Vacancy‐Type Defects in H+6%He Neutral Beam Irradiated WK Alloy Probed by Slow Positron Beam

H+6%He mixed neutral beam with a power density of 8 MV m −2 and fluence of 10 25 m −2 is produced to induce irradiation damage at surface temperatures of 400, 800, and 1300 °C in WK alloy, and at 400 and 800 °C in pure W samples. Doppler‐broadening positron annihilation spectroscopy (DB‐PAS) and X‐ray diffraction (XRD) are used to probe the evolution of vacancy‐type defects in the samples after irradiation. The results show that for the WK alloy, the S‐parameters for the 400, 800, and 1300 °C samples increase sequentially because of the increase in vacancy‐type defect concentrations or volumes. The WK sample at 1300 °C exhibits a wider defect damage layer and more complicated vacancy‐type defects than those in the WK samples at 400 and 800 °C. For pure W, the S‐parameter of the 800 °C samples is larger than that of the 400 °C samples, which is attributed to the aggregation and growth of vacancy‐type defects. Based on a comparison of the WK and pure W samples, the pinning effect of K bubbles in the WK samples has an influence on the transportation of H/He atoms and the migration and aggregation of vacancy‐type defects.