(697c) Evolution of Plasma-Exposed Tungsten Surfaces Due to Helium Diffusion and Bubble Growth

Helium from linear plasma devices and tokamak plasmas is known to cause formation of microscopic features, termed as “fuzz” or “coral,” on the surface of plasma-exposed materials after only a few hours of plasma exposure. The precise details of such surface modifications are as yet uncertain. This study examines the initial and intermediate stages of fuzz formation by large-length-scale molecular dynamics (MD) simulations of helium-implanted tungsten over time scales of up to microseconds using single-crystal and polycrystalline supercell models of tungsten. The large-scale MD simulations employ state-of-the-art many-body interatomic potentials and implantation depth distributions for the insertion of helium atoms into the tungsten matrix constructed based on MD simulations of helium-atom impingement onto tungsten surfaces under prescribed thermal and implantation conditions. The large-scale MD simulations reveal surface features formed via the sequence of helium implantation, diffusion of helium atoms and their aggregation to form bubbles, growth of bubbles and consequent production of tungsten self-interstitial atoms, organization of those atoms into prismatic loops, glide of those loops to the surface, and bubble rupture. The crystallographic orientation of the surface and the grain microstructure of the tungsten matrix, namely, the presence or absence of grain boundaries in tungsten, are found to have strong effects on the resulting surface features. In particular, grain boundaries serve as sinks for helium atoms, resulting in more rapid growth of surface features in the vicinity of grain boundaries. Our findings have significant implications for the surface morphological evolution and the near-surface structural evolution of plasma-facing components in nuclear fusion reactors.