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The evolution of the helium-related defect in polycrystalline tungsten was studied by Doppler Broadening Positron Annihilation Spectroscopy and Scanning Electron Microscope as functions of annealing temperature and implantation fluence. The experimental results showed that the defect type was not changed at lower annealing temperature, and the decline of S parameter manifested that the lower temperature annealing led the decrease of defect concentration. In addition there existed the same defect type with the increment of the implantation fluence, thus the rise of S parameter implied the increase of the defect concentration.

W/Cu multilayer nanofilms and pure W nanofilms were prepared in pure Ar and He/Ar mixing atmosphere by radio frequency magnetron sputtering method. The defect evolution of the samples was characterized by Doppler broadening positron annihilation spectroscopy (DB-PAS).The results show that plenty of defects can be produced by introducing helium (He) into W/Cu multilayer nanofilms. With the natural storage time increasing, the helium located in the near surface of W/Cu multilayer nanofilm would be released gradually and induce the coalescence of the helium related defects due to the diffusion of the helium and defects. In addition, the pure W nanofilms were irradiated by 30 keV helium ions and 40 keV hydrogen (H) ions in sequence at room temperature. From the DB-PAS analysis, it can be shown that a large number of vacancy-type defects are produced due to the He and/or H irradiation. H ions would be trapped by He related defects and produced He-H-V complexes.

The microstructure of He+-irradiation induced defects in tungsten was studied as functions of annealing temperature by means of scanning electron microscope (SEM) and Doppler broadening positron annihilation spectroscopy (DB-PAS). The results show that the S parameter become higher after helium implanted in tungsten. When the annealing temperature increases to 923 K, plenty of large size of holes and new types of defects could be found in this sample.

Molecular dynamics (MD) simulations are performed on the diffusion and coalescence of helium in tungsten. A new method for determining the effective capture radii (ECRs) and the dissociation energies of helium-related defects is proposed in this work. It is observed that the ECR of an interstitial helium atom trapping helium interstitials (denoted as He–Hen, n = 1–3) decreases with increasing temperature, except for He–He2 at T D3 > D4 at T < 500 K, which can be attributed to the disordered structure of He5. The Arrhenius relation describes the diffusion of Hen well in the temperature range from 300 K to 550 K, whereas the diffusion is not a standard thermally activated process at higher temperatures. Taken together, these results help elucidate the initial stage of helium bubble formation in tungsten as well as the requirements of long-term evolution methods such as KMC or RT models.