Hot dense matter systems resist theoretical description by the established approaches of solid state or plasma physics, emphasizing the need for experimental data to help produce a detailed picture for how the atomic, radiative and thermodynamic properties of matter are modified in extreme conditions. Hot dense plasma conditions were created by high-intensity laser irradiation of solid foils containing thin buried Al or Al/Fe tracer layers. The targets were driven by high-contrast 1ω and 2ω laser pulses at focused intensities up to 1 x 1019 W/cm2. A streaked X-ray spectrometer recorded Al Heα thermal line emission from the buried layer with 2-ps temporal resolution and resolving power close to E/ΔE ≈ 1000. The material response to intense heating was inferred from the time-dependent intensity of the Al Heα thermal line using a fully explicit, kinetic and electromagnetic particle-in-cell model. The instantaneous bulk plasma conditions were inferred by comparing the measured Heα-to-satellite intensity ratio and spectral line width to a non-local thermodynamic equilibrium (NLTE) atomic kinetics model. The data are consistent with thermal temperatures exceeding 300 eV and high densities within 80 percent of solid. At high density, the experimental spectra show a linear redshift of the Al Heα thermal line. Numerical ion-sphere model calculations demonstrate broad agreement with the measured redshift over the full range of densities and temperatures studied, providing a new test of dense plasma theories for atomic structure and radiation transport in extreme environments.
