  We report first direct experimental evidence of interspecies ion separation
in direct-drive ICF experiments performed at the OMEGA laser facility via
spectrally, temporally and spatially resolved imaging x-ray-spectroscopy data
[S. C. Hsu et al., EPL 115, 65001 (2016)]. These experiments were designed
based on the expectation that interspecies ion thermo-diffusion would be
strongest for species with large mass and charge difference. The targets were
spherical plastic shells filled with D2 and a trace amount of Ar (0.1% or 1% by
atom). Ar K-shell spectral features were observed primarily between the time of
first-shock convergence and slightly before neutron bang time, using a time-
and space-integrated spectrometer, a streaked crystal spectrometer, and two
gated multi-monochromatic x-ray imagers fielded along quasi-orthogonal lines of
sight. Detailed spectroscopic analyses of spatially resolved Ar K-shell lines
reveal deviation from the initial 1% Ar gas fill and show both Ar-concentration
enhancement and depletion at different times and radial positions of the
implosion. The experimental results are interpreted with radiation-hydrodynamic
simulations that include recently implemented, first-principles models of
interspecies ion diffusion. The experimentally inferred Ar-atom-fraction
profiles agree reasonably with calculated profiles associated with the incoming
and rebounding first shock.
