The intense magnetic field generated by the Z accelerator at Sandia National Laboratories is used as a pressure source for material science studies. A current of ∼20 MA can be delivered to the loads used in experiments on a time scale of ∼100-600 ns. Magnetic fields (pressures) exceeding 1200 T (600 GPa) have been produced in planar configurations. In one application we have developed, the magnetic pressure launches a flyer plate to ultra-high velocity in a plate impact experiment; equation of state data is obtained on the Hugoniot of a material that is shock compressed to multi-megabar pressure. This capability has been enhanced by the recent development of a planar stripline configuration that increases the magnetic pressure for a given current. Furthermore, the cross sectional area of a stripline flyer plate is larger than in previous coaxial loads; this improves the planarity of the flyer thereby reducing measurement uncertainty. Results of experiments and multi-dimensional magneto hydrodynamic (MHD) simulation are presented for ultra-high velocity aluminum and copper flyer plates. Aluminum flyer plates with dimensions ∼25 mm by ∼13 mm by ∼1 mm have been launched to velocities up to ∼45 km/s; for copper the peak velocity is ∼22 km/s. The significance of these results is that part of the flyer material remains solid at impact with the target; an accomplishment that is made possible by shaping the dynamic pressure (current) ramp so that the flyer compresses quasi-isentropically (i.e., shocklessly) during acceleration.
The Z machine is a fast pulsed-power machine at Sandia National Laboratories designed to deliver a 100-ns rise-time, 26-MA pulse of electrical current to Z-pinch experiments for research in radiation effects and inertial confinement fusion. Since 1999, Z has also been used as a current source for magnetically driven, high-pressure, high-strain-rate experiments in condensed matter. In this mode, Z produces simultaneous planar ramp-wave loading, with rise times in the range of 300-800 ns and peak longitudinal stress in the range of 4-400 GPa, of multiple macroscopic material samples. Control of the current-pulse shape enables shockless propagation of these ramp waves through samples 1-2 mm thick to measure quasi-isentropic compression response, as well as shockless acceleration of copper flyer plates to at least 28 km/s for impact experiments to measure ultra-high-pressure (-3000 GPa) shock compression response. This presentation will give background on the relevant physics, describe the experimental technique, and show recent results from both types of experiments.
CHARICE (CHARacteristics-based inverse analysis of Isentropic Compression Experiments) is a computer application, previously documented in SAND2007-4948, that analyzes velocity waveform data from ramp-wave experiments to determine a material's quasi-isentropic loading response in stress and density using an iterative characteristics-based approach. This short report documents only the changes in CHARICE release version 1.1 relative to release version 1.0, and is not intended to stand alone. CHARICE version 1.1 corrects an error in the algorithm of the method, fixes several bugs, improves robustness and performance, provides more useful error descriptions, and adds a number of minor features.
CHARICE is a multi-platform computer application that analyzes velocity waveform data from ramp-wave experiments to determine a material's quasi-isentropic loading response in stress and density using an iterative characteristics-based approach. The application was built using ITT Visual Information Solutions Interactive Data Language (IDL{reg_sign}), and features graphical interfaces for all user interaction. This report describes the calculation method and available analysis options, and gives instructions for using the application.
Using a magnetic pressure drive, an absolute measurement of stress and density along the principal compression isentrope is obtained for solid aluminum to 240 GPa. Reduction of the free-surface velocity data relies on a backward integration technique, with approximate accounting for unknown systematic errors in experimental timing. Maximum experimental uncertainties are +/-4.7% in stress and +/-1.4% in density, small enough to distinguish between different equation-of-state (EOS) models. The result agrees well with a tabular EOS that uses an empirical universal zero-temperature isotherm.
The intense magnetic field generated in the 20 MA Z-machine is used to accelerate metallic flyer plates to high velocity for the purpose of generating strong shocks in equation of state experiments. We present results pertaining to experiments in which a 0.085 cm thick Al flyer plate is magnetically accelerated across a vacuum gap into a quartz target. Peak magnetic drive pressures up to 4.9 Mbar were produced, which yielded a record 34 km/s flyer velocity without destroying it by shock formation or Joule heating. Two-dimensional MHD simulation was used to optimize the magnetic drive pressure on the flyer surface, shape the current pulse to accelerate the flyer without shock formation (i.e., quasi-isentropically), and predict the flyer velocity. Shock pressures up to 11.5 Mbar were produced in quartz. Accurate measurements of the shock velocity indicate that a fraction of the flyer is at solid density when it arrives at the target. Comparison of measurements and simulation results yields a consistent picture of the flyer state at impact with the quartz target.
INVICE (INVerse analysis of Isentropic Compression Experiments) is a FORTRAN computer code that implements the inverse finite-difference method to analyze velocity data from isentropic compression experiments. This report gives a brief description of the methods used and the options available in the first beta version of the code, as well as instructions for using the code.
