(84e) Effects of Surface Anisotropy on the Surface Morphological Response of 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 surface nanostructure, which consists of fragile nanoscale-sized crystalline tendrils. This nanotendril formation is driven by stress-induced surface atomic diffusion, with stress originating from the over-pressurized He bubbles formed due to He implantation in the near-surface region of PFC tungsten, and has adverse effects on the mechanical behavior and structural response of PFC tungsten as well as on the reactor performance. Moreover, several He plasma exposure experiments have reported that during the very early stage of the fuzz formation process, different types of nanostructures are observed to appear on the PFC tungsten surface and this difference in the surface morphologies arises due to the difference in the crystallographic orientation of the plasma-exposed surfaces. 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 on the effects of surface anisotropy on morphological evolution and pattern formation on a PFC tungsten surface of specified crystallographic orientation. 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 surface vacancy-adatom pair formation 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. Moreover, the model accounts for the surface free energy anisotropy, which contributes to facet formation on the surface nanostructural features due to the difference in the surface free energy of tungsten planes with different crystallographic orientations. The surface free energy parameterization was obtained by optimally fitting the surface free energy values for different surface crystallographic orientations predicted by atomic-scale simulations based on a reliable interatomic potential; optimal adatom diffusion pathways have been computed consistently, using the same interatomic potential.

Using the model described above, we found that by incorporating surface anisotropy, we were able to predict the pyramid-type features that are observed experimentally in the surface morphology of a W(100) surface exposed to a helium plasma. We have established that the preferential diffusion of the surface W adatoms generated as a result of the surface-vacancy adatom pair formation effect, combined with the surface free energy anisotropy, are responsible for the detailed topography of the PFC tungsten surface; for the W(100) surface, these effects underlie the formation of mounds on the surface and their precise faceted features. Specifically, the adatom diffusion contributes to the formation of a pattern of distinct mounds on the surface while the surface free energy anisotropy facilitates the facet formation on the mounds, which leads to the formation of the experimentally observed pyramid-type nanostructures. In addition to the surface morphology, the surface nanostructure growth kinetics is investigated in detail and the impact of surface anisotropy on such kinetics is explained.