Characteristics analysis of Isentropic Compression Experiments (ICE)

Several new techniques for high-pressure (quasi-) isentropic compression of materials are becoming available: pulsed-power-driven magnetic pressure, graded-density impactors on gas-guns, and laser drive. These experiments are analysed to give sound speed as a function of particle velocity, which may then be integrated to pressure and volume, giving data for a complete isentrope with one shot. However, to do this the experimentally-measured free-surface - or window-interface - velocities must be corrected to give the in-situ material velocities in the absence of the surface or interface. This may be done by either backward integration (integrate the equations of motion in space rather than time using surface velocity vs time as the boundary conditions) or by a characteristics method. This presentation describes the characteristics method and its application to AWE experiments on the isentropic compression of lead. The experiment has produced results consistent with pre-existing experimental data within errors in sound speed and pressure of similar to 4%. Iterative corrections to in-situ velocity, using the EOS calculated during the previous step, reduce - or do away with - any dependence of the results on an assumed EOS. Simultaneous characteristics calculations of the input pressure-time are used to estimate the errors.