All major design studies of future fusion research and reactor devices employ tungsten (W) on plasma- facing components (PFCs) at least in a part of the divertor region, since in general, the erosion rate for low-Z materials like carbon or beryllium seems to be far too high in a steady state power-producing device. Additionally, the use of large area carbon based materials may absorb tritium, leading to excessive co-deposition of carbon and tritium. Metal walls avoid this problem. The applicability of molybdenum is tested in FTU (Frascati, Italy) and Alcator C-mod (USA). Tungsten, which will be used in ITER, has a low erosion yield and a high sputtering threshold, its use implies the risk of unduly high radiative losses in the central plasma, and concentrations above 10-4 would prevent DT-burn.

However, the experience with tungsten in present-day fusion devices is comparatively small and ASDEX Upgrade is the only major fusion device which uses tungsten as a plasma-facing material on a large scale. Starting from preliminary studies, a full tungsten divertor was installed in ASDEX Upgrade for the 1995/1996 campaign, before it was replaced by the optimized divertor DIV II. Since the experimental campaign 1999 the central column of ASDEX Upgrade has been equipped with tungsten-coated graphite tiles in a step by step approach. In 2002, 14.6 m2 of newly tungsten-coated tiles were mounted in the main chamber, representing about 40% of the total plasma facing components. For the 2003/2004 campaign, another 13.6 m2 of W-coated wall tiles will be added, finally including also the complete upper divertor as well as also the baffles at the lower divertor. In preparation for these experiments, W-coating techniques have been tested in cooperation with commercial manufacturers with regard to their suitability for plasma facing components which are subject to power loads of up to 20 MW/m2. New spectroscopic tools have been developed, which allow measurement of the W-influx and detection of central Wconcentrations (cW) below 10-6. Deposition probe measurements and post mortem analyses of the tiles yielded important results on the erosion and migration of tungsten in a mixed material device. The deposition pattern in the divertor clearly shows that the inner divertor as well as the far scrape-off layer region in the outer divertor strike zone are strongly depositiondominated. The main thrust of the W-experiments in ASDEX Upgrade is the demonstration and documentation of the compatibility of fusion plasmas with tungsten plasma-facing components. The results of several tungsten campaigns can be summarized as follows: most of the relevant discharge scenarios are not hampered in any way and cW remains below 10-5, which would be sufficiently low in a future device. At the same time, regimes were identified which are prone to higher cW or to tungsten accumulation. Unlike with carbon PFCs, a strong difference between limiter and divertor operation is found, and care has to be taken in designing the plasma shape in order to provide adequate wall clearance. Nevertheless, the start-up of the plasma at a W surface is possible without leading to a deterioration of the plasma performance in the subsequent flat top phase. In discharges with peaked density profiles in combination with low diffusive transport the central cW was increased. It could be reduced drastically by applying central heating, as typical for an ignited plasma. This procedure is routinely used and has been verified by other devices in the case of the accumulation of mid-Z impurities. ELM-free discharges or discharges with low ELM frequency, which are produced typically at the L-H confinement mode power threshold, showed increased W-concentrations. These could be overcome by controlling the ELM frequency through pellet injection. In summary, ASDEX Upgrade follows the route to a virtually carbon-free device and will deliver important benchmarks for the use of W in a future device.

The ASDEX Upgrade tokamak (Axially Symmetric Divertor EXperiment) went into operation at the Association Euratom-IPP in Garching (Germany) in 1991. This fusion device, Germany’s largest at present, is for investigating crucial problems in fusion research.

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