets to provide high resolution for particle motion and interactions in the cusp region. Results from the experiment and model agree well and are consistent with existing theory for the "leak radius" at the cusp. © 2012 by Richard E. Wirz.},
 bibtype = {inproceedings},
 author = {Wirz, Richard E. and Araki, Samuel J. and Dankongkakul, Ben},
 doi = {10.2514/6.2012-3948},
 booktitle = {48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit 2012}
}

For this study, we use a combined experimental and computational approach to investigate the near-surface dynamics and structure of plasma confined by a permanent magnet cusp. Improved understanding in this region allows electric propulsion designers to take advantage of cusp confinement for micro-scale discharges (≤ 1 cm), enabling high performance microthrusters that are attractive for both microsatellite and formation flying missions. An electron gun experiment designed specifically for this effort provides detailed data on the plasma for a single permanent magnet cusp for electron-only conditions and multispecies (electron, neutral, and ion) weakly ionized conditions. For these experimental conditions, a multispecies particle-in-cell model is developed that uses an adaptive mesh and analytical solutions for permanent magnets to provide high resolution for particle motion and interactions in the cusp region. Results from the experiment and model agree well and are consistent with existing theory for the "leak radius" at the cusp. © 2012 by Richard E. Wirz.

@inproceedings{
 title = {Ion-neutral collision analysis for a well-characterized plasma experiment},
 type = {inproceedings},
 year = {2012},
 id = {a30c1fa8-0c76-3282-a8e6-bb8a3bfcde02},
 created = {2021-07-30T22:43:32.384Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:32.384Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A simplified plasma experiment was designed to provide data for basic validation of computational models for new and existing electric propulsion devices. For this effort, a simple semi-analytical model was developed to provide first-order analysis of the results and guidance for future experimental approaches. The model examines the momentum-exchange and charge-exchange collisions for a 1500 V xenon ion beam guided through a Test Cell populated with xenon neutrals. The model achieves extremely short runtimes by using a semi-analytical approach that assumes a one-dimensional beam, single collisions, and only heavy species interactions. It treats the scattered particles as discrete point sources located along the axis of the cylindrical domain, where particles are deflected with a distribution that is forwardly weighted for momentum-exchange ions and a combination of orthogonally weighted and isotropic for charge-exchange ions. Using this approach, the model provides a rapid analysis capability that yields useful insight into important mechanisms in the experiment. Results from the model agree well with experimental measurements in the single collision regime and illustrate that the heavy species are almost entirely forward scattered at either large or small angles inside the Test Cell. That is, ions from momentum-exchange collisions and fast neutrals from charge-exchange collisions are scattered at small angles, whereas charge-exchange ions are predominantly forward-scattered but at large angles near 90;°;. © 2012 by Richard E. Wirz.},
 bibtype = {inproceedings},
 author = {Patino, Marlene I. and Chu, Lauren E. and Wirz, Richard E.},
 doi = {10.2514/6.2012-4119},
 booktitle = {48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit 2012}
}

A simplified plasma experiment was designed to provide data for basic validation of computational models for new and existing electric propulsion devices. For this effort, a simple semi-analytical model was developed to provide first-order analysis of the results and guidance for future experimental approaches. The model examines the momentum-exchange and charge-exchange collisions for a 1500 V xenon ion beam guided through a Test Cell populated with xenon neutrals. The model achieves extremely short runtimes by using a semi-analytical approach that assumes a one-dimensional beam, single collisions, and only heavy species interactions. It treats the scattered particles as discrete point sources located along the axis of the cylindrical domain, where particles are deflected with a distribution that is forwardly weighted for momentum-exchange ions and a combination of orthogonally weighted and isotropic for charge-exchange ions. Using this approach, the model provides a rapid analysis capability that yields useful insight into important mechanisms in the experiment. Results from the model agree well with experimental measurements in the single collision regime and illustrate that the heavy species are almost entirely forward scattered at either large or small angles inside the Test Cell. That is, ions from momentum-exchange collisions and fast neutrals from charge-exchange collisions are scattered at small angles, whereas charge-exchange ions are predominantly forward-scattered but at large angles near 90;°;. © 2012 by Richard E. Wirz.

@inproceedings{
 title = {Optimum low-thrust elliptic transfer for power limited spacecraft using numerical averaging},
 type = {inproceedings},
 year = {2011},
 volume = {140},
 id = {6d4331a6-d6b7-3add-aa62-1e4ff57465de},
 created = {2021-07-30T20:26:06.287Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:26:06.287Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Low-thrust electric propulsion is increasingly being used for spacecraft missions due to its high propellant efficiency, which permits larger payloads for a given mission delta-V. As a result, trajectory optimization for low-thrust missions is becoming increasingly important. However, there are few medium and low fidelity low-thrust trajectory tools available for use in preliminary mission design studies. This paper describes a quick method for obtaining such preliminary trajectories for transfers which involve many revolutions about the primary body. This paper considers variable thrust transfers using a first order averaging method to obtain the fuel optimum rates of change of the equinoctial orbital elements in terms of each other and the Lagrange multipliers. The advantages of using variable thrust are analyzed with comparisons to the results of constant thrust methods. Constraints on maximum thrust and power as well as minimum periapsis are implemented and the equations are averaged numerically using a 10th order Gausian quadrature. The method also accounts for J2 effects and shadowing. A shooting method is used to solve the optimization problem numerically based on an initial guess. The use of numerical averaging allows for more complex gravity perturbations to be added in the future without great difficulty. A few example elliptic orbit transfers with the Earth as the central body are analyzed.},
 bibtype = {inproceedings},
 author = {Tarzi, Zahi and Speyer, Jason and Wirz, Richard},
 booktitle = {Advances in the Astronautical Sciences}
}

Low-thrust electric propulsion is increasingly being used for spacecraft missions due to its high propellant efficiency, which permits larger payloads for a given mission delta-V. As a result, trajectory optimization for low-thrust missions is becoming increasingly important. However, there are few medium and low fidelity low-thrust trajectory tools available for use in preliminary mission design studies. This paper describes a quick method for obtaining such preliminary trajectories for transfers which involve many revolutions about the primary body. This paper considers variable thrust transfers using a first order averaging method to obtain the fuel optimum rates of change of the equinoctial orbital elements in terms of each other and the Lagrange multipliers. The advantages of using variable thrust are analyzed with comparisons to the results of constant thrust methods. Constraints on maximum thrust and power as well as minimum periapsis are implemented and the equations are averaged numerically using a 10th order Gausian quadrature. The method also accounts for J2 effects and shadowing. A shooting method is used to solve the optimization problem numerically based on an initial guess. The use of numerical averaging allows for more complex gravity perturbations to be added in the future without great difficulty. A few example elliptic orbit transfers with the Earth as the central body are analyzed.

@article{
 title = {Time-dependent erosion of ion optics},
 type = {article},
 year = {2011},
 volume = {27},
 id = {76192a62-8371-3c56-afcc-39407f6dfda4},
 created = {2021-07-30T20:26:09.469Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:26:09.469Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The accurate prediction of ion thruster life requires time-dependent erosion estimates for the ion optics assembly. Such information is critical to end-of-life mechanisms such as electron backstreaming. A two-dimensional ion optics code, CEX2D, was recently modified to handle time-dependent erosion, double ions, and multiple throttle conditions in a single run. The modified code is called CEX2D-t. Comparisons of CEX2D-t results with the NASA solar electric propulsion technology application readiness (NSTAR) thruster life demonstration test and extended life test results show good agreement for both screen and acceleration grid erosion, including important erosion features such as chamfering of the downstream end of the accelerator grid and reduced rate of accelerator grid aperture enlargement with time. The influence of double ions on grid erosion proved to be important for simulating the erosion observed during the NSTAR life demonstration test and extended life test. Copyright © 2010 by Richard E. Wirz.},
 bibtype = {article},
 author = {Wirz, Richard E. and Anderson, John R. and Katz, Ira},
 doi = {10.2514/1.46845},
 journal = {Journal of Propulsion and Power},
 number = {1}
}

The accurate prediction of ion thruster life requires time-dependent erosion estimates for the ion optics assembly. Such information is critical to end-of-life mechanisms such as electron backstreaming. A two-dimensional ion optics code, CEX2D, was recently modified to handle time-dependent erosion, double ions, and multiple throttle conditions in a single run. The modified code is called CEX2D-t. Comparisons of CEX2D-t results with the NASA solar electric propulsion technology application readiness (NSTAR) thruster life demonstration test and extended life test results show good agreement for both screen and acceleration grid erosion, including important erosion features such as chamfering of the downstream end of the accelerator grid and reduced rate of accelerator grid aperture enlargement with time. The influence of double ions on grid erosion proved to be important for simulating the erosion observed during the NSTAR life demonstration test and extended life test. Copyright © 2010 by Richard E. Wirz.

@inproceedings{
 title = {Aero-structural performance of multiplane wind turbine blades},
 type = {inproceedings},
 year = {2011},
 id = {03a67580-d5a7-32c4-87db-e92c9175b074},
 created = {2021-07-30T22:43:32.991Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:32.991Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {In current wind turbine blade designs, the inboard region suffers performance losses from competing structural and aerodynamic requirements. The objective of this effort is to develop multiplane inboard configurations that provide attractive aero-structural performance for wind turbine blades. A biplane approach may be sufficient to realize the full benefits of this approach. To compare the performance of a conventional inboard section with a biplane inboard section. cross-sectional properties of a thick monoplane and a biplane were measured to obtain their approximate structural and aerodynamic characteristics. Numerical simulations were used to explicitly compare the aerodynamic performance of a thick monoplane to a biplane. Then, several model beams were designed to be simple approximations of a conventional wind turbine blade ("monoplane beam") and the biplane blade approach ("biplane beam"). Three canonical bending loads were applied to each of these model beams and the deflection of each beam was compared. Numerical simulations show that the lift-to-drag ratio of the biplane is significantly greater than the lift-to-drag ratio of the thick monoplane for the angles of attack investigated (0-15.5°). A parametric analysis of different biplane beam configurations shows that tip deflections of the biplane beam configurations are smaller than those of monoplane beams of the same length. For example, a nominal 50 m biplane blade has less than 30% of the deflection of a 50 m monoplane blade. Thus, for a monoplane beam of fixed length, it is possible to construct a longer biplane beam with an equal tip deflection. These combined aerodynamic and structural benefits can lead directly to greater turbine power by three important advantages: (1) improved aerodynamic performance, (2) improved spacing due to the potential for decreased vortex shedding, and, most importantly, (3) potential increases in blade length due to improved inboard blade strength. Therefore, these results suggest that the biplane blade approach is an attractive design for the next-generation of large wind turbine blades. © 2011 by Richard E. Wirz and Perry M. Johnson. Published by the American Institute of Aeronautics and Astronautics, Inc.},
 bibtype = {inproceedings},
 author = {Wirz, Richard E. and Johnson, Perry M.},
 doi = {10.2514/6.2011-3025},
 booktitle = {29th AIAA Applied Aerodynamics Conference 2011}
}

In current wind turbine blade designs, the inboard region suffers performance losses from competing structural and aerodynamic requirements. The objective of this effort is to develop multiplane inboard configurations that provide attractive aero-structural performance for wind turbine blades. A biplane approach may be sufficient to realize the full benefits of this approach. To compare the performance of a conventional inboard section with a biplane inboard section. cross-sectional properties of a thick monoplane and a biplane were measured to obtain their approximate structural and aerodynamic characteristics. Numerical simulations were used to explicitly compare the aerodynamic performance of a thick monoplane to a biplane. Then, several model beams were designed to be simple approximations of a conventional wind turbine blade ("monoplane beam") and the biplane blade approach ("biplane beam"). Three canonical bending loads were applied to each of these model beams and the deflection of each beam was compared. Numerical simulations show that the lift-to-drag ratio of the biplane is significantly greater than the lift-to-drag ratio of the thick monoplane for the angles of attack investigated (0-15.5°). A parametric analysis of different biplane beam configurations shows that tip deflections of the biplane beam configurations are smaller than those of monoplane beams of the same length. For example, a nominal 50 m biplane blade has less than 30% of the deflection of a 50 m monoplane blade. Thus, for a monoplane beam of fixed length, it is possible to construct a longer biplane beam with an equal tip deflection. These combined aerodynamic and structural benefits can lead directly to greater turbine power by three important advantages: (1) improved aerodynamic performance, (2) improved spacing due to the potential for decreased vortex shedding, and, most importantly, (3) potential increases in blade length due to improved inboard blade strength. Therefore, these results suggest that the biplane blade approach is an attractive design for the next-generation of large wind turbine blades. © 2011 by Richard E. Wirz and Perry M. Johnson. Published by the American Institute of Aeronautics and Astronautics, Inc.

