Performance of tungsten coatings as plasma facing components used in ASDEX upgrade

Tungsten coated graphite tiles were mounted in the divertor of the ASDEX upgrade tokamak and exposed to approximately 800 plasma discharges with varying average heat fluxes of up to 6 MW/m2. After the experimental campaign, cracks were detected on almost all tiles in accordance with results of electron beam tests. The W-coated tiles in the inner divertor were found to be covered by deposited layers of low-Z material with thicknesses in the μm range and only a few percent tungsten remaining on the surface. In contrast the outer divertor is dominated by erosion. Here, the tungsten concentration at the surface was found to be close to the initial level. The global W-migration pattern was determined by analyzing the deposition of tungsten on a complete poloidal set of plasma facing wall components. Both divertor regions display broad deposition peaks with deposited layers at least one order of magnitude thicker than the chamber values. (orig.)