Particle drifts and the transition to containment in the LT-3 Tokamak. Hutchinson, I., H. & Strachan, J., D. Nuclear Fusion, 14(5):649, 1974. ![Particle drifts and the transition to containment in the LT-3 Tokamak [pdf]](https://bibbase.org/img/filetypes/pdf.svg "pdf")
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Website abstract bibtex The Canberra Tokamak LT-3 exhibits a delay between the application
of the toroidal electric field and the main rise in gas current indicating
bulk ionization of the gas. During this "prebreakdown" phase the
electric field is high and the current is dominated by runaway electrons.
The rotational transform is very low, however, and the condition
for compensation of transverse particle drifts is not satisfied.
A theoretical model is considered which includes only the curvature
drift of the runaways. This provides estimates of the maximum energy
an electron achieves before it is lost to the walls and a critical
condition on the current when the transition to particle containment
and total gas breakdown should occur. Measurements of runaway electron
energies and gas currents in LT-3 are in agreement, within experimental
and theoretical accuracy, with the predictions of the model. The
effect of a compensating perpendicular magnetic field also confirms
the model. The mechanisms governing the duration of the delay to
breakdown remain obscure. In view of the short particle lifetime
a means of re-introducing electrons, such as secondary emission,
seems indicated. The prebreakdown phase may be avoided by pre-ionization
to densities above a critical value. The particle drifts are then
compensated and no delay occurs before the onset of exponential ionization
increase.
@article{
title = {Particle drifts and the transition to containment in the LT-3 Tokamak},
type = {article},
year = {1974},
pages = {649},
volume = {14},
websites = {http://stacks.iop.org/0029-5515/14/i=5/a=007},
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last_modified = {2017-07-07T04:09:21.758Z},
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starred = {false},
authored = {false},
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citation_key = {Hutchinson1974},
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abstract = {The Canberra Tokamak LT-3 exhibits a delay between the application
of the toroidal electric field and the main rise in gas current indicating
bulk ionization of the gas. During this "prebreakdown" phase the
electric field is high and the current is dominated by runaway electrons.
The rotational transform is very low, however, and the condition
for compensation of transverse particle drifts is not satisfied.
A theoretical model is considered which includes only the curvature
drift of the runaways. This provides estimates of the maximum energy
an electron achieves before it is lost to the walls and a critical
condition on the current when the transition to particle containment
and total gas breakdown should occur. Measurements of runaway electron
energies and gas currents in LT-3 are in agreement, within experimental
and theoretical accuracy, with the predictions of the model. The
effect of a compensating perpendicular magnetic field also confirms
the model. The mechanisms governing the duration of the delay to
breakdown remain obscure. In view of the short particle lifetime
a means of re-introducing electrons, such as secondary emission,
seems indicated. The prebreakdown phase may be avoided by pre-ionization
to densities above a critical value. The particle drifts are then
compensated and no delay occurs before the onset of exponential ionization
increase.},
bibtype = {article},
author = {Hutchinson, I H and Strachan, J D},
journal = {Nuclear Fusion},
number = {5}
}
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During this \"prebreakdown\" phase the\nelectric field is high and the current is dominated by runaway electrons.\nThe rotational transform is very low, however, and the condition\nfor compensation of transverse particle drifts is not satisfied.\nA theoretical model is considered which includes only the curvature\ndrift of the runaways. This provides estimates of the maximum energy\nan electron achieves before it is lost to the walls and a critical\ncondition on the current when the transition to particle containment\nand total gas breakdown should occur. Measurements of runaway electron\nenergies and gas currents in LT-3 are in agreement, within experimental\nand theoretical accuracy, with the predictions of the model. The\neffect of a compensating perpendicular magnetic field also confirms\nthe model. The mechanisms governing the duration of the delay to\nbreakdown remain obscure. In view of the short particle lifetime\na means of re-introducing electrons, such as secondary emission,\nseems indicated. The prebreakdown phase may be avoided by pre-ionization\nto densities above a critical value. 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During this \"prebreakdown\" phase the\nelectric field is high and the current is dominated by runaway electrons.\nThe rotational transform is very low, however, and the condition\nfor compensation of transverse particle drifts is not satisfied.\nA theoretical model is considered which includes only the curvature\ndrift of the runaways. This provides estimates of the maximum energy\nan electron achieves before it is lost to the walls and a critical\ncondition on the current when the transition to particle containment\nand total gas breakdown should occur. Measurements of runaway electron\nenergies and gas currents in LT-3 are in agreement, within experimental\nand theoretical accuracy, with the predictions of the model. The\neffect of a compensating perpendicular magnetic field also confirms\nthe model. The mechanisms governing the duration of the delay to\nbreakdown remain obscure. 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