@article{
 title = {Electron backstreaming determination for ion thrusters},
 type = {article},
 year = {2011},
 volume = {27},
 id = {95821d82-3a72-3e47-b763-a16303ad9565},
 created = {2021-07-30T22:43:33.589Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:33.589Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Electron backstreaming in ion thrusters is caused by the random flux of beam electrons past a potential barrier established by the accelerator grid. A technique that integrates this flux over the radial extent of the barrier reveals important aspects of electron backstreaming phenomena for individual beamlets, across the thruster beam, and throughout thruster life. For individual beamlets it was found that over 99% of the electron backstreaming occurs in a small area at the center of the beamlet that is less than 20% the area of the beamlet at the potential barrier established by the accelerator grid. For the thruster beam it was found that over 99% of the backstreaming current occurs inside of r = 6 cmfor the over 28cm diameterNSTARgrid. Initial validation against extended life test data for the NSTAR thruster shows that the technique provides the correct behavior and magnitude of electron backstreaming limit, jVebsj. From the sensitivity analyses it is apparent that accelerator grid chamfering due to sputter erosion contributes significantly to the sharp rise in electron backstreaming limit observed in the extended life test, but does not explain the rise in grid ion transparency. Reduction of the grid gap over the life of the thruster also contributes to increases in electron backstreaming limit and increases in ion transparency. Screen grid erosion contributes generally to rises in jVebsj and grid ion transparency, but for the assumptions used herein, it appears to not have as much of an effect as chamfering or grid gap change. Overall, it is apparent that accelerator grid chamfering, grid gap change, and screen grid erosion are important to the increase in electron backstreaming observed during the NSTAR extended life test. Copyright © 2010 by Richard E. Wirz.},
 bibtype = {article},
 author = {Wirz, Richard E. and Katz, Ira and Goebel, Dan M. and Anderson, John R.},
 doi = {10.2514/1.46844},
 journal = {Journal of Propulsion and Power},
 number = {1}
}

Electron backstreaming in ion thrusters is caused by the random flux of beam electrons past a potential barrier established by the accelerator grid. A technique that integrates this flux over the radial extent of the barrier reveals important aspects of electron backstreaming phenomena for individual beamlets, across the thruster beam, and throughout thruster life. For individual beamlets it was found that over 99% of the electron backstreaming occurs in a small area at the center of the beamlet that is less than 20% the area of the beamlet at the potential barrier established by the accelerator grid. For the thruster beam it was found that over 99% of the backstreaming current occurs inside of r = 6 cmfor the over 28cm diameterNSTARgrid. Initial validation against extended life test data for the NSTAR thruster shows that the technique provides the correct behavior and magnitude of electron backstreaming limit, jVebsj. From the sensitivity analyses it is apparent that accelerator grid chamfering due to sputter erosion contributes significantly to the sharp rise in electron backstreaming limit observed in the extended life test, but does not explain the rise in grid ion transparency. Reduction of the grid gap over the life of the thruster also contributes to increases in electron backstreaming limit and increases in ion transparency. Screen grid erosion contributes generally to rises in jVebsj and grid ion transparency, but for the assumptions used herein, it appears to not have as much of an effect as chamfering or grid gap change. Overall, it is apparent that accelerator grid chamfering, grid gap change, and screen grid erosion are important to the increase in electron backstreaming observed during the NSTAR extended life test. Copyright © 2010 by Richard E. Wirz.

@inproceedings{
 title = {Comparison of charged particle tracking methods for non-uniform magnetic fields},
 type = {inproceedings},
 year = {2011},
 id = {2c8119eb-c020-37de-96d9-868710a66d38},
 created = {2021-07-30T22:43:34.922Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:34.922Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Five particle tracking methods are compared for their ability to determine trajectory and conserve energy. The trajectory integrators include: the Boris particle tracking method (a widely used method for charged particles), the classical fourth order Runge-Kutta (a ubiquitous high order integrator), Störmer-Verlet as a partitioned Runge-Kutta (a second order symplectic integrator), fourth order Gauss Runge-Kutta (a fourth order symplectic integrator), and Wirz's modified Boris method (a new particle tracker constructed to handle large magnetic gradients). In general, it is the goal of this study to compare the symplectic integrators with the classical methods, as well as benchmark the Wirz's algorithm against these proven particle trackers. The symplectic integrators control their overall energy error, but at the cost of being implicit, where as the classical "Newtonian" methods are explicit but slowly accumulate errors in energy over time. Wirz's modified Boris is shown to strike the balance between the two classes with explicit implementation and conservation of total energy. © 2011 by Hann-Shin Mao.},
 bibtype = {inproceedings},
 author = {Mao, Hann Shin and Wirz, Richard E.},
 doi = {10.2514/6.2011-3739},
 booktitle = {42nd AIAA Plasmadynamics and Lasers Conference}
}

Five particle tracking methods are compared for their ability to determine trajectory and conserve energy. The trajectory integrators include: the Boris particle tracking method (a widely used method for charged particles), the classical fourth order Runge-Kutta (a ubiquitous high order integrator), Störmer-Verlet as a partitioned Runge-Kutta (a second order symplectic integrator), fourth order Gauss Runge-Kutta (a fourth order symplectic integrator), and Wirz's modified Boris method (a new particle tracker constructed to handle large magnetic gradients). In general, it is the goal of this study to compare the symplectic integrators with the classical methods, as well as benchmark the Wirz's algorithm against these proven particle trackers. The symplectic integrators control their overall energy error, but at the cost of being implicit, where as the classical "Newtonian" methods are explicit but slowly accumulate errors in energy over time. Wirz's modified Boris is shown to strike the balance between the two classes with explicit implementation and conservation of total energy. © 2011 by Hann-Shin Mao.

@inproceedings{
 title = {Collision modeling for high velocity ions in a quiescent gas},
 type = {inproceedings},
 year = {2011},
 id = {20b68a2b-93a5-3bf7-babc-176c27b5092f},
 created = {2021-07-30T22:43:35.533Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:35.533Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A particle-in-cell Monte Carlo collision model is developed to explore dominant collisional effects on high velocity ions incident to a quiescent gas, as examined in an ion gun experiment being carried out by UCLA researchers. Two different collision mechanisms are implemented in the computational model: elastic and charge exchange (CEX) colli- sions. Because of very low ionization level observed in the ion beam experiment, Coulomb collision is not incorporated in the model. The two collision types are approximated by two different models depending on incident particle energy: variable hard sphere model and classical scattering with repulsive interaction potential. The classical scattering model predicts a high probability in small angle scattering at large ion energies of interest. Four different species (The primary and CEX ions, fast neutrals, and secondary electrons) are tracked separately, and their destinations within a well-defined domain are determined. Predicted currents as a function of the test cell pressure are compared with electrode cur- rents measured in the ion gun experiment. The simulation results agree well with the experiment up to test cell pressure of 1 mTorr. Below this test cell pressure, each primary ion experiences one or two collisions on average. The simulation result starts to digress from the experimental result as an increasing number of collisions are involved at higher pressures. The computational results suggest that the primary ion energy can be reduced significantly after many collisions, however, repulsive interaction potential is a good ap- proximation only at high energies. Therefore, a better interaction potential function valid for lower energies is needed to resolve the disagreement. © 2011 by Samuel J. Araki.},
 bibtype = {inproceedings},
 author = {Araki, Samuel J. and Wirz, Richard E.},
 doi = {10.2514/6.2011-3740},
 booktitle = {42nd AIAA Plasmadynamics and Lasers Conference}
}

A particle-in-cell Monte Carlo collision model is developed to explore dominant collisional effects on high velocity ions incident to a quiescent gas, as examined in an ion gun experiment being carried out by UCLA researchers. Two different collision mechanisms are implemented in the computational model: elastic and charge exchange (CEX) colli- sions. Because of very low ionization level observed in the ion beam experiment, Coulomb collision is not incorporated in the model. The two collision types are approximated by two different models depending on incident particle energy: variable hard sphere model and classical scattering with repulsive interaction potential. The classical scattering model predicts a high probability in small angle scattering at large ion energies of interest. Four different species (The primary and CEX ions, fast neutrals, and secondary electrons) are tracked separately, and their destinations within a well-defined domain are determined. Predicted currents as a function of the test cell pressure are compared with electrode cur- rents measured in the ion gun experiment. The simulation results agree well with the experiment up to test cell pressure of 1 mTorr. Below this test cell pressure, each primary ion experiences one or two collisions on average. The simulation result starts to digress from the experimental result as an increasing number of collisions are involved at higher pressures. The computational results suggest that the primary ion energy can be reduced significantly after many collisions, however, repulsive interaction potential is a good ap- proximation only at high energies. Therefore, a better interaction potential function valid for lower energies is needed to resolve the disagreement. © 2011 by Samuel J. Araki.

@inproceedings{
 title = {Cubesat lunar mission using a miniature ion thruster},
 type = {inproceedings},
 year = {2011},
 id = {2bae84d0-9e81-3445-9e04-23fefe3361f2},
 created = {2021-07-30T22:43:36.068Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:36.068Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The feasibility of CubeSats utilizing the Miniature Xenon Ion (MiXI) thruster for lunar missions is the focus of this investigation. The successful heritage, simplicity, and low cost of CubeSats make them attractive candidates for scientific missions to the Moon. This investigation presents the first-order design process for developing a lunar mission CubeSat. The results from this process are then applied to a 3U CubeSat equipped with a MiXI thruster and specifically designed to reach the lunar surface from a low Earth orbit. The small, 3cm diameter MiXI thruster utilized is capable of producing 0.1-1.553mN of thrust with a specific impulse of over 3000s and is projected to be capable of generating over 7000m/s of ΔV for a CubeSat mission. With all margins and contingencies considered, the total power required for the CubeSat using a 40W ion thruster is approximately 92W during daylight operations. With an assumed solar incidence angle of 45°, this power can be generated from a Spectrolab UTJ solar array of 0.35m2. Thermal calculations show an average spacecraft surface temperature of approximately 59°C during daylight operations and 0°C during eclipse, which are within the operational and survivable temperature ranges of the spacecraft subsystems. A low-thrust trajectory model is utilized to calculate and plot Earth-Moon trajectories. © 2011 by Ryan Conversano. Published by the American Institute of Aeronautics and Astronautics, Inc.},
 bibtype = {inproceedings},
 author = {Conversano, Ryan W. and Wirz, Richard E.},
 doi = {10.2514/6.2011-6083},
 booktitle = {47th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit 2011}
}

The feasibility of CubeSats utilizing the Miniature Xenon Ion (MiXI) thruster for lunar missions is the focus of this investigation. The successful heritage, simplicity, and low cost of CubeSats make them attractive candidates for scientific missions to the Moon. This investigation presents the first-order design process for developing a lunar mission CubeSat. The results from this process are then applied to a 3U CubeSat equipped with a MiXI thruster and specifically designed to reach the lunar surface from a low Earth orbit. The small, 3cm diameter MiXI thruster utilized is capable of producing 0.1-1.553mN of thrust with a specific impulse of over 3000s and is projected to be capable of generating over 7000m/s of ΔV for a CubeSat mission. With all margins and contingencies considered, the total power required for the CubeSat using a 40W ion thruster is approximately 92W during daylight operations. With an assumed solar incidence angle of 45°, this power can be generated from a Spectrolab UTJ solar array of 0.35m2. Thermal calculations show an average spacecraft surface temperature of approximately 59°C during daylight operations and 0°C during eclipse, which are within the operational and survivable temperature ranges of the spacecraft subsystems. A low-thrust trajectory model is utilized to calculate and plot Earth-Moon trajectories. © 2011 by Ryan Conversano. Published by the American Institute of Aeronautics and Astronautics, Inc.

