Reduced Net Erosion of High-Z PFC Materials in DIII-D Divertor

DiMES samples featuring 1 cm and 1 mm diameter W films deposited on a Si substrate were exposed in DIII-D near the attached outer strike point of LSN L-mode discharges. The measured net and gross erosion rates of W, determined from post-mortem ion beam analysis (IBA) of 1 cm and 1 mm samples, were 0.14 and 0.48 nm/s, respectively, giving net/gross erosion ratio of 0.29. REDEP/WBC modeling of this experiment yielded a very close ratio of 0.33. Projection of the modeling results to ITER shows very low net erosion of W. In another experiment Mo-coated samples were exposed with $^{13}$CH$_4$ gas injected $\sim$2 cm upstream of DiMES. Reduction of Mo erosion was evidenced $in-situ$ by the suppression of MoI line radiation. Post-mortem IBA showed that the net erosion of Mo was below the measurement resolution of 0.5 nm, corresponding to a rate of $\leq$0.07 nm/s. Compared to the previously measured erosion rates, this constitutes a reduction of more than 10X.