Plasma operation with tungsten tiles at the central column of ASDEX Upgrade

Two rows of the graphite tiles (≈1.2 m²) at the lower end of the central column of ASDEX Upgrade were coated with tungsten. Since no high heat fluxes are present in this region, the tiles were coated by a physical vapour deposition technique to a thickness of only 500 nm. The experimental campaign was started without wall conditioning by coating of the vacuum vessel. This allowed to measure the W-erosion and the W-concentration with an almost pure tungsten surface exposed to the plasma. The influx as well as the concentration were monitored by spectroscopic methods. The penetration probability for tungsten from the central column is deduced from direct laser ablation of the coating. The measurements were complimented by deposition probe measurements in the midplane and in the divertor. In all important discharge scenarios, which were performed already during the phase without wall conditioning, no concentrations above the detection limit of about 5×10^-6 were found. This result is supported by the deposition probe measurements, which showed no or very small amounts of W deposited in single discharges. The concentrations are at least a factor of 10 below the acceptable concentration in ASDEX Upgrade. Consequently, no influence on the overall plasma behaviour was found and W seems also to be suited for use on larger surfaces in ASDEX Upgrade. Extrapolation to ITER conditions yields concentrations, which will not prohibit successful operation.