@inproceedings{
 title = {Well-characterized plasma experiments for validation of computational models},
 type = {inproceedings},
 year = {2011},
 pages = {1-12},
 id = {36977eba-e594-3126-ae21-4fbf1a31ae71},
 created = {2021-07-30T22:59:33.088Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:59:33.088Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 source_type = {CONF},
 private_publication = {false},
 bibtype = {inproceedings},
 author = {Wirz, Richard E and Chu, Lauren and Patino, Marlene and Mao, Hann-Shin and Araki, Samuel},
 booktitle = {32nd International Electric Propulsion Conference, Wiesbaden, Germany}
}

@inproceedings{
 title = {Mission enabling and enhancing spacecraft capabilities with microNewton electric propulsion},
 type = {inproceedings},
 year = {2010},
 id = {0c474fd5-93b1-3eca-ae82-4736f6735373},
 created = {2021-07-30T22:43:36.599Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:36.599Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The capability to significantly improve current spacecraft pointing, precision orbit maintenance and disturbance mitigation were considered using precision, quiescent microNewton electric propulsion systems.1,2The results of the analyses showed that electric propulsion systems operating in the microNewton to hundreds of microNewtons thrust range can offer significant improvements over state-of-the-art mission capabilities to enable 30 m Earth-fixed orbital tubes, constellation spacecraft position control to within nanometers and exoplanet observatory pointing with 0.1 milliarcsecond precision. Electric propulsion thrust characteristics required to support these capabilities are discussed. ©2010 IEEE.},
 bibtype = {inproceedings},
 author = {Marrese-Reading, Colleen M. and Ziemer, John K. and Scharf, Daniel P. and Martin-Mur, Tomas J. and Thompson, Paul and Mueller, Juergen and Wirz, Richard},
 doi = {10.1109/AERO.2010.5447032},
 booktitle = {IEEE Aerospace Conference Proceedings}
}

The capability to significantly improve current spacecraft pointing, precision orbit maintenance and disturbance mitigation were considered using precision, quiescent microNewton electric propulsion systems.1,2The results of the analyses showed that electric propulsion systems operating in the microNewton to hundreds of microNewtons thrust range can offer significant improvements over state-of-the-art mission capabilities to enable 30 m Earth-fixed orbital tubes, constellation spacecraft position control to within nanometers and exoplanet observatory pointing with 0.1 milliarcsecond precision. Electric propulsion thrust characteristics required to support these capabilities are discussed. ©2010 IEEE.

@inproceedings{
 title = {Ground effects for widely spaced, supersonic, vertical retrorockets},
 type = {inproceedings},
 year = {2009},
 volume = {46},
 issue = {3},
 id = {e4dc5c78-4641-39d5-b6f0-74a602fff4cf},
 created = {2021-07-30T20:26:12.530Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:26:12.530Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A scenario for creating acceptable touchdown velocity on land for the Orion Crew Excursion Vehicle employs retrorockets for final landing Δ V. To capture the ground effects due to the interaction of the retrorockets and the vehicle, detailed computational modeling was used to determine the effective thrust at several different firing heights. These results were then used to determine the change in impulse for a wide range of possible firing altitudes. For a Crew Excursion Vehicle retrorocket firing time of 0.5 s, the steady-state effective vertical thrust of the module changes from -13.2 to +11.8% for altitudes from 152 to 15.2 cm, respectively. A simple descent analysis shows that ground effects will impart a net decrease or increase in impulse, depending on ignition altitude. In this analysis, the ground effects serve to increase the optimal firing height and increase the ignition altitude margin for a given maximum landing velocity. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.},
 bibtype = {inproceedings},
 author = {Wirz, Richard E. and Shariff, Shaun S.},
 doi = {10.2514/1.36299},
 booktitle = {Journal of Spacecraft and Rockets}
}

A scenario for creating acceptable touchdown velocity on land for the Orion Crew Excursion Vehicle employs retrorockets for final landing Δ V. To capture the ground effects due to the interaction of the retrorockets and the vehicle, detailed computational modeling was used to determine the effective thrust at several different firing heights. These results were then used to determine the change in impulse for a wide range of possible firing altitudes. For a Crew Excursion Vehicle retrorocket firing time of 0.5 s, the steady-state effective vertical thrust of the module changes from -13.2 to +11.8% for altitudes from 152 to 15.2 cm, respectively. A simple descent analysis shows that ground effects will impart a net decrease or increase in impulse, depending on ignition altitude. In this analysis, the ground effects serve to increase the optimal firing height and increase the ignition altitude margin for a given maximum landing velocity. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.

@inproceedings{
 title = {Orion spacecraft nominal and contingency earth landing retro rocket system options},
 type = {inproceedings},
 year = {2009},
 id = {c9a7ce05-01c5-3b3f-8cbf-6468a29d7805},
 created = {2021-07-30T22:43:37.151Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:37.151Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Retro rocket Earth landing system architectures for the Orion crew module were developed for the Constellation Program during the Landing System Advanced Development Project design trades. The architectures include both variable and fixed impulse base-mounted solid-propellant retro rocket motors for vertical and horizontal landing velocity control. Monte Carlo performance analyses were conducted, detailed motors were designed and support system configurations were developed for mass and performance characterization. Preliminary penetration schemes for blow-out ports through the TPS were designed for the motor exhaust plumes. The requirements, configurations, elements, mass and performance of the retro rocket landing system architectures for multiple landing scenarios are discussed. ©2009 IEEE.},
 bibtype = {inproceedings},
 author = {Marrese-Reading, Colleen and St. Vaughn, Joshua and Prakash, Ravi and Singh, Guru and Frisbee, Robert and Mandic, Milan and Corliss, James and Tutterow, Robin and Pain, Rob and Slade, William and Rooney, Daniel and Leipold, Daniel and Ramos, Amadi and True, William and Robbins, Rick and Barr, Dustin and Stephens, John and Pierce, Dave and Wirz, Richard},
 doi = {10.1109/AERO.2009.4839349},
 booktitle = {IEEE Aerospace Conference Proceedings}
}

Retro rocket Earth landing system architectures for the Orion crew module were developed for the Constellation Program during the Landing System Advanced Development Project design trades. The architectures include both variable and fixed impulse base-mounted solid-propellant retro rocket motors for vertical and horizontal landing velocity control. Monte Carlo performance analyses were conducted, detailed motors were designed and support system configurations were developed for mass and performance characterization. Preliminary penetration schemes for blow-out ports through the TPS were designed for the motor exhaust plumes. The requirements, configurations, elements, mass and performance of the retro rocket landing system architectures for multiple landing scenarios are discussed. ©2009 IEEE.

@inproceedings{
 title = {Time-dependent erosion of ion optics},
 type = {inproceedings},
 year = {2008},
 id = {c4658e2c-711d-38ed-acee-6d173cbc64e1},
 created = {2021-07-30T20:26:10.853Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:26:10.853Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The accurate prediction of thruster life requires time-dependent erosion estimates for the ion optics assembly. Such information is critical to end-of-life mechanisms such as electron backstreaming. A two-dimensional ion optics code, CEX2D, was recently modified to handle time-dependent erosion, double ions, and multiple throttle conditions in a single run. The modified code is called "CEX2D-t". Comparisons of CEX2D-t results with the NSTAR thruster life demonstration test (LDT) and extended life test (ELT) results show good agreement for both screen and accel grid erosion including important erosion features such as chamfering of the downstream end of the accelerator grid and reduced rate of accelerator grid aperture enlargement with time. © 2008 by the American Institute of Aeronautics and Astronautics, Inc.},
 bibtype = {inproceedings},
 author = {Wirz, Richard E. and Anderson, John R. and Katz, Ira and Goebel, Dan M.},
 doi = {10.2514/6.2008-4529},
 booktitle = {44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit}
}

The accurate prediction of thruster life requires time-dependent erosion estimates for the ion optics assembly. Such information is critical to end-of-life mechanisms such as electron backstreaming. A two-dimensional ion optics code, CEX2D, was recently modified to handle time-dependent erosion, double ions, and multiple throttle conditions in a single run. The modified code is called "CEX2D-t". Comparisons of CEX2D-t results with the NSTAR thruster life demonstration test (LDT) and extended life test (ELT) results show good agreement for both screen and accel grid erosion including important erosion features such as chamfering of the downstream end of the accelerator grid and reduced rate of accelerator grid aperture enlargement with time. © 2008 by the American Institute of Aeronautics and Astronautics, Inc.

@article{
 title = {Decel Grid Effects on Ion Thruster Grid Erosion},
 type = {article},
 year = {2008},
 volume = {36},
 id = {371d73c6-d4d4-3f91-a1cf-c40f84d17ba8},
 created = {2021-07-30T20:26:14.029Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:26:14.029Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Jet Propulsion Laboratory (JPL) is currently assessing the applicability of the 25-cm Xenon Ion Propulsion System (XIPS) as part of an effort to infuse low-cost technically mature commercial ion thruster systems into NASA deep space missions. Since these mission require extremely long thruster lifetimes to attain the required mission ΔV, this paper is focused on understanding the dominate wear mechanisms that effect the life of the XIPS three-grid system. Analysis of the XIPS three-grid configuration with JPL’s CEX3D grid erosion model shows that the third “decel” grid effectively protects the accel grid from pits and grooves erosion that is commonly seen with two-grid ion thruster grid systems. For a three-grid system, many of the charge-exchange ions created downstream of the grid plane will impact the decel grid at relatively low energies (∼25 V), instead of impacting the accel grid at high energies (∼200 V), thus reducing overall erosion. JPL’s CEX3D accurately predicts the erosion patterns for the accel grid, although it appears to overpredict the pits and grooves erosion rates due, mainly, to uncertainties in incident energies and angles for sputtering ions and since it does not account for local redeposition of sputteredmaterial. Since the model accurately simulates the erosion pattern but tends to overpredict the erosion rates for both the two- and three-grid sets, this comparative analysis clearly shows how the decel grid significantly suppresses the erosion of the downstream surface of the accel grid as observed in experimental tests. The results also show that the decel grid has a relatively small effect on barrel erosion (erosion of the aperture wall) and erosion of the upstream surface of the accel grid. Decreasing the accel grid voltage of the XIPS can reduce barrel (and total) erosion of the accel grid and should be considered for high-ΔV missions. © 2008, IEEE. All rights reserved.},
 bibtype = {article},
 author = {Wirz, Richard E. and Anderson, John R. and Goebel, Dan M. and Katz, Ira},
 doi = {10.1109/TPS.2008.2001041},
 journal = {IEEE Transactions on Plasma Science},
 number = {5}
}

Jet Propulsion Laboratory (JPL) is currently assessing the applicability of the 25-cm Xenon Ion Propulsion System (XIPS) as part of an effort to infuse low-cost technically mature commercial ion thruster systems into NASA deep space missions. Since these mission require extremely long thruster lifetimes to attain the required mission ΔV, this paper is focused on understanding the dominate wear mechanisms that effect the life of the XIPS three-grid system. Analysis of the XIPS three-grid configuration with JPL’s CEX3D grid erosion model shows that the third “decel” grid effectively protects the accel grid from pits and grooves erosion that is commonly seen with two-grid ion thruster grid systems. For a three-grid system, many of the charge-exchange ions created downstream of the grid plane will impact the decel grid at relatively low energies (∼25 V), instead of impacting the accel grid at high energies (∼200 V), thus reducing overall erosion. JPL’s CEX3D accurately predicts the erosion patterns for the accel grid, although it appears to overpredict the pits and grooves erosion rates due, mainly, to uncertainties in incident energies and angles for sputtering ions and since it does not account for local redeposition of sputteredmaterial. Since the model accurately simulates the erosion pattern but tends to overpredict the erosion rates for both the two- and three-grid sets, this comparative analysis clearly shows how the decel grid significantly suppresses the erosion of the downstream surface of the accel grid as observed in experimental tests. The results also show that the decel grid has a relatively small effect on barrel erosion (erosion of the aperture wall) and erosion of the upstream surface of the accel grid. Decreasing the accel grid voltage of the XIPS can reduce barrel (and total) erosion of the accel grid and should be considered for high-ΔV missions. © 2008, IEEE. All rights reserved.

