Extreme compression of planetary gases: High-accuracy pressure-density measurements of hydrogen-helium mixtures above fourfold compression

Hydrogen $({\mathrm{H}}_{2})$ and helium (He), the most abundant elements in the universe, pose a unique challenge in measuring the equation of state of the mixture, owing to their differing physical properties. There remains a need for data with high enough precision to discriminate between existing equation of state (EOS) mix models in order to understand the internal structure of gas-giant planets. Here, we have measured the EOS of precompressed ${\mathrm{H}}_{2}\text{\ensuremath{-}}\mathrm{He}$ mixtures at conditions directly relevant to the planetary interiors using hypervelocity gas guns and Sandia's Z machine with less than 10% uncertainty in density, enabling validation of mixture models. We precompressed 50:50 molar mixtures of ${\mathrm{H}}_{2}$-He to 0.1--0.2 GPa and directly measured particle velocity (in gas-gun experiments) and shock velocities (in Z-machine experiments). To complement the experimental efforts, we also computed the Hugoniots of precompressed ${\mathrm{H}}_{2}$-He mixtures using density-functional-theory-based molecular dynamics. We observe approximately 3- to 4.3-fold density compression at pressures up to 44 GPa.