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 (rho(1)) for each experiment was tuned to a value between rho(0L) < rho(1) < 3.3 rho(0L), where rho(0L) is the zero-pressure density of the cryogenic liquid (rho(0L)=0.123 g/cm(3)). The maximum observed shock-compression ratios range from rho/rho(1) = 6 for rho(1) = rho(0L) to rho/rho(1) = 4 for rho(1) >= 3 rho(0L) (i.e., rho/rho(0L) >= 12). Data show an increase in compressibility at the onset of ionization, similar to theoretical predictions.