Impurity control experiments in the ASDEX divertor tokamak

ASDEX is a large tokamak (R=1.65m, a=0.40m) which was brought into operation in February 1980 to study the mechanisms of impurity production and control by a poloidal divertor. To elucidate the contribution of different processes to the impurity behaviour, four modes of tokamak operation are compared keeping the plasma parameters as similar as possible. Limiter (L), Limiter with gettering in the divertor chamber (LP), Divertor without getter pumping (D) and Divertor with getter pumping (DP). In limiter discharges (L), light (oxygen) and heavy (iron) impurities radiate a large fraction (approximately 80%) of the Ohmic power input. Retracting the limiter and switching on the divertor coils (D) reduces the iron content by, at least, a factor of 10. On the other hand, oxygen is reduced by almost an order of magnitude when titanium is gettered in the divertor chambers (LP and DP). In diverted discharges with gettering (DP) radiation losses account for only 20 to 40% of the Ohmic power input. Only part of the remaining power is found on the neutralizer plates, suggesting that a sizeable fraction of the input power is lost inside the divertor chambers. Radial profiles of O IV to VIII taken in DP discharges indicate a diffusion coefficient for oxygen ions of approximately 8000cm2.s-1, which is of the order of the Bohm diffusion. This anomalous inward diffusion impairs the shielding efficiency of the scrape-off layer. In general, divertor operation considerably extends the parameter range, allowing e.g. anti nsub(e)<=5x1013cm-3, tausub(E)<=50ms, Zsub(eff) approximately 1, qsub(a)>=2.0 and discharge durations of up to 3s. (author)