Compressing Pre-Compressed Hydrogen-Helium Mixtures using Z-Machine and Hypervelocity Gas-Guns

Hydrogen and helium are the most abundant elements in the universe. Understanding the properties of hydrogen-helium mixtures is essential to study the origin and structural evolution of Jovian planets. However, the equation of state of hydrogen-helium mixtures remains ill determined. While there has been extensive theoretical work on the phase diagram relevant for H+He demixing, there are to date few experimental studies exploring this important, yet, ill-determined region of the H+He equation of state. Here, we have measured the equation of state of pre-compressed hydrogen-helium (H₂-He) mixtures using hypervelocity gas-guns and Sandia’s Z machine up to 86 GPa. We pre-compressed 50:50 molar mixtures of H₂-He to 0.1 -0.2 GPa using a sapphire anvil cell. We directly measured particle velocity (in gas-gun experiments) and shock velocities (in Z-machine experiments). To complement experimental efforts, we have also computed the Hugoniots of pre-compressed hydrogen-helium samples using density functional theory based molecular dynamics. The current Hugoniot data can be used to distinguish between the proposed controversial mixing models.