Properties of deuterium (D) supersaturated surface layers (DSSLs) formed in tungsten (W), such as thickness, internal microstructures, and D retention, are experimentally investigated as a function of the incident ion energy, Ei. W samples were exposed to D plasmas in the PISCES-A linear plasma device in a range of Ei ∼ 45–175 eV, while other plasma exposure parameters were fixed: sample temperature, Ts, ∼423 K, ion flux, Γi, ∼1.2 × 1021 m−2s−1, and fluence, Φi, ∼3.0 × 1024 m−2. High-resolution, cross-sectional, transmission electron microscopy observations confirm that (1) a DSSL forms even at the lowest Ei ∼ 45 eV, (2) the DSSL thickness, ΔtDSSL, is found to decrease with decreasing Ei from ∼11–12 nm at Ei ∼ 175 eV to ∼5–6 nm at ∼45 eV, and to agree with approximately the maximum implantation depth calculated using SDTrimSP, and (3) high-density D nanobubbles with a diameter of ∼1 nm or less exist inside the DSSL, which is deemed to validate a theory-predicted vacancy stabilization process due to trapping of a solute D atom(s). Utilizing a D areal density of ∼4.2 × 1019 m−2 in the first 14 nm from the surface at Ei ∼ 75 eV from nuclear reaction analysis and the measured Ei dependence of ΔtDSSL, our previous laser-induced breakdown spectroscopy data is updated: both dynamic and static D retention increase with decreasing Ei, and the D/W atomic fraction during plasma exposure reaches ∼0.3 at Ei ∼ 45 eV. A possible DSSL formation mechanism is proposed.
The effect of He-induced surface microstructure on D plasma-driven permeation through RAFM steel