Bubble evolution and fuzz formation in plasma-facing materials: from ridges to tendrils

Abstract Fuzz formation in plasma-facing materials remains insufficiently understood. We correlate subsurface helium-bubble evolution with surface morphological transitions in tungsten–rhenium (25 atomic %) exposed to high-flux, 90 eV He plasma at ∼1173 K. Microstructural characterization reveals three sequential surface stages—incubation (ridges), nucleation (bundles of nanocrystalline protrusions, ‘tendril embryos’), and growth (tendrils)—and three corresponding types of bubbles: substrate bubbles beneath wavy or smooth surfaces, embryo bubbles at grain boundaries, and tendril bubbles within the fuzz. Substrate bubbles attain size and density saturation prior to nucleation, indicating a dynamic subsurface bubble layer under continued plasma impact; embryo bubbles undergo anomalous growth that drives surface roughening and protrusion bifurcation, while polycrystalline tendrils emanate from a nanocrystalline near-surface layer, suggesting outward tip-driven growth. We show that elevated local surface temperature and a reduced surface melting point at protrusion and fiber tips generate capillary forces which, together with adatom surface diffusion, sustain fuzz nucleation and growth. The proposed thermodynamic, capillary-driven framework integrates subsurface bubble dynamics, surface diffusion, and plasma–wall interactions into a unified model of fuzz growth that accounts well for the experimental observations.