(247d) Analysis of Helium Segregation on Surfaces of Plasma-Exposed Tungsten

Plasma facing components (PFCs) as the first wall materials in nuclear fusion reactors are exposed to intense plasma heat and particle fluxes. The implantation of helium (He) atoms into these materials impacts significantly their surface morphological and near-surface structural evolution. Tungsten (W) is a promising PFC material because of its thermomechanical properties. In tungsten, such interstitial He atoms are very mobile and aggregate to form clusters of various sizes. Small, mobile helium clusters (He_n, 1≤n≤7) are attracted to the tungsten surface due to an elastic interaction force that provides the thermodynamic driving force for surface segregation and their diffusional transport mediates the evolution of surface morphology and near-surface microstructure.

In this presentation, using molecular-statics (MS) computations and molecular-dynamics (MD) simulations based on a reliable many-body interatomic potential, we explore helium segregation on tungsten surfaces for tungsten that has been exposed to different levels of helium implantation. The cluster-surface elastic interaction force induces drift fluxes of these mobile He_n clusters, which increase substantially as the migrating clusters approach the surface, facilitating helium segregation on the surface. Moreover, the clusters’ drift toward the surface enables cluster reactions in the near-surface region at rates much higher than those in the bulk material; the most important of these reactions is trap mutation, which produces W surface adatoms and immobile complexes of helium clusters surrounding W vacancies located within the lattice planes at a short distance from the surface that evolve to form He bubbles. With higher levels of helium implantation at higher fluence, these mobile clusters are subjected to cluster-cluster and cluster-bubble interactions in addition to the more dominant cluster-surface interactions, which complicate cluster dynamics beyond the dilute limit of helium content in the PFC material. These near-surface cluster dynamics have significant effects on the surface morphology, near-surface defect structures, and the amount of helium retained in the material upon plasma exposure.

We characterize in detail helium cluster dynamics and their effects on helium surface segregation based on analysis of a large number of MD trajectories for such mobile clusters near W(100), W(110), W(111), and W(211) surfaces. Furthermore, we analyze the effects of varying helium fluence due to increased plasma exposure of the tungsten PFC by performing systematic MS computations of small helium cluster energetics near low-Miller-index W surfaces as a function of distance (depth) of the cluster center from the surface on a grid of lateral locations on the surface. We analyze the defect interactions that mediate the energetics of small He clusters migrating to the surface away from the dilute limit of He in W taking into account that the migrating cluster also is subjected to the stress field generated by larger He bubbles, as well as other He clusters, and quantify the strength of these interactions for different levels of He implantation. The outcome of this analysis is the systematic parameterization of helium mobile cluster energetics at varying levels of He implantation through functional forms that include contributions from cluster-cluster and cluster-bubble interactions as well as cluster-surface interactions. Such parameterizations are important for developing atomistically informed, hierarchical multiscale models of helium cluster dynamics in PFC materials.