Speaker: Anurag Maan, University of Tennessee, Knoxville
Abstract: Lithium is an attractive low-Z plasma facing component (PFC), due to its ability to reduce ra-diative power loss and recycling by retaining hydrogen. 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 evapo-ratively 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-β. The SEP is equipped with a vacuum suitcase capable of transporting samples representative of LTX-β outer mid-plane PFCs under high vacuum to a stand-alone high resolution X-ray Photoelectron Spectroscopy (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 Li2O 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-β like vacuum conditions. It is further indicated that Li2O improves plasma performance in comparison to LiOH by both sequestering oxygen and increasing hydrogen retention.