Off-Hugoniot States at MultiMegabar Pressures on a Gas Gun from Multi-Shock Loading

Off-Hugoniot states are critical for exploring new pressure-temperature regimes and investigating material response beyond the principal Hugoniot. We report reproducible methods for the reliable generation and observation of off-Hugoniot pressure states up to 4.9 Mbar in tantalum using a two-stage gas gun with layered impactors. This work provides detailed methods for the preparation of multi-layer impactors using low-impedance polymer front layers that are designed to prevent delamination at impact velocities up to 7.4 km/s, enabling robust multi-shock loading. Using simultaneous multi-probe Photonic Doppler Velocimetry (PDV) measurements and a statistical averaging procedure, we obtain high precision interface velocity histories and quantify the uniform region of the drive. Combined with one- and two-dimensional hydrodynamic simulations, these data are used to assess lateral release, validate the one-dimensional loading in the central region of the target, and determine the equation-of-state-predicted pressure–temperature paths for each experiment. We compare these loading paths to those produced by graded density impactors and laser or magnetic ramp platforms and show that multi-shock gas-gun loading can access similar multimegabar, relatively low-temperature states using a simple and accessible driver. This work establishes layered impactors on a gas gun as a practical platform for future high-pressure studies of strength, phase transitions, and other off-Hugoniot phenomena.