SURFACE AND IMPURITY STUDIES IN ORMAK AND ISX

The ORMAK vacuum liner is constructed of stainless steel, overcoated with a thin platinum diffusion barrier and a final layer of gold. Gold was selected as the vacuum surface because it is chemically inert to the adsorption of common gases. However, gold surfaces do adsorb hydrocarbons, and carbon (along with oxygen) was the principal plasma contaminant during the first two years of ORMAK operation. Upon switching discharge cleaning gases from hydrogen to oxygen, carbon levels dropped until carbon is no longer a significant contaminant. Residual hydrocarbons can now be controlled by either hydrogen or oxygen discharge cleaning. The principal measured plasma contaminant in ORMAK is now oxygen. Samples taken from the ORMAK liner and analyzed by Auger electron spectroscopy reveal the presence of iron and oxygen. There is evidence from a SXAPS (Soft X-ray Appearance Potential Spectroscopy) probe of iron and chromium diffusion from the stainless steel through the gold surface in spite of the platinum diffusion barrier. The Fe and Cr provide surface oxidation sites, and SXAPS analysis shows that these metals exist as oxides. In order to investigate tokamak impurity problems further, the ISX (Impurity Study Experiment) tokamak is presently under construction. It will provide a cleaner and more flexible vacuum system in which to conduct studies of surfaces and plasma impurities. The operating characteristics will be much the same as those of ORMAK (with ohmic heating) in terms of size, plasma current, and plasma temperature.