Stabilization of defects by the presence of hydrogen in tungsten: simultaneous W-ion damaging and D-atom exposure

Abstract

The possible mutual influence and synergistic effect between defect production and the presence of hydrogen isotopes in the crystal lattice of tungsten is studied. For this purpose, we perform modelling of experimental data where polycrystalline tungsten samples were in one case sequentially irradiated by 10.8 MeV tungsten ions followed by low energy deuterium exposure and in the other case simultaneously irradiated by tungsten ions while exposed to deuterium atoms. Modeling of the measured deuterium depth profiles and thermal-desorption spectra for different irradiation temperatures is performed by the MHIMS (migration of hydrogen isotopes in materials) code. A model of trap creation due to tungsten ion irradiation during the deuterium atom exposures is implemented. In both experimental series, the deuterium desorption peaks corresponding to defects induced by tungsten irradiation are described by the same two de-trapping energies of 1.83 eV and 2.10 eV. The experiments give and unambiguous proof that the presence of deuterium increases the overall trap density. The modelling reveals that the two trap concentrations are affected differently by the temperature and the presence of deuterium: The concentration of the low energy trap is significantly higher in the case of simultaneous exposure as compared to sequential exposure especially at high temperature (2.2 times higher at 1000 K). The concentration of the high energy trap is only weakly affected by the presence of hydrogen.

Displacement damage stabilization by hydrogen presence under simultaneous W ion damage and D ion exposure

“…We can conclude from this that no effect was observed in the atom case at 450 K due to the shallow penetration depth (Fig. 14b) which was also the conclusion from modeling the atom data in [7]. Therefore, high concentration of mobile atoms increases the speed of filling of the available traps and consequently increases stabilization of the defects.…”

“…where R is the reflection coefficient, Γ is the ion flux, d is the average ion range, D is the diffusion constant and δ W is the tungsten volume density. [7]. At 450 K c S is two orders of magnitude smaller than for the ion case.…”

“…Recently, we performed first experiments where W samples were simultaneously damaged by high energy W ions and exposed to D atoms [6]. By comparing simultaneous W ion irradiation and D atom exposure to sequential W ion irradiation and D atom exposure, it was shown that the presence of deuterium has an influence on fuel retention and consequently on defect stabilization [6,7]. In that case the sub-eV energy D atoms, need to overcome a large energy barrier to migrate from the surface into the bulk [8,9] to be able to further diffuse in the bulk and populate defects produced by the W irradiation.…”

“…As a consequence, in the ion exposure case, the vacancies can be occupied because of lower exposure temperature and hence we should be able to see the effects of the presence of D on vacancy production. In addition, due to the direct penetration of ions into the bulk, processes at the surface are not the rate limiting ones as is the case for atom exposure with sub eV energy [7]. Therefore, the concentration of mobile hydrogen atoms that diffuse through interstitial positions in the tungsten lattice is different for the two cases as a function of temperature.…”

Influence of grain size on deuterium transport and retention in self-damaged tungsten

“…This is in good agreement from past results reported by Hodille et al [13] but still lower than the results reported by Frauenfelder. This can be explained by the fact that Frauenfelder conducted his experiment at a much higher temperature of about a 1000 K where the surface D coverage was much lower than in our case where the experiment was done at 600 K. Some DFT calculations [36] and also our own experimental results [37] indicate that the chemisorption energy of the surface depends on the D coverage, increasing with falling coverage. From our recent results [37] we see that indeed at higher exposure temperatures E ef f bulk + E ch = 1.9 eV which is much closer to the value reported by Frauenfelder.…”