Real-time helium bubble growth in tungsten by in-situ GISAXS

Nuclear fusion, as a carbon-free energy source, is actively pursued by the international community. However, the irradiation by helium (He) of the walls facing the plasma can lead to important damages at the surface and deeper in the bulk, affecting the properties and life span of the materials. To address the effects of He irradiation, we have studied in real time, during He bombardment at 1273 K, the nucleation and growth of He bubbles nearby the surface of tungsten by grazing incidence small angle X-ray scattering (GISAXS). We could disentangle the surface and bubble contributions on the measured GISAXS patterns by adjusting the X-ray incident angle below and above the critical angle of total external reflection. The occurrence of tilted diffuse scattering rods proves the presence of facetted bubbles buried inside the tungsten matrix. The time evolution of the X-ray diffuse scattering has been used to evaluate the growth kinetics. A close comparison with analytical modelling of the growth process suggests a growth kinetics dominated by the migration-coalescence of bubbles during which the bubble motion is hindered by the nucleation of ledges at the bubble facet surface.