@article{
 title = {Effects of magnetic field topography on ion thruster discharge performance},
 type = {article},
 year = {2008},
 volume = {17},
 id = {6f3cc4d3-1874-37ea-a974-9e2fb80a9f44},
 created = {2021-07-30T20:26:16.216Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:26:16.216Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Traditional magnetic field design techniques for dc ion thrusters typically focus on closing a sufficiently high maximum closed magnetic contour, B cc, inside the discharge chamber. In this study, detailed computational analysis of several modified NSTAR thruster 3-ring and 4-ring magnetic field geometries reveals that the magnetic field line shape as well as Bcc determines important aspects of dc ion thruster performance (i.e. propellant efficiency, beam flatness and double ion content). The DC-ION ion thruster model results show that the baseline NSTAR configuration traps the primary electrons on-axis, which leads to the high on-axis plasma density peak and high double ion content observed in experimental measurements. These problems are further exacerbated by simply increasing Bcc and not changing the field line shape. Changing the field line shape to prevent on-axis confinement (while maintaining the NSTAR baseline Bcc) improves thruster performance, improves plasma uniformity and lowers double ion content. For these favorable field line geometries, we observe further improvements to performance with increased Bcc, while maintaining plasma uniformity and low double ion content. These improvements derive from the fact that the field lines guide the high-energy primaries to regions where they are most efficiently used to create ions while a higher Bcc prevents the loss of ions to the anode walls. Therefore, it is recommended that the ion thruster designer first establish a divergent field line shape that ensures favorable beam flatness, low double ion content and reasonable performance; then the designer may adjust the Bcc to attain desirable performance and stability for the target discharge plasma conditions. © 2008 IOP Publishing Ltd.},
 bibtype = {article},
 author = {Wirz, Richard and Goebel, Dan},
 doi = {10.1088/0963-0252/17/3/035010},
 journal = {Plasma Sources Science and Technology},
 number = {3}
}

Traditional magnetic field design techniques for dc ion thrusters typically focus on closing a sufficiently high maximum closed magnetic contour, B cc, inside the discharge chamber. In this study, detailed computational analysis of several modified NSTAR thruster 3-ring and 4-ring magnetic field geometries reveals that the magnetic field line shape as well as Bcc determines important aspects of dc ion thruster performance (i.e. propellant efficiency, beam flatness and double ion content). The DC-ION ion thruster model results show that the baseline NSTAR configuration traps the primary electrons on-axis, which leads to the high on-axis plasma density peak and high double ion content observed in experimental measurements. These problems are further exacerbated by simply increasing Bcc and not changing the field line shape. Changing the field line shape to prevent on-axis confinement (while maintaining the NSTAR baseline Bcc) improves thruster performance, improves plasma uniformity and lowers double ion content. For these favorable field line geometries, we observe further improvements to performance with increased Bcc, while maintaining plasma uniformity and low double ion content. These improvements derive from the fact that the field lines guide the high-energy primaries to regions where they are most efficiently used to create ions while a higher Bcc prevents the loss of ions to the anode walls. Therefore, it is recommended that the ion thruster designer first establish a divergent field line shape that ensures favorable beam flatness, low double ion content and reasonable performance; then the designer may adjust the Bcc to attain desirable performance and stability for the target discharge plasma conditions. © 2008 IOP Publishing Ltd.

@article{
 title = {Effects of internally mounted cathodes on Hall thruster plume properties},
 type = {article},
 year = {2008},
 volume = {36},
 id = {ede3b1dd-c9aa-3f78-962d-b1b8ca019356},
 created = {2021-07-30T20:26:20.288Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:26:20.288Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The effects of cathode position on the operation and plume properties of an 8-kW Hall thruster are discussed. Thruster operation was investigated at operating conditions ranging from 200 to 500 V of discharge voltage, 10-40 A of discharge current, and 2-8 kW of discharge power, with a cathode positioned either in the traditional externally mounted configuration outside the outer magnetic pole piece or in an internally mounted configuration central to the inner magnetic core. With the external cathode, substantial emission in the visible spectrum that follows magnetic field lines surrounds the exterior pole pieces of the thruster. With the internal cathode, the emission is largely absent while the cathode plume is compressed and elongated in the axial direction by the strong axial magnetic field on the thruster centerline. Discharge current oscillation and ion species fraction measurements were found to be similar for the cathode locations, whereas the operation with the internal cathode was found to favor an improved coupling of the cathode plume with the thruster discharge. Ion current density measurements show that with respect to externally mounted designs, internally mounted cathodes reduce plume divergence and increase the symmetry of the near-field plume. The impacts of internally mounted cathodes on thruster physics and spacecraft integration activities are assessed. © 2008 IEEE.},
 bibtype = {article},
 author = {Hofer, Richard R. and Johnson, Lee K. and Goebel, Dan M. and Wirz, Richard E.},
 doi = {10.1109/TPS.2008.2000962},
 journal = {IEEE Transactions on Plasma Science},
 number = {5 PART 1}
}

The effects of cathode position on the operation and plume properties of an 8-kW Hall thruster are discussed. Thruster operation was investigated at operating conditions ranging from 200 to 500 V of discharge voltage, 10-40 A of discharge current, and 2-8 kW of discharge power, with a cathode positioned either in the traditional externally mounted configuration outside the outer magnetic pole piece or in an internally mounted configuration central to the inner magnetic core. With the external cathode, substantial emission in the visible spectrum that follows magnetic field lines surrounds the exterior pole pieces of the thruster. With the internal cathode, the emission is largely absent while the cathode plume is compressed and elongated in the axial direction by the strong axial magnetic field on the thruster centerline. Discharge current oscillation and ion species fraction measurements were found to be similar for the cathode locations, whereas the operation with the internal cathode was found to favor an improved coupling of the cathode plume with the thruster discharge. Ion current density measurements show that with respect to externally mounted designs, internally mounted cathodes reduce plume divergence and increase the symmetry of the near-field plume. The impacts of internally mounted cathodes on thruster physics and spacecraft integration activities are assessed. © 2008 IEEE.

@inproceedings{
 title = {Electron backstreaming determination for ion thrusters},
 type = {inproceedings},
 year = {2008},
 id = {9e9d500e-ed75-37ff-a3a6-aa4e13b7b658},
 created = {2021-07-30T22:43:34.301Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:34.301Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Electron backstreaming in ion thrusters is caused by the random flux of beam electrons past a potential barrier established by the accel grid. A technique that integrates this flux over the radial extent of the barrier reveals important aspects of electron backstreaming phenomena for individual beamlets, across the thruster beam, and throughout thruster life. For individual beamlets it was found that over 99% of the electron backstreaming occurs in a small annulus at the center of the beamlet that is less than 20% the area of the beamlet at the potential barrier established by the accel grid. For the thruster beam it was found that over 99% of the backstreaming current occurs inside of r = 6 cm for the over 28 cm diameter NSTAR grid. Initial validation against ELT data shows that the technique provides the correct behavior and magnitude of electron backstreaming limit, Vebs. From the sensitivity analyses it is apparent that accel grid chamfering may be the dominant mechanism contributing to the sharp rise in |Vebs| observed in the ELT but does not explain the rise in ion transparency. Grid gap change also contributes to |Vebs| rise and large rises in ion transparency with thruster life for the center gridlet. Screen grid erosion contributes generally to rises in |Vebs| and ion transparency, but for the assumptions used herein, it appears to not have as much of an effect as chamfering or grid gap change. Overall, it is apparent that accel grid chamfering, grid gap change, and screen grid erosion are important to the increase in electron backstreaming observed during the ELT. © 2008 by the American Institute of Aeronautics and Astronautics, Inc.},
 bibtype = {inproceedings},
 author = {Wirz, Richard E. and Katz, Ira and Goebel, Dan M. and Anderson, John R.},
 doi = {10.2514/6.2008-4732},
 booktitle = {44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit}
}

Electron backstreaming in ion thrusters is caused by the random flux of beam electrons past a potential barrier established by the accel grid. A technique that integrates this flux over the radial extent of the barrier reveals important aspects of electron backstreaming phenomena for individual beamlets, across the thruster beam, and throughout thruster life. For individual beamlets it was found that over 99% of the electron backstreaming occurs in a small annulus at the center of the beamlet that is less than 20% the area of the beamlet at the potential barrier established by the accel grid. For the thruster beam it was found that over 99% of the backstreaming current occurs inside of r = 6 cm for the over 28 cm diameter NSTAR grid. Initial validation against ELT data shows that the technique provides the correct behavior and magnitude of electron backstreaming limit, Vebs. From the sensitivity analyses it is apparent that accel grid chamfering may be the dominant mechanism contributing to the sharp rise in |Vebs| observed in the ELT but does not explain the rise in ion transparency. Grid gap change also contributes to |Vebs| rise and large rises in ion transparency with thruster life for the center gridlet. Screen grid erosion contributes generally to rises in |Vebs| and ion transparency, but for the assumptions used herein, it appears to not have as much of an effect as chamfering or grid gap change. Overall, it is apparent that accel grid chamfering, grid gap change, and screen grid erosion are important to the increase in electron backstreaming observed during the ELT. © 2008 by the American Institute of Aeronautics and Astronautics, Inc.

@inproceedings{
 title = {Miniature valve for Xenon thrusters},
 type = {inproceedings},
 year = {2008},
 id = {67b97567-0146-3217-a275-d2a03879384c},
 created = {2021-07-30T22:43:37.692Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:37.692Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {NASA is continuing its development of electric propulsion engines for various applications for both large and small thrusters1,2,3. A current program is focused on the development of a miniature thruster. This program is for a space based interferometer that requires separation control of microns to millimeters, and angular control to within arcseconds. Development of a Miniature Xenon Ion (MiXI) thruster will enable precision spacecraft positioning and formation maneuvers for formation-flying spacecraft 4,5. The current MiXI thruster prototype will provide 0.5 - 3 mN thrust at 3000 sec specific impulse and efficiencies around 50% or better. The MiXI thruster will use xenon propellant, a noble gas, minimizing spacecraft contamination. One of the challenges of such a small thruster is to be able to accurately control the flow of propellant at low flow rates using a small flow control assembly. The focus of this effort was to develop a miniature valve that could control the very low propellant flow rates required for these small thrusters. A valve was designed, developed, and a rough prototype was tested to ensure the requirements were met. The significance of the success of this effort is that fine flow control of xenon has been demonstrated. This enables small thrusters, such as the MiXI thruster, to continue development and progress toward use on small satellites for very fine precision control. With the advent of smaller satellites and formation flying systems, the need for small thrust control is critical to mission success. The development of Midé's Miniature Xenon Valve (MXV) helps make these thrusters a reality. © 2008 by the American Institute of Aeronautics and Astronautics, Inc.},
 bibtype = {inproceedings},
 author = {Lengyel, Attila J. and Wirz, Richard E.},
 doi = {10.2514/6.2008-5085},
 booktitle = {44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit}
}

NASA is continuing its development of electric propulsion engines for various applications for both large and small thrusters1,2,3. A current program is focused on the development of a miniature thruster. This program is for a space based interferometer that requires separation control of microns to millimeters, and angular control to within arcseconds. Development of a Miniature Xenon Ion (MiXI) thruster will enable precision spacecraft positioning and formation maneuvers for formation-flying spacecraft 4,5. The current MiXI thruster prototype will provide 0.5 - 3 mN thrust at 3000 sec specific impulse and efficiencies around 50% or better. The MiXI thruster will use xenon propellant, a noble gas, minimizing spacecraft contamination. One of the challenges of such a small thruster is to be able to accurately control the flow of propellant at low flow rates using a small flow control assembly. The focus of this effort was to develop a miniature valve that could control the very low propellant flow rates required for these small thrusters. A valve was designed, developed, and a rough prototype was tested to ensure the requirements were met. The significance of the success of this effort is that fine flow control of xenon has been demonstrated. This enables small thrusters, such as the MiXI thruster, to continue development and progress toward use on small satellites for very fine precision control. With the advent of smaller satellites and formation flying systems, the need for small thrust control is critical to mission success. The development of Midé's Miniature Xenon Valve (MXV) helps make these thrusters a reality. © 2008 by the American Institute of Aeronautics and Astronautics, Inc.

