Multishock compression of dense cryogenic hydrogen-helium mixtures up to 60 GPa: Validating the equation of state calculated from first principles

Multishock compression experiments on hydrogen-helium (H₂-He) mixtures have been performed via a two-stage light-gas gun for providing equations of state (EOS) covering a wide pressure range. The initial gaseous H₂-He sample was precompressed to 20--30 MPa and cooled down to around 90 K to gain high initial sample density. Up to three shock compressions were clearly observed from the time-resolved light radiance of the shocked sample recorded by a multichannel optical pyrometer. The measured EOS data of the H₂-He mixture reached an unexplored range of pressure up to 60 GPa, which is well in the molecular-to-atomic transition regime. The wide-range experimental data are used to validate the state-of-the-art first-principles simulation methods. It is found that the density functional theory molecular dynamics (DFT-MD) simulations underestimate the dissociation of hydrogen and therefore predict the H₂-He EOS to be stiffer than the experimental data in the molecular-to-atomic transition regime. A careful analysis of the pair correlation functions and comparison with the results of pure hydrogen revealed that DFT-MD might overestimate the effects of helium on the bond of the hydrogen molecule when hydrogen is mixed with helium. Finally, the current measurements validate a linear-mixing ab initio EOS [Astrophys. J. Suppl. S. 215, 21 (2014)] widely used in astrophysics.