Equation of State Measurements at Ultra-High Pressures on the Sandiaz Accelerator

Abstract

Summary form only given. A method has been developed to launch macroscopic flyer plates using the intense magnetic pressure produced by the Sandia Z accelerator. The accelerator is capable of producing >20 MA current pulses with ~200 ns rise-time into a short circuit load, generating intense magnetic fields within the anode cathode (AK) gap. The resulting Lorentz force enables the Z accelerator to be used very effectively in accelerating plates to ultra-high velocity. These experiments have been directed toward highly accurate dynamic material studies. In particular, emphasis has been placed on the launching of planar, solid density flyer plates to velocities exceeding 30 km/s for use in equation of state (EOS) studies at high-pressure. Velocities up to 34 km/s have been obtained with aluminum flyer plates several mm in lateral dimensions and a few hundred microns in thickness, resulting in constant pressure drive times up to ~30 ns in plate impact, shock wave experiments. Initial experiments were performed on aluminum, a widely studied metal, to validate the technique for use in high-pressure EOS experiments. Highly accurate, absolute Hugoniot measurements of aluminum have been obtained in the pressure range of 1-12 Mbar. Velocity interferometry (VISAR) was used to monitor the velocity of the aluminum flyer plate from launch to impact. The resulting shock speed was obtained from shock break out measurements of stepped targets. Accuracy of the measurements are ~1-2% in both shock and particle speed. This technique has also been used to perform high-pressure shock and release experiments. Direct impact experiments were performed on a low shock impedance, 200 mg/cc silica aerogel to determine the aerogel Hugoniot in the pressure range of ~30-150 GPa. Release experiments were then performed in which the aerogel was mounted onto an aluminum base plate, and the initial shock in the base plate was transmitted to the aerogel sample. In this way release states on release adiabats of aluminum from ~240-830 GPa states on the principal Hugoniot were ascertained. The results were compared to several EOS models for aluminum, and a rigorous statistical analysis was performed. The statistical analysis enabled the suitability of these various models in describing the release response of hot, liquid states to be determined. This study enhances our understanding of the release response of aluminum for high-pressure states on the Hugoniot and lends confidence to the use of aluminum as a standard material in impedance matching experiments