Extreme Atomic Physics in Plasma Mixtures at Gbar Pressure

Observable changes in atomic spectra at very high pressures offer a test of our fundamental understanding of matter in extreme conditions -- and by extension the spectroscopic interpretation of dense astrophysical objects. However, detailed spectroscopic data at pressures comparable to stellar interior conditions are rare, severely limiting the information available to guide the development of atomic and plasma physics models. In this talk, we report both time-integrated and time-resolved x-ray spectroscopy data at several billion atmospheres (~Gbar) with a laser-driven spherical implosion. Using a Cu-doped layer located inside a stagnating plastic shell, detailed Cu K_α-emission and 1s-2p absorption spectra are measured in CH-Cu plasma mixtures. The spectral observations, augmented by experimentally constrained radiation-hydrodynamic predictions of the imploded plasma conditions, are in good agreement with a self-consistent treatment of the dense-plasma environment based on density-functional theory (DFT). Compared to the DFT-based approach, the similarities and differences of a traditional collisional-radiative equilibrium (CRE) treatment, which uses an isolated atomic database with ad hoc continuum lowering models, are revealed. These results open a path towards developing a deeper understanding of dense-plasma mixtures at ultra-high pressure.