Prompt vs Local Redeposition: Model Refinement and Experimental Design for Understanding High-Z Net Erosion in Magnetic Confinement Fusion*

Here, we present a high-Z net erosion model discriminating three primary redeposition mechanisms for sputtered material: prompt (geometric-driven), local (sheath-driven), and far (scrape-off-layer-driven). Using these distinctions, we show predictive modeling for high-Z net erosion of plasma facing components (PFCs) in magnetic-confinement fusion over a matrix of key plasma-material interaction parameters. With global sensitivity scans using Sobol’ methods we assess critical limits of each mechanism and show that a prompt-vs-local tradeoff can explain a commonly seen underprediction in redeposition losses (of up to two orders of magnitude) for shallow magnetic-field-to-PFC pitch angles. Finally, we report a “design-of-experiment” study exploring the measurability of prompt vs local distinctions in current experimental facilities in order to validate these new findings. For this effort, we use a combination of synthetic diagnostics and plasma-parameter optimization to propose a method of measurement using an isotopically enriched high-Z surface and exploiting robust in situ and ex situ techniques. In principle, this approach provides a path forward for refining PFC models with the expectation that it could aid in PFC design for future reactors.