Thermal stability of Li films on a polycrystalline W substrate

Abstract

The thermal stability of thin Li films (≤7 nm) on a polycrystalline W substrate is reported in the temperature range of 300–1800 K to advance understanding of Li plasma-facing components (PFCs) in fusion devices. High steady-state and transient heat loads on the PFCs of fusion devices can yield surface temperatures exceeding 2200 K, so the thermal stability of Li in proposed Li-W hybrid PFCs requires testing for potential “dry-out” conditions at such high temperatures. Li films were deposited on a polycrystalline W foil in ultrahigh vacuum conditions, and their thermal stability was probed with a combination of temperature programmed desorption (TPD), low-energy ion scattering (LEIS), and X-ray photoelectron spectroscopy (XPS). With TPD, Li desorption became measurable starting at 460 K, and Li did not fully desorb from the W substrate until 1125–1200 K. LEIS and XPS results agreed with the findings from TPD. At 455 K, increased surface diffusion of Li was indicated by the XPS results before the onset of Li multilayer desorption. This investigation of the thermal stability of high-purity Li thin films on a clean, W-based substrate will inform design, operation, and performance of future hybrid PFC systems with liquid Li on a solid W substrate.