Plasma Interactions and Electron Dynamics for Volumetrically Complex Materials

Book excerpt: Electron dynamics and plasma-infusion are crucial for understanding plasma and materialrecycling, cooling, and the plasma-material interactions (PMI) for plasma facing surfaces for applications such as fusion energy and space propulsion. Notably, we have shown that new materials such as volumetrically complex materials (VCMs) can improve plasma device performance and lifetime and may provide a versatile design space for plasma-facing components such as inner walls and high-power electrodes. The objective of this dissertation is to investigate the key PMI of VCMs by examining and characterizing the effects of electron dynamics occurring at the near-surface plasma region with focus on SEE and ion-induced sputtering. This work uses a combination of experimental, computational, and analytical methods. Much of this work uses reticulated foams as a representative material architecture to investigate PMI behavior across a wide range of the VCM design space. Secondary electron emission (SEE) yield analyses using a new scanning electron microscope (SEM) method revealed up to 43% suppression from foam compared with at. Foam ligament-to-pore aspect ratio showed the presence of an optimal geometric configuration, which is in agreement with current and past analytical models. Using SEM techniques, the importance of sample transparency and backplate yield contributions on overall target yield in plasma-facing regimes was also established. These results showed that geometric xxxi transparency can be used to assess the effective transparency to plasma species. Angular dependence revealed multi-scale behavior in that foams with um features exhibit loss of angular dependence much like fuzzes, while large foams (mm) are directional much like fibers and velvets. Electron spectroscopy showed that backscattered electron suppression is 80% more than low energy SEE suppression in carbon foams, while low energy SE generation may be enhanced. An analytical model for ion-sputtering of foams was modi ed to employ SEE physics and calculate SEE yields, which were then compared with experimental results. A dedicated, compact, hollow-cathode generated plasma facility was developed to expose biased foam (-100 to -600 V) to investigate plasma infusion regimes using electrostatic probing, optical emission spectroscopy, and in-situ sputter yield monitoring. A new diagnostic developed for real-time and in-situ surface profilometry is used to monitor the surface morphological evolution of foams during plasma exposure. Combined with ex-situ surface analysis techniques, plasma-foam experiments and analysis of inter-foam deposition and sputterant transport within pore layers have shown that plasma infusion is key for predicting back vs forward material sputtering. In addition, user facility was used in a collaboration project with UCLA Physics to investigate the effect of biased tungsten foams in a pulsed He plasma on material erosion and arcing in comparison with a planar surface. It was found that tungsten-based VCMs can withstand up to 600 V negative bias in a continuous and pulsed plasma environment, with significant reduction of arcing events when comparing to planar tungsten.