Simulation of the Effect of Hydrogen Repulsion on Clustering and Retention in Tungsten

Prior studies have shown that helium plasma exposure simulations with molecular dynamics can be sped up using direct implantation of helium for energies well below the sputtering threshold, which eliminates the time necessary to simulate atoms that reflect off the surface. In this study, we attempt to use the same approximation for hydrogen. However, because hydrogen atoms in tungsten are generally repelled from each other, and hydrogen is therefore not self-clustering, we also conducted several simulations in which hydrogen directly bombards the surface for comparison. The bombardment and implantation simulations significantly differed, even at short times. These differences include deeper penetration of hydrogen into tungsten in the bombardment case. We also observed that hydrogen trapped beneath the surface will, if it diffuses to the surface, generally have enough energy to desorb without forming an adsorbed layer, but that hydrogen directly impacting on the surface can be captured and adsorb on the surface. These observations suggest not only that hydrogen behavior in tungsten is very different than that of helium, but that hydrogen may require much more time and effort to achieve realistic, tractable models of hydrogen retention and diffusion in plasma-facing materials.