J.-P. Davis

Strength of lithium fluoride under shockless compression to 114 GPa

A magnetic loading technique was used to ramp load single-crystal [100] lithium fluoride specimens to peak stresses of 5–114 GPa. Waveanalysis of in situparticle velocity profiles was used to estimate the compressive strength of LiF at peak stress. It was found that the strength increased with peak stress and showed two distinct regions of hardening; the first is believed to be governed by strain hardening and the second by pressure hardening. The quasielastic strain obtained from the initial part of the unloading was shown to saturate at about 1.3% for peak stresses greater than approximately 30 GPa. Over the studied pressure range, the measured strength of LiF varied from its initial value of 0.08 to about 1.1 GPa at the highest compressed state of 114 GPa. Comparison of the measured strength to results from two strength models showed good agreement. It was demonstrated that the strength of LiF introduces systematic error of about 10% when used as an interferometer window for measurements of material strength in isentropic compression experiments.

Genesis: A 5-MA Programmable Pulsed-Power Driver for Isentropic Compression Experiments

Enabling technologies are being developed at Sandia National Laboratories to improve the performance and flexibility of compact pulsed power drivers for magnetically driven dynamic materials properties research. We have designed a modular system capable of precision current pulse shaping through the selective triggering of pulse forming components into a disk transmission line feeding a strip line load. The system is comprised of two hundred and forty 200 kV, 60 kA modules in a low inductance configuration capable of producing 250–350 kbar of magnetic pressure in a 1.75 nH, 20 mm wide strip line load. The system, called Genesis, measures approximately 5 meters in diameter and is capable of producing shaped currents greater than 5 MA. This performance is enabled through the use of a serviceable solid dielectric insulator system which minimizes the system inductance and reduces the stored energy and operating voltage requirements. Genesis can be programmed by the user to generate precision pulse shapes with rise times of 220–500 ns, allowing characterization of a range of materials from tungsten to polypropylene. This paper provides an overview of the Genesis design including the use of genetic optimization to shape currents through selective module triggering.

Genesis: A 5 MA programmable pulsed power driver for Isentropic Compression Experiments

Enabling technologies are being developed at Sandia National Laboratories to improve the performance and flexibility of compact pulsed-power drivers for magnetically driven dynamic materials properties research. We have designed a modular system that is capable of precision current pulse shaping through the selective triggering of pulse-forming components into a disk transmission line feeding a strip line load. The system is composed of 240 200-kV 60-kA modules in a low-inductance configuration that is capable of producing 250-350 kbar of magnetic pressure in a 1.75-nH 20-mm-wide strip line load. The system, called Genesis , measures approximately 5 m in diameter and is capable of producing shaped currents that are greater than 5 MA. This performance is enabled through the use of a serviceable solid-dielectric insulator system which minimizes the system inductance and reduces the stored energy and operating voltage requirements. Genesis can be programmed by the user to generate precision pulse shapes with rise times of 220-500 ns, allowing characterization of a range of materials from tungsten to polypropylene. This paper provides an overview of the Genesis design, including the use of genetic optimization to shape currents through selective module triggering.

Status of genesis a 5 MA programmable pulsed power driver

Genesis is a compact pulsed power platform designed by Sandia National Laboratories to generate precision shaped multi-MA current waves with a rise time of 200–500 ns. In this system, two hundred and forty, 200 kV, 80 kA modules are selectively triggered to produce 280 kbar of magnetic pressure (>500 kbar pressure in high Z materials) in a stripline load for dynamic materials properties research. This new capability incorporates the use of solid dielectrics to reduce system inductance and size, programmable current shaping, and gas switches that must perform over a large range of operating conditions. Research has continued on this technology base with a focus on demonstrating the integrated performance of key concepts into a Genesis-like prototype called Protogen. Protogen measures approximately 1.4 m by 1.4 m and is designed to hold twelve Genesis modules. A fixed inductance load will allow rep-rate operation for component reliability and system lifetime experiments at the extreme electric field operating conditions expected in Genesis.

