EXPERIMENTS WITH MOLYBDENUM AND TUNGSTEN LIMITERS IN TEXTOR

摘要

Experiments with molybdenum and tungsten test limiters have been carried out on TEXTOR under various conditions with ohmic and auxiliary heating. A fraction of up to about 8% of the total convective energy in the plasma was deposited onto the high Z limiters. Mo and W impurities are predominantly produced through physical sputtering by impact of carbon and oxygen impurities. Under ohmic heating conditions significant accumulation of Mo and W impurities in the plasma centre has been observed when the line averaged density was increased up to a critical value near (3-3.2) × 10~(13) cm~(-3), leading to centrally peaked radiation, hollow temperature profiles and an internal disruption. Under NBI heating (co-NBI, 1.3 MW), the high Z impurity content in the plasma generally decreased with increasing plasma density as did the relative impurity release from the limiter. At the highest plasma densities investigated (n_e = 5 × 10~(13)/cm~3), high Z plasma contamination and its influence on plasma performance became negligible. This is attributed to screening of released impurities due to local redeposition. In general, no impurity accumulation was observed under NBI heating conditions.