The effect of O2 impurity on surface morphology of polycrystalline W during low-energy and high-flux He+ irradiation

Abstract The interaction between the impurities (such as carbon, nitrogen, oxygen) and the plasma-facing materials (PFMs) can profoundly influence the performance and service of the PFMs. In this paper, we investigated the influence of oxygen (O 2 ) impurity in the helium radio frequency (RF) plasma on the surface morphology of polycrystalline tungsten (W) irradiated at the surface temperature of 1450 ± 50 K and the ion energy of 100 eV. The pressure ratio of O 2 to He (R) in RF source varied from 4.0 × 10 −6 to 9.0 × 10 -2 . The total irradiation flux and fluence were ˜1.2 × 10 22 ions·m -2 ·s -1 and ˜1.0 × 10 26 ions·m -2 , respectively. After He + irradiation, the specimen surface morphology was observed by scanning electron microscopy. It was found that with increasing R from 4.0 × 10 −6 to 9.0 × 10 -2 the thickness of nano-fuzz layer at the W surface was thinner and thinner, accompanied by the formation of rod-like structures. The erosion yield increased from 5.2 × 10 -4 to 2.3 × 10 -2 W/ion when R varied from 4.0 × 10 -6 to 9.0 × 10 -2 . The X-ray diffraction analysis shows that tungsten oxides were formed at the near surface of specimens when R exceeded 1.8 × 10 -2 . The erosion yield measurements revealed that in addition to surface physical sputtering process, the chemical erosion process could occur due to the interaction between oxygen-containing species and W at the surface. The results indicated that the presence of O 2 impurity in He plasma can obviously affect the surface microstructure of W. The study suggested that O 2 impurity can effectively reduce the growth of nano-fuzz structures.