He bubble-driven growth of W fuzz during the interaction between H_(2)/He plasmas and W materials

The tungsten(W)material as the divertor of fusion reactors is exposed to low-energy and high-flux He/H isotope irradiation,leading to the growth of fuzz layers.The W fuzz growth does not show any dependence on the plasma irradiation devices used;however,it is strongly dependent on He^(+)fluence and energy,irradiation temperature,and impurity level.When the incident He ions collide inside the dense fuzz layers with the extremely high specific area,their mean free path can be up to 690 nm.Up to now,the He bubble-driven W fuzz growth process is not entirely understood;however,it can be closely related to the surface bursting of He bubbles in the W surface layer and W surface erosion by He^(+)implantation.The formation of He bubbles can be attributed to the solute He diffusion into defects or bubbles,which is strongly affected by the temperature and He^(+)fluence.The W fuzz grows over the W surface,where the micro-stress caused by He bubbles in the W surface layer acts as the driving force.The W fuzz layer inhibits He^(+)implantation into W bulk,and provides an entire protection against the He^(+)erosion into W bulk beneath the fuzz layer.In this review article,the current status of experiment and theory are presented,and some of the remaining issues are discussed.