Investigation Of Surface Blistering On Tungsten Induced By Helium Implantation

Energy is one of the key factors restricting the development of society,and the use of fusion energy has been considered as a promising way to solve the energy crisis.However,there are still a lot of challenges to overcome before a fusion reactor can be put into commercial operation.One of the major technical bottlenecks is the issue of materials.The plasma-facing components in Tokamak must withstand high temperatures,high thermo-mechanical loads,and a variety of radiation damage in the fusion reactor.Due to the low sputtering yield,low hydrogen isotopes retention,and high thermal conductivity.Tungsten(W)is considered as a primary candidate material for the PFCs in future nuclear fusion devices.However,the irradiation will cause a series of degradation on tungsten.One of these is the surface blistering induced by helium implantation(helium blister),which is investigated in this thesis.During the operation of the fusion reactor,tungsten will suffer large fluxes and low-energy helium(He)ions escaping from the fusion plasma on the diverter.These He atoms trends to be trapped at grain defects and boundaries to form clusters,bubbles and eventually cause surface modifications,including the formation of blisters and exfoliation.The helium bubbles will increase the hydrogen isotopes retention,while the exfoliations will cause the mass loss of the divertor armor and may eventually disturb the stabilization of the plasma.Thus,it is important to understand the surface blistering on tungsten induced by helium implantation.This thesis is based on experiments,which have used helium implantation to studied three factors(grain orientation,polishing methods,and implantation temperature)to the surface blistering formation.Transmission electron microscopy(TEM),scanning electron microscope(SEM),atomic force microscope(AFM),Focusion beam facility(FIB)were involved in characterization.Experimental results were compared with molecular dynamics(MD)simulation.Results are list as follow:(1)The effect of grain orientation to helium blistersExperiment results show that the size of helium blisters was related to the grain orientation.In the marked five grains,smaller blisters were observed on {111} while lager blisters on {100}.Molecular dynamics simulation(LAMMPS)was used to build tungsten grain models in 3 directions.Simulated results showed that they have different ductile fracture threshold.The minimum threshold was on {100} orientation at ~13 GPa.Stress-strain curve showed that large deformation will occur on this grain with lower inner gas pressure,corresponding to the lager average size of blisters.(2)The effect of polishing pretreatments to helium blistersIt was found that the size and density of helium blisters on tungsten samples pretreated by mechanical polishing and electropolishing were different.The blisters on the electropolished sample were evenly distributed,while blisters with less density and smaller size were only observed on some specific grains on the mechanically polished sample.Further characterization indicated that helium atoms in mechanically polished samples were concentrated in the grain boundaries.These mechanical damage structure can contain the helium atoms and connect to the surface.As a result,it releases the inner gas pressure.This indicates that the mechanically polished sample has similar properties to ultrafine tungsten.(3)The effect of implantation temperature to helium blistersElectro-polished tungsten was implanted by helium ions at different temperatures.Helium blisters were observed on the sample surface in all temperatures.When the temperature is above 1073 K,large areas of exfoliation occurred.This indicates that temperature would promote the evolution of surface blistering.This thesis has not only obtained SEM/TEM images of helium blister from initial formation to finally rupture,but also has found some special void structures.Besides,experimental results showed that helium blister is slight in some specific crystal directions and at low implantation temperature,and the mechanical polishing pretreatment can inhibit the helium blistering to some extent.These results are helpful in understanding the formation and evolution of helium blister,and can provide an essential guide to the material design in the fusion reactor.