Absolute Hugoniot Measurements for CH Foams in the 2-9 MBAR Range

Absolute Hugoniot measurements for empty plastic foams at ~10% of solid polystyrene density and supporting rad-hydro simulation results are reported. Planar foam slabs, $\sim 400\mu \mathrm{m}$ thick and $\sim 500\mu \mathrm{m}$ wide, some of which were covered with a 10 $\mu \mathrm{m}$ solid plastic ablator, were directly driven with 4 ns long Nike krypton-fluoride 248 nm wavelength laser pulses that produced strong shock waves in the foam. The shock and mass velocities in our experiments were up to 104 km/s and 84 km/s, respectively, and the shock pressures up to ~9 Mbar. The motion of the shock and ablation fronts was recorded using side-on monochromatic x-ray imaging radiography. The steadiness of the observed shock and ablation fronts within $\sim$ 1 % has been verified. The Hugoniot data inferred from our velocity measurements agree with the predictions of the SESAME and CALEOS equation-of-state models near the highest pressure ~9 Mbar and density compression ratio ~5. In the lower pressure range 2 to 5 Mbar, a lower shock density compression is observed than that predicted by the models, see the Figure. Possible causes for this discrepancy are discussed.