He cluster dynamics in fusion related plasma facing materials

The theoretical model describing spatiotemporal dynamics of He clusters in tungsten, which takes into account He trap generation associated with the growth of He clusters, is presented. This model is characterized by only one fitting dimensionless parameter gamma, which describes the number of new He traps generated per one He atom absorbed in existing traps. Application of this model to the formation of the layer of nano-bubbles underneath of the surface of thick He irradiated sample before surface morphology starts to change gives a very good agreement with all available experimental data: (i) surface modification starts to be visible when He ion fluence exceeds the critical value similar to 2-3 x 10(20) cm (2), which does not depend on the magnitude of He ion flux to the surface; (ii) the thickness of the layer similar to 30 nm; (iii) the characteristic size of nano-bubbles in the layer is similar to 2 nm and remains the same within the whole layer. All these parameters were fitted simultaneously with a reasonable value gamma similar to 10(-3).