Visco-elastic model of the ``fuzz'' growth*

Recent experiments on the irradiation of Tungsten with helium-hydrogen plasma [1] have shown the formation of ``fuzz,'' filled with nano-bubbles, on the front surface of the sample. We present theoretical model describing ``fuzz'' growth. This model is based on plastic deformation of the ``skin'' of the ``fuzz'' fibers caused by newly growing nano-bubble on the tip of the fiber. Newly glowing bubble having high helium pressure inside creates an excessive force on surrounding tungsten ``skin'' of the fiber and forming pressure difference between base and nose of the fiber. As a result, tungsten ``flows'' through the ``skin'' from the base to the nose. This model predicts all main features observed in experiments: t to 1/2 growth of length of the fibers, strong temperature dependence of the growth rate, and the saturation of the growth with ion helium flux to the substrate.\\[4pt] [1] S. Takamura, et al., Plasma Fusion Res., 1, 051 (2006); M. J. Baldwin, and R. P. Doerner, Nucl. Fusion 48, 035001 (2008); R. S. Kajita et al., Nucl. Fusion 49, 095005 (2009)