David Barton Reisman

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.

Isentropic compression of irradiated stainless steel on the Z accelerator

We have performed quasi-isentropic compression experiments on radiation-damaged stainless steel. The samples were dynamically loaded by Sandia National Laboratory’s Z accelerator with a ramp compression wave. Sample/window interface velocities were recorded with VISAR. The velocity histories suggest a sudden volume reduction of the material above 40 kbar caused by the collapse of nanosized voids. This is predicted by a theoretical model of void collapse based on the emission of vacancy-type dislocations loops. We compare the results of these experiments to hydrodynamic calculations performed using a constitutive model which is derived from the atomistic void collapse mechanism.

Isentropic compression of cyclotetramethylene tetranitramine (HMX) single crystals to 50GPa

Single crystals of cyclotetramethylene tetranitramine (HMX) were isentropically compressed perpendicular to (010) and (011) faces at the Sandia Z-Machine. A 50GPa ramped magnetic pressure load of about 200ns rise time loaded four specimens of each orientation. HMX specimens were from 300–600μm thick. Velocity histories at the rear of each crystal were measured by Doppler velocimetry. Although a phase change in HMX at 27GPa has been proposed based upon isothermal data, no evidence of this change is seen in our analyses between 5 and 50GPa along the isentrope. Previous isentropic loading experiments on HMX had not shown evidence of a phase change either, but those experiments were complicated by the use of NaCl interferometer windows that have a phase change near the pressure of interest. The experiments described in this paper employed LiF interferometer windows that are known to be absent phase changes in the regime of application. Accurate determination of isentropic compressibility for HMX was not possi...

Dynamic strength of metals in shock deformation

The Hugoniot and critical shear strength of shock-compressed metals can be obtained directly from molecular dynamics simulations without recourse to surface velocity profiles and their analyses. Results from simulations in aluminum containing an initial distribution of microscopic defects are shown to agree with experimental results.

Kinetics of Propagating Phase Transformation in Compressed Bismuth

We observed dynamically driven phase transitions in isentropically compressed bismuth. By changing the stress loading conditions we explored two distinct cases: one in which the experimental signature of the phase transformation corresponds to phase-boundary crossings initiated at both sample interfaces, and another in which the experimental trace is due to a single advancing transformation front in the bulk of the material. We introduce a coupled kinetics-hydrodynamics model that for this second case enables us, under suitable simplifying assumptions, to directly extract characteristic transition times from the experimental measurements.

Isentropic compression loading of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and the pressure-induced phase transition at 27GPa

The 27GPa pressure-induced epsilon–phi phase transition in octahydro-1,3,5,7-tetranitro-l,3,5,7-tetrazocine (HMX) is explored using the isentropic compression experiment (ICE) technique at the Sandia National Laboratories Z-machine facility. Our data indicate that this phase transition is sluggish and if it does occur to any extent under the time scales (200–500ns) and strain rates (5×105) typical of ICE loading conditions, the amount of conversion is small.

The advanced helical generator

A high explosive pulsed power generator called the advanced helical generator (AHG) has been designed, built, and successfully tested. The AHG incorporates design principles of voltage and current management to obtain a high current and energy gain. Its design was facilitated by the use of modern modeling tools as well as high precision manufacture. The result was a first-shot success. The AHG delivered 16 MA of current and 11 MJ of energy to a quasistatic 80 nH inductive load. A current gain of 160 times was obtained with a peak exponential rise time of 20 micros. We will describe in detail the design and testing of the AHG.

Determining the equation-of-state isentrope in an isochoric heated plasma

A novel method for determining the equation-of-state (EOS) along the release isentrope in an isochoric (constant volume) heated plasma is presented. This approach is demonstrated using simulations of a solid density, 10 eV expanding Al plasma. Determining the material EOS data is validated to pressures near 80 Mbar, much higher than current isentropic compression experiments allow. Limitations at very high temperature (Te⩾100 eV), due to the formation of a radiative conduction layer near the rarefaction interface, are also discussed.

Isentropic compression experiments to 1 Mbar using magnetic pressure

Isentropic compression experiments that utilize intense magnetic fields to compress samples have been designed, developed and performed. The technique has been shown to work to pressures of more than 1 Mbar on Sandia National Laboratorys Z pulsed power machine. We are extending the technique to use high-explosive pulsed power.