Transmission Measurement and Investigation of Li Diffusion on B/C under ultrahigh vacuum*

Lithium is a strong candidate for a plasma-facing material because of its low hydrogen recycling, which improves plasma performance in fusion devices. We investigated the transmission of lithium films for visible light wavelength range and the diffusion of lithium in boron (B) and carbon (C), where an accurate control of lithium deposition is crucial. A lithium doser setup with a W heating coil with a quartz crystal microbalance (QCM) was constructed to enable the deposition of lithium films of precise thicknesses with a high deposition rate (~1 nm/s). An ultrahigh vacuum chamber was constructed with a lithium doser setup and two optical windows to measure the transmission of visible light through lithium films. A continuous light source and a spectrometer were utilized to measure the optical transmission through the lithium film for various film thickness deposited on the optical windows. A lithium doser setup is also installed on another ultrahigh vacuum chamber, the Sample Exposure Station (SES), to deposit lithium on a B film, >10 nm thickness, or a bulk graphite sample. The lithiated film was etched and analyzed using the secondary ion mass spectrometry (SIMS) technique in the SES chamber to conduct a depth profile of the Li-B/C layers for the lithium diffusion study. A lithium doser with accurate thickness control is beneficial for laboratory investigation of the chemical evolution of surfaces, deuterium retention, sputtering yields, and related phenomena.