Characterization of Vacuum Improvement in Proto-Lite via Mass Spectroscopy*

Tungsten targets exposed to plasma within Proto-MPEX (Prototype Material Plasma Exposure eXperiment) were observed to have significant accumulation of impurity-atom coatings. These coatings primarily consisted of Al and O atoms. It was determined that the source of the Al atoms was sputtering from the AlN helicon window, and the O atoms were likely from water vapor due to poor vacuum conditions. Impurity accumulation at this level is a concern for MPEX operations and mitigation strategies are currently being explored on the Proto-Lite device (the successor to Proto-MPEX). This work focuses on the efforts towards improving the base vacuum. To do so, we will use a combination of vessel baking and glow discharge cleaning of the vessel. Here we will show a comparison of the vacuum quality from the baseline Proto-Lite vacuum before and after the glow discharge cleaning of the vessel. We relate the change in deposition of impurities on the target surface to the improvement of the vacuum. Vacuum quality is quantified through mass spectroscopy and base pressure measurements. A residual gas analyzer (RGA) is used as a mass spectrometer to determine gas partial pressures; thus, we can measure the reduction in molecules (primarily water) responsible for the O atoms.