Liquid Metal Surface Properties and Plasma Material Interactions for Plasma-Facing Component Development in NSTX-U

The aim of this project is to establish the physics and engineering basis of slow-flow or 'hybrid' plasma-facing component (PFC) materials that can provide a transformational enabling capability to the National Spherical Torus Experiment Upgrade (NSTX-U) portfolio to investigate particle exhaust and plasma material interaction (PMI) control via lithium (Li) pumping for density and impurity control. Lithium-based nanostructured porous tungsten (W) PFCs can provide one possible platform to introduce lithium in a controlled approach compared to fast-flowing liquid lithium approaches that carry with them intrinsic risk and safety concerns. Research to control the introduction of lithium into the plasma will investigate a robust architected porous tungsten interface that can introduce small amounts of lithium whereby the first few microns is covered by 100% wetting of W surfaces provided by nanopatterned structures designed to maximize Li wettability. A multi-institutional team consisting of three universities will advance the science of liquid-metal (LM) research in NSTX-U for architected porous refractory metal substrates used with liquid lithium PFCs.The focus of this project is on surface science research and liquid metal plasma-facing component development in the NSTX-U. Lithium-based LMs (e.g. Li, Li-Sn alloy) and candidate porous W substrate materials will be studied and developed over the course of the project. This research will also identify optimized porous W architectures ultimately providing sample modules for testing in NSTX-U plasma environments and future designs for possible deployment in NSTX-U. The collaborative team is organized as follows: 1) Dr. J. P. Allain of Penn State University will lead the development of novel engineered refractory-metal substrates that can enhance LM interaction and study the dynamics of wettability of the LM on these substrates; 2) Dr. B. E. Koel of Princeton University will lead studies of fundamental properties of LM PFCs including surface chemistry at temperatures seen and foreseen in NSTX-U; and 3) Dr. K. Woller of the Massachusetts Institute of Technology will lead studies of compositional changes and hydrogenic retention under high-fluence plasma exposure.This project addresses key aspects of the NSTX-U mission and plans by the study of so-called hybrid or 'slow-flow' technologies providing liquid lithium (low-Z) pre-filled refractory metal (high-Z) plasma-facing component materials and establishing an understanding of their surface and PMI properties for use in NSTX-U. In addition to establishing a physics basis for these PFCs, this proposal will help establish the engineering basis of future PFC divertor upgrade(s) preparing for the next NSTX-U Five-Year Plan (2026-2030) whose central element would be a Liquid Lithium program.