Measurement and modification of first-wall surface composition in the Oak Ridge tokamak (ORMAK)

Impurities coming into the plasma from the walls of present-day toroidal plasma confinement devices modify plasma behavior substantially. Small fractions of high-Z ions in the plasma greatly decrease plasma temperatures and increase plasma energy losses. We are attempting to identify and control the sources of impurities from the ''first-wall'' in ORMAK. Auger electron spectroscopy, soft x-ray appearance-potential spectroscopy, and other surface-sensitive techniques have been used to characterize the surface composition of the first wall and to develop methods to remove carbon and oxygen. Oxygen glow-discharge cleaning has been shown, in the laboratory, to be an effective way of removing carbon from gold films (simulated ORMAK liner material) and the use of oxygen-discharge cleaning in ORMAK has resulted in a decrease in plasma contamination, a 50% increase in plasma current, and an accompanying increase in plasma temperature. In spite of these improvements the walls of ORMAK are far from clean. Substantial amounts of carbon, oxygen, iron, and other elements remain