Full-orbit particle modeling of erosion and redeposition of mixed-material PFC targets on the WEST divertor and first wall

During long-pulse WEST operations, thick surface deposits were observed on the high-field side lower divertor. Understanding the possible sources and processes affecting the formation of such deposits is an important step for qualifying tungsten as a viable plasma-facing material. Detailed surface analysis revealed rather complex surface chemistry with significant amounts of oxygen, carbon, boron, and other elements in addition to tungsten. A hybrid particle-in-cell code, hPIC2 linked with the sputtering code RustBCA has been used to determine the gross and net erosion including mixed materials along the full extension of the divertor and first wall. It is found that despite the large prompt redeposition fraction (>98%), the lower divertor is the main contributor to SOL contamination. Screening efficiency of each region is analyzed in parts as prompt and non-local redeposition and the calculated screening efficiency is used in the contamination quantification. Further, we observed that light impurities tend to have a higher net-redeposition fraction along the high-field side of the device, a possible reason for the observed buildup.