Atomic physics at atomic pressures: Line shifts of inner-shell transitions at electron density 10²⁵ cm^-3

The frontier of atomic physics lies in the dense plasma regime. At electron densities N_e ≈ 10²⁵ cm^-3 characteristic of stellar interiors and inertial fusion plasmas, atomic transition energies shift due to electrostatic interactions between the radiator and nearby charged particles. Though predicted by theory, experimentally isolating such line shifts and identifying the dependence on the thermodynamic state constitutes a step change in understanding and would enable the benchmarking of dense plasma diagnostics. In plastic shells hosting a Cr tracer layer imploded at the OMEGA-60 laser facility, 1s−2p absorption lines from L-shell Cr ions present a decreasing red-shift as the stagnated shell releases from peak compression N_e ≈ 10²⁵ cm^-3. These measurements are at nearly an order of magnitude greater density than previous studies where lineshape models have not been benchmarked. Constraints of Doppler and satellite-driven shifts and their uncertainties are fully consistent with analysis of the spectra, being derived solely from a Bayesian forward model of the implosion geometry, plasma conditions, and opacity. Temperature and density are primarily constrained by the ionization balance encoded in the relative depths of the absorption lines. The residual shift after accounting for the Doppler and satellite contributions is consistent with a sophisticated plasma polarization shift model evaluated at the inferred conditions. More broadly, measurements of this type will be critical to guide development of modern atomic theory into the dense plasma regime.