Helium aggregation and surface morphology near grain boundaries in plasma-facing tungsten

Abstract

We conduct molecular dynamics simulations of helium in tungsten to study the interaction of helium with grain boundaries. Model systems with grain boundary planes perpendicular to the surface and parallel to the surface are considered. The net attraction of mobile helium to the grain boundary results in a “depleted region” within approximately 3.5 nm of the grain boundary plane at low fluence, and once on the plane of the grain boundary, helium transport slows considerably. Helium retention is also strongly affected by the grain boundaries and their density: grain boundary planes approximately 6 nm beneath the plasma-facing surface and parallel to the surface tend to reduce the maximum bubble size due to the attraction of mobile clusters to the grain boundary plane, which lowers the concentration of helium near the surface (where it is being implanted); grain boundaries perpendicular to the surface tend to increase retention due to retention on the grain boundary plane. For grain boundaries parallel to the surface, the strong gettering effect of the grain boundaries on helium results in essentially no helium penetration through the grain boundary during the first 1.5 μs of plasma exposure at a flux of 1.6×1025 m−2s−1, corresponding to fluences on the order of 1020 m−2. Coarse-grained simulations capable of capturing the long-term dynamics of helium aggregation near grain boundaries would be required to determine whether these effects would have any measurable impact on phenomena, such as tungsten fuzz growth.