Lithium PFC Characterization and Plasma Performance for LTX-beta

Lithium coatings on high-Z plasma facing components (PFCs) in the Lithium Tokamak eXperiment (LTX) led to flat temperature profiles. The flat temperature profiles were observed along with a hot low density edge, implying a broad, collisionless Scrape-Off Layer (SOL). Additionally, in-vacuo analysis of PFCs indicated that evaporatively deposited lithium coatings appeared to be oxidized, while the ability to achieve good plasma performance was retained. Theory attributes flat temperature profiles to low recycling walls, which was assumed to be due to hydrogen binding with elemental lithium to form lithium hydride. The presence of oxidized lithium, however, raises questions regarding the exact mechanism of hydrogen retention in LTX. To investigate these questions, a new Sample Exposure Probe (SEP) for detailed in-vacuo analysis of PFC samples was designed and commissioned for LTX-$\beta$. The SEP is equipped with a vacuum suitcase capable of transporting samples representative of LTX-$\beta$ outer mid-plane PFCs under high vacuum to a stand-alone high resolution XPS system. Surface analysis using the SEP was performed with sufficient energy resolution to identify for the first time, the compounds that grow on evaporative lithium coatings inside a tokamak. This was the first demonstration that a vacuum suitcase can afford a solution that is simpler in design and affords more flexibility than building material characterization test stands for installation on a tokamak. The results indicate that Li$_2$O and LiOH are prime surface constituents of Li PFCs. Their presence substantiates the hypothesis that lithium oxide grows on elemental lithium before the growth transitions to lithium hydroxide for LTX-$\beta$ like vacuum conditions. It is further indicated that Li$_2$O improves plasma performance in comparison to LiOH by both sequestering oxygen and increasing hydrogen retention.