The effects of impurity gas seeding on the growth of fuzzy tungsten

Abstract Using the linear plasma device NAGDIS-II operating with gas mixtures of helium (He) and impurity (Ne, N 2 , Ar), fibre-form nanostructures known as “fuzz” have been grown on tungsten (W) samples. In this system, fuzzy W was grown over a range of sample temperatures and ion fluences (containing He and impurity ion mixtures), but with operator control over the impurity to helium percentage (from 5 – 10%). The ion energies were held constant at 60 eV throughout the experiments. Close inspection of the surface morphologies showed that a variation in fuzzy structure was produced on W surfaces depending on the impurity species used. Specifically, Ne impurity produced cone-like fuzz structures and Ar and N 2 caused more random tendril growth. Using a constant fluence of He and impurity ion species, the effective fuzzy layer growth rate is reduced for higher impurity atomic masses and higher concentrations. For 10 % N 2 within the He plasma, little fuzz growth was visible on the W surface. Where 7.5 – 10% of Ar was used, fuzz growth was terminated completely. From mass loss measurements the net erosion yields of W surfaces exposed to 95 % He + 5% impurity (Ne or N 2 ) discharges were measured to be lower than bulk W sputtering yields for each impurity species, decreasing sharply with increasing fuzz thickness. Under high ion fluences up to 10 27 m − 2 , steady-state thicknesses of W fuzz were attained due to the combined effect of fuzz growth and sputter erosion caused by the impurity species in the plasma. The measured steady-state thicknesses were found to be consistent with an analytical model developed for the simultaneous growth and erosion of fuzz. SEM imaging showed the presence of larger fuzzy structures, Nano-tendril bundles (NTBs), on surfaces where fuzz grew under the presence of impurity species. The formation of NTBs were characterised in terms of their surface temperature threshold ranges.