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

Abstract

• Exposures of nanostructured W coatings and bulk W was performed to high energy D plasmas in GyM. • Formation of nanostructures was observed after exposures. • Nanostructures strongly depend on the structure of W. • Development of blisters was observed on compact coatings. In this work, we discuss the exposures of polycrystalline tungsten (W) and nanostructured W coatings to high energy ( ~ 30 ÷ 300 eV ) medium-flux D ( ~ 10 20 m - 2 s - 1 ) deuterium plasmas of the linear machine GyM at an overall fluence of 10 24 m - 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.