Discovering Key Unknowns for Tungsten-Hydrogen-Helium Plasma Material Interactions Using Molecular Dynamics

Molecular dynamics simulations have been used to study plasma material interactions to better understand the performance of a tungsten divertor. A tendril-like geometry was modeled to study the diffusion of helium in nanotendrils and its relation to fuzz growth. The tendrils remain stable throughout the simulation and a modified helium release mechanism is found that allows the helium retention to reach a steady state within the tendril. The helium retention within the tendril inversely depends on the surface to volume ratio. There is limited diffusion deep into the tendril and extrapolating the flux calculated to experimentally relevant time scales indicates that helium diffusion is not sufficient to drive fuzz growth. Helium implantation near a grain boundary, but not directly on the grain boudary itself, was performed. Helium behavior within the implantation zone is consistent with previous simulations of helium in defect-free tungsten. Some helium diffuses to the grain boundary where it forms small helium clusters but virtually no helium atoms diffuse over the grain boundary. The sink strength of the grain boundary and helium bubbles are calculated and the values are comparable, indicating that the grain boundary sink strength only matters at the beginning of the simulation before the helium bubbles form. Simulations of hydrogen and helium were performed to assess the interaction between the two gas atom species in tungsten. Simulations of small subsurface mixed hydrogen-helium bubbles indicate that hydrogen diffuses to the helium bubble periphery region and becomes trapped there. A binding energy of 2 eV is calculated. Modeling of hydrogen implantation in helium pre-implanted tungsten were performed and the presence of helium modifies the depth distribution and blocks the deeper diffusion of hydrogen when compared with hydrogen implantation in pure tungsten. This could potentially have a significant impact on tritium retention and material performance.