1 IAEA-CN-149/EX/3-3Ra Tungsten as First Wall Material in ASDEX Upgrade

Abstract In ASDEX Upgrade, nearly all plasma facing components (PFC) besides the strike point area of the lower divertor are tungsten coated. The main, newly added components are the low field side (LFS) poloidal guard limiters, the roof baffle in the lower divertor and the lower passive stabiliser loop. The increased W-area resulted in higher W concentrations in the plasma. Especially the LFS limiters represent a significant W source mainly during ICRF operation, which is probably due to a rise of the sheath potential. NBI leads to sputtering by fast D+ ions. The fast ion loss is enhanced by ELMs. In H-modes with type-I ELMs, about 70 % of the local W-influx at the middle of the limiter appears during ELMs. During the ELM the mean energy of the ions hitting the W-surface is increased, which is the predominant contribution to the rise of the tungsten influx. A clear reduction of the carbon content has now become visible. The slow decrease of the C concentration with increasing W coverage can be explained by the strong main chamber recycling. Now, the divertor carbon source contributes significantly to the core C content. 1