PLASMA SURFACE INTERACTIONS OF GRAPHITE AS NUCLEAR-FUSION MATERIAL

As nuclear fusion devices are being scaled to larger size with substantially increased plasma stored energy, the engineering tasks associated with plasma-surface interactions of the plasma facing material referred to as the first wall have become increasingly important. In large tokamaks, the energy confinement of the plasma has been substantially improved by the use of graphite or carbon material. Graphite, however, has several disadvantages, e.g. large capacity to absorb gases due to its porous structure and a large erosion rate due to bombardment by hydrogen ions. Therefore, various problems must be solved before graphite can be used as the first wall material of next generation fusion devices. In this paper, we first explain plasma-surface interactions. In particular, the problem associated with impurities is described. We than show the results of our characterization study for several kinds of graphite as the first wall material. These include (1) surface and vacuum properties of graphites, i.e. gas desorption and micro surface area (effective surface area), and (2) interactions of graphite with hydrogen ions, i.e. chemical sputtering, radiation-enhanced sublimation and hydrogen retention. Required conditioning of, and modifications to graphite intended for use as the first wall material are also discussed.