Session E3: ED-1b: Isentropic Compression and Planar Loadings

A new strip-line design for multi-megabar dynamic loading experiments on the Z-machine

The challenges associated with isentropically compressing material samples to multi-megabar pressures in experiments on the refurbished Z accelerator (ZR) are discussed. Due to the higher stored energy on ZR relative to Z, the peak current delivered to a low inductance ($\sim $1.3 nH) flyer load has increased by a factor of $\sim $1.3 to $\sim $26 MA. It should be possible to achieve peak pressures of $\sim $10 Mb in dynamic ramps for isentropic compression experiments (ICE), and shockless acceleration of flyer plates. We have developed a new strip-line load for dynamic materials experiments on ZR that increases the drive pressure relative to what was possible with the rectangular slab loads used exclusively on Z. The isentrope of Ta has been measured to 4 Mb using a strip-line load. Predictive MHD simulations of the strip-line have been performed to characterize this load and to scope out designs for flyer plates and ICE that utilize the full pressure available on ZR; results indicate that a peak flyer velocity of $\sim $40 km/s is possible. We discuss these results and related experiments.\\[4pt] In collaboration with M.D. Knudson, J.P. Davis, T.A. Haill, D.B. Seidel, and W.L. Langston, Sandia National Laboratories.   

Measurements of Multi-Megabar Quasi-Isentropes for Several Materials

Quasi-isentropic ramp-wave experiments promise accurate equation-of-state (EOS) data in the solid phase at relatively low temperatures and multimegabar pressures. In this range of pressure, isothermal diamond-anvil techniques have limited accuracy due to reliance on theoretical EOS of calibration standards, thus accurate quasi-isentropic compression data would help immensely in constraining EOS models. Isentropic compression experiments (ICE) using the Z Machine at Sandia as a magnetic drive have recently seen significant improvements in accuracy and pressure range, due to (1) a major refurbishment and upgrade of the accelerator, (2) the development of a stripline target configuration, and (3) new experiment design and data analysis approaches. After a brief discussion of these improvements, new data will be presented on tantalum, beryllium, and aluminum metals as well as lithium fluoride crystal. Comparisons will be made to several independently developed EOS as well as recent quantum molecular dynamics (QMD) results.   

Light Initiated High Explosives (LIHE) Test Technique and Capabilities

The Light Initiated High Explosives (LIHE) test facility has been re-established and chartered to impart impulsive loads to a variety of targets. This loading is achieved through the detonation of a primary explosive applied directly to the target surface using a robotic spraying system. Using light as the initiating mechanism ensures virtually simultaneous loading. Uniform, discontinuous, or graded explosive loading conditions are achievable over complex shapes with the LIHE process. This direct detonation technique is a demonstrated capability at the LIHE facility. Test results will be presented. In addition to the direct detonation technique, the LIHE facility is developing the capability to explosively accelerate a thin flyer plate to impact various test targets. This explosively accelerated flyer plate (X-Flyer) will enable pressure control during impulsive loading. By controlling flyer density (material), thickness, velocity, and acceleration gap, the impact pressure amplitude and pulse duration can be controlled. Similar to the direct detonation technique, a primary explosive is robotically sprayed onto the flyer plate and subsequently detonated using an intense flash of light. Through the control of the explosive deposition and flyer gap, virtually simultaneous impact is achievable for either uniform or graded loading conditions. X-Flyer test results will be presented.   

Deformation Response of Copper Single Crystals under Shockless Loading Conditions

Recovery based observations of high pressure material behavior generated under laser based quasi-isentropic loading conditions are reported. Material, recovered from a high pressure, high strain rate laser based platform, was characterized to infer the deformation response of copper to changes in the loading path from the shock Hugoniot to one near the isentrope. Whereas the temperature, pressure and strain states along these two paths are essentially equivalent, the strain rates are considerably different and a significant difference in the active deformation mechanism is observed. Material loaded quasi-isentropically shows a residual dislocation cell structure at pressure that is well above the slip twin transition under shock loading. At even higher pressures the quasi-isentropic deformation mechanism transitions to twin formation. These results show that the high pressure slip-twin transition can be strongly affected by the loading path. The observations are rationalized by using a pressure dependent twinning threshold stress along with a strain rate dependent constitutive model.   

Explicit time dependence of the Lagrangian velocity and ramp waves

In recent years new experimental techniques have been developed to study the propagation of nonlinear compression waves as they steepen and eventually approach a stationary Hugoniot state. In general the wave evolves from a low entropy state to a high entropy state with increasing Lagrangian coordinate. It is of interest to quantify how far the wave has deviated from an isentrope for accurate determinations of the equation of state. A new theoretical technique is being developed to exhibit the explicit time dependence of the Lagrangian velocity and therefore irreversibility in a wide range of experimental ramp waves. The technique is also demonstrated for theoretical nonlinear waves with varying degrees of irreversibility. The technique can be used to estimate corrections to the Lagrangian velocity for the determination of a more accurate equation of state.   

In-plane propagation and focusing of laser-driven shock waves

Typically, laser induced shock research involves focusing an intense laser pulse into a thin layer which launches a shock pulse into the underlying material that includes the sample of interest. A novel approach to optical shock generation has been demonstrated that permits direct real-time visualization of the shock front. Our approach opens up new possibilities for controlling the shock parameters and for a wide range of spectroscopic measurements of shock propagation and sample response. In this approach, a shock wave is generated that propagates \textit{laterally} in the plane of the sample (perpendicular to the direction of the optical beam) rather than through the sample plane. The optical configuration and sample geometry make shock wave formation and propagation directly accessible to optical imaging and spectroscopic probes with wavelengths ranging from UV to far-IR. With proper shaping of the optical shock generation pulse, focusing of the shock response can be initiated to provide increased shock pressure. Preliminary results will be shown that illustrate some of the possibilities for shock generation, control, and measurement.   

Theory for Isentropic Compression of Ta

In high-pressure isentropic compression experiments (ICE), the pressure is dominated by the cold curve. In order to obtain an accurate semi-empirical cold curve for Ta, we calculate the thermal pressure from {\em ab initio} phonon and electronic excitation spectra. The cold curve is then inferred from ultrasonic and shock data to pressures in the 2-3 Mbar range, and extended to higher pressures using electronic structure calculations. We predict the principal isentrope to 5 Mbar on this basis. We also make estimates of the contribution of shear strength, both directly and via dissipation of plastic work, in Mbar-range ICE.