(323c) Helium Bubble Size Effects 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. Based on several experimental observations [1, 2], in our previous studies on surface morphological evolution of PFC tungsten, we considered the average He bubble radius to be in the range of 0.75 nm to 1.25 nm. However, recent experimental findings [3] have revealed a broader size range of He bubbles formed in the near-surface region of PFC tungsten as a result of He implantation with average bubble radii extending up to 3 nm.

Motivated by these recent experimental results, we report here a simulation study on the effects of He bubble size on the surface morphological evolution and pattern formation of PFC tungsten. Our analysis focuses on the W(110) surface orientation and 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 [4].

Using the model described above, we find that varying the average He bubble radius has a direct impact on the kinetics of the surface morphological response of PFC tungsten and the resulting surface topography. Specifically, increasing the He bubble size leads to a decrease in the growth rate of the nanotendrils emanating from the PFC surface, which is interpreted based on the corresponding decrease in the stress level in the tungsten nanobubble region. Moreover, the separation distance between the resulting surface features increases with an increase in the average He bubble size. We analyze this coarsening effect systematically and find that it is a thermally activated process with the average surface feature separation depending on temperature according to an Arrhenius relation. The results of our computational analysis are in good agreement with theoretical predictions based on linear stability analysis of the PFC surface morphological response.

[1] K. Wang, R. P. Doerner, M. J. Baldwin, F. W. Meyer, M. E. Bannister, A. Darbal, R. Stroud, and C. M. Parish, Sci. Rep. 7, 42315 (2017).

[2] M. Thompson, P. Kluth, R. P. Doerner, N. Kirby, and C. Corr, Nucl. Fusion 55, 042001 (2015).

[3] M. Ialovega, E. Bernard, M. Barthe, R. Bisson, A. Campos, M. Cabie, T. Neisus, R. Sakamoto, A. Kreter, C. Grisolia, T. Angot, and C. Martin, Nucl. Fusion 62, 126022 (2022).

[4] C.-S. Chen, D. Dasgupta, A. Weerasinghe, K. D. Hammond, B. D. Wirth, and D. Maroudas, Nucl. Fusion 63, 026033 (2023).