Fundamental surface interactions of vacuum-level contaminants with lithium coatings on plasma-facing materials

The National Spherical Torus Experiment Upgrade (NSTX-U) has been used to investigate the effect of wall tile surface conditioning on plasma performance during operation. Previous campaigns have demonstrated the enhanced suppression of edge-localized modes and achievement of high confinement (H-mode) conditions when reactor walls were conditioned with lithium. In addition to lithium coatings, binary alloys of lithium with other metals, such as tin, have been proposed as potential liquid metal plasma facing materials. In order to better understand the surface chemistry of lithium and its interaction with vacuum-level species, various lithium coatings were exposed to controlled levels of water vapor, atomic hydrogen radicals, and energetic deuterium during irradiation. The systems analyzed in this study included lithium coatings on both ATJ graphite and tungsten, in addition to a liquid phase Sn-Li alloy. The Ion-Gas-Neutral Interactions with Surfaces (IGNIS) In-situ Surface Science Facility was used to analyze the systems using X-ray photoelectron spectroscopy (XPS), and the Angle-Resolved Ion Energy Spectrometer (ARIES) facility was used to analyze the surface interactions using low energy ion scattering spectroscopy (LEISS) and Fourier-transform infrared spectroscopy (FTIR).