New results from the tungsten programme at ASDEX upgrade

Since 1998 an increasing area of tungsten coated tiles has been installed at the central column of ASDEX Upgrade, reaching 7.1 m² in 2001/2002. The tiles were coated commercially by plasma arc deposition to a thickness of 1 μm. Post mortem analysis of the W-coating showed a strong erosion, which is attributed to ion sputtering, due to its two-dimensional variation. During the campaigns with W-coated central column, almost no negative influence on the plasma performance was found. Only during direct plasma wall contact or for reduced clearance in divertor discharges spectroscopic evidence for tungsten influx could be found. Effective sputtering yields of about 10^-3 were derived, pointing to a strong contribution by light intrinsic impurities to the total W-sputtering. The increased W-content during plasma current ramp up decreases after X-point formation and the flux consumption during current ramp is only marginally enhanced compared to the graphite wall case. The W concentrations ranged from below 10^-6 up to a few times 10^-5. In discharges with increased density peaking, a tendency for impurity accumulation was observed, which affected only a closely localized central region. Central heating led to a strong reduction of the central impurity content, consistent with neoclassical impurity transport.