Helium bubble size effects on the surface morphological response of plasma-facing tungsten

Abstract

We report a simulation study on the effects of helium (He) bubble size on the morphological evolution and pattern formation on the surface of tungsten used as a plasma-facing component (PFC) in nuclear fusion devices. We have carried out a systematic investigation based on self-consistent dynamical simulations of surface morphological evolution according to an atomistically-informed, 3D continuum-scale model that captures well the relevant length and time scales of surface nanostructure formation in PFC tungsten. The model accounts for PFC surface diffusion, driven by the biaxial compressive stress originating from the over-pressurized He bubbles in the near-surface region of PFC tungsten as a result of He plasma exposure, combined with the formation of self-interstitial atoms in tungsten that diffuse toward the PFC surface and the flux of surface adatoms generated as a result of surface vacancy-adatom pair formation upon He implantation; this transport of surface adatoms 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. Our detailed analysis reveals that varying the average He bubble size plays an important role in the PFC surface growth kinetics as well as the resulting surface topography. Specifically, we find that an increase in the He bubble size leads to a deceleration in the growth rate of the tungsten nanotendrils that emanate from the PFC surface. We also find that the separation distance between the resulting surface features increases with increasing He bubble size, as well as over time. This coarsening effect is a thermally activated process resulting in an accurate description of the temperature dependence of the average surface feature separation by an Arrhenius relation.

Focus on plasma-facing materials in nuclear fusion reactors

“…While the materials science community has a long history of conducting fundamental and applied research on plasma-materials interactions [6][7][8], this collection focuses on the same type of physics with respect to a nuclear fusion environment. The topics covered are unique to 'Plasma-Facing Materials in Nuclear Fusion Reactors' such as: deuterium and tritium retention in PFCs [9][10][11][12][13]; fundamental processes at the plasma-surface interface [10,[14][15][16][17][18][19][20]; evolution of structure and properties under fusion-reactor-relevant heat loads [21]; material degradation under ion exposure [15,16,19]; material degradation under neutron irradiation [9,21]; material erosion, migration, and deposition [14,15,18,20,22]; plasma fueling [12]; and diagnostics for plasmamaterials interactions [23]. Although the details of the underlying mechanisms that govern the above phenomena remain largely unresolved, the results presented here will drive the emergence of engineering solutions to the amelioration of plasma-facing materials degradation.…”