Retention/reflection of hydrogen and surface evolution during cumulative bombardment of low-energy hydrogen on tungsten: A molecular dynamics study

Tungsten (W) is a primary candidate for plasma-facing materials (PFM) in future fusion devices because of its excellent properties. The knowledge of the initial deposition of low-energy hydrogen (H) on W surfaces from the atomic perspective is still deemed as the key to understanding the mechanism of radiation damage induced by the retained H in W. Therefore, it is necessary to carefully study the cumulative bombardment process of low-energy H on W surfaces. In this work, the retention and reflection of implanted H atoms were simulated by molecular dynamics (MD) and the evolution of these surfaces in the cumulative implantation process was analysed. The temperature effect (diffusion effect) and the time evolution were the main focus in this work. The cumulative simulation extended to times on the order of 0.1 μs, longer than in most previous MD simulations. And the implanted fluxes are lower than previous MD works, thus closer to the real fluxes in fusion reactors. The channelling effect on the H implantation and H retention was dominant at low temperatures while the diffusion effect was notable at high temperatures. The binary collision theory can be used to describe the distribution of the reflected H atoms in the high-energy range due to the first couple of collisions. The retention of H atoms can induce surface expansion and some W atoms occupy a larger atomic volume than the normal volume. In the MD simulation with lower flux, no point defects (including sputtering atoms) were observed throughout the cumulative bombardment process especially at higher temperature. The defects induced by the implanted H could occur at lower temperature when the amount of the retained H reached a certain threshold. These results may be important for understanding the long-term micro-structural evolution of materials under irradiation.