L. Gao

Influence of nitrogen pre-implantation on deuterium retention in tungsten

The influence of nitrogen (N) pre-implantation on the deuterium (D) retention in tungsten (W) at different temperatures was investigated. Bulk W samples were exposed to D plasma with a fluence of 1uf0b410 24 D/m 2 with or without nitrogen pre-implantation at 300 K and 500 K, respectively. Nuclear reaction analysis was applied for the determination of N content and D retention in the near surface. Optical microscopy was used to investigate the surface modification by blistering after implantation. It is shown that, the W:N layers formed during the N preimplantation play very different roles on D retention and blistering in the samples at different temperatures. At 500 K, the W:N layer seems to enhance D diffusion into the bulk by suppressing D loss from the surface, which results in a much higher D concentration in the bulk and larger blisters than without N pre-implantation. At 300 K, the effect of this layer is much less pronounced than that at 500 K.

Deuterium retention in tungsten films deposited by magnetron sputtering

Deuterium retention in tungsten films deposited on polycrystalline bulk tungsten substrates was investigated and compared with deuterium retention in W films deposited on silicon and in polycrystalline bulk W alone. The structure of the deposited films was investigated by x-ray diffraction and scanning electron microscopy combined with focused ion beam cutting. D retention after implantation was measured by nuclear reaction analysis and temperature programmed desorption (TPD). The W films deposited on bulk W show a typical columnar epitaxial growth. After D implantation, high densities of blisters with diameters of about a hundred µm were formed. Interestingly, the blisters are located within the W substrate well below the interface of deposited film and substrate. TPD spectroscopy reveals two D2 release peaks at 510 and 700K, indicating at least two different trap energies.

Deuterium supersaturation in low-energy plasma-loaded tungsten surfaces

Fundamental understanding of hydrogen–metal interactions is challenging due to a lack of knowledge on defect production and/or evolution upon hydrogen ingression, especially for metals undergoing hydrogen irradiation with ion energy below the displacement thresholds reported in literature. Here, applying a novel low-energy argon-sputter depth profiling method with significantly improved depth resolution for tungsten (W) surfaces exposed to deuterium (D) plasma at 300 K, we show the existence of a 10 nm thick D-supersaturated surface layer (DSSL) with an unexpectedly high D concentration of ~10 at.% after irradiation with ion energy of 215 eV. Electron back-scatter diffraction reveals that the W lattice within this DSSL is highly distorted, thus strongly blurring the Kikuchi pattern. We explain this strong damage by the synergistic interaction of energetic D ions and solute D atoms with the W lattice. Solute D atoms prevent the recombination of vacancies with interstitial W atoms, which are produced by collisions of energetic D ions with W lattice atoms (Frenkel pairs). This proposed damaging mechanism could also be active on other hydrogen-irradiated metal surfaces. The present work provides deep insight into hydrogen-induced lattice distortion at plasma–metal interfaces and sheds light on its modelling work.

Suppression of hydrogen-induced blistering of tungsten by pre-irradiation at low temperature

Blistering of tungsten (W) surfaces due to deuterium (D) implantation was investigated by a sequence of exposures at two different temperatures—230 and 450xa0K—and by the reversed sequence. A single exposure at 230xa0K leads to a high areal density of small dome-shaped blisters (up to 3xa0µm in diameter) together with much smaller flat-topped structures, while a single 450-K exposure produces large dome-shaped blisters up to 40 µm in diameter without the flat-topped structures. Most of the small dome-shaped blisters from 230xa0K exposure disappeared after annealing at 450xa0K for 17xa0h, but survived and even grew in size if the surface was exposed to D plasma during annealing. Sequential exposure at the two temperatures reveals a non-commutative behaviour: after a first exposure at 450xa0K the second exposure at 230xa0K leads to superposition of the observed blister structures without changing the large blisters from the first exposure. By contrast, a first exposure at 230xa0K almost completely suppresses the formation of large blisters during a second exposure at 450xa0K. Obviously, the presence of the small blisters strongly influences the penetration of D into the W bulk.

Interaction of Deuterium Plasma with Sputter-deposited Tungsten Nitride Films

Magnetron-sputtered tungsten nitride (WNx) films were used as a model system to study the behaviour of re-deposited WNx layers which could form in fusion devices with tungsten (W) wall during nitrogen seeding. The interaction of such WNx layers with deuterium (D) plasmas was investigated in dedicated laboratory experiments. D retention and N removal due to D plasma exposure (D flux: 9.9 × 1019 D m−2 s−1, ion energy 215 eV) at different temperatures were measured with ion beam analysis (IBA). Low-energy argon sputtering followed by IBA was applied to resolve the D distribution in the top-most surface of WNx with significantly improved depth resolution compared with the standard D depth profiling method by nuclear reaction analysis. Experimentally determined thicknesses for the penetration of D in WNx were compared with the penetration depth for D calculated in SDTrimSP simulations. Results show that D is only retained within the ion penetration range for samples exposed at 300 K. In contrast to the 300 K case, D diffuses beyond the implantation depth in a sample exposed at 600 K. However, the D penetration depth is much lower than in pure W at comparable conditions. The total amount of retained D in WNx at 600 K is by 50% lower than for implantation at 300 K with the same D fluence. Nitrogen is removed only within the D ion range.