TY - JOUR
T1 - Impurity behaviour in the ASDEX upgrade divertor tokamak with large area tungsten walls
AU - Neu, R.
AU - Dux, R.
AU - Geier, A.
AU - Kallenbach, A.
AU - Pugno, R.
AU - Rohde, V.
AU - Bolshukhin, D.
AU - Fuchs, J. C.
AU - Gehre, O.
AU - Gruber, O.
AU - Hobirk, J.
AU - Kaufmann, M.
AU - Krieger, K.
AU - Laux, M.
AU - Maggi, C.
AU - Murmann, H.
AU - Neuhauser, J.
AU - Ryter, F.
AU - Sips, A. C.C.
AU - Stäbler, A.
AU - Stober, J.
AU - Suttrop, W.
AU - Zohm, H.
PY - 2002/6
Y1 - 2002/6
N2 - At the central column of ASDEX Upgrade, an area of 5.5 m2 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 ΓW stayed below the detection limit only during direct plasma wall contact or for reduced clearance in divertor discharges spectroscopic evidence for ΓW 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 W-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.
AB - At the central column of ASDEX Upgrade, an area of 5.5 m2 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 ΓW stayed below the detection limit only during direct plasma wall contact or for reduced clearance in divertor discharges spectroscopic evidence for ΓW 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 W-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.
UR - http://www.scopus.com/inward/record.url?scp=0036600847&partnerID=8YFLogxK
U2 - 10.1088/0741-3335/44/6/313
DO - 10.1088/0741-3335/44/6/313
M3 - Conference article
AN - SCOPUS:0036600847
SN - 0741-3335
VL - 44
SP - 811
EP - 826
JO - Plasma Physics and Controlled Fusion
JF - Plasma Physics and Controlled Fusion
IS - 6
M1 - 313
T2 - IAEA Technical Committee Meeting on Divertor Concepts
Y2 - 11 September 2001 through 14 September 2001
ER -