Effects of residual species in vacuo on plasma-induced surface phenomena observed on first-wall materials

The chemical sputtering effect and the enhanced evaporation, caused by the existence of trace amounts of the residual impurities O-, C- and H-bearing species in vacuo, were studied. Specimens of Fe, Ni, Cr, Mo and preoxidized Mo were bombarded by O2+ and A+ (2 keV) in a SIMS apparatus. The chemical enhancement of sputtering was shown to reflect the magnitude of enthalpy change for possible chemical reactions between the bombarding species and the substrates. The modified surfaces contained metastable phases, and the chemical reactions among the activated species in gaseous phases near the surface were found to be the controlling step of the enhancement. The surfaces of TiC-coated Mo, exposed to plasma in JFT-2M, were analysed by ESCA/Auger. The formation of the metastable phases was found to cause the enhanced evaporations. The role of trace amounts of oxygen-bearing species in the plasma-wall interactions is stressed. (orig.)