SURFACE SCIENCE STUDIES OF PLASMA-MATERIAL INTERACTIONS OF LITHIUM IN A TOKAMAK ENVIRONMENT

Abstract

Fusion energy holds a crucial role as a potential power source in the future, offering a clean and virtually limitless energy supply. At the crux of successful fusion device operation lie plasma-material interactions, which significantly influence the plasma performance. Lithium has emerged as a promising plasma-facing material candidate in fusion devices, but further research efforts are required to better understand the intricacies of how lithium surfaces evolve under hydrogen plasma exposure and affect plasma performance. This thesis utilized a surface science approach to study the plasma-material interactions of lithium plasma-facing materials in tokamaks. Experiments followed two approaches: the use of a sample insertion device to expose material samples to tokamak environments and laboratory-based experiments where tokamak environments could be modeled. The lithium wall coatings in the Lithium Tokamak eXperiment-beta (LTX-beta) were a focus of this research, where surface analysis was facilitated by the Sample Exposure Probe (SEP). The SEP was a sample insertion device that could interface with LTX-beta and transfer a sample in-vacuo for subsequent characterization by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). The development of TPD capability for the SEP was a key accomplishment of this thesis work. Initial XPS studies with the SEP on LTX-beta were able to resolve, for the first time, Li2O and LiOH species in the evaporated lithium films in LTX-beta. Metallic lithium was identified in the evaporated lithium films within the XPS probe depth within 1.5 hours after deposition. TPD results from the evaporated lithium films in LTX-beta revealed impurity co-deposition during the lithium evaporation process. Rather than lithium-lithium interactions being dominant in the freshly evaporated lithium films in LTX-beta, the TPD experiments suggested a mixture of compounds containing lithium, hydrogen, oxygen, nitrogen, and carbon. The impurity-rich lithium films still retained incident hydrogen from plasma exposure in LTX-beta. In laboratory-based experiments, high purity lithium thin films did not fully desorb from polycrystalline tungsten until temperatures of 1125-1200 K. Ex-situ surface characterization of graphite samples exposed to lithium conditioning techniques in the RFX-mod fusion device revealed improved spatial uniformity of lithium deposition in RFX-mod compared to a single lithium evaporator.