Isentropic compression experiments on the Sandia Z accelerator

A long-standing goal of the equation of state (EOS) community has been the development of a loading capability for direct measurement of material properties along an isentrope. Previous efforts on smooth bore launchers have been somewhat successful, but quite difficult to accurately reproduce, had pressure limitations, or tended to be a series of small shocks as opposed to a smoothly increasing pressure load. A technique has recently been developed on the Sandia National Laboratories Z accelerator which makes use of the high current densities and magnetic fields available to produce nearly isentropic compression of samples that are approximately 1 mm in thickness over approximately 120 ns. Velocity interferometry is used to measure the rear surface motion of these samples. The resulting time resolved velocity profiles from multiple sample thicknesses provide information about mechanical response under isentropic loading conditions and phase transition kinetics. Feasibility experiments have been performed to pressures of approximately 130 kbar in copper and 300 kbar in iron with effects of the alpha-epsilon phase change kinetics in iron clearly observed. Work is in progress to achieve 1%-2% accuracy in P-upsilon space along an isentrope, provide uniaxial strain, and to eliminate magnetic field and current diffusion within the sample of interest. (C) 2000 American Institute of Physics. [S1070-664X(00)93705-4].