J. R. Asay

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...

Effects of Point Defects on Elastic Precursor Decay in LiF

Experimental data for shock propagation along a 〈100〉 direction in single‐crystal LiF show that elastic precursor decay is critically dependent on the origin of the sample. The most obvious differences among samples used are in their concentrations of impurities. It is suggested that divalent cation impurities are responsible for variations in precursor decay, and this is supported by results from a set of samples irradiated with γ rays to produce F centers. For the observed range of defect concentrations, quasistatic yield stresses varied monotonically with concentration from 0.02 kbar for pure crystals to 1.0 kbar for the hardest material studied. In the shock loading experiments both hard and soft crystals showed an initial rapid decay of the precursor to near‐equilibrium values of about 2 kbar for the softest crystals and about 6 kbar for the hardest. For crystals of intermediate hardness the decay was much slower. From observed effects of annealing before shocking it is inferred that dislocation mechanisms in shock differ from those believed to operate at low strain rates. Impact stress for all experiments was about 28.6 kbar and sample thicknesses ranged from 0.27 to 15.44 mm.

A self‐consistent technique for estimating the dynamic yield strength of a shock‐loaded material

A technique is described for estimating the dynamic yield stress in a shocked material. This method employs reloading and unloading data from a shocked state along with a general assumption of yield and hardening behavior to estimate the yield stress in the precompressed state. No other data are necessary for this evaluation, and, therefore, the method has general applicability at high shock pressures and in materials undergoing phase transitions. In some special cases, it is also possible to estimate the complete state of stress in a shocked state. Using this method, the dynamic yield strength of aluminum at 2.06 GPa has been estimated to be 0.26 GPa. This value agrees reasonably well with previous estimates.

Rise-time measurements of shock transitions in aluminum, copper, and steel

Time‐resolved measurements of shock‐wave rise times have been accomplished for aluminum, copper, and steel to stress levels of 41, 96, and 139 GPa, respectively, using velocity‐interferometer techniques. To within the time resolution of the technique, the shock transition is found to occur within 3 ns in all materials. Based on this upper limit for the transition time, limiting viscosity coefficients of 1000, 3000, and 4000 P are obtained for 6061‐T6 aluminum, OFHC copper, and 4340 steel, respectively, at strain rates above 108 s−1. It is found that the effective viscosity can be expressed as parameters in a Maxwellian relation for an elastic‐plastic solid, in which the viscosity is related to an effective relaxation time. It is also shown that viscosity is inversely proportional to mobile‐dislocation density, which implies that the density of mobile dislocations obtained during shock compression in these materials is well over 109/cm2.

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...

Reshock and release of shock‐compressed 6061‐T6 aluminum

The release and reshock behavior of aluminum from an initial shock stress of 2 GPa (20 kbar) has been examined. It is found that a two‐wave structure characterizes both release and recompression, although a definite elastic‐plastic structure is not obtained in either case. The velocity of the initial disturbance for both recompression and release agrees with the extrapolated ultrasonic longitudinal velocity, which implies initial elastic response from the precompressed state. The present results are discussed in terms of a rate‐independent model which incorporates a distribution of yield states in the precompressed material. Reasonable agreement with experimental reshock and release wave profiles is obtained with this model. A brief discussion of rate effects estimated from an acceleration wave analysis is also presented.

Interferometric measurement of shock‐induced internal particle velocity and spatial variations of particle velocity

Methods of applying laser interferometry to measure particle velocity history at the interface between a shocked specimen and a transparent window material are discussed. Particular emphasis is given to diffusely reflecting interface surfaces, to shock‐induced light polarization shifts which can occur in the window material, and to a transient loss of fringe contrast which occurs whenever the reflecting surface velocity is spattially nonuniform. It is shown that the loss of finge contrast can be used to provide time‐resolved measurements of the spatial variations in particle velocity.

Pressure and Temperature Dependence of the Acoustic Velocities in Polymethylmethacrylate

The acoustic velocities in polymethylmethacrylate have been measured with an ultrasonic pulse‐echo technique as functions of frequency, temperature, and pressure. At atmospheric pressure, data on the velocities and attenuation coefficients were obtained for the temperature range of 22°–75°C in the frequency range of 6–30 MHz. For the measurements of velocity and attenuation as a function of frequency, the complex adiabatic bulk modulus was calculated at room temperature and atmospheric pressure for the above frequencies. At temperatures of 25°, 40°, 55°, and 75°C, the pressure dependence of the longitudinal and shear velocity was determined to 150 kpsi at a frequency of 6 MHz. It was found that the measured velocities under increasing pressure conditions were generally lower than those of decreasing pressure by about 0.5% for the longitudinal measurements and about 1% for the shear measurements. However, measurements of the velocities at atmospheric pressure after the specimens had been exposed to 150 kps...