Tungsten as plasma-facing material in ASDEX Upgrade

Since 1993, a tungsten programme is run at ASDEX Upgrade, leading to the installation of a complete W-divertor in 1995 followed by 1 year of operation. From 1998 onwards, an increasing area of tungsten-coated tiles has been installed at the central column of ASDEX Upgrade, reaching an area of 7.1 m² in 2001/2002 representing about 85% of the total area of the central column. The programme comprises not only the operation with large area W-coatings but also the qualification and testing of the coated tiles, W-erosion and -migration investigations, development of spectroscopic diagnostics and interpretation of the observed W-behaviour by impurity transport calculations. During the campaigns with the W-divertor as well as with W-coated central column, almost no negative influence on the plasma performance was found. The W-erosion is dominated by light intrinsic impurities and typical tungsten concentrations in the main plasma range from below 10^-6 up to a few times 10^-5, which would be sufficiently low for a burning plasma.