Plasma-wall interaction and first-wall materials in ASDEX Upgrade

Experiments dealing with plasma-wall interactions and first-wall materials comprise a significant part of the work program of ASDEX Upgrade. To elucidate carbon chemical erosion under reactor-relevant conditions, dedicated spectroscopic measurements were performed. These investigations are complemented with long-term erosion and deposition probes consisting of various materials, which are mounted at numerous locations inside the vacuum vessel. The codeposition of hydrogen with carbon below the divertor is studied in detail with long-term samples as well as with quartz microbalance measurements, which allow a discharge-resolved measurement of the layer growth. In parallel to the investigations on carbon, the behavior of tungsten plasma facing components (PFCs) and their influence on plasma performance is studied. In several experimental campaigns, the divertor as well as large parts of the PFCs in the main chamber were equipped with tungsten-coated tiles. Surface conditioning by applying a silicon layer (siliconization) was performed as a preexperiment of the tungsten program, and the results are compared to those of surface conditioning with boron (boronization)