Energetics and Dynamics of Mobile Helium Clusters in Near-Surface Regions of Plasma-Exposed Tungsten

The implantation of helium (He) atoms has significant implications for the surface morphological evolution and the near-surface structural evolution of plasma-facing components in nuclear fusion reactors. In tungsten (W), such interstitial He atoms are very mobile and aggregate to form clusters; the smaller of these clusters are mobile and their migration mediates the evolution of the surface morphology and the near-surface microstructure of the plasma-exposed material. In this presentation, we report results of a systematic atomic-scale analysis of the energetics and dynamics of mobile He clusters in near-surface W regions based on carefully parameterized many-body potentials. The analysis combines molecular-statics computations of the energies of structurally relaxed He-cluster configurations as a function of their distance from the surface with molecular-dynamics simulations of mobile cluster migration in the near-surface region. The cluster size n (ranging from 1 to 7 He atoms) and the surface crystallographic orientation are important parameters in the study. This atomic-scale analysis also is extended to the migration of small mobile He clusters near sinks other than surfaces, such as grain boundaries (GBs), with emphasis on GBs in near-surface regions.