Synergistic Hydrogen Damaging with Sub-threshold Ion Energy I: Deuterium Supersaturated Surface Layer

In our previous work (Nucl. Fusion 57 016026), a deuterium-supersaturated surface layer (DSSL) has been observed and resolved in plasma-exposed tungsten (W) with ion energy below the threshold for creating permeant lattice damage. In the present work, we will follow the previous observation and propose a new physical model for DSSL formation and experimentally validate it via reproducing a HSSL in hydrogen-irradiated W under conditions predicted by the proposed model. Post-exposure scanning electron microscopy and also the H depth profile in the HSSL measured by 15N NRA show both comparable results as for DSSL, fitting to the prediction of the proposed model. By this, we establish a new physical mechanism for hydrogen-induced distortion in tungsten lattice with sub-threshold ion energy, which applies most likely also to other crystalline materials under hydrogen irradiation. Further TEM characterization on the defects structure will also be shown and discussed followed by an outlook on the future work and the implication of such DSSL on fuel retention in future fusion devices.