Hosted by: Meifeng Lin

Currently, tungsten (W) is considered the leading candidate plasma-facing material for divertor regions of future tokamak reactors. However, recent investigations have suggested that W may undergo dramatic surface morphology changes ("fuzz" formation) under high-flux helium ion irradiation expected in these reactors. In light of these results, the present experiments systematically study the response of alternative refractory metals (Ta, Nb, Mo, and V) under similar helium ion loading conditions to assess their relevance as plasma-facing materials. High-flux helium ion irradiations (up to > 1025 ions m-2 s-1) and simultaneous sample heating up to 1223 K are used to replicate fusion-like conditions. Resulting surface morphology response is characterized with scanning electron microscopy (SEM) along with various other complementary materials characterization techniques. While all tested materials show some degree of helium-induced surface morphology changes, temperature ranges over which these changes occur vary with material properties, hinting at underlying driving mechanisms. Furthermore, Ta was found to have an increased resilience to surface nanostructure formation compared to W. Although hydrogen retention characteristics are notably inferior for Ta, thermal desorption spectroscopy (TDS) studies on deuterium-irradiated Ta reveal (for the first time) discrete desorption energies for implanted deuterium. While desorption energies for Ta are indeed higher than these for W (implying higher retention in Ta), the high temperature expected in the divertor region may provide sufficient energy for deuterium desorption and mitigate the effect of these inferior retention characteristics.