Effect of Helium Flux on Helium Accumulation in the Near-surface Region of Plasma-Exposed Tungsten*

Tungsten is a candidate material for plasma-facing components (PFCs) in nuclear fusion reactors. During its operation, PFCs are subjected to temperatures > 800 K and a low-energy ( eV), high-flux (on the order of ) of He+ ion irradiation, resulting in subsurface He accumulation. To understand the subsurface accumulation of implanted helium as a function of its flux, object kinetic Monte Carlo (OKMC) simulations were carried 100 eV — helium implantation of tungsten (100) surface were performed. These simulations were performed at a te2mperature of 933 K for He implantation fluxes ranging from  to . In the near-surface region, helium clusters elastically interact with the free surface. The interaction is attractive and becomes increasingly stronger, thereby lowering the activation energy barriers for helium clusters to hop towards the surface and trap mutate with decreasing depth. Furthermore, the lowering of the trap mutation barrier with decreasing depth also results in the kick-out of multiple tungsten atoms, which sometimes is followed by the partial dissociation of the mutated helium cluster. Therefore, these simulations were performed using not only the depth-dependent helium cluster migration and trap mutation barriers but also the depth-dependent trap mutation processes. The types of trap mutation processes and their energetics were obtained using molecular dynamics simulations. A detailed discussion on the influence of helium flux on subsurface helium and, helium bubble accumulation as a function of depth and fluence will be presented.