Accumulation and growth of Lithium Hydride in bulk Lithium at Low Hydrogen Concentrations

A significant obstacle to the realization of a fusion power plant is the development of Plasma Facing Components (PFCs) that can withstand the extreme heat and particle flux incident on the first wall and divertor region. Under the intense particle and energy flux present at the divertor, solid PFCs suffer damage such as microstructure growth, sputtering and melting. Due to the issues with solid PFCs, liquid metals have gained popularity. Lithium is commonly considered one of the most promising liquid PFC candidates due to its low atomic number, ability to suppress ELMs while promoting a uniform temperature profile and its capability to getter impurities and cold fuel that reach the divertor. Lithium's use in a fusion device requires a closed loop system for the molten lithium. Due to lithium's ability to getter the fuel it is important to understand how the build up of hydrogen influences a lithium loop system designed to circulate lithium into and out of the chamber. To that end a pair of vacuum systems are being designed and built at the Center for Plasma Material Interaction (CPMI) on the University of Illinois at Urbana-Champaign (UIUC) campus. The first system injects clean liquid lithium at 300 Celsius into hot stainless-steel sample vials before a hydrogen plasma source is used to expose the lithium surface to hydrogen, with primary focus around 5% hydrogen in the lithium. After exposure the chamber will facilitate controlled cooldown of the lithium/lithium hydride samples before evaporatively coating them in gold. The gold coating will be limited to 100 nm or less to permit safe transport and to enable exsitu analysis of samples, mitigating contamination from the atmosphere. The primary analytical technique planned is X-Ray Diffraction (XRD) to measure concentration ratio and crystal size, with other techniques such as Electron Microscopy being considered. The second system planned is designed to measure the diffusion rate of the LiH precipitate through molten lithium. This work shows the systems that are currently being built and the initial results of the proof-of-concept testing, results from the planned analytical techniques and initial testing of the chamber systems.