Research On Micro-Nano Scale Damage On Tungsten Surface Under Helium Irradiation

Tungsten(W)material is the main candidate for plasma facing materials(PFMs)of tokamak device in fusion reactor,which undergoes high heat flux and plasma exposure.Helium(He)is a product of the fusion reaction of deuterium(D)and tritium(T),which is widely present in the reactor.There is an extremely strong interaction between He plasma and W on the surface of divertor.In this research,W material was irradiated by He ion and He plasma.The damage behavior of W material under He irradiation condition was investigated.The thermal conductivity of the damaged layer was analyzed by a selfbuilt micro-nano scale thermal test device.The formation mechanism of the damage structure and the mechanism of thermal conductivity decrease were studied.A thermal conductivity measurement device for damage surface was constructed and a method for evaluating the damage degree of the material was established.Combined with the study of the microscopic damage mechanism,the thermal conductivity reduction mechanism was analyzed.The surface thermal conductivity of He ion irradiated W decreased by one order of magnitude and decreased with the increase of irradiation dose/damage.The decrease in the thermal conductivity of the damaged layer is due to the high density of He clusters,which strongly scatter the electrons,resulting in enhanced electron-electron interaction in the W material and a decrease of the mean-free-path.The thermal conductivity of He plasma irradiated W reduced by 2 orders of magnitude and decreased with the increase of irradiation temperature and dose.On one hand,the descending mechanism is that the electrons are strongly scattered and the heat transfer is inhibited.On the other hand,more He bubbles are generated on the W surface,making the W damage layer into a porous structure,which further inhibits heat transfer.The study reveals the evolution mechanism of the W surface damage structure under He irradiation conditions.Under medium and low temperature,the surface of the W material formed a nano-scale damage structure,which was orientation-dependent.A serious damage layer within 10 nm near the surface layer was formed on the W surface.The formation of the nano-scale structure is resulted from the bubble growth mechanism.During the growth of the He bubble in the damaged layer,the {001} surface is easily formed,and the stress gradient is preferentially grown in the plane,making the large-scale He bubble shape anisotropic.Therefore,traces parallel to the <001> direction are generated on different orientation grains,resulting in orientation dependence of the surface morphology of the material.Under high temperature conditions,a fuzz structure was produced on the W surface.The growth of He bubbles still preferentially forms the {001} plane,which makes the structure of the fuzz orientation-dependent.As the surface temperature of irradiation increases,the He bubble migration ability increases and the orientation dependence decreases.Damaged W material was reloaded by a transient high heat flux,resulting in a decrease of melting threshold and a more severe melting behavior.Surface temperature rises with a reduced thermal conductivity of the surface submicron-scale damaged layer.It can be deduced that W materials are therefore easily melt and recrystallize.The results of the thermal conductivity tests are consistent with reloading performance.Thermal conductivity can be utilized as a standard for estimating the performance of PFMs and it has guiding significance for evaluating the damage degree of materials and predicting service life in the future.