(37c) Modeling and Simulation of Surface Morphological Response of Plasma-Facing Tungsten

Commercial fusion energy is one of the grand challenges for engineering in the 21^st century. The growth of damaged surface nanostructure in fusion plasma-facing materials is a major obstacle to achieving commercial fusion energy. Tungsten (W) is the chosen material for plasma-facing components (PFCs), including the exhaust system of the fusion reactor known as the divertor, in the International Thermonuclear Experimental Reactor (ITER), which has to withstand extreme conditions of heat load and particle fluxes. While tungsten’s high melting point, low sputtering yield, and low tritium retention are advantageous for its use as a PFC, experimental studies have demonstrated that severe material degradation of PFC tungsten in a fusion reactor environment can cause significant operational challenges. Specifically, helium (He) implantation above a threshold ion energy of ~35 eV, within the temperature range from 900 K to 2000 K causes the formation of a ‘fuzz’-like fragile nanostructure on the PFC tungsten surface. Previous studies have established that the formation of nanotendril-like structures, which evolve into fuzz, is largely driven by stress-induced surface atomic diffusion, with stress originating from over-pressurized He bubbles resulting from He implantation in the near-surface region of PFC tungsten. 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.

We have conducted systematic computer simulation studies on the surface morphological response of PFC tungsten. Our analysis is based on an atomistically-informed, hierarchically developed continuum-scale surface evolution model that captures the spatiotemporal scales relevant to fuzz formation. The model accounts for atomic diffusion on the PFC surface driven by biaxial compressive stress developed in the near-surface PFC region from over-pressurized He bubbles, as well as a diffusive flux toward the surface of W self-interstitial atoms, and the softening of tungsten’s elastic properties in the near-surface region of the PFC material. It also includes surface adatom fluxes generated through surface vacancy-adatom pair formation upon He implantation, contributing to anisotropic surface nanostructure growth due to different adatom diffusion rates along and across surface terrace step edges. Additionally, surface free energy anisotropy is incorporated into the model and is responsible for driving planar facet formation on surface features. Simulation results for He irradiation of W(111), W(110), and W(100) surfaces show the growth of tetrahedral, striped, and pyramid-shaped surface features, respectively, consistent with experimental observations. We have analyzed the growth kinetics of the damaged surface layer on the PFC tungsten surface, and our predictions are in excellent agreement with the available experimental results on PFC tungsten surface layer thickness evolution. We further examine how He bubble size affects the morphological evolution and pattern formation on the surface of PFC tungsten. We find that an increase in the He bubble size leads to a deceleration in the growth rate of the tungsten nanotendrils while increasing the separation distance between the resulting surface features. Such studies and model developments contribute to a fundamental understanding toward quantitative prediction of the structural and morphological response of plasma-facing materials under conditions representative of a fusion pilot plant.