Cleanup and gettering during the Beryllium Limiter Experiment in Impurity Study Experiment-B

Abstract

Changes in plasma impurities and radiated power due to cleanup and gettering during the Beryllium Limiter Experiment are related to changes of the surface composition of the torus walls and residual gases in the torus. Pulse discharge cleaning was very effective in cleaning the ISX-B vacuum vessel after installation of the beryllium limiter. As a result of discharge cleaning, areas near the limiter were soon covered with a beryllium rich deposit containing some iron, nickel, chromium, and titanium as well as oxygen, carbon, sulfur, and nitrogen. Oxygen levels on the wall were reduced to below 10 at. %. Materials transport during discharge cleaning was apparently due to sputtered neutrals. Evaporation of beryllium from the limiter and subsequent deposition on the torus walls caused by high power tokamak shots produced gettering of the machine equal or better than that previously obtained with titanium or chromium. This gettering by beryllium eliminated the need for further discharge cleaning or other gettering. Beryllium transport in this case appears to be mostly as ions in the plasma edge.