Deuterium supersaturated surface layer in tungsten: ion energy dependence

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, E-i. W samples were exposed to D plasmas in the PISCES-A linear plasma device in a range of E-i similar to 45-175 eV, while other plasma exposure parameters were fixed: sample temperature, T-s, similar to 423 K, ion flux, Gamma(i), similar to 1.2 x 10(21) m(-2)s(-1), and fluence, Phi(i), similar to 3.0 x 10(24) m(-2). High-resolution, cross-sectional, transmission electron microscopy observations confirm that (1) a DSSL forms even at the lowest E-i similar to 45 eV, (2) the DSSL thickness, Delta t(DSSL), is found to decrease with decreasing E-i from similar to 11-12 nm at E-i similar to 175 eV to similar to 5-6 nm at similar to 45 eV, and to agree with approximately the maximum implantation depth calculated using SDTrimSP, and (3) high-density D nanobubbles with a diameter of similar to 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 similar to 4.2 x 10(19) m(-2) in the first 14 nm from the surface at E-i similar to 75 eV from nuclear reaction analysis and the measured E-i dependence of Delta t(DSSL), our previous laser-induced breakdown spectroscopy data is updated: both dynamic and static D retention increase with decreasing E-i, and the D/W atomic fraction during plasma exposure reaches similar to 0.3 at E-i similar to 45 eV. A possible DSSL formation mechanism is proposed.