Hydrogen behaviors at the near-surface region of tungsten: A first-principles study

As a plasma facing material, tungsten (W) is exposed to high-flux hydrogen isotope plasma, enduring unwanted surface blistering. Here, we performed first-principles calculations on the interaction between H and W surfaces, aiming to understand the surface bubble formation. We calculated the energetics of H at the top-surface and sub-surface regions of W(110) and W(001), in together with the migration energy barriers of H at different near-surface sites. We found that H prefers to be adsorbed on the top-surface rather than dissolving into the sub-surface regions, where H atoms generally exhibit a repulsive repulsion between each other to prevent the formation of H clusters. While migration of H from top-surface into sub-surface is difficult, the reverse migration is extremely easy. These findings suggest that retained H species during irradiation at sub-surface region of W are energetically unstable, and surface H bubble formation would not occur without the presence of defects.