The tungsten experiment in ASDEX Upgrade

Tungsten coated tiles, manufactured by plasma spray on graphite, were mounted in the divertor of the ASDEX Upgrade tokamak and cover almost 90% of the surface facing the plasma in the strike zone. Over 500 plasma discharges, among which around 300 were heated with heating powers up to 10 MW, were performed up to now. The tungsten flux in the divertor was monitored by a WI line at 400.8 nm. In the plasma centre an array of spectral lines at 5 nm emitted by ionisation states around W XXX was monitored. Under normal discharge conditions W-concentrations of around 10-5 or even lower were found. The influence on the main plasma parameters was negligible. In a few low power discharges accumulation of tungsten occurred and the temperature profile was flattened. The concentrations of the intrinsic impurities carbon and oxygen are comparable to the discharges with graphite divertor. Furthermore, the density-limits and the β-limits remained unchanged and no negative influence on the energy confinement as well as on the H-mode threshold was found. (orig.)