SURFACE IMPURITY STUDIES IN THE ISX-A TOKAMAK

The Impurity Study Experiment (ISX-A) was a tokamak designed to study impurity sources, plasma wall interactions, impurity behavior within the plasma, and related phenomena. An Auger electron spectrometer, attached to the tokamak by means of a UHV sample transfer system, has been the primary tool used for surface studies of the first wall. Stainless steel samples positioned at the wall edge were used to follow the progressive ''cleanup'' of the torus wall. Cleaning consisted of low energy hydrogen discharge plasmas, and later titanium gettering. The principal contaminants initially were carbon, oxygen, and sulfur. As cleaning proceeded, the surface oxide layers were reduced to substoichiometric levels, and the carbon deposits appeared to be converted to metallic carbides. The sulfur was largely removed. A decrease in the effective nuclear charge of the plasma could be correlated with these surface changes. Discharge cleaning reduced Z//e//f//f from similar 4 to 1. 6 over a period of several months while titanium gettering produced values approaching 1. 0.