Investigation Of Bubbles Behavior In Tungsten By Hydrogen And Helium Implantation

Along with the implementation of International Thermonuclear Experimental Reactor?ITER?project,tungsten is considered as a leading candidate material for the divertor upper baffle in future nuclear fusion reactors,due to its low sputtering yield,low solubility for hydrogen and good thermal properties.However,the interactions of hydrogen?H?and helium?He?particles with tungsten,such as the hydrogen bubble problem in tungsten under plasma irradiation,are unavoidable and remain a challenging issue for fusion engineering and design.Thus,it is crucial for the design of fusion materials to investigate the behavior of hydrogen and helium in tungsten.In this study,in order to investigate the hydrogen and helium behavior in tungsten,the polished specimens were irradiated by hydrogen and helium ions,and well-defined hydrogen or helium bubbles were observed by ex-situ Transmission Electron Microscope?TEM?and Scanning Electron Microscope?SEM?.We studyed the bubbles behavior with fluence,flux,ion energy and annealling temperature.In order to acquire the inner information of the microstructure,the specimens were cut by Focused Ion Beam?FIB?.Finally,we compared the two polishing methods,that electro-chemical polishing and mechanical polishing.According to the studies above,we can make several conclusions as following :?1?The TEM specimens were implanted by 18 keV H₃⁺ with different fluence at the fixed flux at room temperature.It is observed that,the hydrogen bubbles formed at the fluence of 1.5×1018 ions/cm² and grow up with the total hydrogen fluence increasing.?2?The hydrogen bubbles' diameter appeared larger with the larger irradiation flux with a fixed fluences of 2.25×1018 ions/cm²,and the bubbles' size saturated when the flux was higher than 45 ??/cm².?3?The annealling experiment shows that,there's no changing for the hydrogen bubbles at low temperatures.However,hydrogen bubbles began to grow up when the annealing temperature exceeded 600?,and the bubbles' diameter became larger with the increasing temperature.?4?To investigate the effects of hydrogen energy on the bubbles' behavior,the samples were implanted by hydrogen ions with different energys at the same fluence.As a result,the bubbles' diameters became larger with the increasing of hydrogen energy,but the bubbles' diameter increased slightly when the energy exceeded 7.5 ke V.?5?To investigate the irradiation damage of helium ions in tungsten,the SEM specimens were irradiated by hellium ions with different fluences at a flux of 60 ?A/cm²,the energy of the helium ions is 18 ke V.Shows that,with the increasing fluence of helium ions,there will be four processes appeared on the same surface :bubble formation,growth,burst,and delamination.?6?In order to acquire the inner information of the microstructure,the embossing on the surface is cut by FIB.As a result,we found a cavity under the embossing,which means that the embossing was result from the migraton and collection of helium.The depth of the cavity is about 140 nm at the fluence of 1.35×1018 ions/cm².?7?Compared with the mechanical polishing and chemical polishing specimens,It is found that,the shape and distribution of the helium bubbles were not regular,which means that mechanical polishing method would induce new damages on the surface of tungsten.