| In the course of commercialization fusion energy,the research on plasma-facing materials(PFMs)has become a top priority for the application of fusion energy.Due to its high thermal conductivity,low sputtering,low hydrogen retention and other physical properties,tungsten and its alloy are considered as one of the most viable PFMs.In a fusion environment,tungsten materials will be exposed to the fusion reactants(deuterium,tritium)and products(helium,neutron)directly.Therefore,it has a great scientific significance and engineering value to explore the interaction between materials and fusion plasma.In this paper,the high density plasma generated by a self-development high-power radio frequency(RF)ion source is used to simulate the interaction between materials and plasma.The radiation damage behavior of tungsten and its alloy materials is systematically studied under various experimental conditions.The radiation damage mechanisms of materials are explored by means of advanced material analysis methods.The research content of this thesis mainly includes the following aspects:(1)Design of high-power radio frequency(RF)ion source.Three different-sized ion sources are designed by RF technology.Finally,a high-power RF ion source with a 10 cm internal diameter and a water-cooled faraday screen protection is selected to generate the high density plasma,which is utilized to simulate the harsh plasma environment where the PFMs are located.And then,a set of low-energy,high-flux ion irradiation material system based on high-power RF plasma was established under a steady-state operation.A invention patent of the system has been authorized.(2)The surface damage behavior of pure W specimens exposed to Helium(He)plasma was systematically investigated.It is found that the surface morphology of specimens after irradiation varied from the change of irradiation temperature.Under low temperature(less then 1100 K)irradiation,specimens exhibited a pinhole(10-20 nm)structure;the dense nano-fuzz structure was formed at W surface under high temperature(1200-1900 K)irradiation;after irradiation at 2200 K,many holes with hundred nanometers in size appeared at the near-surface of specimens.The analysis suggests that the irradiation damage of W specimens exposed to He+ is mainly dominated by the diffusion and migration behavior of implanted He and self-interstitial W atoms(SIAs)induced by irradiation.Compared to low and extraordinarily high temperature irradiation,the dense nano-fuzz structures formed at medium-high temperature has a protective effect on the sputtering of surface W atoms.Conversely,the bursting of over-pressure helium bubble could cause serious material loss,resulting in the formation of holes with tens to hundreds nanometers at W surface.(3)The formation,growth and physical properties of nano-fuzz structures were studied in detail.The analysis suggests that the formation and growth of nano-fuzz could be treated as a complex surface modification process induced by the aggregation and diffusion behavior of He atoms and SIAs.Our measurements show that corrosion effects occurred with the growth of fuzz,which means that the nano-fuzz structures are unstable compared to bulk W due to the reduction of binding energy between W atoms(～8.6 eV).The results indicate that the sputtering threshold of the nano-fuzz is lower than that of bulk W.From the perspective of corrosion of the whole materials,the existence of nano-fuzz reduced the overall corrosion rate of the material,as described in content(2)above.(4)The evolution process of nano-fuzz structures under the irradiation of hydrogen(H)and deuterium(D)was explored.The experimental results show that nano-fuzz structures are indeed an unstable.Helium could be released from the nano-fuzz under H or D ions irradiation.At the same time,W atoms could tend to gather together.Thermal desorption experiments suggest that He could enter the near-surface of W during irradiation even if the thickness of fuzz layer increases to several micrometers(1-4 μm),resulting in gas retention in bulk W.Significantly,the nano-fuzz structures have a strong absorption effect on D2 during low-temperature irradiation(400 K),which will greatly affect the recycling utilization of D2 or T2 during the operation of device.(5)The irradiation effects of oxide dispersion strengthened(lanthanum and yttrium oxides)and solid solution(rhenium)strengthened W alloy with various doping content were studied under different irradiation temperature.It was found that the second phase doping could effectively inhibit the directed diffusion behavior of He atoms in near-surface of materials under 1100 K irradiation,resulting in a random distribution of damages.Meanwhile,oxides doping can suppress the growth of nano-fuzz on W surface under high temperature(1550 K)irradiation.However,compared with W matrix,doped oxides could be sputtered preferentially,resulting in a large number of holes forming in material surface after irradiation,accompanied by the generation of impurities.At the same time,it was found that the addition of yttrium oxides has a significant effect on the refinement of W gain size.But compared with lanthanum oxides doped W alloy with larger grain size,the yttrium oxides doped W alloy exhibited a local grain growth effect under high temperature irradiation,which will cause inevitable damage to the physical and mechanical properties of materials.(6)The surface damage behavior of W materials exposed to oxygen-seeded He or H plasma was studied.The results show that a trace of O2 doping effectively inhibited the growth of nano-fuzz on W surface without affecting the erosion rate of materials.However,the x-ray diffraction analyses suggest that a large amount of WOx was formed on W surface with the increase of O2 content,which significantly increased the erosion rate due to the evaporation and sputtering of WOx.H ion irradiation experiments show that obvious grain growth or recrystallization occurred after irradiation,which would cause the damage to the mechanical and physical properties of materials.Meanwhile,it was found that a trace of O2 doping did not affect the surface microstructure evolution of specimens under H ions irradiation,but the erosion rates increased gradually with increasing O2 content in H plasma.X-ray photoelectron spectroscopy analyses indicate that the increase of erosion rates could be attributed to the formation of complex WOx at W surface during irradiation. |
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