Hydrogen and helium constitute the main components of Jovian planets. Under the warm dense conditions inside gas giants, helium droplets settle deeper into the planet, forming a helium gradient. Consequently, thermodynamic properties, such as the equation of state (EoS) and sound speed, of hydrogen-helium mixtures with varying atomic helium mole fractions $\left(x_{\mathrm{He}}\right)$ under warm dense conditions are crucial for evaluating EoS models used in astrophysical modeling. To this end, we conducted multiple reverberation compression experiments on gaseous deuterium-helium $\left({\mathrm{D}}_2\text{−}\mathrm{He}\right)$ mixtures with two distinct $x_{\text{He}}$ of 0.14 and 0.60. The multishock pressures reached 110 GPa, and reshock temperatures reached 7620 K, conditions directly relevant to planetary interiors. The obtained EoS and sound velocity are used to evaluate ${\mathrm{H}}_2\text{−}\mathrm{He}$ EoS models constructed with or without taking into account nonideal mixing (NIM) effects. It is found that the state-of-the-art $\mathrm{CMS}19+\mathrm{HG}23$ EoS model, which incorporates NIM effects for arbitrary $x_{\text{He}}$ values, can reasonably reproduce the measured pressure-density EoS for both mixtures but underestimates the shock temperature of the $x_{\text{He}}=0.14$. First-principles molecular dynamics (FPMD) simulations reveal that the NIM effect, arising from the strengthening effect of helium on deuterium molecule bonds, is more prominent for the mixture with $x_{\text{He}}=0.60$. Our findings impose new constraints on the EoS models applicable to the mixtures with arbitrary $x_{\text{He}}$, which are crucial for modeling the structure and evolution of gas giants.

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• Electrical conductivity
• Equations of state
• Laboratory studies of space & astrophysical plasmas
• Optical interferometry
• Plasma production & heating by shock waves & compression
• Shock waves
• Warm-dense matter
• First-principles calculations
• Molecular dynamics