C. A. Hall

Pulsed-power-driven high energy density physics and inertial confinement fusion research

The Z accelerator [R. B. Spielman, W. A. Stygar, J. F. Seamen et al., Proceedings of the 11th International Pulsed Power Conference, Baltimore, MD, 1997, edited by G. Cooperstein and I. Vitkovitsky (IEEE, Piscataway, NJ, 1997), Vol. 1, p. 709] at Sandia National Laboratories delivers ∼20MA load currents to create high magnetic fields (>1000T) and high pressures (megabar to gigabar). In a z-pinch configuration, the magnetic pressure (the Lorentz force) supersonically implodes a plasma created from a cylindrical wire array, which at stagnation typically generates a plasma with energy densities of about 10MJ∕cm3 and temperatures >1keV at 0.1% of solid density. These plasmas produce x-ray energies approaching 2MJ at powers >200TW for inertial confinement fusion (ICF) and high energy density physics (HEDP) experiments. In an alternative configuration, the large magnetic pressure directly drives isentropic compression experiments to pressures >3Mbar and accelerates flyer plates to >30km∕s for equation of state ...

Experimental configuration for isentropic compression of solids using pulsed magnetic loading

A capability to produce quasi-isentropic compression of solids using pulsed magnetic loading on the Z accelerator has recently been developed and demonstrated [C. A. Hall, Phys. Plasmas 7, 2069 (2000)]. This technique allows planar, continuous compression of materials to stresses approaching 1.5 Mbar. In initial stages of development, the experimental configuration used a magnetically loaded material cup or disk as the sample of interest pressed into a conductor. This installation caused distortions that limited the ability to attach interferometer windows or other materials to the rear of the sample. In addition, magnetic pressure was not completely uniform over sample dimensions of interest. A new modular configuration is described that improves the uniformity of loading over the sample surface, allows materials to be easily attached to the magnetically loaded sample, and improves the quality of data obtained. Electromagnetic simulations of the magnetic field uniformity for this new configuration will a...

Magnetically driven isentropic compression experiments on the Z accelerator

Isentropic compression experiments (ICE) have been performed on the Z accelerator facility at Sandia National Laboratory. We describe the experimental design that used large magnetic fields to slowly compress samples to pressures in excess of 400 kbar. Velocity wave profile measurements were analyzed to yield isentropic compression equations of state (EOS). The method can also yield material strength properties. We describe magnetohydronamic simulations and results of experiments that used the “square short” configuration to compress copper and discuss ICE EOS experiments that have been performed with this method on tantalum, molybdenum, and beryllium.

Near-absolute Hugoniot measurements in aluminum to 500 GPa using a magnetically accelerated flyer plate technique

Hugoniot measurements were performed on aluminum (6061-T6) in the stress range of 100–500 GPa (1–5 Mbar) using a magnetically accelerated flyer plate technique. This method of flyer plate launch utilizes the high currents, and resulting magnetic fields produced at the Sandia Z Accelerator to accelerate macroscopic aluminum flyer plates (approximately 12×25 mm in lateral dimension and ∼300 μm in thickness) to velocities in excess of 20 km/s. This technique was used to perform plate-impact shock-wave experiments on aluminum to determine the high-stress equation of state (EOS). Using a near-symmetric impact method, Hugoniot measurements were obtained in the stress range of 100–500 GPa. The results of these experiments are in excellent agreement with previously reported Hugoniot measurements of aluminum in this stress range. The agreement at lower stress, where highly accurate gas gun data exist, establishes the magnetically accelerated flyer plate technique as a suitable method for generating EOS data. Furth...

A compact strip-line pulsed power generator for isentropic compression experiments.

