Correlating defect microstructures and hydrogen retention in supersaturated layer of tungsten surface

Fuel retention in tungsten (W) as a plasma-facing material, especially of the radioactive hydrogen isotope (HI) tritium, presents severe concerns for operation cost and safety of future fusion devices. In tungsten with very low intrinsic H solubility, HI retention is dominated by trapping at irradiation-induced defects. In our previous work, strong lattice distortion was observed in W surfaces after deuterium (D) plasma exposure with kinetic ion energies significantly below the thresholds for production of stable Frenkel pairs, which caused formation of a D-supersaturated surface layer(D-SSL) containing ~10 at.% of retained D. We recently proposed and experimentally verified a physical model for the SSL production by HI plasmas at sub-threshold ion energy based on hydrogen atom-ion synergy effects. However, the connection between the observed defect microstructures and the unexpectedly high concentrations of retained HIs in the SSL has remained unestablished. In the present work, we exposed W samples to HI plasma and characterized them with transmission electron microscopy (TEM) to determine the defect microstructures in the HI-SSL. High quality TEM thin foil specimens were preparedby adopting a back-thinning electropolishing approach. In planar view, contrast images in kinematical two-beam bright-field conditions confirmed the formation of “black-spot” clusters and their raft structures after a 1×1024 m-2 exposure to D ions of215 eV at 300 K. The average defect size and number density measured 4-5 nm and 1022 m-3, respectively. Since the same SSL defect microstructure forms under H plasma with doubled ion energy as for D plasma exposure, both bulk and foil Wsamples were simultaneously exposed to a series of H ion fluences to track the evolution of the defect microstructure in the HSSL. In order to clarify the correlation between the microstructure and the concentration of retained H, hydrogen depthprofiles were acquired with 1H-15N nuclear reaction analysis on the bulk W samples. The analysis of the defect nature and cluster geometry in the HSSL under different H ion fluences is ongoing. The present work is expected to provide an indepth understanding of HI retention in W materials upon injection of energetic projectiles (ions, charge-exchange neutrals, neutrons) in future fusion devices.