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Abstract. The material for plasma facing components of a future fusion
device is still not decided. At present most experiments use graphite,
because of its good thermo mechanical properties and the low radiation
potential of carbon. Due to the high erosion yield and, especially, due to
the codeposition with tritium, its use in a fusion reactor is still
questionable. Based on the good experience using tungsten as divertor
material in ASDEX Upgrade, which demonstrated that a divertor tokamak can be
operated with a tungsten divertor without reduction of the performance, a
step by step strategy was followed. Main sources of the carbon are predicted
at the inner heat shield, which covers the central column. Tungsten test
tiles confirm the erosion at this position due to charge exchange neutral,
but also a non negligible ion sputtering component. A first step was done by
siliconisation. In ASDEX Upgrade the maximal silicon concentration was
0.002. Consequently the performance of the experiment was not influenced by
silicon radiation. A second step was done by tungsten coating of
1.2m2
of the inner heat shield. Experiments are done without subsequent wall
coating, which would cover the tungsten. Spectroscopically measured central
tungsten densities are always below
5*10-6 and mostly below the
detection limit. Again no influence on the plasma performance parameters are
found. Extrapolation to ITER conditions yields concentrations, which will
not prohibit successful operation. The next step in ASDEX Upgrade will be a
mostly tungsten covered inner heat shield at the next experimental
campaign.
IAEA 2001
