Exposures of bulk W and nanostructured W coatings to medium flux D plasmas

In this work, we discuss the exposures of polycrystalline tungsten (W) and nanostructured W coatings to high energy (~30÷300eV) medium-flux D (~1020m-2s-1) deuterium plasmas of the linear machine GyM at an overall fluence of 1024m-2. The energy and particle flux are tailored to that of charge-exchange neutrals at the vessel main chamber of present-day tokamaks and expected in ITER. Scanning Electron Microscopy measurements show the formation of surface nanostructures that are found to strongly depend on the W morphology and crystallinity, as well as on the energy of the ion species. The obtained nanostructures are few nanometres deep and similar to the one obtained after exposure of bulk W to divertor-like conditions. Dedicated annealing experiments are performed on selected samples, showing that the nanostructures developed on W coatings are thermally stable up to their recrystallisation temperature. The development of micrometric-sized blisters is also observed and found to strongly depend on the features of the W coatings, the substrate and the applied bias voltage.