Oxygen Opacity Measurements at High-Energy-Density Conditions

We present the first benchmark measurements of oxygen opacity at high-energy-density, crucial for understanding stellar interiors. A 3-5-μm thick SiO₂ sample was heated using a Z facility x-ray source, and opacity was measured in the 5-19.5 Å range with multiple spectrometers. The electron temperature and density inferred from Si K-shell spectra were 148±4 eV and 8.6±1.4× 10²¹ cm⁻³, respectively. Six experiments showed opacity reproducibility within ±15% and 5%-25% opacity uncertainty. Measured oxygen opacity and seven different opacity models exhibit extraordinary overall agreement. Discrepancies near the H-like photon ionization threshold and in certain spectral lines suggest the need for model refinements.