Although W fuzz is formed in the divertor region of the fusion reactor, no theory may clearly explain the W fuzz growth mechanism. In this study, we observe the growth process of W fuzz over W crystal under ITER-relevant He ion irradiations. We propose the tensile stress-driven cracking of nano-structured fuzz during the initial growth of W fuzz. We demonstrate that the existence of tensile stress is due to the swelling of He nano-bubbles in the fuzz. After this cracking, the W fuzz breaks away from the planar network and grows over the W surface, where the micro-stress in the W surface layer acts as the driving force.

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Low energy, high flux He exposure of tungsten surface leads to the formation of surface nanostructures and severe morphology changes, which may eventually erode tungsten divertor and threaten the operation of the reactor. In this study, the response of polycrystalline W under low energy He+ irradiation at different temperatures has been investigated in order to analyze the early stage of nanostructure formation. It is found that the interactions of high-density over-pressured He bubbles result in the formation of surface stress layers. The significant effect of temperature on the surface stress leads to differences in the incubation dose of W fuzz growth. The interaction between the planar network and the underlying W matrix is weak under surface stress, and the W fuzz grows on the surface once the stress reaches a threshold. Thereafter, the tensile stress-driven cracking causes the three-dimensional growth of W nanofibers.