Hugoniot Data for Helium in the Ionization Regime

Hugoniot data were obtained for fluid He in the 100 GPa pressure range by shock compression of samples statically precompressed in diamond-anvil cells. The initial (precompressed) He density (ρ₁) for each experiment was tuned to a value between ρ0L<ρ₁<3.3ρ0L, where ρ0L is the zero-pressure density of the cryogenic liquid (ρ0L=0.123 g/cm³). The maximum observed shock-compression ratios range from ρ/ρ₁=6 for ρ₁=ρ0L to ρ/ρ₁=4 for ρ₁≥3ρ0L (i.e., ρ/ρ0L≥12). Data show an increase in compressibility at the onset of ionization, similar to theoretical predictions.