First-principles Studies On The Dissolution And Diffusion Of Hydrogen In The Surface Of Tungsten With Helium

In nuclear fusion devices,tungsten（W）-based divertors will expose to low-energy,high-flux hydrogen（H）isotope and helium（He）ions,which will cause surface damages,such as void formation,blisters,bubbles and surface sputtering,and further affect the mechanical and thermodynamic performance of plasma-facing materials（PFMs）.A lot of experiments have shown that the presence of inert gas atoms such as He can reduce the total H retention and surface blistering.In addition,surface damages are also related to the crystal orientation.These results provide a way to reduce surface damage by W surface pretreatment.Therefore,it is necessary to explore the physical mechanism of H/He aggregation,retention,migration and bubble formation in W surface and the influence of crystal orientation on H,He dissolution and diffusion behavior.In this paper,we use the density functional theory（DFT）to systematically explore the effect of He on the dissolution and diffusion of H from two aspects,and the formation of the Helium-Vacancy clusters complex（He₃-V）in different crystal orientation W surfaces.The results are shown in the following:1)In W（110）surface,we found that H is more easily to dissolve around V by comparing H dissolution and diffusion around vacancy（V）/helium-vacancy complex（He-V）,but there are more capture sites around the He-V.The binding energy of H with V is larger than that of H with He-V,and the H around He-V is more likely to diffuse towards the surface.Thus,in W surface,He-V acting as the capture center of H can hinders the formation of H₂ and inhibits the diffusion of H into deeper,as well as makes the H more easily desorb to the surface,thereby reduces the formation of H bubbles and the total H retention.2)The effect of He on the dissolution and diffusion of H in different crystal orientation W surfaces（W（110）,W（100）and W（111））have studied and found that in the W（110）surface,the formation of He-V is different in different surfaces and the He-V in the third layer of the W（111）surface is very unstable.It is most difficult for H in dissolving around He-V in the W（110）surface.Once H dissolved in W surfaces,the binding energy of H and He-V is lower in the W（110）and W（100）surfaces than that in the W（111）surface.This is mainly due to the plastic deformation of W（111）,which results in the lower charge density at the stable position of H.In the kinetic processes,for the three different crystal orientation surfaces,when the depth of He-V is below 4?,the H captured by He-V is more likely to diffuse into the deeper.When the depth of He-V is above 4?,the H trapped by He-V is more likely to diffuse towards the surface,with the values of dissociation energies following W（111）<W（110）<W（100）.Therefore,in the W（111）surface,H is more easily to diffuse towards the surface,and in the W（100）surface,H is most difficult in diffusing towards the surface.However,since the W（111）surface is an open surface and He-V in the third layer is very unstable,He easily moves towards the vacuum layer under the irradiation of H to form others serious surface damage.For the W（110）and W（100）surfaces,H in the W（110）surface is easier to escape when considering the effect of temperature.Therefore,the W（110）surface with He-V has the best properties for inhibiting H retention and H bubbles formation.3)The dissolution behavior of He₃-V clusters in different crystal orientation W surfaces affects the damage morphology of surface.In the W（111）and W（112）surfaces,He₃-V clusters can be formed much deeper than that in the W（110）and W（100）surfaces.Therefore,in the W（111）and W（112）surfaces,there are more W atoms aggregated to form surface damage,and more He₃-V clusters formed in the sub-surface.With He irradiation,He₃-V clusters increase gradually to form more He bubbles,resulting in serious surface damage.In the W（100）surface,the depth of He₃-V is the shallowest,so surface damage caused by He irradiation is the minimum.This result classifies the difference of He₃-V cluster formation in different surfaces under He irradiation and provides fundamental data supports for the simulation and analysis of Molecular dynamics（MD）.Our results show that the presence of He-V in W surfaces can reduce the retention of H and surface blistering.For different crystal orientation W surfaces with He-V,the W（111）surface is more likely to increase H retention and form various surface damages than the W（100）and W（110）surfaces.The surface of W（110）is beneficial to the anti-hydrogen bubbles formation.Under the irradiation of He,the He₃-V clusters in the W（111）and W（112）surfaces will form more He bubbles and cause higher W concentration on the surface and interstitials than that in the W（100）and W（110）surfaces.In the W（100）surface,the surface damage caused by He irradiation is the minimum.The above conclusions are helpful to understand the interaction of H and He in W surface and the influence of crystal orientation on the retention and bubbles of H and He,as well as provide new ideas for the design of anti-irradiation PFMs.