Microscopic mechanism of nucleation and growth of helium bubbles in monovacancy in tungsten: helium regulates the charged states of tungsten atoms

In fusion reactor, tungsten (W) has been selected as a candidate for plasma-facing materials due to its excellent properties. However, W-PFMs suffer from helium (He) bubbles where He atoms are produced during deuterium tritium fusion in fusion reactors. To date, there have been few contributions to uncovering the formation of He bubbles from the perspective of the microscopic electronic structure of He-mediated tungsten. In this work, we develop a tight-binding potential model for the W–He interaction to study He atom aggregation and nucleation in the electronic ground state as well as in different electronic excited states. The most important finding of this paper is that caused by the He atoms in the vacancy, some d-orbital electrons of the W atoms at the inner wall of the vacancy are transferred to the W atoms farther away from the vacancy, leading to the feature of positively charged W ions at the inner wall of the vacancy. As the number of He atoms in the vacancy increases, these W ions become more cationic. Under the repulsion between these adjacent cationic ions, the volume of vacancies increases, and more He atoms tend to gather and nucleate there. At the same time, the enhancement of the electronic excitation can also promote the abovementioned electron transfer between W atoms and further increase the vacancy volume, which increases the self-aggregation of the He atoms in the vacancy. Our results shed new light on understanding He self-aggregation in many different metal materials.