Subsurface Gas Dynamics in the ARC Divertor Under Prolonged Exposure to Detached Plasma Conditions

Plasma-facing components of future fusion systems will be expected to withstand years of operations in an extremely harsh environment. Long-term exposure to this environment will alter the material properties and therefore performance of the components. Using an integrated plasma-material interaction model that includes descriptions of the edge plasma, ion-surface interactions, and subsurface gas dynamics, we assess the evolution of 10 distinct spatial locations across the tungsten outer divertor target of the current ARC design during 10 pulses of deuterium-tritium (D-T) plasma exposure, for a total exposure time of 150 min, as well as a longer exposure of 4+ h. Our simulations show no erosion due to the extremely low ion energies in the detached plasma conditions, even with neon seeding of the plasma. The lowimpact energies also lead to a constant reflection coefficient, and therefore, the D-T content accumulation is primarily driven by particle flux, as more D-T is implanted. The content increases with each pulse. Gas diffusion is not affected by changes in substrate temperature that are caused by heat fluxes. When these simulations continue beyond the 10 initial pulses until D-T reaches the bulk boundary, the gases permeate through the backside of the W substrate at the same rate as they are implanted. Thus, the gas content accumulated in the material saturates, and the depth-resolved concentrations remain constant, at least within the ideal microstructure assumptions for the tungsten divertor. These results provide input for modeling the crystal plasticity evolution of the tungsten and the polycrystal evolution and grain growth dynamics under in-service conditions, from which critical stresses, temperatures, and doses can be extracted for use in device design optimization.