Metallic impurities in the ASDEX scrape-off layer

Abstract

An experimental investigation of metallic impurities has been made in the ASDEX divertor in order to better understand their likely sources and production mechanisms. The impurities were collected on rotatable carbon collector strips which were exposed to the scrape-off layer plasma near the upper outer divertor plate under various discharge conditions. Quantitative ion beam analysis of exposed samples was subsequently carried out by Rutherford backscattering spectroscopy, proton induced X-ray emission and nuclear reaction spectroscopy. Time resolved measurements and spatial profiles of collected impurity fluxes reveal an increase in collected metallic impurities during auxiliary neutral beam heating of plasma discharges causing strong erosion of the copper divertor plate. To a rough approximation, 0.2% of this auxiliary heating power is invested in the production of stainless steel impurities. Measurements made for single-null divertor discharges show a copper flux of 6 × 1017 atoms·s−1 in the scrape-off layer which is not directly sputtered from the target plates. Collected impurity fluxes in the divertor chamber are found to increase with both the main plasma current and the density. Measurements made after boronization reveal an initial reduction in collected stainless steel impurities although boronization does not have any long term effect on the suppression of such impurities. Some limitations of the collector probe technique are also pointed out, and planned improvements of the existing setup are outlined.