Strategies to reduce impurity production and transport in linear plasma devices used for plasma material interaction research

The Material Plasma Exposure eXperiment (MPEX), under construction at ORNL, will be a high plasma and heat flux device that will be used to test materials at fusion-relevant divertor conditions. Previous work on the Proto-MPEX device, which served as the physics-basis experiment for MPEX, showed that plasma interactions on the window of a helicon plasma source could generate and transport impurities downstream and deposit onto a materials target. Several strategies are being explored to reduce this impurity production, including using high-Z refractory coatings on the helicon window to reduce sputtering, a Faraday shield to reduce the sheath potential on the window surface, and improving the vacuum conditions to reduce oxygen impurities, which contribute to the window erosion. These approaches are being tested on the Proto-Lite device, which is a modified version of Proto-MPEX. Current experiments are focusing on determining the effectiveness of a Ta₂O₅ plasma-sprayed coating on the silicon nitride helicon window and improving vacuum conditioning using glow discharge cleaning (GDC) and vacuum chamber baking. Impurity deposition on the target is determined by X-ray photoelectron spectroscopy. Improvements in the vacuum conditions are determined by mass spectroscopy and optical emission spectroscopy. The impact of these approaches on impurity deposition will be presented.