Impurity production and transport in the divertor tokamak ASDEX

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Abstract
The complete heavy impurity cycle of the divertor tokamak ASDEX is presented. The impurity sources, i.e. sputtering at the vessel wall and at the divertor plates, have been investigated spectroscopically or with special erosion probes. The measured erosion fluxes agree with data from sputtering calculations using measured incident charge exchange (CX) neutral and ion fluxes. The impurity transport in the plasma boundary has been simulated with a time dependent one-dimensional computer code. Central impurity confinement has been investigated with a laser blow-off technique. Investigations on impurity transport from the divertor plates back to the main plasma have shown a surprisingly low retention capability of the divertor for plate produced impurities at low densities, but no adequate code calculation is available. Finally, the deposition of heavy impurities has been investigated with time resolving probes and long term wall probes as well as through a post mortem analysis of the limiters and divertor plates. The results show clearly that CX sputtering is the main source of wall impurities, while the erosion of the divertor plates is mainly caused by ionic sputtering. Despite the fact that, in regimes of high confinement, impurity radiation often terminates the discharge, the present investigation shows a decrease in impurity production in ASDEX during these phases. The deposition pattern of heavy impurities indicates that impurities produced in the divertor are redeposited mainly on the divertor walls rather than on the divertor plates.