(697d) Analysis of Drift and Diffusional Transport 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 of different sizes; the smaller of these clusters also are mobile and their diffusional transport mediates the evolution of surface morphology and the structural evolution of the near-surface regions of the plasma-exposed material.

In this presentation, we report results of a systematic, comprehensive study of mobile He-cluster transport in W.  Our analysis bridges atomistic and continuum modeling and simulation for a rigorous and quantitative description of drift and diffusional transport of such He clusters in near-surface W regions.  In addition to Fickian diffusion, the analysis takes into account drift fluxes for cluster transport driven by surface segregation forces; such thermodynamic driving forces become significant and induce substantial drift fluxes in near-surface regions. Our modeling approach links hierarchically atomic-scale computations with continuum drift-diffusion models for the evolution of the cluster concentration fields in the near-surface region; the atomic-scale modeling consists of molecular-dynamics (MD) simulations of mobile cluster diffusion, molecular-statics (MS) computations of the energies of structurally relaxed He-cluster configurations as a function of their distance from the surface, and computations of the corresponding optimal cluster migration pathways employing the climbing-image nudged elastic band (NEB) method.  The cluster size n (1 ≤ n ≤ 8) and the surface crystallographic orientation are important parameters in the study. In addition to the fundamental and quantitative understanding of mobile He-cluster mass transport in near-surface W regions, this study provides the required closure relations for the drift and diffusive fluxes in continuum cluster mass transport (drift-diffusion) models through predictions of diffusion coefficients and segregation potentials as a function of distance from the surface. This transport modeling also is extended to the analysis of cluster transport near sinks other than surfaces, such as grain boundaries (GBs), with emphasis on GBs in near-surface regions and the corresponding combined (multi-sink) drift effects due to segregation forces.