(84h) 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 PFCs, 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, experimental studies have shown that, under typical operating conditions expected in the ITER divertor, He implantation above a threshold of incident ion energy of approximately 35 eV causes formation of a ‘fuzz’-like fragile nanostructure on the PFC tungsten surface. We have previously established that formation of a nanotendril-like structure, which would ultimately evolve to ‘fuzz’, 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. ‘Fuzz’ 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 has significantly limited 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, we focus here on understanding the very early stage of the fuzz formation process, during which several experiments have reported that tungsten surfaces with different crystallographic orientations exhibit different periodic patterns, including pyramidal, triangular, and stripe-shaped features. 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 atomic diffusion on the PFC surface driven by the biaxial compressive stress due to the over-pressurized helium bubbles in a thin layer, which forms in the near-surface region of PFC tungsten as a result of He implantation, as well as a diffusive flux toward the surface of W self-interstitial atoms. Furthermore, the flux of surface adatoms generated through surface vacancy-adatom pair formation upon He implantation is accounted for; such surface adatom fluxes contribute to the anisotropic growth of surface nanostructural features due to the different adatom diffusion rates along and across step edges of islands on the tungsten surface. Importantly, the model also accounts for the difference in the surface free energy of tungsten planes with different crystallographic orientations, which incorporate into the analysis surface free energy anisotropy effects that give rise to facet formation on the surface nanostructural features. In our study, optimal diffusion adatom pathways have been computed by atomic-scale simulation based on a reliable interatomic potential. Surface free energy parameterization was obtained by optimally fitting the surface free energy values for different surface crystallographic orientations predicted consistently by atomic-scale computations using the same interatomic potential.

Using the model described above, we attempt to obtain a fundamental understanding of the mechanisms of the stripe- and triangle-shaped feature formation that are experimentally observed in the W(110) and W(111) surfaces, respectively, when exposed to a helium plasma. Moreover, significant efforts have been made to extend the predictive capabilities of our surface evolution model to successfully predict the surface orientation dependent nanostructure formation; a detailed characterization of the predicted surface topographies will be discussed as well. 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 for all surface orientations examined. In addition to the detailed surface morphology and specific morphological features for the various surface crystallographic orientations studied, the growth kinetics of the surface nanostructure is investigated in detail and the impact of surface crystallographic orientation on such kinetics is discussed and explained.