(84f) Helium Bubble Coalescence in Plasma-Facing Tungsten

Predicting surface nanostructure formation on tungsten (W) under plasma exposure conditions is of utmost importance when tungsten is used as a plasma-facing component (PFC) in nuclear fusion devices. Subsurface helium (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 as well as in refueling/recycling of fuel. However, in spite of its importance to the dynamical response of the PFC material, a fundamental understanding of the He bubble coalescence mechanism in PFC tungsten remains incomplete. To this end, here, we report the results of a systematic study of He bubble coalescence in PFC tungsten based on atomic-scale modeling and simulations. Our PFC tungsten models consist of He nanobubbles embedded in the tungsten matrix with bubble sizes and He density relevant to the conditions of plasma exposure in nuclear fusion devices. 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 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 through a stress-driven transport process. 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 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/segments 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 advance our fundamental understanding of helium bubble coalescence in PFC tungsten and contribute to an atomistically derived database for further improving the predictive capabilities of coarse-grained models of near-surface structural evolution and surface morphological evolution of PFC tungsten under fusion reactor operating conditions.