Helium Bubble Coalescence in Plasma-Facing Tungsten

We present a systematic computational study of helium bubble coalescence in plasma-facing component (PFC) tungsten under helium retention conditions relevant to fusion reactor operation. The thermodynamics and kinetics of bubble coalescence are examined over a multidimensional parameter space encompassing the sizes, separation distance, internal pressure, and growth rate of two interacting helium bubbles. Targeted molecular-statics and molecular-dynamics simulations are used to identify the governing interaction energetics and coalescence mechanisms. The interaction between two helium bubbles is found to be well described by an elastic perturbation from a finite-width square-well potential, whose width defines a capture radius that correlates directly with bubble pressure. At short separation distances, the defective tungsten regions surrounding the adjacent bubbles merge, forming a characteristic dumbbell-shaped configuration. When the tungsten ligament separating the two bubbles is reduced to approximately one atomic layer, stress-driven displacement of tungsten atoms creates an open channel that connects the bubbles, enabling helium gas flow between the two bubbles and initiating coalescence. Localized strain in the narrowing ligament promotes Frenkel-pair formation, facilitating channel opening through vacancy creation and tungsten self-interstitial accommodation. Continued helium implantation after coalescence increases bubble pressure, leading to the emission of $\frac{1}{2}⟨111⟩$ and ⟨100⟩ dislocation line segments from the bubble surface. For sufficiently large helium bubbles separated by distances exceeding the capture radius, dislocation emission narrows the interbubble tungsten ligament, thereby accelerating the coalescence process.