(369b) Plasma-Induced Evolution of Surfaces

Tungsten is a candidate material for the divertor in ITER and other plasma devices for fusion power. Chemical sputtering is virtually absent, activation is low, and physical sputtering is not expected at the relevant ion potentials (tens of Volts). However, helium plasma exposure experiments have demonstrated that fuzz-like and even coral-like surface features form on tungsten surfaces after a few hours. We investigate possible formation mechanisms behind these surface features using molecular dynamics and kinetic Monte Carlo simulations. Mechanisms under study include sub-surface bubble formation and growth resulting in surface ad-atom formation via loop-punching and interstitial migration; ad-atom diffusion to form “islands”; and bubble rupture creating craters. Understanding these mechanisms is crucial to the long-term use of tungsten in plasma-facing devices. Our results suggest it is possible that loop-punching (creation of interstitial atoms near an expanding helium bubble) combined with surface diffusion and bursting/cratering create a possible pathway to surface fuzz formation.