@inproceedings{
 title = {Qualification of commercial XIPS® ion thrusters for NASA deep space missions},
 type = {inproceedings},
 year = {2008},
 id = {5633f760-f3c6-3511-9337-d27bf22ed8fc},
 created = {2021-07-30T22:43:38.293Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:38.293Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Electric propulsion systems based on commercial ion and Hall thrusters have the potential for significantly reducing the cost and schedule-risk of Ion Propulsion Systems (IPS) for deep space missions. The large fleet of geosynchronous communication satellites that use SEP, which will approach 40 satellites by year-end, demonstrates the significant level of technical maturity and spaceflight heritage achieved by the commercial electric propulsion systems. A program to delta-qualify XIPS® ion thrusters for deep space missions is underway at JPL. This program includes modeling of the thruster grid and cathode life, environmental testing of a 25-cm EM thruster over anticipated vibe and thermal requirements for deep space missions, and wear testing of the thruster cathodes to demonstrate the life and benchmark the model results. This paper will present the delta-qualification status of the XIPS thruster and discuss the life and reliability with respect to known failure mechanisms. © 2008 by the American Institute of Aeronautics and Astronautics, Inc.},
 bibtype = {inproceedings},
 author = {Goebel, Dan M. and Polk, James E. and Wirz, Richard E. and Snyder, John Steven and Mikellides, Ioannis G. and Katz, Ira and Anderson, John},
 doi = {10.2514/6.2008-4914},
 booktitle = {44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit}
}

Electric propulsion systems based on commercial ion and Hall thrusters have the potential for significantly reducing the cost and schedule-risk of Ion Propulsion Systems (IPS) for deep space missions. The large fleet of geosynchronous communication satellites that use SEP, which will approach 40 satellites by year-end, demonstrates the significant level of technical maturity and spaceflight heritage achieved by the commercial electric propulsion systems. A program to delta-qualify XIPS® ion thrusters for deep space missions is underway at JPL. This program includes modeling of the thruster grid and cathode life, environmental testing of a 25-cm EM thruster over anticipated vibe and thermal requirements for deep space missions, and wear testing of the thruster cathodes to demonstrate the life and benchmark the model results. This paper will present the delta-qualification status of the XIPS thruster and discuss the life and reliability with respect to known failure mechanisms. © 2008 by the American Institute of Aeronautics and Astronautics, Inc.

@inproceedings{
 title = {Design study for a planet-finding space interferometer},
 type = {inproceedings},
 year = {2008},
 id = {93634e31-c439-32fa-b8c3-e02dce53bc42},
 created = {2021-07-30T22:43:38.917Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:38.917Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The characterization of earthlike planets that may be orbiting nearby stars will require large observatories capable of detecting a small number of planet photons and separating them from the much larger flux from the parent star. One approach to this is to employ nulling interferometry on a space-based platform utilizing a number of spacecraft At the Jet Propulsion Laboratory, a linear dual chopped Bracewell array design was studied in depth while in Europe various two-dimensional designs were discussed. This study looks in a little more depth at the design issues for one of the European concepts and concludes that the concept has promise for a significant reduction in spacecraft complexity and mass and therefore would result in a lower cost mission. ©2008 IEEE.},
 bibtype = {inproceedings},
 author = {Martin, Stefan and Scharf, Daniel P. and Wirz, Richard and Lay, Oliver and McKinstry, David and Mennesson, Bertrand and Purcell, George and Rodriguez, Jose and Scherr, Laurence and Smith, James R. and Wayne, Leonard},
 doi = {10.1109/AERO.2008.4526410},
 booktitle = {IEEE Aerospace Conference Proceedings}
}

The characterization of earthlike planets that may be orbiting nearby stars will require large observatories capable of detecting a small number of planet photons and separating them from the much larger flux from the parent star. One approach to this is to employ nulling interferometry on a space-based platform utilizing a number of spacecraft At the Jet Propulsion Laboratory, a linear dual chopped Bracewell array design was studied in depth while in Europe various two-dimensional designs were discussed. This study looks in a little more depth at the design issues for one of the European concepts and concludes that the concept has promise for a significant reduction in spacecraft complexity and mass and therefore would result in a lower cost mission. ©2008 IEEE.

@article{
 title = {Hollow cathode and low-thrust extraction grid analysis for aminiature ion thruster},
 type = {article},
 year = {2008},
 volume = {2008},
 id = {63070c85-859c-39ec-8820-febd5a4e3892},
 created = {2021-07-30T22:43:41.132Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:41.132Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Miniature ion thrusters are well suited for future space missions that require high efficiency, precision thrust, and low contamination in the mN to sub-mN range. JPL's miniature xenon Ion (MiXI) thruster has demonstrated an efficient discharge and ion extraction grid assembly using filament cathodes and the internal conduction (IC) cathode. JPL is currently preparing to incorporate a miniature hollow cathode for the MiXI discharge. Computational analyses anticipate that an axially upstream hollow cathode location provides the most favorable performance and beam profile; however, the hot surfaces of the hollow cathode must be sufficiently downstream to avoid demagnetization of the cathode magnet at the back of the chamber, which can significantly reduce discharge performance. MiXI's ion extraction grids are designed to provide < 3mN of thrust; however, previous to this effort, the low-thrust characteristics had not been investigated. Experimental results obtained with the MiXI-II thruster (a near replica or the original MiXI thruster) show that sparse average discharge plasma densities of ∼ 5 × 10 15 - 5 × 1016 m-3 allow the use of very low beamlet focusing extraction voltages of only ∼ 250 -500 V, thus providing thrust levels as low as 0.03 mN for focused beamlet conditions. Consequently, the thrust range thus far demonstrated by MiXI in this and other tests is 0.03-1.54 mN.},
 bibtype = {article},
 author = {Wirz, Richard and Sullivan, Regina and Przybylowski, Johanna and Silva, Mike},
 doi = {10.1155/2008/693825},
 journal = {International Journal of Plasma Science and Engineering}
}

Miniature ion thrusters are well suited for future space missions that require high efficiency, precision thrust, and low contamination in the mN to sub-mN range. JPL's miniature xenon Ion (MiXI) thruster has demonstrated an efficient discharge and ion extraction grid assembly using filament cathodes and the internal conduction (IC) cathode. JPL is currently preparing to incorporate a miniature hollow cathode for the MiXI discharge. Computational analyses anticipate that an axially upstream hollow cathode location provides the most favorable performance and beam profile; however, the hot surfaces of the hollow cathode must be sufficiently downstream to avoid demagnetization of the cathode magnet at the back of the chamber, which can significantly reduce discharge performance. MiXI's ion extraction grids are designed to provide < 3mN of thrust; however, previous to this effort, the low-thrust characteristics had not been investigated. Experimental results obtained with the MiXI-II thruster (a near replica or the original MiXI thruster) show that sparse average discharge plasma densities of ∼ 5 × 10 15 - 5 × 1016 m-3 allow the use of very low beamlet focusing extraction voltages of only ∼ 250 -500 V, thus providing thrust levels as low as 0.03 mN for focused beamlet conditions. Consequently, the thrust range thus far demonstrated by MiXI in this and other tests is 0.03-1.54 mN.

@inproceedings{
 title = {A survey of micro-thrust propulsion options for microspacecraft and formation flying missions},
 type = {inproceedings},
 year = {2008},
 id = {caa5502c-b5e2-373e-9daf-a6a5535d33c3},
 created = {2021-07-30T22:59:33.771Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:59:33.771Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 source_type = {CONF},
 private_publication = {false},
 bibtype = {inproceedings},
 author = {Mueller, Juergen and Ziemer, John and Hofer, Richard and Wirz, Richard and O’Donnell, Timothy},
 booktitle = {5th Annual CubeSat Developers Workshop San Luis Obispo, CA}
}

@article{
 title = {Analytical ion thruster discharge performance model},
 type = {article},
 year = {2007},
 volume = {23},
 id = {be550364-c2cd-3b33-bda9-f18060fb525c},
 created = {2021-07-30T20:26:22.527Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:26:22.527Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A particle and energy balance model of the plasma discharge in magnetic ring-cusp ion thrusters has been developed. The model follows prior work in the development of global zero-dimensional discharge models that use conservation of particles into and out of the thruster, conservation of energy into the discharge and out of the plasma in the form of charged particles to the walls and beam, and plasma radiation. The present model is significantly expanded over the prior art by closing the set of equations with self-consistent calculations of the internal neutral pressure, electron temperature, primary electron density, electrostatic ion confinement (due to the ring-cusp fields), plasma potential, discharge stability, and time-dependent behavior during recycling. The model only requires information on the thruster geometry, ion optics performance, and electrical inputs, such as discharge voltage and currents, to produce accurate performance curves of discharge loss vs mass utilization efficiency. The model has been benchmarked against several ion thrusters, and successfully predicts the thruster discharge loss as a function of mass utilization efficiency for a variety of thrusters. The discharge performance model will be described and results showing ion thruster performance and stability presented. Copyright © 2007 by Stanford University.},
 bibtype = {article},
 author = {Goebel, Dan M. and Wirz, Richard E. and Katz, Ira},
 doi = {10.2514/1.26404},
 journal = {Journal of Propulsion and Power},
 number = {5}
}

A particle and energy balance model of the plasma discharge in magnetic ring-cusp ion thrusters has been developed. The model follows prior work in the development of global zero-dimensional discharge models that use conservation of particles into and out of the thruster, conservation of energy into the discharge and out of the plasma in the form of charged particles to the walls and beam, and plasma radiation. The present model is significantly expanded over the prior art by closing the set of equations with self-consistent calculations of the internal neutral pressure, electron temperature, primary electron density, electrostatic ion confinement (due to the ring-cusp fields), plasma potential, discharge stability, and time-dependent behavior during recycling. The model only requires information on the thruster geometry, ion optics performance, and electrical inputs, such as discharge voltage and currents, to produce accurate performance curves of discharge loss vs mass utilization efficiency. The model has been benchmarked against several ion thrusters, and successfully predicts the thruster discharge loss as a function of mass utilization efficiency for a variety of thrusters. The discharge performance model will be described and results showing ion thruster performance and stability presented. Copyright © 2007 by Stanford University.