Veloce is a medium-voltage, high-current, compact pulsed power generator developed for isentropic and shock compression experiments. Because of its increased availability and ease of operation, Veloce is well suited for studying isentropic compression experiments (ICE) in much greater detail than previously allowed with larger pulsed power machines such as the Z accelerator. Since the compact pulsed power technology used for dynamic material experiments has not been previously used, it is necessary to examine several key issues to ensure that accurate results are obtained. In the present experiments, issues such as panel and sample preparation, uniformity of loading, and edge effects were extensively examined. In addition, magnetohydrodynamic simulations using the ALEGRA code were performed to interpret the experimental results and to design improved sample/panel configurations. Examples of recent ICE studies on aluminum are presented.

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 ...

Characterization of magnetically accelerated flyer plates

The intense magnetic field generated by the 20 megaampere Z machine [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories is being used as a pressure source for material science studies. An application we have studied in great detail involves using the intense magnetic field to accelerate flyer plates (small metal disks) to very high velocities (>20 km/s) for use in shock loading experiments. We have used highly accurate velocity interferometry measurements (error ∼1%) in conjunction with one-dimensional magnetohydrodynamic (MHD) simulation to elucidate details of the flyer dynamics. One-dimensional MHD simulations are able to produce experimental results with a high degree of accuracy, thereby revealing otherwise unobtainable, but useful information about magnetically accelerated flyers on Z. Comparisons of simulation results with time-resolved measurements of velocity from a shock loading experiment involving a 925 μm aluminum flyer are presented. Results show that Joule ...

Shock response of a glass-fiber-reinforced polymer composite

Abstract The present work describes the compression and release response of a glass-fiber-reinforced polyester composite (GRP) under shock loading to 20 GPa. Shock experiments in GRP were performed at Sandia National Laboratories and the US Army Research Laboratory. GRP is a heterogeneous material. The diagnostic measurements fluctuate beyond the precision of the experimental measurements but they do permit determination of an average response of the material at the end state. These experiments show that: (i) GRP deforms elastically in compression to at least 1.3 GPa; (ii) the deformation coordinates of shocked and re-shocked GRP lie on the deformation locus of initially shocked GRP to 4.3 GPa; (iii) and the release path of GRP shocked to varying magnitudes of stresses indicate that the GRP expands such that its density when stresses are released in the range of 3–5 GPa from a peak compressive stress of 9 GPa and above is lower than the initial density of GRP. Possible reasons for the observed lower density remain to be investigated.

Measurement of the compression isentrope for 6061-T6 aluminum to 185GPa and 46% volumetric strain using pulsed magnetic loading

The Z accelerator at Sandia National Laboratories was used to measure the compression isentrope of 6061-T6 aluminum to 185GPa. The isentropic compression experimental technique uses a rapidly increasing, planar magnetic field to simultaneously subject multiple planar aluminum samples of different thicknesses to a ramped magnetic stress load. This magnetic stress load causes a ramped compression wave to propagate in the aluminum. Motion histories at the rear surface of each aluminum sample are measured through a LiF window using laser velocity interferometry. Backward and forward integration of the one-dimensional equations of motion are used to analyze the data. Imposing the requirement that each motion history comes from the same magnetic stress load is sufficient to determine both the stress load and the stress-strain behavior of the aluminum. Because of shocks that grow in the LiF, the usual VISAR interferometer analysis was modified. The measured compression curve obtained on different aluminum sample...

Isentropic loading experiments of a plastic bonded explosive and constituents

The plastic bonded explosive PBX 9501 and its constituents [cyclotetramethylene tetranitramine (HMX) crystals, nitroplasticized Estane ®5703 and a fine-crystallite HMX laden binder mixture] were subjected to a ramped quasi-isentropic compression load using the Z machine at Sandia National Laboratories to determine equation of state and constitutive property data. Various sample thicknesses of these materials were subjected to an identical ramp loading history up to 4.5 GPa over 350 ns and particle velocities were measured using a velocity interferometry technique to assess material response. Upon defining appropriate constitutive relationships for the individual constituents, a topologically disordered model of the composite material was numerically simulated and details of the mesoscale simulation indicate that much of the plastic deformation first occurs locally at the large HMX crystal contacts points and subsequently by the deformation of the interstitial fine-crystallite/binder material.