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In Tokamak fusion devices, tungsten divertor will be bombarded with low-energy high-flux helium ions, resulting in the formation of fuzzy nanostructures or pinhole-like surface damage on tungsten surface, which degrades the tungsten material properties and affects the steady-state operation of the plasma. It is now generally accepted that helium-induced surface damages are closely related to the formation and growth of helium bubbles under the tungsten surface. After helium irradiation, tungsten will form a high density of small helium bubbles near surface. The coalescence of helium bubbles is one of the major ways for the formation of large helium bubbles. In order to understand the effects of relative position, temperature, He/V and initial spacing of helium bubbles on helium bubble fusion, molecular dynamics method was used to simulate the fusion process of helium bubbles in tungsten. The results show that the coalescence of the helium bubbles is affected by the relative positions of the helium bubbles, the temperature, the helium-to-vacancy ratio (He/V), and the distance between helium bubbles, but the influence mechanism is not the same. Specifically, the relative position of the helium bubbles is the key factor affecting the coalescence of the helium bubbles. When the helium bubbles are arranged along the 〈100〉 direction, they tend to coalesce. In contrast, when they are arranged along the 〈111〉 direction, coalescence is not easy to happen. This is because there is an anisotropic stress field near the helium bubbles. The higher the temperature is, the faster and more sufficient relaxation of the helium bubbles will be obtained, resulting in promoted coalescence. Helium bubbles with higher He/V have higher pressure, so they are more likely to coalesce. When the temperature is 1 500 K, the maximum distance of coalescence is 0.96 nm for two helium bubbles with a radius of 1 nm and a He/V of 3, while the interaction distance between them can reach 1.28 nm or more. This study can promote the understanding of the coalescence mechanism of helium bubbles in tungsten and provide a possible explanation for the formation of large helium bubbles in tungsten. In addition, the results of this study can provide relevant input parameters for large-scale simulations (such as kinetic Monte Carlo, cluster dynamics) to study the long-time evolution of high-density helium bubbles. |
