Recycling of gaseous impurities in ASDEX

The closed divertor configuration of ASDEX isolates the zone of dominant plasma-wall interaction in a rather well defined manner from the plasma in the main chamber. Studies have shown that for recycling gases the effective conductance between divertor and plasma chamber is closely represented by the values for molecular flow also during a discharge. Hence, from partial pressure measurements of contaminant gases in the divertor impurity recycling fluxes can be obtained. In deuterium discharges the problem of mass peak interferences, especially for methane and watervapour, has to be resolved. Data are shown for various ASDEX scenarios: stainless steel walls, carbon wall elements, Ti-gettering and boronization. The results expose the production of CO as main culprit, as long as no gettering or boronization is employed. Then, however, with carbon still present in the machine, the hydrocarbons limit the attainment of optimum performance parameters. What are the conclusions? (orig.)