Plasma Interactions with Complex Surfaces*

Complex material surfaces (i.e., material surfaces with feature sizes from the micron to mm scale) interact with plasmas in unique ways that can lead to significantly improved plasma conditions and material lifetime. Since these complex material surfaces offer a broad design space across material composition and surface geometry, there are many opportunities to optimize surfaces for plasma applications from advanced space propulsion to fusion energy. Efficiently navigating this multi-variable design space requires understanding the mutual plasma-surface interactions (PMI) for a range of material and surface designs. This research has required a combination of experiments, computational models, and theory to accurately describe the coupled behavior of the material, surface and feature erosion, ion-induced and secondary electron yield, sputtering yield, geometric trapping, and plasma response. Our experiments and models have shown that properly designed complex surfaces can significantly lower both sputtering and electron yield, leading to improved plasma performance via reductions to both plasma contamination and plasma cooling. However, since these surface features erode away with time, persistent yield reduction for complex surfaces offers a significant challenge. In response to this challenge, we recently reported in Physical Review Letters the first-ever demonstration of persistent sputtering yield reduction of up to 80% throughout tens of hours of plasma exposure by using “volumetrically complex” metallic foams. An exciting result of this research effort was the discovery of a new “plasma‑infused” regime, which causes significant changes to the plasma-material interactions because the plasma infuses into the surface and transitions the plasma material interactions from superficial to volumetric phenomena. The applicability of this research in aerospace, clean energy, medicine, and related opportunities will be discussed.