Pure tungsten (W) is a primary plasm-facing material (PFM) candidate because of its superior properties, but it still has some drawbacks. In order to solve these problems, various methods have been used to improve the performances of tungsten-based materials. Potassium (K) doping, as one of the typical dispersion-strengthening methods for W materials, improves low temperature brittleness, reduces the ductile-brittle transition temperature, and suppresses the recrystallization. Meanwhile, it also improves the thermal shock resistance and fracture toughness of the material by introducing nano-sized K bubbles. However, this method brings a large number of defects inevitably. In fact, the K bubbles and the dislocations which are pinned by these K bubbles can affect the morphology and evolution of hydrogen (H) and helium (He) atoms in the alloys. As a very sensitive method to detect vacancy-type defects in materials, positron annihilation spectroscopy is used to study the irradiation damage caused by H and He atoms in this paper. The calculation of positron lifetime shows that positrons are more sensitive to the vacancy-type defects. Bounding of H and He with vacancies reduces the positron lifetime because of the increase of valence electron density. Combining the calculated results with the positron annihilation lifetime spectrum (PLAS) results shows that the W-K alloy is easier to promote the H atoms to release. Besides, it also more likely to form larger He bubbles which can be estimated by positron lifetime values. The result is also confirmed by the measurements from the scanning electron microscope (SEM) and slow positron Doppler broadening spectroscopy (DBS). The defects in the W-K alloy such as K bubbles and their pinned dislocations can act as diffusion channels to promote the H atoms to release, which gives rise to a smoother surface under the pure H irradiation. Meanwhile, under the condition of the H+6%He irradiation, the K bubbles and their pinned dislocations in the W-K alloy become the capture center of He atoms, promote their nucleation and grow into larger He bubbles. Moreover, under the action of stress and temperature gradient, some of the He bubbles migrate to the surface and release, this process is conducive to the recovery of related defects and the reduction of radiation damage.

Tungsten alloy is known as a promising plasma-facing material (PFM) in IETR because of high strength, high-temperature stability, low sputtering erosion, low tritium retention, etc. However, tungsten has some disadvantages, such as high ductile-brittle transition temperature, low temperature brittleness, and radiation embrittlement. For the severe environment of PFM, various techniques have been adopted to improve W-based materials, among which the potassium doping is an effective bubble strengthening method, it can bring in nano-sized K bubbles, and enhance the toughness and strength, thermal shock performance, irradiation resistance of the materials. The K bubbles, which can pin grain boundaries (GBs) and dislocations, are the most characteristic defects in W-K alloy and have been widely reported. However, little attention is paid to other defects such as vacancies, GBs and dislocations. In fact, high-density dislocations exist in W-K alloy and vacancies play a considerable role in forming the K bubbles. Thus, positron annihilation technique (including the positron annihilation lifetime spectrum and slow positron beam Doppler broadening spectrum), which is a useful technique for detecting defects in solids, can be used to study these defects in W-K alloy samples. The positron lifetime of potassium bulk is about 376 ps and the positron lifetime of tungsten bulk is about 110 ps. But by simulating positron lifetime of defects in tungsten, it is found that potassium atoms in tungsten lattice do not exhibit the characteristic positron lifetime. Therefore, potassium is not considered in analyzing positron annihilation lifetime spectra of W-K alloy samples with different potassium content (46, 82, 122, 144 ppm). Three-state capture model is established in this paper, the dislocation density and vacancy cluster concentration of these samples are obtained. From the results, the dislocation densities in all samples are very high, but vacancy cluster concentrations are relatively low, and the vacancy cluster concentration in the sample with 82 ppm potassium content is the lowest in all samples. The behavior of potassium atoms in the sintering process is also discussed. Then the slow positron beam Doppler broadening spectra of W-K alloy samples and pure tungsten samples are measured and the obtained data are fitted by VEPFIT. It is noted that the defects in W-K alloy samples are much more than those in pure tungsten sample, and are distributed homogeneously with depth. The positron diffusion length information simultaneously obtained is compared with these values computed by dislocation density and vacancy cluster concentration, confirming the positrons trapped by potassium bubbles and grain boundaries are existent.

H+6%He mixed neutral beam with a power density of 8 MV m−2 and fluence of 1025 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.