Using a magnetic pressure drive, an absolute measurement of stress and density along the principal compression isentrope is obtained for solid aluminum to 240 GPa. Reduction of the free-surface velocity data relies on a backward integration technique, with approximate accounting for unknown systematic errors in experimental timing. Maximum experimental uncertainties are {+-}4.7% in stress and {+-}1.4% in density, small enough to distinguish between different equation-of-state (EOS) models. The result agrees well with a tabular EOS that uses an empirical universal zero-temperature isotherm.
The magnetic-pressure drive technique allows single-shot measurements of compression isentropes. We have used this method to measure the isentropes in the pressure-volume space of bulk and single-crystal lead, and lead-antimony alloy to {approx}400 kbar. The isentrope pressure-volume curves were found from integration of the experimentally deduced Lagrangian sound speed as a function of particle velocity. A characteristics calculation method was used to convert time-resolved free-surface velocity measurements to corresponding in situ particle-velocity histories, from which the Lagrangian sound speed was determined from the times for samples of different thicknesses to reach the same particle velocity. Use of multiple velocity interferometry probes decreased the uncertainty due to random errors by allowing multiple measurements. Our results have errors of from 4% to 6% in pressure, {approx}1% to 1.5% in volume, depending on the number of measurements, and are consistent with existing isotherm and Hugoniot data and models for lead.
The intense magnetic field generated in the 20 MA Z-machine is used to accelerate metallic flyer plates to high velocity (peak velocity {approx}20-30 km/s) for the purpose of generating strong shocks (peak pressure {approx}5-10 Mb) in equation of state experiments. We have used the Sandia developed, 2D magneto-hydrodynamic (MHD) simulation code ALEGRA to investigate the physics of accelerating flyer plates using multi-megabar magnetic drive pressures. Through detailed analysis of experimental data using ALEGRA, we developed a 2D, predictive MHD model for simulating material science experiments on Z. The ALEGRA MHD model accurately produces measured time dependent flyer velocities. Details of the ALEGRA model are presented. Simulation and experimental results are compared and contrasted for shots using standard and shaped current pulses whose peak drive pressure is {approx}2 Mb. Isentropic compression of Al to 1.7 Mb is achieved by shaping the current pulse.
Isentropic compression experiments were performed on molten tin (initial temperature 500-600 K), using the Sandia Z Accelerator to generate magnetically driven, planar ramp waves compressing the tin across the equilibrium liquid-solid phase boundary. Velocity interferometry measured time-resolved wave profiles at the tin/window interface. The experiments exhibit a departure from expected liquid response, time-dependent behavior above 8 GPa, and, at higher pressure, reduced wave speed relative to calculations using a nonequilibrium phase-mixture model. These phenomena may be due to a nonequilibrium solidification process, but verification of this conjecture will require further work.
Recently an innovative technique known as the Isentropic Compression Experiment (ICE) was developed that allows the dynamic compressibility curve of a material to be measured in a single experiment. Hence, ICE significantly reduces the cost and time required for generating and validating theoretical models of dynamic material response. ICE has been successfully demonstrated on several materials using the 20 MA Z accelerator, resulting in a large demand for its use. The present project has demonstrated its use on another accelerator, Saturn. In the course of this study, Saturn was tailored to produce a satisfactory drive time structure, and instrumented to produce velocity data. Pressure limits are observed to be approximately 10-15 GPa (''LP'' configuration) or 40-50 GPa (''HP'' configuration), depending on sample material. Drive reproducibility (panel to panel within a shot and between shots) is adequate for useful experimentation, but alignment fixturing problems make it difficult to achieve the same precision as is possible at Z. Other highlights included the useful comparison of slightly different PZT and ALOX compositions (neutron generator materials), temperature measurement using optical pyrometry, and the development of a new technique for preheating samples. 28 ICE tests have been conducted at Saturn to date, including the experiments described herein.
In order to provide real-time data for validation of three dimensional numerical simulations of heterogeneous materials subjected to impact loading, an optically recording velocity interferometer system (ORVIS) has been adapted to a line-imaging instrument capable of generating precise mesoscopic scale measurements of spatially resolved velocity variations during dynamic deformation. Combining independently variable target magnification and interferometer fringe spacing, this instrument can probe a velocity field along line segments up to 15 mm in length. In high magnification operation, spatial resolution better than 10 {micro}m can be achieved. For events appropriate to short recording times, streak camera recording can provide temporal resolution better than 0.2 ns. A robust method for extracting spatially resolved velocity-time profiles from streak camera image data has been developed and incorporated into a computer program that utilizes a standard VISAR analysis platform. The use of line-imaging ORVIS to obtain measurements of the mesoscopic scale dynamic response of shocked samples has been demonstrated on several different classes of heterogeneous materials. Studies have focused on pressed, granular sugar as a simulant material for the widely used explosive HMX. For low-density (65% theoretical maximum density) pressings of sugar, material response has been investigated as a function of both impact velocity and changes in particle size distribution. The experimental results provide a consistent picture of the dispersive nature of the wave transmitted through these samples and reveal both transverse and longitudinal wave structures on mesoscopic scales. This observed behavior is consistent with the highly structured mesoscopic response predicted by 3-D simulations. Preliminary line-imaging ORVIS measurements on HMX as well as other heterogeneous materials such as foam and glass-reinforced polyester are also discussed.
