(457e) Simulation of Helium Transport Near Prismatic Dislocation Loops in Tungsten

We analyze the effect of sub-surface prismatic dislocation loops on the surface morphology and helium clustering behavior of plasma-facing tungsten through the use of molecular dynamics simulations. These simulations are moderately large, 25 nm x 25 nm x 24 nm, consisting of approximately 830,000 atoms and simulated to times on the order of one microsecond. This approach eliminates some finite-size effects common in smaller simulations and reduces the flux to approximately 5.5 x 10^26/m^2/s, including ions that reflect back into the plasma. This flux is approximately a factor of 15 lower than is typically used in smaller simulations. These results indicate that prismatic loops with radii of approximately 3 nm that are centered 10 nm below the surface with Burgers vectors parallel to the surface initially cause helium atom clusters to accumulate at the edges of the dislocation core relatively quickly (within 100-150 ns of the onset of plasma exposure). Subsequent growth of these clusters, however, is relatively minimal. This is partially explained by the relatively high helium implantation flux, which causes bubbles to accumulate 0-7 nm below the surface and "block" the region of the metal containing the dislocation. Another effect results from the strain field around the loop itself: the compressive regions along the direction of the Burgers vector repel helium, but the tensile region initially attracts helium and traps it. However, we think that the attractive tensile stress region is effectively shielded by the formation of helium clusters above it, and these bubbles subsequently experience relatively slow growth.