A sputtering/redeposition analysis of alkali-based tungsten composites for limiter/divertor applications

Abstract

Composites of porous tungsten infiltrated with alkali metal-bearing alloys have been projected as a means of reducing plasma impurities and sputter erosion in magnetic fusion devices. Self-sustaining alkali metal overlayers have been observed to inhibit erosion of the underlying structural substrate by 2× to 10×. The alkali metal itself, insofar as it sputters as a secondary ion, is trapped at the surface by sheath potential and tangential magnetic fields. Self-regeneration of the alkali metal coating is obtained by thermal and radiation-induced segregation from the bulk. The sputtering and redeposition properties of several tungsten-based composites are analyzed for limiter and divertor plasma regimes in comparison to pure tungsten, Composites consisting of 20% to 80% porous tungsten infiltrated with binary alloys of Cu, Al, or Si and with alkali metal solutes have been identified for their high-temperature strength, high thermal conductivity, melt layer stability, and alkali metal replenishment characteristics. It has been found that plasma edge temperatures in excess of 200 eV may be obtainable for a number of tungsten composites in contrast to pure tungsten for which the plasma edge temperature must be kept ⩽ 50 eV to minimize plasma impurities and suppress runaway self-sputtering. In addition, the constraint of a low- Z surface in contact with the plasma may be unnecessary since for a given overlayer density, high- Z alkali metal overlayers are shown to reduce the substrate sputter erosion more efficiently than low- Z alkali metal overlayers.