The sound velocities of materials under high pressures and high temperatures are important in characterizing the elasticities and phase changes, and especially crucial for our understanding of the seismic structures in planetary interiors. As the simplest elements, deuterium and helium can be well used as the standard test cases for the sound velocity studies. Moreover, deuterium and helium are major constituents of many astrophysical objects such as stars and giant planets, and also the primary constituents in inertial confinement fusion (ICF). The knowledge of their physical properties such as sound velocities, equations of state (EOSs), electrical conductivities at extreme pressure-temperature conditions play a vital role in the constructions of giant planets interiors models and the understanding of fundamental physical processes in ICF. In this work, we perform a serious of reverberating shocks experiments on deuterium-helium (D₂-He) mixtures, yielding for the first time measurements of the isentropic sound velocity and the shock equation of state (EOS) in a wide pressure regime. The measured sound velocity and EOS of the D₂-He mixtures reached an unexplored range of pressure up to 120 GPa, which is direct relevant to the molecular region of the giant planet interiors. The wide-range experimental data are used to validate the state-of-the-art first-principle simulation methods and chemical models. Finally, measurements of sound velocity of the D₂-He mixtures in megabar range can provide more direct constraints on the seismic structure of the molecular region of giant planets.

This work was supported by the National Natural Science Foundation of China (Grands No. 11802280 and 11674292) and the National Postdoctoral Program for Innovative Talents (Grand No. BX201700215)