@inproceedings{
 title = {Ground Effects Due to CEV Retrorockets},
 type = {inproceedings},
 year = {2007},
 id = {6da4a248-b3c3-3897-86fe-316053c2afc8},
 created = {2021-07-30T22:43:39.493Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:39.493Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A scenario for creating acceptable touchdown velocity on land for the Orion Crew Excursion Vehicle employs retrorockets for final landing ?V. To capture the ground effects due to the interaction of the retrorockets and the vehicle, detailed computational modeling was used to determine the effective thrust at several different firing heights. These results were then used to determine the change in impulse for a wide range of possible firing altitudes. For a Crew Excursion Vehicle retrorocket firing time of 0.5 s, the steady-state effective vertical thrust of the module changes from?13:2to?11:8%for altitudes from 152 to 15.2 cm, respectively.Asimple descent analysis shows that ground effects will impart a net decrease or increase in impulse, depending on ignition altitude. In this analysis, the ground effects serve to increase the optimal firing height and increase the ignition altitude margin for a given maximum landing velocity.},
 bibtype = {inproceedings},
 author = {Wirz, Richard},
 doi = {10.2514/6.2007-5758}
}

A scenario for creating acceptable touchdown velocity on land for the Orion Crew Excursion Vehicle employs retrorockets for final landing ?V. To capture the ground effects due to the interaction of the retrorockets and the vehicle, detailed computational modeling was used to determine the effective thrust at several different firing heights. These results were then used to determine the change in impulse for a wide range of possible firing altitudes. For a Crew Excursion Vehicle retrorocket firing time of 0.5 s, the steady-state effective vertical thrust of the module changes from?13:2to?11:8%for altitudes from 152 to 15.2 cm, respectively.Asimple descent analysis shows that ground effects will impart a net decrease or increase in impulse, depending on ignition altitude. In this analysis, the ground effects serve to increase the optimal firing height and increase the ignition altitude margin for a given maximum landing velocity.

@inproceedings{
 title = {TPF-Emma: concept study of a planet finding space interferometer},
 type = {inproceedings},
 year = {2007},
 volume = {6693},
 id = {3ae41f17-04f0-3895-85de-7a8870536581},
 created = {2021-07-30T22:43:40.048Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:40.048Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A novel space interferometer design originating in Europe has been studied. The interferometer uses the technique of starlight nulling to enable detection of earth-like planets orbiting nearby stars. A set of four telescope spacecraft flying in formation with a fifth, beam-combiner spacecraft forms the interferometer. This particular concept shows potential for reducing the mission cost when compared with previous concepts by greatly reducing the complexity of the telescope spacecraft. These spacecraft have no major deployable systems, have simplified propulsion and a more rugged construction. The formation flying geometry provides for greater average separation between the spacecraft with commensurate risk reduction. Key aspects of the design have been studied at the Jet Propulsion Laboratory with a view to collaborations between NASA and the European Space Agency. An overview of the design study is presented with some comparisons with the TPF-FFI concept.},
 bibtype = {inproceedings},
 author = {Martin, Stefan R. and Scharf, Daniel and Wirz, Richard and Lay, Oliver and McKinstry, David and Mennesson, Bertrand and Purcell, George and Rodriguez, Jose and Scherr, Laurence and Smith, James R. and Wayne, Leonard},
 doi = {10.1117/12.734835},
 booktitle = {Techniques and Instrumentation for Detection of Exoplanets III}
}

A novel space interferometer design originating in Europe has been studied. The interferometer uses the technique of starlight nulling to enable detection of earth-like planets orbiting nearby stars. A set of four telescope spacecraft flying in formation with a fifth, beam-combiner spacecraft forms the interferometer. This particular concept shows potential for reducing the mission cost when compared with previous concepts by greatly reducing the complexity of the telescope spacecraft. These spacecraft have no major deployable systems, have simplified propulsion and a more rugged construction. The formation flying geometry provides for greater average separation between the spacecraft with commensurate risk reduction. Key aspects of the design have been studied at the Jet Propulsion Laboratory with a view to collaborations between NASA and the European Space Agency. An overview of the design study is presented with some comparisons with the TPF-FFI concept.

@inproceedings{
 title = {Production of high energy ions near an ion thruster discharge hollow cathode},
 type = {inproceedings},
 year = {2006},
 volume = {3},
 id = {205a09b6-5c4e-3c72-ba72-657deba6c6ff},
 created = {2021-07-30T18:06:25.516Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T18:06:25.516Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Several researchers have measured ions leaving ion thruster discharge chambers with energies far greater than the measured discharge chamber potentials. A new mechanism for the generation of high energy ions is proposed in this paper. The mechanism relies on the charge exchange neutralization of xenon ions accelerated radially into the potential trough in front of the discharge cathode. The fast neutrals are then re-ionized and gain potential energy. Simplified numerical simulations have been performed to support the hypothesis and the results are compared with measured ion spectra. The measured high energies are the sum of the kinetic and potential energies. Previous researchers have identified the importance of charge exchange in this region as a mechanism for protecting discharge cathode surfaces from ion bombardment. This paper is the first to identify how charge exchange in this region can lead to ion energy enhancement. It is also found however that the high-energy ions produced by this mechanism are not likely to cause any significant erosion of the keeper because of their low density, and their mainly radially directed motion. Copyright © 2006 The American Institute of Aeronautics and Astronautics, Inc.},
 bibtype = {inproceedings},
 author = {Katz, Ira and Mikellides, Ioannis G. and Goebel, Dan M. and Jameson, Kristina K. and Wirz, Richard and Johnson, Lee K.},
 doi = {10.2514/6.2006-4485},
 booktitle = {Collection of Technical Papers - AIAA/ASME/SAE/ASEE 42nd Joint Propulsion Conference}
}

Several researchers have measured ions leaving ion thruster discharge chambers with energies far greater than the measured discharge chamber potentials. A new mechanism for the generation of high energy ions is proposed in this paper. The mechanism relies on the charge exchange neutralization of xenon ions accelerated radially into the potential trough in front of the discharge cathode. The fast neutrals are then re-ionized and gain potential energy. Simplified numerical simulations have been performed to support the hypothesis and the results are compared with measured ion spectra. The measured high energies are the sum of the kinetic and potential energies. Previous researchers have identified the importance of charge exchange in this region as a mechanism for protecting discharge cathode surfaces from ion bombardment. This paper is the first to identify how charge exchange in this region can lead to ion energy enhancement. It is also found however that the high-energy ions produced by this mechanism are not likely to cause any significant erosion of the keeper because of their low density, and their mainly radially directed motion. Copyright © 2006 The American Institute of Aeronautics and Astronautics, Inc.

@inproceedings{
 title = {Discharge hollow cathode and extraction grid analysis for the MiXI ion thruster},
 type = {inproceedings},
 year = {2006},
 volume = {3},
 id = {fbc22acf-ec05-34ec-a7a0-36de70210d02},
 created = {2021-07-30T20:26:18.545Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T20:26:18.545Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Miniature ion thrusters are well-suited for future space missions that require highly efficient and controllable thrust in the mN to sub-mN range, and a benign plume. JPL's MiXI ion thruster has demonstrated an efficient discharge and ion extraction grid assembly using filament cathodes and the Internal Conduction (IC) cathode. JPL is currently preparing to incorporate a miniature hollow cathode for the MiXI discharge. Computational analyses anticipate that an axially upstream hollow cathode location provides the most favorable performance and beam profile; however, the hot surfaces of the hollow cathode must be sufficiently downstream to avoid demagnetization of the cathode magnet at the back of the chamber (or else suffer a possible ∼10% loss in propellant efficiency). MiXI's ion extraction grids are designed to provide >3 mN of thrust; however, previous to this effort, the low thrust characteristics had not been investigated. Experimental results obtained with the MiXI-II thruster (a near replica or the original MiXI thruster) show that sparse average discharge plasma densities of ∼5e15 - 5e16 m-3 allow very low focused extraction voltage of only ∼250-500 V, thus providing thrust levels as low as 0.03 mN for focused beamlet conditions. Consequently, the thrust range thus far demonstrated by MiXI in this and other tests is 0.03-1.54mN.},
 bibtype = {inproceedings},
 author = {Wirz, Richard and Sullivan, Regina and Przybylowski, Johanna and Silva, Mike},
 doi = {10.2514/6.2006-4498},
 booktitle = {Collection of Technical Papers - AIAA/ASME/SAE/ASEE 42nd Joint Propulsion Conference}
}

Miniature ion thrusters are well-suited for future space missions that require highly efficient and controllable thrust in the mN to sub-mN range, and a benign plume. JPL's MiXI ion thruster has demonstrated an efficient discharge and ion extraction grid assembly using filament cathodes and the Internal Conduction (IC) cathode. JPL is currently preparing to incorporate a miniature hollow cathode for the MiXI discharge. Computational analyses anticipate that an axially upstream hollow cathode location provides the most favorable performance and beam profile; however, the hot surfaces of the hollow cathode must be sufficiently downstream to avoid demagnetization of the cathode magnet at the back of the chamber (or else suffer a possible ∼10% loss in propellant efficiency). MiXI's ion extraction grids are designed to provide >3 mN of thrust; however, previous to this effort, the low thrust characteristics had not been investigated. Experimental results obtained with the MiXI-II thruster (a near replica or the original MiXI thruster) show that sparse average discharge plasma densities of ∼5e15 - 5e16 m-3 allow very low focused extraction voltage of only ∼250-500 V, thus providing thrust levels as low as 0.03 mN for focused beamlet conditions. Consequently, the thrust range thus far demonstrated by MiXI in this and other tests is 0.03-1.54mN.

@inproceedings{
 title = {Analytical ion thruster discharge performance model},
 type = {inproceedings},
 year = {2006},
 volume = {3},
 id = {603e7f15-dbcf-32d7-bd85-32bece282abd},
 created = {2021-07-30T20:26:27.865Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
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 last_modified = {2021-07-30T20:26:27.865Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A particle and energy balance model of the plasma discharge in magnetic ring-cusp ion thrusters has been developed. The model follows the original work of Brophy in the development of global 0-D discharge models that utilize conservation of particles into and out of the thruster and conservation of energy into the discharge and out of the plasma in the form of charged particles to the walls and beam and plasma radiation. The present model is significantly expanded over Brophy's original work by closing the set of equations with self-consistently calculations of the internal neutral pressure, electron temperature, primary electron density, electrostatic ion confinement (due to the ring-cusp fields), plasma potential, discharge stability, and time dependent behavior during recycling. The model only requires information on the thruster geometry, ion optics performance and electrical inputs such as discharge voltage and currents, etc. to produce accurate performance curves of discharge loss versus mass utilization efficiency. The model has been bench marked against NSTAR and NEXIS thrusters, and successfully predicts the thruster discharge loss as a function of mass utilization efficiency for a variety of thrusters. The discharge performance model will be described and results showing ion thruster performance and stability presented.},
 bibtype = {inproceedings},
 author = {Goebel, Dan M. and Wirz, Richard E. and Katz, Ira},
 doi = {10.2514/6.2006-4486},
 booktitle = {Collection of Technical Papers - AIAA/ASME/SAE/ASEE 42nd Joint Propulsion Conference}
}

A particle and energy balance model of the plasma discharge in magnetic ring-cusp ion thrusters has been developed. The model follows the original work of Brophy in the development of global 0-D discharge models that utilize conservation of particles into and out of the thruster and conservation of energy into the discharge and out of the plasma in the form of charged particles to the walls and beam and plasma radiation. The present model is significantly expanded over Brophy's original work by closing the set of equations with self-consistently calculations of the internal neutral pressure, electron temperature, primary electron density, electrostatic ion confinement (due to the ring-cusp fields), plasma potential, discharge stability, and time dependent behavior during recycling. The model only requires information on the thruster geometry, ion optics performance and electrical inputs such as discharge voltage and currents, etc. to produce accurate performance curves of discharge loss versus mass utilization efficiency. The model has been bench marked against NSTAR and NEXIS thrusters, and successfully predicts the thruster discharge loss as a function of mass utilization efficiency for a variety of thrusters. The discharge performance model will be described and results showing ion thruster performance and stability presented.

