The behavior of small helium clusters near free surfaces in tungsten

The results of a computational study of helium–vacancy clusters in tungsten are reported. A recently developed atomistic kinetic Monte Carlo method employing empirical interatomic potentials was used to investigate the behavior of clusters composed of three interstitial-helium atoms near {111}, {110} and {100} free surfaces. Multiple configurations were examined and the local energy landscape was characterized to determine cluster mobility and the potential for interactions with the surface. The clusters were found to be highly mobile if far from the surface, but were attracted and bound to the surface when within a distance of a few lattice parameters. When near the surface, the clusters were transformed into an immobile configuration due to the creation of a Frenkel pair; the vacancy was incorporated into what became a He3–vacancy complex. The corresponding interstitial migrated to and became an adatom on the free surface. This process can contribute to He retention, and may be responsible for the observed deterioration of the plasma-exposed tungsten surfaces.