Modeling W fuzz growth over polycrystalline W due to He ion irradiations at an elevated temperature

Based on the physical processes, such as the collisions of energetic He ions in a fuzz layer, He+ implantations into W bulk, and surface erosion of W fuzz layer, we have modeled the growth of W fuzz layer over polycrystalline W targets under fusion-relevant He/D ion irradiations. From this modeling, we have found that the W fuzz growth results from He+ implantations into W bulk beneath the fuzz layer, and their subsequent erosion into W bulk. The fast-growing, slow-growing, and equilibrium regions are clearly observed during the growth of fuzz layer. In the equilibrium region, the maximum fuzz layer thickness is formed while the rate of W fuzz growth resulting from He+ implantations into W bulk is quite close to the erosion rate. Our study shows that the W fuzz layer with a large surface-to-volume ratio provides an entire protection against the He+ erosion into W bulk. In this study, we have predicted the maximum fuzz layer thickness at the fusion-relevant He+ dose of ~1030/m2, which is strongly dependent on the free distance of He+ ions in fuzz layer and the surface erosion yield of fuzz layer.