@inproceedings{
 title = {Ion thruster discharge performance per magnetic field topography},
 type = {inproceedings},
 year = {2006},
 volume = {3},
 id = {e25e87df-8fcf-3c04-90de-442c44dbbde3},
 created = {2021-07-30T22:43:40.593Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:40.593Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Traditional magnetic field design techniques for DC ion thrusters typically focus on closing a sufficiently high maximum closed magnetic contour, B cc, inside the discharge chamber. In this study, detailed computational analysis of several modified NSTAR thruster 3-ring and 4-ring magnetic field geometries reveals that the magnetic field line shape, as well as Bcc determines important aspects of DC ion thruster performance (i.e. propellant efficiency, beam flatness, and double ion content). The DC-ION ion thruster model results show that the baseline NSTAR configuration traps the primary electrons on-axis, which leads to the high on-axis plasma density peak and high double ion content observed in experimental measurements. These problems are further exacerbated by simply increasing Bcc and not changing the field line shape. Changing the field line shape to prevent on-axis confinement (while maintaining the NSTAR baseline Bcc) improves thruster performance, improves plasma uniformity, and lowers double ion content. For these favorable field line geometries, we observe further improvements to performance with increased Bcc, while maintaining plasma uniformity and low double ion content. These improvements derive from the fact that the field lines guide the high energy primaries to regions where they are most efficiently used to create ions while a higher Bcc prevents the loss of ions to the anode walls. Therefore, it is recommended that the ion thruster designer first establish a divergent field line shape that ensures favorable beam flatness, low double ion content, and reasonable performance; then the designer may adjust the Bcc to attain desirable performance and stability for the target discharge plasma conditions.},
 bibtype = {inproceedings},
 author = {Wirz, Richard and Goebel, Dan},
 doi = {10.2514/6.2006-4487},
 booktitle = {Collection of Technical Papers - AIAA/ASME/SAE/ASEE 42nd Joint Propulsion Conference}
}

Traditional magnetic field design techniques for DC ion thrusters typically focus on closing a sufficiently high maximum closed magnetic contour, B cc, inside the discharge chamber. In this study, detailed computational analysis of several modified NSTAR thruster 3-ring and 4-ring magnetic field geometries reveals that the magnetic field line shape, as well as Bcc determines important aspects of DC ion thruster performance (i.e. propellant efficiency, beam flatness, and double ion content). The DC-ION ion thruster model results show that the baseline NSTAR configuration traps the primary electrons on-axis, which leads to the high on-axis plasma density peak and high double ion content observed in experimental measurements. These problems are further exacerbated by simply increasing Bcc and not changing the field line shape. Changing the field line shape to prevent on-axis confinement (while maintaining the NSTAR baseline Bcc) improves thruster performance, improves plasma uniformity, and lowers double ion content. For these favorable field line geometries, we observe further improvements to performance with increased Bcc, while maintaining plasma uniformity and low double ion content. These improvements derive from the fact that the field lines guide the high energy primaries to regions where they are most efficiently used to create ions while a higher Bcc prevents the loss of ions to the anode walls. Therefore, it is recommended that the ion thruster designer first establish a divergent field line shape that ensures favorable beam flatness, low double ion content, and reasonable performance; then the designer may adjust the Bcc to attain desirable performance and stability for the target discharge plasma conditions.

@inproceedings{
 title = {Ion Thruster Life Models},
 type = {inproceedings},
 year = {2005},
 id = {b67abbb2-eb6c-301f-bb5b-63fe6197234c},
 created = {2021-07-30T22:43:41.664Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:41.664Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Many of the missions considered under the Space Vision demand electric propulsion lifetimes that far exceed what has been demonstrated. Some missions for example, such as the recently studied Prometheus 1, require thruster operation life, including margin, in excess of a decade. That length of thruster operations essentially precludes relying exclusively on testing for life validation. A coordinated program of experimental measurement and computer simulation has produced analytical models of several of the most important ion thruster physics and life limiting mechanisms. In this paper we review the most recent 2-D and 3-D plasma simulation models for ion thruster hollow cathodes, discharge chambers, and ion optics.},
 bibtype = {inproceedings},
 author = {Katz, Ira and Mikellides, Ioannis and Wirz, Richard and Anderson, John and Goebel, Dan},
 doi = {10.2514/6.2005-4256}
}

Many of the missions considered under the Space Vision demand electric propulsion lifetimes that far exceed what has been demonstrated. Some missions for example, such as the recently studied Prometheus 1, require thruster operation life, including margin, in excess of a decade. That length of thruster operations essentially precludes relying exclusively on testing for life validation. A coordinated program of experimental measurement and computer simulation has produced analytical models of several of the most important ion thruster physics and life limiting mechanisms. In this paper we review the most recent 2-D and 3-D plasma simulation models for ion thruster hollow cathodes, discharge chambers, and ion optics.

@inproceedings{
 title = {Dynamic simulations for the Terrestrial Planet Finder interferometer},
 type = {inproceedings},
 year = {2005},
 volume = {5899},
 id = {8445324b-205a-3407-96f4-37e004e2ccb5},
 created = {2021-07-30T22:43:42.544Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:42.544Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 bibtype = {inproceedings},
 author = {Li, Ying-Yong and Hamlin, Louise A. and Wirz, Richie and Adams, Douglas and Moore, Greg and Coppolino, Robert and Peng, Chia-Yen and Levine, Marie},
 doi = {10.1117/12.620012},
 booktitle = {UV/Optical/IR Space Telescopes: Innovative Technologies and Concepts II}
}

@inproceedings{
 title = {Plasma processes of DC ion thruster discharge chambers},
 type = {inproceedings},
 year = {2005},
 id = {68e3bca0-67c6-3ec2-83e7-3d3ac5c77994},
 created = {2021-07-30T22:43:43.119Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:43.119Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {This study reveals important aspects of discharge plasma processes and advances state-of-the-art discharge modeling. A multi-component hybrid 2-D computational Discharge Chamber Model (DCM) was developed to help identify important ion thruster discharge processes and aid in thruster design and optimization. The model accounts for the five major chamber design parameters (chamber geometry, magnetic field, discharge cathode, propellant feed, and ion extraction grid characteristics) and self-consistently tracks the effects of the four discharge plasma species (neutral propellant atoms, secondary electrons, primary electrons, and ions). Good agreement with NSTAR thruster data was found by weighting the influence of Bohm-type non-classical diffusion per the relative level of ion-centered electron collisions. According to DCM results, ion thrusters operate in an intermediately ionized plasma regime that is between fully and weakly ionized approximations. The model analyses show that the peak observed in the NSTAR beam profile is due to double ions that are created by over-confinement of primary electrons on the thruster axis. A design analysis performed with DCM shows that simply increasing the strength of the middle magnet ring of the NSTAR thruster leads to remarkable improvements in thruster efficiency and beam flatness. These improvements are due to the propensity of this modified magnetic field to evenly distribute the primary electrons, which decreases double ion content on-axis, increases primary electron utilization, and promotes beam flatness. Simply increasing primary confinement does not guarantee better performance. The modified design also closes a higher magnetic field contour (-40 Gauss, instead of ∼30 Gauss) and thus shows better plasma confinement between the cusps. ©2005 by the American Institute of Aeronautics and Astronautics, Inc.},
 bibtype = {inproceedings},
 author = {Wirz, Richard and Katz, Ira},
 doi = {10.2514/6.2005-3690},
 booktitle = {41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit}
}

This study reveals important aspects of discharge plasma processes and advances state-of-the-art discharge modeling. A multi-component hybrid 2-D computational Discharge Chamber Model (DCM) was developed to help identify important ion thruster discharge processes and aid in thruster design and optimization. The model accounts for the five major chamber design parameters (chamber geometry, magnetic field, discharge cathode, propellant feed, and ion extraction grid characteristics) and self-consistently tracks the effects of the four discharge plasma species (neutral propellant atoms, secondary electrons, primary electrons, and ions). Good agreement with NSTAR thruster data was found by weighting the influence of Bohm-type non-classical diffusion per the relative level of ion-centered electron collisions. According to DCM results, ion thrusters operate in an intermediately ionized plasma regime that is between fully and weakly ionized approximations. The model analyses show that the peak observed in the NSTAR beam profile is due to double ions that are created by over-confinement of primary electrons on the thruster axis. A design analysis performed with DCM shows that simply increasing the strength of the middle magnet ring of the NSTAR thruster leads to remarkable improvements in thruster efficiency and beam flatness. These improvements are due to the propensity of this modified magnetic field to evenly distribute the primary electrons, which decreases double ion content on-axis, increases primary electron utilization, and promotes beam flatness. Simply increasing primary confinement does not guarantee better performance. The modified design also closes a higher magnetic field contour (-40 Gauss, instead of ∼30 Gauss) and thus shows better plasma confinement between the cusps. ©2005 by the American Institute of Aeronautics and Astronautics, Inc.

@inproceedings{
 title = {Computational Modeling of a Miniature Ion Thruster Discharge},
 type = {inproceedings},
 year = {2005},
 id = {6606fb3d-d2ba-3a47-b582-f5722c38aa2f},
 created = {2021-07-30T22:43:43.720Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:43.720Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 bibtype = {inproceedings},
 author = {Wirz, Richard},
 doi = {10.2514/6.2005-3887}
}

@inproceedings{
 title = {2-D Discharge Chamber Model for Ion Thrusters},
 type = {inproceedings},
 year = {2004},
 id = {771d41bc-5423-39a3-805b-e81d73674b1b},
 created = {2021-07-30T22:43:44.361Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:44.361Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 bibtype = {inproceedings},
 author = {Wirz, Richard and Katz, Ira},
 doi = {10.2514/6.2004-4107}
}

@inproceedings{
 title = {Miniature Ion Thruster for Precision Formation Flying},
 type = {inproceedings},
 year = {2004},
 id = {2b7fe702-3230-3861-8884-2e0fa5538f85},
 created = {2021-07-30T22:43:44.954Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:44.954Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 bibtype = {inproceedings},
 author = {Wirz, Richard and Mueller, Juergen and Gale, Michael and Marrese, Colleen},
 doi = {10.2514/6.2004-4115}
}

@inproceedings{
 title = {Development of cathode technologies for a miniature ion thruster},
 type = {inproceedings},
 year = {2003},
 id = {18e346b4-d545-329c-9c1f-bcc400bb6f48},
 created = {2021-07-30T22:43:45.482Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:45.482Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {Ion thrusters have traditionally been thought to be infeasible at smaller sizes; however, recent investigations at JPL using a 3cm Miniature Xenon Ion (MiXI) thruster have shown that a potential exists for an efficient miniature ion thruster that is capable of providing thrust from 0.1 to >1.5 mN, Isp in excess of 3000 seconds, and propellant utilization efficiency greater than 80%. MiXI, thruster tests have thus far used filament cathodes as placeholders for low-power cathodes. This paper discusses the cathode technologies that are candidates for the MiXI thruster. A discussion about RF/microwave sources for MiXI is also given. Comparison of the interrelationship of thruster and cathode performance show the need for cathodes that do not require propellant beyond what is required for the discharge. In an effort to avoid the issue of excess propellant flow, and due to the low discharge current requirements of the MiXI discharge, direct emission cathodes are considered. Initial results demonstrated that direct emission cathodes are possible candidates for the MiXI discharge but vacuum facility and cathode assembly issues must be resolved before an accurate comparison of cathode technologies can be completed. © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.},
 bibtype = {inproceedings},
 author = {Wirz, Richard and Goebel, Dan and Marrese, Colleen and Mueller, Juergen},
 doi = {10.2514/6.2003-4722},
 booktitle = {39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit}
}

Ion thrusters have traditionally been thought to be infeasible at smaller sizes; however, recent investigations at JPL using a 3cm Miniature Xenon Ion (MiXI) thruster have shown that a potential exists for an efficient miniature ion thruster that is capable of providing thrust from 0.1 to >1.5 mN, Isp in excess of 3000 seconds, and propellant utilization efficiency greater than 80%. MiXI, thruster tests have thus far used filament cathodes as placeholders for low-power cathodes. This paper discusses the cathode technologies that are candidates for the MiXI thruster. A discussion about RF/microwave sources for MiXI is also given. Comparison of the interrelationship of thruster and cathode performance show the need for cathodes that do not require propellant beyond what is required for the discharge. In an effort to avoid the issue of excess propellant flow, and due to the low discharge current requirements of the MiXI discharge, direct emission cathodes are considered. Initial results demonstrated that direct emission cathodes are possible candidates for the MiXI discharge but vacuum facility and cathode assembly issues must be resolved before an accurate comparison of cathode technologies can be completed. © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

