Improving helium bubble models in tungsten: Refined structural and energetic insights

Tungsten, a key plasma-facing material for fusion reactors, forms bubbles under helium ion fluxes, causing compromising reactor stability. The nucleation and growth of bubbles are linked to helium aggregation at nanovoids, but critical atomistic information, such as the bubble structure and energetics, remains poorly understood, which hinders a thorough understanding of bubbling behavior. Here, we conducted a systematic investigation on the structural and energetic properties of helium-vacancy complexes in tungsten using ab initio molecular dynamics and first-principal static calculations. The structure of helium clusters in nanovoids was characterized in detail using liquid structure analysis techniques. Our energetic calculations validate the robustness of the existing physical model whilst also revealing the inherent shortcomings of the model. By adjusting the gap width and revising the void formation energy, our revised model shows better agreement with DFT calculations, especially for larger nanovoids. Based on this revised model, we can conclude that as the size increases, the He/V ratios of thermodynamically stable bubbles range between 1.5 and 3, whereas the critical He/V ratios for trap mutation lie between 5 and 6.5. Furthermore, our assessment of the available empirical potentials for the W-He system highlights the limitations of these potentials. These findings provide critical insights into bubble nucleation and growth, offer essential parameters for mesoscopic-scale simulations and advance the development of new W-He empirical potentials.