(497c) Helium Bubble Dynamical Phenomena in Plasma-Facing Tungsten

The performance and durability of plasma-facing components (PFCs) in nuclear fusion reactors are dependent on the ability of the materials to endure extreme conditions, such as high-energy neutron irradiation and low-energy helium (He) implantation. Helium implantation in tungsten, the leading candidate for PFC materials, leads to formation of He bubbles below the tungsten surface under a prototypical fusion reactor environment. Subsurface He transport and He bubble dynamical processes, such as bubble coalescence and bubble bursting, play crucial roles in the near-surface structural evolution and surface morphological response of the PFC material. In this work, we present the findings of a systematic study on He bubble coalescence in PFC tungsten, based on atomic-scale modeling and simulations, toward a fundamental understanding of the mechanism of He bubble coalescence. Based on molecular-statics (MS) and molecular-dynamics (MD) simulations, we explore the governing thermodynamics of bubble coalescence over a multi-dimensional parameter space, with parameters that include bubble size, bubble separation distance, bubble pressure, bubble growth rate, and bubble location (near the PFC surface or in the bulk material), and design targeted dynamical simulations to study the underlying kinetics and characterize the governing coalescence mechanisms.

We find that the interaction energetics between two He bubbles in tungsten (W) can be described as an elastic interaction perturbation to a finite-width square-well potential. The width of the square-well potential has a direct correlation to the bubble pressure and corresponds to the capture radius of the two-bubble system, beyond which the bubbles interact strongly to facilitate their coalescence. In general, smaller bubbles tend to be captured by the larger bubbles due to the higher bubble pressure (He/Vacancy ratio) in the smaller bubbles. When the two bubbles are sufficiently close to each other, the defective W regions around the spherical bubble surfaces merge at the narrow W gap that separates the two He bubbles forming a defective region with a characteristic “peanut” or “dumbbell” shape. Furthermore, we find that when the W gap between the two He bubbles becomes sufficiently narrow (about one W atomic layer), the W atoms in that narrow layer are pushed away from each other to create a channel between the two bubbles, triggering the coalescence mechanism that allows He atoms to migrate/flow between the two bubbles. As the gap narrows, the local strain in the tungsten matrix drives the formation of Frenkel pairs to initiate the channel opening through formation of W vacancies with the corresponding W self-interstitial atoms occupying sites near the channel. Moreover, we find that continuing He implantation into the bubbles increases the pressure of the post-coalescence bubble configuration, which causes emissions of ½<111> and <100> dislocation loops from the bubble surface. When the two He bubbles are sufficiently large and far apart from each other (at a distance larger than the capture radius), dislocation emissions between the two bubbles cause a narrowing of the gap between them, thus accelerating the bubble coalescence mechanism.

These analyses are part of an ongoing effort to develop hierarchical multiscale models that describe the near-surface structural evolution and surface morphological evolution of PFC tungsten under fusion reactor operating conditions. As an example, this presentation will discuss a key component of the hierarchical multiscale framework: the calculation of the bubble-matrix (He-W) interfacial free energy per unit area, which is an essential material property for predicting PFC surface evolution.