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 helium clusters, containing n helium atoms with n = 1-7, also are mobile and their diffusional transport mediates the evolution of surface morphology and the sub-surface helium gas bubble structure and dynamics.
In this presentation, we report the results of a systematic atomic-scale analysis of the reactions of small mobile helium clusters near low-Miller-index tungsten surfaces, aiming at a fundamental understanding of the near-surface dynamics of helium-carrying species in plasma-exposed tungsten. These small mobile helium clusters are attracted to the surface and migrate to the surface by Fickian diffusion and drift due to the thermodynamic driving force for surface segregation. As the clusters migrate toward the surface, trap mutation (TM) and cluster dissociation reactions are activated at rates higher than in the bulk. TM produces W adatoms and immobile complexes of helium clusters surrounding W vacancies located within the lattice planes at a short distance from the surface. These reactions are identified and characterized in detail based on analysis of a large number of molecular-dynamics (MD) trajectories for each such mobile cluster near W(100), W(110), and W(111) surfaces. TM is found to be the dominant cluster reaction for all cluster and surface combinations, except for the clusters with n = 4 and n = 5 near W(100) where cluster partial dissociation following TM dominates. We find that there exists a critical cluster size, n = 4 near W(100) and W(111) and n = 5 near W(110), beyond which formation of multiple W adatoms and vacancies in TM reactions is observed. The identified cluster reactions are responsible for important structural, morphological, and compositional features in plasma-exposed tungsten, including surface adatom populations, near-surface immobile helium-vacancy complexes, and retained helium content, which are expected to influence the amount of hydrogen re-cycling and tritium retention in fusion tokamaks.
We have also carried out a systematic atomic-scale analysis of reactions of small mobile helium clusters near a symmetric tilt grain boundary (GB) based on MD simulations. We found that TM reactions are dominant for clusters with n = 4-7, and only a single vacancy is formed in each of the TM reactions identified regardless of cluster size n. The displaced W atom generated from the TM reaction near the GB forms an extended W interstitial configuration on the GB. We also found that, for n = 1-4, the helium clusters (including single He interstitial atoms) that segregate on the GB are rendered practically immobile. However, the mobility of the extended W interstitial configuration on the GB depends on the exact location where the TM reactions take place, with the W interstitial being highly mobile if the TM reaction occurs on the GB. Although this study is limited to a simple, prototypical model GB structure, the insights obtained are very useful for understanding surface morphology and helium retention in plasma-exposed polycrystalline tungsten.