Surface damage in tungsten induced by high heat flux helium irradiation at high temperatures across melting point

Understanding the behavior of tungsten (W) surface damage under the synergistic effects of high heat flux (HHF) loading and helium (He) irradiation is essential for predicting material performance during off-normal operations in ITER. In this study, surface modifications occurring at high temperatures (>2200 K) up to the melting point were investigated by conducting experiments involving two campaigns of vertical displacement events like HHF He neutral beam pulse irradiation on polycrystalline W samples at the test facility Garching LArge DIvertor Sample. As the surface temperature of W increased due to irradiation (2253-3683 K), pinholes appeared on the surface, showing a trend of increasing size and decreasing number density, indicating severe lattice damage. Accordingly, we proposed a model for pinhole growth under high-temperature He irradiation based on thermal activation diffusion of He. The calculated activation energy for He diffusion in this process was found to be 0.51 eV, which is considerably higher than the results obtained from previous simulations (0.021-0.157 eV) (Zhou et al 2010 Nucl. Fusion 50 115010; Becquart and Domain 2006 Phys. Rev. Lett. 97 1-4; Shu et al 2013 Nucl. Instrum. Methods Phys. Res. B 303 84-6; Fu et al 2021 J. Nucl. Mater. 543 152599). This suggests that extensive defects in the matrix have a significant impact on the diffusion of He in high-temperature environments, which is distinct from diffusion behavior at lower temperatures. However, as the surface temperature further increased beyond the melting point, the melting and re-solidification process nearly completely repaired almost all defects induced by He ion irradiation. The re-solidified grains were characterized by being intact, damage-free, and having lower residual stress. This study establishes a foundation for the quantitative analysis of helium migration mechanisms under high-temperature helium irradiation, which lays the foundation for understanding material structural damage behavior under off-normal operations for ITER.