Panorama of “fuzz” growth on tungsten surface under He irradiation

Abstract Nanostructure formation on tungsten (W) surfaces under helium (He) irradiation is a unique and interesting phenomenon that directly impacts the performance and lifetime of W-based materials as plasma-facing materials (PFMs) in nuclear fusion reactors. The evolution of He bubbles beneath the W surface and its effect on surface morphology are essential for understanding the deterioration of the surface properties of PFMs. In this work, the morphology of the W surface under different irradiation conditions was investigated using molecular dynamics in order to extract essential factors that determine the nanostructure of the W surface. The design concerned with investigating the W surface morphology encompassed: (1) the morphology of the W-free surface without He bubbles vs. recoil energy; (2) the morphology of the W surface in the presence of He bubbles vs. recoil energy; and (3) sputtering vs. surface roughness in the absence of He bubbles. A mass of adatoms generated primarily by loop punching of sub-surface He bubbles slightly roughens the surface. Bubble bursting further heightens surface protrusions. In this case, the fiber-like structure of the surface is attributable to two processes involved in He bubble evolution: He diffusion plus aggregation and bubble expansion plus bursting. A high temperature window expedites He diffusion and aggregation beneath the protruded surface. Once the He bubbles burst regardless of spontaneous bursting (when the bubble pressure is beyond the critical value), or bursting due to external irradiation, the protrusions will be further heightened, thus serving as precursors of the fiber-like structure. Continuous aggregation, followed by bursting, should constitute the recursive process of protrusion upgrowth. The panoramic view of the fiber-like structure growth described here is expected to provide a clue for designing tungsten-based materials with high radiation-resistance performance.