Measurements of Sound Speed and Gr\"{u}neisen Parameter in Polystyrene Shocked to 8.5 Mbar

The high-pressure behavior of polymers is important to fundamental high-energy-density studies and inertial confinement fusion experiments. The sound speed affects shock timing and determines the amplitude of modulations in unstable shocks. The Gr\"{u}neisen parameter provides a means to model off-Hugoniot behavior, especially release physics. We use laser-driven shocks and a nonsteady wave analysis to infer sound speed in shocked material from the arrival times of drive-pressure perturbations at the shock front. Data are presented for CH shocked to 8.5 Mbar and compared to models. The Gr\"{u}neisen parameter is observed to drop significantly near the insulator--conductor transition---a behavior not predicted by tabular models but is observed in quantum molecular dynamic simulations. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.