(497b) Effects of Surface Crystallographic Orientation on the Surface Morphological Response of Plasma-Facing Tungsten

Tungsten (W) is the chosen material for plasma-facing components (PFCs), including the divertor, in the International Thermonuclear Experimental Reactor (ITER). While the properties of tungsten, such as its high melting point, low sputtering yield, and low tritium retention, are advantageous for its use as PFC, a large body of experimental evidence has established that severe material degradation of PFC tungsten in a fusion reactor environment can lead to significant operational challenges. Specifically, high fluxes of low-energy (> 35 eV) helium (He) 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 as a result of 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. On the other hand, the extreme fusion environment limits significantly the material choices for PFCs. Therefore, it is essential to mitigate this surface damage toward enabling the use of fusion reactors on the power grid where a reactor would be expected to operate continuously for many months, namely, a period much longer than the typical time scale for fuzz formation, which is on the order of hours. Given this predicament, several He plasma exposure experiments have reported that during the very early stage of the fuzz formation process, different types of nanostructure patterns, including patterns with pyramidal, tetrahedral, and stripe-shaped features, are observed to appear on the PFC tungsten surface and this difference in the observed 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 crystallographic orientation on the surface morphological response of PFC tungsten. Our analysis is based on an atomistically-informed, hierarchically developed continuum-scale surface evolution model that 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 difference in the surface free energy of tungsten planes with different crystallographic orientations, which incorporates into the analysis surface free energy anisotropy effects that give rise to planar facet formation on the surface nanostructural features. In our study, atomic-scale computation of optimal adatom diffusion pathways has been based on a reliable interatomic potential. 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 using the same interatomic potential.

Using the model described above, we were able to predict the surface nanostructure patterns that consist of pyramidal, tetrahedral, and stripe-shaped surface features that are observed experimentally in the surface morphology of plasma-facing W(100), W(111), and W(110) surfaces, respectively. We find that adatom diffusion plays a key role in determining the main qualitative features of the surface topography, such as formation of mounds or striped features on the surface, while the surface free energy anisotropy facilitates the faceting of such mounds or striped features, resulting in the full complexity of the experimentally observed surface morphologies. In addition to the surface morphology, the surface nanostructure growth kinetics is investigated in detail and the model predictions are shown to be in good agreement with experimental data for the surface nanostructure layer thickness evolution. The impact of surface crystallographic orientation on such surface growth kinetics is explained.