@inproceedings{
 title = {Plasma Generation Near an Ion Thruster Discharge Chamber Hollow Cathode},
 type = {inproceedings},
 year = {2003},
 id = {bc390bf7-92d4-3183-abb9-4e8cdf4a56ac},
 created = {2021-07-30T22:43:46.072Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:46.072Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {In grided electrostatic thrusters, ions are produced by electron bombardment in the discharge chamber. In most of these thrusters, a single, centrally located hollow cathode supplies the ionizing electrons. An applied magnetic field in the discharge chamber restricts the electrons leaving the hollow cathode to a very narrow channel. In this channel, the high electron current density ionizes both propellant gas flowing from the hollow cathode, and other neutrals from the main propellant flow from the plenum. The processes that occur just past the hollow cathode exit are very important. In recent engine tests, several cases of discharge cathode orifice plate and keeper erosion have been reported. In this paper we present results from a new 1-D, variable area model of the plasma processes in the magnetized channel just downstream of the hollow cathode keeper. The model predicts plasma densities and temperatures consistent with those reported in the literature for the NSTAR engine, and preliminary results from the model show a potential maximum just downstream of the cathode. © 2003 by the American Institute of Aeronautics and Astronautics, Inc.},
 bibtype = {inproceedings},
 author = {Katz, Ira and Anderson, John R. and Goebel, Dan M. and Wirz, Richard and Sengupta, Anita},
 doi = {10.2514/6.2003-5161},
 booktitle = {39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit}
}

In grided electrostatic thrusters, ions are produced by electron bombardment in the discharge chamber. In most of these thrusters, a single, centrally located hollow cathode supplies the ionizing electrons. An applied magnetic field in the discharge chamber restricts the electrons leaving the hollow cathode to a very narrow channel. In this channel, the high electron current density ionizes both propellant gas flowing from the hollow cathode, and other neutrals from the main propellant flow from the plenum. The processes that occur just past the hollow cathode exit are very important. In recent engine tests, several cases of discharge cathode orifice plate and keeper erosion have been reported. In this paper we present results from a new 1-D, variable area model of the plasma processes in the magnetized channel just downstream of the hollow cathode keeper. The model predicts plasma densities and temperatures consistent with those reported in the literature for the NSTAR engine, and preliminary results from the model show a potential maximum just downstream of the cathode. © 2003 by the American Institute of Aeronautics and Astronautics, Inc.

@inproceedings{
 title = {A preliminary 2-D computational model of an ion thruster discharge chamber},
 type = {inproceedings},
 year = {2003},
 id = {8dfa5465-1159-31bd-9769-7602a1795467},
 created = {2021-07-30T22:43:46.694Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:46.694Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A better understanding of ion thruster discharge chamber physics will be possible if a computational model that describes the non-uniform properties and behavior of the discharge can be developed. To date, the design and optimization for ion thruster discharge chambers has been almost entirely based on empirical investigations and has benefited little from computational modeling. The twodimensional model currently under development at JPL is designed to predict the behavior of an axisymmetric ion thruster discharge chamber and will eventually integrate with existing models of the discharge cathode and ion extraction grids. Recent investigations at JPL have shown that the plasma behavior within the discharge chamber may be best understood by considering the "unmagnetized" transport and cross-transport on an orthogonal computational mesh aligned with the magnetic field and magnetic potential lines. This model uses such a mesh with classical fluid transport for the plasma electrons that includes parallel and perpendicular behavior with respect to the magnetic field. To describe the non-uniform properties and ionization rates of the neutral xenon propellant atoms the model uses a steady-state 2.5-D model by borrowing techniques used for thermal transport view factors. Future versions of the model will incorporate primary electrons and ion motion. Performance of the components of this preliminary model is verified with detailed NSTAR experimental data for a 30 cm diameter ion thruster. Preliminary results of the electron model demonstrate that electrons in the NSTAR discharge cathode plume are likely confined to a magnetic channel in the cathode plume. Preliminary results from the neutral atom model are consistent with NSTAR performance data and show that a large drop in neutral atom density along the thruster centerline is likely. © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.},
 bibtype = {inproceedings},
 author = {Wirz, Richard and Katz, Ira},
 doi = {10.2514/6.2003-5163},
 booktitle = {39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit}
}

A better understanding of ion thruster discharge chamber physics will be possible if a computational model that describes the non-uniform properties and behavior of the discharge can be developed. To date, the design and optimization for ion thruster discharge chambers has been almost entirely based on empirical investigations and has benefited little from computational modeling. The twodimensional model currently under development at JPL is designed to predict the behavior of an axisymmetric ion thruster discharge chamber and will eventually integrate with existing models of the discharge cathode and ion extraction grids. Recent investigations at JPL have shown that the plasma behavior within the discharge chamber may be best understood by considering the "unmagnetized" transport and cross-transport on an orthogonal computational mesh aligned with the magnetic field and magnetic potential lines. This model uses such a mesh with classical fluid transport for the plasma electrons that includes parallel and perpendicular behavior with respect to the magnetic field. To describe the non-uniform properties and ionization rates of the neutral xenon propellant atoms the model uses a steady-state 2.5-D model by borrowing techniques used for thermal transport view factors. Future versions of the model will incorporate primary electrons and ion motion. Performance of the components of this preliminary model is verified with detailed NSTAR experimental data for a 30 cm diameter ion thruster. Preliminary results of the electron model demonstrate that electrons in the NSTAR discharge cathode plume are likely confined to a magnetic channel in the cathode plume. Preliminary results from the neutral atom model are consistent with NSTAR performance data and show that a large drop in neutral atom density along the thruster centerline is likely. © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

@inproceedings{
 title = {Experimental and computational investigation of the performance of a micro-ion thruster},
 type = {inproceedings},
 year = {2002},
 id = {c4bd189c-83d5-3993-a17c-4b3a4deba38c},
 created = {2021-07-30T22:43:47.319Z},
 file_attached = {false},
 profile_id = {8372bdd0-d979-3265-b6ea-94796483bea8},
 group_id = {03394242-3f71-3138-9666-6151a94a5c8b},
 last_modified = {2021-07-30T22:43:47.319Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {A micro-ion thruster with a characteristic diameter of 3-cm has been developed at JPL to study the scalability of ion engine technologies. The ability of the magnetic field in a small discharge chamber to create a uniform plasma profile that fully utilizes the capabilities of the ion extraction grids is investigated in this paper. Experimentally obtained beam profiles are used to determine the ion density profile just inside the chamber. A method is presented for determining the beam profile at the accelerator grid exit from beam profiles measured at two different distances downstream of the grids. The experimental and computational results show that the micro-ion thruster design discussed herein yields favorable beam and ion density profiles. The calculated values of ion and neutral density in the chamber, along with preliminary computational and experimental results, show that considerable increases to thruster performance may be attained with relatively minor design and operational modifications. © 2002 by the American Institute of Aeronautics and Astronautics, Inc.},
 bibtype = {inproceedings},
 author = {Wirz, Richard and Polk, James and Marrese, Colleen and Mueller, Juergen},
 doi = {10.2514/6.2002-3835},
 booktitle = {38th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit}
}

A micro-ion thruster with a characteristic diameter of 3-cm has been developed at JPL to study the scalability of ion engine technologies. The ability of the magnetic field in a small discharge chamber to create a uniform plasma profile that fully utilizes the capabilities of the ion extraction grids is investigated in this paper. Experimentally obtained beam profiles are used to determine the ion density profile just inside the chamber. A method is presented for determining the beam profile at the accelerator grid exit from beam profiles measured at two different distances downstream of the grids. The experimental and computational results show that the micro-ion thruster design discussed herein yields favorable beam and ion density profiles. The calculated values of ion and neutral density in the chamber, along with preliminary computational and experimental results, show that considerable increases to thruster performance may be attained with relatively minor design and operational modifications. © 2002 by the American Institute of Aeronautics and Astronautics, Inc.

@article{
 title = {Development and Testing of a 3cm Electron Bombardment Micro-Ion Thruster},
 type = {article},
 year = {2001},
 id = {e86cf10d-b956-3e20-9f96-4cd8087a1f8b},
 created = {2021-07-30T22:43:47.884Z},
 file_attached = {false},
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 last_modified = {2021-07-30T22:43:47.884Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {false},
 hidden = {false},
 private_publication = {false},
 abstract = {The current development of many microspacecraft concepts, as well as constellation and inflatable spacecraft missions, has increased the need for effective micropropulsion systems. A micro-ion thruster assembly has been developed at JPL for testing and optimization of various system parameters. Some of the design and scaling optimization techniques that have been used for larger ion thrusters are tested in the micro-ion thruster environment. Multiple discharge chambers of approximately 3cm diameter are experimentally tested to determine configuration performance trends. Several magnetic field configurations, including ring cusp and divergent axial are being tested to determine their behavior at this smaller operating scale. Numerical models were used for the initial design and will be used for comparison with experimental results. The performance of a two-grid accelerator system is also being assessed. The accelerator system uses two micromachined molybdenum grids. Concurrent efforts in field emission array (FEA) cathodes and the implementation of small hollow cathodes should supply a large increase in the overall integrity of the design. Similar operating parameters are used for all configurations so that they may be compared primarily on propellant utilization, ion production cost, and total efficiency. This technique allows for a comparison of the relative behavior trends of the configurations but at this time is not an effort to determine optimal operating conditions. The primary system parameters considered for each configuration are the beam current, chamber length, and xenon propellant feed rate. A range of magnetic field designs was tested at chamber length-to- diameter ratios (L/D) of 1.0, 0.75, and 0.5. Tests are performed in a vacuum chamber. From the experimental results, it is clear that the discharge chamber performance is an important issue at the micro-ion scale due the large surface-to-volume ratio of the chamber and an inherently small primary electron containment length. The best overall performance was found using divergent axial and double ring cusp magnetic field configurations at L/D = 0.75 and with a double ring cusp at L/D = 1.0. Improvements in the magnetic field and anode designs have shown increased performance over preliminary tests; however, further improvements to these, and other designs, are necessary.},
 bibtype = {article},
 author = {Wirz, Richard and Polk, James and Marrese, Colleen Marie and Mueller, Juergen and Escobedo, Jesse and Sheehan, Patrick},
 journal = {27th International Electric Propulsion Conference}
}

The current development of many microspacecraft concepts, as well as constellation and inflatable spacecraft missions, has increased the need for effective micropropulsion systems. A micro-ion thruster assembly has been developed at JPL for testing and optimization of various system parameters. Some of the design and scaling optimization techniques that have been used for larger ion thrusters are tested in the micro-ion thruster environment. Multiple discharge chambers of approximately 3cm diameter are experimentally tested to determine configuration performance trends. Several magnetic field configurations, including ring cusp and divergent axial are being tested to determine their behavior at this smaller operating scale. Numerical models were used for the initial design and will be used for comparison with experimental results. The performance of a two-grid accelerator system is also being assessed. The accelerator system uses two micromachined molybdenum grids. Concurrent efforts in field emission array (FEA) cathodes and the implementation of small hollow cathodes should supply a large increase in the overall integrity of the design. Similar operating parameters are used for all configurations so that they may be compared primarily on propellant utilization, ion production cost, and total efficiency. This technique allows for a comparison of the relative behavior trends of the configurations but at this time is not an effort to determine optimal operating conditions. The primary system parameters considered for each configuration are the beam current, chamber length, and xenon propellant feed rate. A range of magnetic field designs was tested at chamber length-to- diameter ratios (L/D) of 1.0, 0.75, and 0.5. Tests are performed in a vacuum chamber. From the experimental results, it is clear that the discharge chamber performance is an important issue at the micro-ion scale due the large surface-to-volume ratio of the chamber and an inherently small primary electron containment length. The best overall performance was found using divergent axial and double ring cusp magnetic field configurations at L/D = 0.75 and with a double ring cusp at L/D = 1.0. Improvements in the magnetic field and anode designs have shown increased performance over preliminary tests; however, further improvements to these, and other designs, are necessary.