Atomic-scale modeling toward enabling models of surface nanostructure formation in plasma-facing materials

Abstract

Designing plasma-facing components (PFCs) that can tolerate the extreme heat and particle flux exposure conditions inside a fusion reactor core is one of the major obstacles toward the practical realization of nuclear fusion. In this article, atomic-scale simulation findings are reviewed that provide a fundamental understanding of the dynamical response of tungsten, an important PFC material, to reactor-relevant plasma exposure conditions leading to helium implantation. This hierarchy of simulations include molecular-statics computations to establish helium-surface interaction energetics and the origin of helium segregation on tungsten surfaces, targeted molecular-dynamics (MD) simulations of near-surface helium cluster reactions, and large-scale MD simulations of implanted helium evolution in plasma-exposed tungsten conducted in leadership-scale computing facilities. The simulations reveal that cluster-surface elastic interactions induce drift fluxes of small mobile helium clusters in the tungsten toward the plasma-exposed tungsten surface, which facilitate helium segregation on the surface and activate cluster reactions, most importantly trap mutation, which generates a flux of self-interstitial tungsten atoms to the surface. Such near-surface cluster dynamics has significant effects on PFC surface morphological evolution, near-surface defect structure formation, especially the nucleation and growth of helium nanobubbles, and the amount of helium retained in the PFC material upon plasma exposure. The above mechanistic understanding enables the development of atomistically-informed coarse-grained models of surface nanostructure formation in PFC materials, a crucial step toward predicting PFC surface degradation and improving PFC operating lifetime and reactor performance.