EROSION AND REDEPOSITION IN THE ASDEX UPGRADE DIVERTOR

The erosion and redeposition of silicon and tungsten evaporated onto the divertor plates of ASDEX Upgrade have been investigated. For comparison with the experimental results the computer program, ERO, has been used. The erosion of Si markers is smaller than expected from plasma ion sputtering, but still exceeds the W erosion by more than an order of magnitude. The observed erosion can only be explained by assuming a protective effect of carbon ions coming from the plasma which are deposited on the marker surfaces, thus reducing the marker erosion. A redeposition of the sputtered tungsten atoms has been found close to the original marker spots mainly because of prompt redeposition during the first gyration.