The role of chemical interactions in sputtering with formation of redeposition layers in a plasma testing device

Surface analyses using a variety of spectroscopic techniques including XPS and Auger were performed on the redeposition layers formed in the specimens placed near the plasma facing wall of an experimented tokamak device, JFT-2M. The sequence of the layer formation as well as a possible mechanism for the accelerated plasma contamination are discussed. The study yielded the following conclusions; (1) The wall erosion is controlled mainly by the chemically accelerated sputtering causes by the existence of several percent of oxygen-bearing gaseous species. (2) Redeposition layers are formed on the monitoring test coupons, on which of metastable compounds, i.e. some complex mixtures MOx, M(CO)x, M(CHx)y, M(OH)x etc. are condensed. (3) The compounds in the layers are generally less protective against further sputtering and evaporation, thus cause enhanced plasma contamination. (4) Magnitude, composition and chemical staste of the redeposited compounds are different, depending on the location within the device, namely, the distance from the plasma edge. (5) A possible mechanism for the self-accelerated plasma contamination is the repeated cycles of the enhanced sputtering from the redeposition layers and the associated chemical sputtering of the wall materials, both yielding as high rate of erosion. (orig.)