Plasma Physics and Controlled Fusion, 44(6):811, 2002. Paper Website abstract bibtex
At the central column of ASDEX Upgrade, an area of 5.5 m^2 of graphite tiles was replaced by tungsten-coated tiles representing about two-thirds of the total area of the central column. No negative influence on the plasma performance was found, except for internal transport barrier limiter discharges. The tungsten influx Γ W stayed below the detection limit only during direct plasma wall contact or for reduced clearance in divertor discharges spectroscopic evidence for Γ W could be found. From these observations a penetration factor of the order of 1% and effective sputtering yields of about 10^-3 could be derived, pointing to a strong contribution by light intrinsic impurities to the total \mboxW-sputtering. The tungsten concentrations ranged from below 10^-6 up to a few times 10^-5 . Generally, in discharges with increased density peaking, a tendency for increased central tungsten concentrations or even accumulation was observed. Central heating (mostly) by ECRH led to a strong reduction of the central impurity content, accompanied by a very benign reduction of the energy confinement. The observations suggest that the W-source strength plays only an inferior role for the central W-content compared to the transport, since in the discharges with increased W-concentration neither an increase in the W-influx nor a change in the edge parameters was observed. In contrast, there is strong experimental evidence, that the central impurity concentration can be controlled externally by central heating.