Effects of surface anisotropy on the surface morphological response of plasma-facing tungsten

Surface free energy has been hypothesized as the cause behind the experimental evidence (Parish et al., 2014) of a strong correlation between crystallographic orientation and surface morphology changes in helium ion-irradiated tungsten at the early stages of ‘fuzz’ growth in fusion plasma-facing tungsten. Here, this hypothesis is tested through self-consistent dynamical simulation based on an atomistically informed model of surface evolution in plasma-irradiated tungsten. Low-energy helium plasma irradiation of a tungsten surface, even when the ion energy is below the sputtering threshold, generates surface vacancy-adatom pairs in addition to adatoms from the flux of self-interstitial atoms to the surface resulting from the growth of high-pressure helium bubbles. Along with the stress-driven surface morphological instability as the primary driving force, preferential diffusion of these surface adatoms modulated by the associated Ehrlich–Schwoebel (ES) barriers, combined with the surface free energy anisotropy drive the anisotropic surface nanostructure growth. Simulation results for helium irradiation on W(100) show growth of pyramidal surface features, akin to experimental observations. This work confirms the hypothesis on the effect of tungsten surface crystallographic orientation on helium-ion-induced surface morphology changes, where surface mound formation is the outcome of a stress-driven surface morphological instability, while the anisotropic growth of the mounds is controlled by the ES-barrier-modulated anisotropic surface adatom diffusion and faceting of the mounds is controlled by the surface free energy anisotropy.