Effect of elevated temperature and D presence on defect creation in W

M. Pečovnik^a,*, S. Markelj^a, T. Schwarz-Selinger^b, E. A. Hodille^c

^aJožef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia
^bMax-Planck-Institut für Plasmaphysik, Boltzmannstrasse 2, D-85748 Garching, Germany
^cDepartment of Physics, University of Helsinki, P. O. Box 43, FI-00014, Finland

The effect of D presence on the amount of displacement damage created in W during self-damaging is investigated. We have employed a macroscopic rate equation (MRE) model to analyse the results obtained in experiments where W was sequentially or simultaneously irradiated by high-energy W ions and exposed to low-energy D ions. The model includes fill-level-dependent D atom trapping in different defects and a novel damage creation, annihilation and stabilization model based on spontaneous recombination of Frenkel pairs and on stabilization of traps that are occupied by D atoms.

The MRE model was first applied on the sequential experiment where the samples were first irradiated by high energy W ions at elevated temperatures to create displacement damage and afterwards loaded with deuterium ions. Such an experiment serves as a comparison with other experiments that simulate damage evolution at high temperatures by employing room temperature damaging and subsequent damage annealing [1, 2]. This distinction in damage creation is important to consider, as displacement damage in future tokamaks will be created at elevated temperatures and not at room temperature. By simulating the experimental results, we deduced that three different defect types with several fill levels retain the majority of D in the sample. The defect densities show a linear decrease with rising temperature in the entire 300-800 K damaging temperature range with slightly different slopes for defects one and two. Defect three is independent of temperature.

The new stabilization model was developed to understand the simultaneous experiment, where damaging was done at the same temperatures as in the sequential experiment but with the addition of a simultaneous D ion exposure. To consider the effect of the W ion irradiation, kinetic de-trapping was also included as it was shown [3] that it could play an important role in the dynamics of D diffusion and trapping. A clear effect of the D presence was observed as the defect densities were higher than the values obtained in the sequential case in the region where D was present during the damaging. This effect was reproduced in our simulations by the addition of a lowered defect annihilation probability due to the presence of trapped D in the annihilating defects. The effect of D presence on additional damage created is very large when damaging at 450 K, as a two-fold increase in created defect fraction is observed, while at 800 K almost no increase is observed. This is expected as less and less defects are occupied by D with rising temperatures and therefore stabilization by trapped D becomes less and less efficient.

[1] A. Založnik, S.Markelj, T. Schwarz-Selinger, et al., Phys. Scr. T167, 014031 (2016)
[2] E. Markina, M. Mayer, A. Manhard, et al., J. Nucl. Mat. 463, 329 (2015)
[3] T. Schwarz-Selinger, J. Bauer, S. Elgeti, et al., Nucl. Mater. Energy. 17, 228 (2018)