Liquid Metal surface properties and plasma material interactions for plasma-facing component development in NSTX-U

We aim 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 investigate particle exhaust and plasma-material interaction (PMI) control via lithium (Li) pumping in the National Spherical Torus Experiment Upgrade (NSTX-U) portfolio. Nanostructured porous tungsten (W) PFCs can provide one possible platform to deliver Li to the surface in a controlled manner compared to fast-flowing liquid Li approaches that carry with them intrinsic risk and safety concerns. We will investigate a robust architected porous W interface that can introduce small amounts of Li whereby the first few microns is permeated with Li 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 Li 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. B. 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 slow-flow or 'hybrid' technologies providing liquid Li (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.