Tungsten as plasma facing material in the ASDEX Upgrade tokamak
Research report (imported) 2003 - Max-Planck-Institut für Plasmaphysik
Whether, or to what degree tungsten (W) can be used as plasma facing material is a key question for all future fusion devices, having in mind the complex chemistry carbon exhibits in the presence of hydrogen isotopes. Since 1999 the fusion experiment ASDEX Upgrade pursues the progressive increase of W plasma facing components (PFCs). Meanwhile 65 per cent of all PFCs are equipped with W coated tiles. The incremental approach in the transition to a W dominated device provides the opportunity to explore the influence of different W-components and to identify major carbon sources. New spectroscopic tools have been developed which allow detection of W concentrations below 10-6. Poloidally resolved measurements of the W and C influx reveal that the W erosion is dominated by light impurities. At the same time a fast redistribution of C onto the tungsten surfaces is observed. The W concentrations remained well below 10-5 in the reference scenario for a future reactor. The central impurity accumulation could be efficiently reduced by central heating and repetitive injection of cryogenic deuterium pellets. The results obtained so far open up optimistic prospects for the use of W in a reactor and future studies in ASDEX Upgrade will enlighten details of the operation of a carbon free device.