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Molecular dynamics simulations reveal sub-surface mechanisms likely involved in the initial formation of nanometre-sized'fuzz'in tungsten exposed to low-energy helium plasmas. Helium clusters grow to over-pressurized bubbles as a result of repeated cycles of helium absorption and Frenkel pair formation. The self-interstitials either reach the surface as isolated adatoms or trap at the bubble periphery before organizing into prismatic< 1 1 1> dislocation loops. Surface roughening occurs as single adatoms migrate to the surface, prismatic loops glide to the surface to form adatom islands, and ultimately as over-pressurized gas bubbles burst.