Calculation of Incident Ion Angle Distributions, Surface Erosion, and Material Migration at the NSTX-U Divertor­­­¹

Plasma-material interactions (PMI) pose a significant threat to the viability of future tokamak fusion reactors. Surface roughness of plasma-facing components (PFCs) can trap impurity materials and T, posing plasma confinement degradation and radioactive inventory concerns. The surface structure also affects PFC erosion due to the ion incident angle dependence of physical sputtering, trapping of sputtered materials, and ion shadowing effects. In this research, a collisionless kinetic model was used to calculate the incident polar and azimuthal ion angle distributions (IADs) under sheath potential profiles expected for the NSTX-U divertor. Calculated IADs were input into a Monte Carlo micro-patterning and roughness (MPR) code to simulate the surface erosion and material migration. MPR input geometries ranged from analytically produced rough surfaces to optical confocal microscopy data of a NSTX-U graphite tile surface, which was exposed to 1138 L- and H-mode discharges (845 s total exposure time) [1]. The calculation results will be verified with Al concentration maps measured by scanning Auger microscopy (SAM). These calculations of IADs, surface erosion, and surface migration will be extended to the latest NSTX-U plasma and divertor configurations, such as fish-scale surfaces. [1] C.H. Skinner, et al., Nucl. Mater. Energy 18 (2019) 35.