(388d) Effects of Competing Kinetics and Material Property Anisotropies on Surface Morphological Evolution in Plasma-Facing Tungsten

Tungsten (W) is the chosen plasma-facing component (PFC) material in the International Thermonuclear Experimental Reactor (ITER) due to its superior thermomechanical properties and relatively low sputtering yield. A large body of experimental evidence has established that PFC tungsten suffers severe surface degradation as a result of exposure to high fluxes of helium (He) and extreme heat loads. Specifically, experiments have shown that high particle fluxes of low-energy helium implantation in tungsten within the temperature range from 900 K to 2000 K is responsible for the formation of the so-called “fuzz” 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.

Here, we report results on the surface morphological evolution of PFC tungsten and examine a number of factors that impact such evolution. Our analysis is based on self-consistent dynamical simulations according to an atomistically-informed continuous-domain 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 compressive stress originated from the over-pressurized He bubbles in a thin nanobubble region, which forms in the near-surface region of PFC tungsten as a result of helium implantation, in conjunction with formation of self-interstitial atoms in tungsten that diffuse toward the surface. The model also accounts for the softening of the elastic moduli of PFC tungsten, both thermal softening at high temperature and softening due to He accumulation in tungsten upon implantation, and the elastic moduli are time dependent and evolve until the He content in tungsten reaches its saturation level. We explore the competing kinetics between surface diffusion processes, the rates of which vary with varying surface crystallographic orientation and temperature, and of helium accumulation to a saturation level in the irradiated tungsten, which have significant effects on the onset of fuzz formation. We find that the characteristic time scale for He accumulation may be comparable to the characteristic time scale for stress-driven surface diffusion on surfaces with low activation barriers for diffusion, such as the W(110) surface, at high temperatures that further accelerate the surface diffusion rates; the former time scale is controlled by the He flux in the experiment and the material temperature for low-energy implantation. Such competition is crucial in determining the plasma exposure period required to lead to nanotendril growth.

Furthermore, we explore the effects on the resulting PFC surface morphology of anisotropy in the PFC material properties, including crystal elastic anisotropy, surface free energy anisotropy, and surface diffusional anisotropy, all of which play a role in determining the geometrical aspects of the features appearing in the evolving surface morphology of He-implanted tungsten under realistic irradiation conditions. All of these material property anisotropies are properly incorporated into our PFC surface evolution model and improve its predictive capabilities. The model is further validated by comparisons of its predictions with detailed PFC surface characterization experiments.