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)<3.3rho_(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)> or =3rho_{0L} (i.e., rho/rho_(0L)> or =12). Data show an increase in compressibility at the onset of ionization, similar to theoretical predictions.