First-principles Investigations Of Dissolution And Diffusion Behaviors Of H/He On W Surface

Tungsten(W)-based divertors will undergo the irradiation of low-energy and high flux hydrogen(H)and isotope as well as helium(He)ions plasma,which can induce surface damages such as voids formation,blisters,bubbles and sputtering erosion.Surface damages are affected by the surface orientation,which may provide a way to optimize the plasma-facing materials(PFMs)by control of the crystallographic texture.Therefore,it is still necessary to investigate the behaviors of H/He accumulation,retention and bubble formation as well as orientation effect on W surface.The dissolution and diffusion behaviors of H/He on the four low-Miller-index W surfaces[(110),(112),(100)and(111)]are systematically studied by the density functional approach to understand the orientation dependence of H/He bubble distribution on surface.The results show that H accumulation on surface is influenced by vacancy formation energy and H formation energy.Vacancy is easier to form on surface due to the lower formation energies of vacancy comparing to that in bulk,which causes that H is easier to accumulate due to vacancy trapping H.H atoms are easy to escape from surface due to that the barriers of diffusion towards surfaces are small.Therefore,diffusion behavior has less influence on H aggregation.H is preferred to accumulate on the W(111)surface due to the lower formation energies of vacancy and H comparing to that other surfaces.While He accumulation on surface is mainly affected by self-trapping,vacancy and He-induced vacancy.The results found that dissolved He atom can induce vacancy formation on surface.The accumulation of He on the surfaces of W(111),W(100)and W(112)is driven by the He-induced vacancy.The He-vacacny provide trapping sites for He.And specially for W(111)surface,He accumulation is contributed by He-induced vacancy trapping hinders He escaping from surface.Based on our studies,W(111)surface is easier to form H/He bubble than other surfaces,while W(110)surface is a resistant surface for H/He bubble due to higher formation energies of vacancy as well as H/He.The results about formation of H/He bubble on surface are helpful for understanding the orientation dependence of surface damages on W surface and designing new irradiation-resistant plasma-facing materials.