(332c) Effect of Surface Vacancy-Adatom Pair Formation on Surface Morphological Response in Plasma-Facing Tungsten

Tungsten (W) is the chosen plasma-facing component (PFC) material for the divertor in the International Thermonuclear Experimental Reactor (ITER) due to its superior properties, including exceptional thermomechanical properties and low sputtering yield. Numerous experimental studies have established that PFC tungsten suffers severe surface degradation as a result of exposure to high fluxes of helium (He) and extremely high heat loads. Specifically, high fluxes of low-energy helium ions implanted in tungsten within the temperature range from 900 K to 2000 K are responsible for the formation of a “fuzz”-like nanostructure, which consists of fragile nanometer-sized tendrils. The formation of such surface nanostructure has adverse effects on the mechanical behavior and structural response of PFC tungsten as well as on the reactor performance. Moreover, several experimental observations of PFC tungsten surface morphology also have revealed that, during He plasma exposure, aligned stripe patterns may form on the plasma-exposed surface depending on the surface crystallographic orientation, surface temperature, and He plasma exposure conditions. Understanding how such surface patterns form has significant implications for improving the structural and morphological response of PFC materials.

Toward this end, we report here a simulation study of the effect of He-irradiation-induced surface vacancy-adatom pair formation on the surface morphological evolution of PFC tungsten and examine a number of factors that impact such evolution. This surface Frenkel pair equivalent formation has been observed in large-scale molecular-dynamics (MD) simulations of PFC tungsten surface and near-surface evolution upon He implantation at incident energies of He atoms that are not sufficiently high to cause sputtering. Our analysis is based on self-consistent dynamical simulations according to an atomistically-informed, continuum-scale surface evolution model that has been developed following a hierarchical multiscale modeling strategy and can access the spatiotemporal scales of relevance to fuzz formation. The model accounts for PFC surface diffusion driven by the biaxial compressive stress originating from the over-pressurized helium bubbles in a thin nanobubble layer, which forms in the near-surface region of PFC tungsten as a result of He implantation, in conjunction with formation of self-interstitial atoms in tungsten that diffuse toward the surface. The model also accounts for the flux of surface adatoms generated as a result of the surface vacancy-adatom pair formation effect upon He implantation, which contributes to the anisotropic growth of surface nanostructural features due to the different rates of adatom diffusion along and across step edges of islands on the tungsten surface. Using state-of-the-art interatomic potentials, we have computed optimal diffusion pathways along and across island step edges on the W(110) surface and established Ehrlich-Schwoebel (ES) barriers in adatom diffusion across a step edge. Such step-edge diffusion of surface adatoms and ES barriers have been incorporated into our PFC surface evolution model, contributing to surface atomic fluxes, namely, step and terrace diffusive currents, which introduce preferred directions in surface mass transport and give rise to surface features that align to form nanoridge stripe patterns on the PFC surface. We establish that these anisotropic diffusive currents accelerate both nanotendril growth on the PFC surface and the onset of surface nanostructure pattern formation. We also explore systematically the dependence of the PFC surface morphological response on the He ion incident flux and surface temperature, characterize in detail the resulting surface topographies, and compare the predicted surface morphologies with experimental observations. Our simulation predictions for the emerging surface nanostructure patterns under certain plasma exposure conditions are consistent with experimental findings in the literature.