Plasma-wall interaction at the ASDEX Upgrade tungsten heat shield

The suitability of tungsten as plasma facing material for main chamber and baffle components was investigated in ASDEX Upgrade. In an initial step four W-coated graphite test tiles were employed at different poloidal positions of the inner column heat shield structure for a full experimental campaign. Tungsten erosion rates were determined by ion beam analysis of the test tiles before and after the campaign. The results are compared to predictions assuming CX sputtering as main erosion mechanism. In a second step the machine was operated with two toroidal rows of W-coated graphite tiles. The experimental campaign was started without wall conditioning by siliconization to ensure a clean tungsten surface exposed to the plasma. Tungsten influx and plasma concentration were measured spectroscopically. Tungsten migration was determined by ion beam analysis of collector probes exposed in the outer midplane and the outer divertor plate. In all important discharge scenarios the W-concentration stayed below the detection limit of ≈5 × 10^-6. Consequently no influence on plasma performance parameters was found, which encourages the use of W for even larger surface areas in the ASDEX Upgrade main chamber.