Experience With High- $Z$ Plasma-Facing Materials and Extrapolation to Future Devices

The use of refractory metal plasma-facing components (PFCs) requires intensive research in all areas, i.e., in plasma-wall interaction, in the physics of the confined plasma, diagnostic, and in material development. Only a few present-day divertor tokamaks-mainly Alcator C-Mod (C-Mod) and ASDEX Upgrade (AUG)-gained experience with the refractory metals molybdenum and tungsten, respectively. AUG was stepwise converted from graphite to tungsten PFCs. In line with this transition, a reduction of the deuterium retention by almost a factor of ten has been observed due to the strong suppression of D codeposition with carbon. The deuterium retained in W is in line with laboratory results in contrast to C-Mod, where the D retention in Mo is more than a factor of ten larger than that in corresponding laboratory experiments. As expected from the sputtering threshold of Mo and W, negligible erosion by the thermal divertor background plasma is found in these experiments under low-temperature divertor conditions. However, erosion by fast particles and intrinsic impurities, which additionally might be accelerated in rectified electrical fields observed during ion cyclotron frequency heating, plays an important role. The Mo and W concentrations in the plasma center are strongly affected by plasma transport, and variations up to a factor of 50 are observed for similar influxes. However, it could be demonstrated that sawteeth and turbulent transport driven by central heating can suppress central accumulation. The inward transport of high-Z ions at the edge can be efficiently reduced by ?flushing? the pedestal region caused by frequent edge instabilities. Extrapolations to ITER and DEMO are difficult since the physics of the plasma transport is not yet completely understood, the particle and energy fluxes are orders of magnitude higher, and the technical boundary conditions in DEMO strongly differ from those of present-day devices.