Radiation damage of tungsten surfaces by low energy helium atom bombardment – A molecular dynamics study

Molecular dynamics (MD) simulations were performed to study the radiation damage of tungsten surfaces bombarded by low energy (20–200 eV) helium atoms at various temperatures. Although the incident energy of the helium atoms is lower than the threshold energy that was thought to be required to induce damage on tungsten surfaces, our simulation results indicate that some W atoms are displaced from their initial lattice sites and stack on the top of the surfaces, leaving behind vacancies or helium occupants. Primary knock-on and the replacement sequence along the 〈1 1 1〉 direction are the two main mechanisms inducing this stacking of tungsten atoms. The threshold energy that is needed to initiate a knock-on W atom at different depths below the surface and produce a stacking W atom atop the surface has been calculated to better understand the observations. Studies on the effects of the substrate temperature and the incident energy indicate that increasing the substrate temperature increases the probability of generating stacking W atoms on the surface, and the most probable range of the incident energy for generating such surface damage is between 50 eV and 80 eV.