Properties of nitrogen-implanted beryllium and its interaction with energetic deuterium

Abstract

Recent successful experiments with nitrogen as a seeding gas in fusion plasma devices, together with the decision to use beryllium as an armor material in the international fusion experiment ITER, have triggered interest in interactions of energetic N ions with Be and the influence of possible compound formation on parameters relevant to reactor operation and safety. Laboratory experiments are performed to investigate the properties of the ‘mixed material’ formed upon bombardment of bulk Be with energetic (keV) nitrogen ions. The formation of beryllium nitride within the implantation zone of a few nanometres is observed by x-ray photoelectron spectroscopy and Rutherford backscattering spectrometry. Upon implantation of N at 1.5 keV per atom, saturation of the Be surface with N occurs at a fluence of 2 × 1018 N cm−2 and a retained areal density of approximately 4 × 1016 N cm−2. The nitride undergoes an ordering process upon annealing, but does not decompose at temperatures up to 1000 K. The influence of such nitride layers on the retention and release of D implanted with different fluences is investigated by nuclear reaction analysis and temperature-programmed desorption. The nitride layer does not act as a diffusion barrier for out-diffusing hydrogen isotopes. A partial sputter yield of 0.013 N/D as a lower limit is measured upon bombardment of the nitride layer with D at 2 keV. These erosion measurements are influenced by the strong tendency of the nitride to oxidize.