Flux Effects on Helium-Induced Surface Evolution in Tungsten*

We investigate the effects of flux on helium behavior in plasma-facing tungsten using molecular dynamics. The simulations span several orders of magnitude and achieve simulation times of up to 2.5 μs, revealing concerted bubble-bursting events that are responsible for significant and very sudden changes in surface morphology. The helium depth distribution is very strongly flux-dependent, as helium becomes trapped near the surface at high flux because of the near-surface bubbles that form. Helium retention is also much lower at low flux than at high flux. Surface features correlate with bubble locations at low fluence, but at high fluence, bubbles merge together, venting to the plasma at one or more locations and leaving large interconnected cavities below the surface. Such ruptured bubbles may serve as pathways deeper into the material, allowing helium to bypass the near-surface layer of bubbles and form deeper and potentially much larger bubbles that can produce more sizeable surface features.