Influence of ion energy on hydrogen-induced defect stabilization of vacancies in tungsten

The effect of deuterium on vacancy formation in tungsten was studied by combining molecular dynamics and SRIM simulations to investigate why deuterium depth profiles of some previous experiments showed notable increase of retention at the deep end of the retention profile while others did not. Molecular dynamics revealed that deuterium lowers the threshold of displacement energy in tungsten, and that this reduction is strongly angle-dependent: crystallographic directions that have high threshold of displacement energy experienced the highest reductions. Information of the lowered energies was used for SRIM simulations, which show that at incident energies of 5 keV, deuterium ions can cause vacancy formation to extend deeper in the material if the TDE is lowered by solute deuterium. This effect gradually disappears at higher energies. We also investigated vacancy formation at higher energies up to 1500 eV in presence of solute deuterium, and found that D increases defect production at all energies but the strongest relative effect was at low energies of 100 eV. This information combined with SRIM simulations shows that incident ions in the tens-to-hundreds keV range produce relatively higher increase of vacancy production at greater depths, whereas 10 MeV tungsten ions yields a nearly depth-uniform increase. The results show that when investigating defect stabilization effect in laboratory conditions, the choice of ion species and energies can change not only the overall defect density but also the depth-distribution of traps. These findings are also important for possible fusion device defect production estimates as the plasma-facing materials may experience similar conditions of having solute hydrogen isotopes and energetic collision events near their surfaces.