Growth of multifractal tungsten nanostructure by He bubble induced directional swelling

Helium (He) plasma irradiation to tungsten (W) leads to morphology changes in nanometer scale by the formation and growth of He bubbles. Initially pinholes and protrusions are formed on the surface followed by the formation of nanostructures. In this study, based on experimental observation, the growth process of the fiberform nanostructures are revisited and the swelling process of the structure is discussed. The novel nanostructures are analyzed from the viewpoint of fractality. It is found that the number of the initially formed pinholes and its sizes have a fractal relation, indicating that the size and number of bubbles formed near the surface have fractality. The fractal dimension is estimated from the brightness variation of a transmission electron microscope (TEM) micrograph and gas adsorption property. Moreover, it is revealed from TEM image analysis that the nanostructure has multifractal feature, probably because of the fractality identified between the number and the size of bubbles near the surface.