(28f) Transport and Reactions of Mobile Helium Clusters Near Surfaces and Grain Boundaries of Plasma-Exposed Tungsten

The evolution of the surface morphology and the near-surface structure of plasma facing components (PFCs) in nuclear fusion reactors is impacted significantly by the implantation of helium (He) atoms.  Tungsten (W) is an important PFC material due to its thermomechanical properties.  In tungsten, such interstitial He atoms are very mobile and aggregate to form clusters of different sizes.  The smallest of these aggregates, that is, helium clusters with up to seven helium atoms, also are mobile; the diffusional transport of such small helium clusters mediates the evolution of surface morphology and sub-surface gas bubble structure and dynamics near the surface.

In this presentation, we report the results of a systematic atomic-scale analysis of the interactions of small mobile helium clusters (He_n) with sinks, such as free surfaces and grain boundaries (GBs), and the cluster reactions near these sinks in tungsten toward development of continuum drift-diffusion-reaction models for the dynamics of mobile helium clusters in plasma-exposed tungsten.  Molecular-statics (MS) simulations based on reliable many-body interatomic potentials are carried out for He_n (1 ≤ n ≤ 7) clusters near sinks (surfaces and GBs) to obtain the potential energy profiles of the He_n clusters as a function of the clusters’ center-of-mass distance from a sink.  Other sinks that have been investigated include regions in the vicinity of junctions where GBs intersect free surfaces.  Elastic interaction potentials based on elastic inclusion theory provide an excellent description of the MS results for the cluster-sink interactions.  The key parameter in the elastic models is the sink segregation strength, which is found to increase with increasing cluster size.  Such cluster-sink interactions are responsible for the migration of small helium clusters by drift and for helium segregation on surfaces and grain boundaries in tungsten.  As the clusters migrate toward the sinks, trap mutation and cluster dissociation reactions are activated at rates higher than in the bulk.  These kinetic processes are identified and characterized based on analysis of many molecular-dynamics (MD) trajectories for each such mobile He_n cluster near surfaces and GBs.  These cluster reactions are found to be responsible for important structural, morphological, and compositional features in plasma-exposed tungsten, including surface adatoms, near-surface immobile helium-vacancy complexes, and retained helium content.  Such helium segregation on sinks is observed in large-scale MD simulations of helium aggregation in model polycrystalline tungsten at 933 K upon helium implantation.