Preferential sputtering of TiC and TiB2 coatings under D+ and 4He+ bombardment: Partial yields

Abstract

The coating of first wall components in fusion devices with materials of low to medium atomic number Z such as TiB2 and TiC, can help improve plasma impurity control and reduce surface erosion. One of the important tests of coatings for fusion applications deals with measurements of plasma impurity release by sputtering. In the sputtering of two component systems such as TiC, the preferential emission of one of the components, e.g., C, may occur. This paper reports experimentally determined partial sputtering yields for B, C, and Ti for D+ and 4He+ irradiations of TiB2 and TiC coatings with energies ranging from 3 to 60 keV. The coatings were irradiated at ambient temperature and at normal incidence. Chemical vapor deposited TiB2 and TiC coatings on Poco graphite substrates have been studied for surfaces which were mechanically polished or left in their as-deposited state. The equivalent B, C, and Ti thicknesses of the sputter deposits were determined by the Rutherford backscattering technique. The partial yields for both polished and unpolished coating surfaces are observed to decrease with increasing ion energy. This trend is in agreement with theoretical predictions. The partial yields for B and C are found to be larger than for Ti. This can lead to improved plasma impurity control since a larger concentration of impurities with low Z can be tolerated than with high Z.