Recent Advances in Quasi‐isentropic Compression Experiments (ICE) on the Sandia Z Accelerator

The Z Accelerator is a pulsed power machine capable of delivering currents to loads of ∼20 MA over times of 100–300 ns. This current produces smoothly increasing, time dependant magnetic pressures that can be applied to specimens allowing quasi‐isentropes for these materials to be inferred. A new load design has been developed that allows this pressure to be uniformly applied to as many as 8 samples simultaneously. Diagnostics have recently been fielded that have resulted in an increased understanding of the magneto‐hydrodynamic effects and our confidence in the utility of this experimental configuration for EOS measurements. Efforts are also underway on Z to provide a capability for shaping the pressure profile applied to the samples which should increase useful sample thicknesses to > 1 mm by eliminating the formation of low‐level shocks. In addition to direct measurements of quai‐isentropic material response, the impulse from this loading technique has been demonstrated to launch macroscopic flyer plat...

Impact of time-varying loads on the programmable pulsed power driver called genesis

The success of dynamic materials properties research at Sandia National Laboratories has led to research into ultra-low impedance, compact pulsed power systems capable of multi-MA shaped current pulses with rise times ranging from 220–500 ns. The Genesis design consists of two hundred and forty 200 kV, 80 kA modules connected in parallel to a solid dielectric disk transmission line and is capable of producing 280 kbar of magnetic pressure (>500 kbar pressure in high Z materials) in a 1.75 nH, 20 mm wide stripline load. Stripline loads operating under these conditions expand during the experiment resulting in a time-varying load that can impact the performance and lifetime of the system. This paper provides analysis of time-varying stripline loads and the impact of these loads on system performance. Further, an approach to reduce dielectric stress levels through active damping is presented as a means to increase system reliability and lifetime.

Isentropic Compression of Lead and Lead Alloy Using the “Z” Machine

We have performed experiments on the quasi‐isentropic compression of lead (both bulk and single‐crystal) and lead‐antimony alloy to ∼500kbar using magnetic‐pressure drive from Sandia National Laboratory’s Z machine. Multiple point VISARs were used to record free‐surface velocities of multiple thicknesses of the three materials — analysis is by calculating sound‐speed as a function of particle velocity from the different times at which the samples reach the same velocity. This apparently simple process is complicated by: formation of shocks, the time‐dependent pressure profile, including surface reflections of ramp waves, and calculation of in‐situ particle velocity from free‐surface velocity. This article will describe the experiments and present a preliminary analysis of the data.

Comparison of the performance of the upgraded Z with circuit predictions

Since the completion of the ZR upgrade of the Z accelerator at the Sandia National Laboratories in the fall of 2007, many shots have been taken on the accelerator, and there has been much opportunity to compare circuit-code predictions of the performance of the machine with actual measurements. We therefore show comparisons of measurements, and describe a full-circuit, 36-line Bertha circuit model of the machine. The model has been used for both short-pulse and long-pulse (tailored pulse) modes of operation. We also present the as-built circuit parameters of the machine and indicate how these were derived. We discuss enhancements to the circuit model that include 2D effects in the water lines, but show that these have little effect on the fidelity of the simulations. Finally, we discuss how further improvements can be made to handle azimuthal coupling of the multiple lines at the vacuum insulator stack.

HIGH‐PRESSURE QUASI‐ISENTROPIC LOADING AND UNLOADING OF INTERFEROMETER WINDOWS ON THE VELOCE PULSED POWER GENERATOR

The Isentropic Compression Experiment (ICE) technique has proven to be a valuable complement to the well‐established method of shock compression of condensed matter. However, whereas the high‐pressure compression response of interferometer window materials has been studied extensively under shock loading, similar knowledge of these materials under quasi‐isentropic loading is limited. We present recent experimental results on the quasi‐isentropic compression of the high‐pressure window LiF on the Veloce pulsed power generator. While it is a frequently used window material in quasi‐isentropic loading and unloading experiments, the unloading response of LiF is usually neglected. It will be shown how the strength of LiF may influence the wave profile analysis and thus the inferred compressive strength of the material being studied.

Circuit-Code Modeling of the Refurbished Z Accelerator: Comparison of Measurements with Predictions

With the successful completion of its refurbishment the Z machine at Sandia is now routinely operating with currents over 26 MA into various loads. Now that the machine is operating we can measure current and voltage at various locations throughout the machine and compare with circuit code predictions. These measurements have led to improvements in the model that provide a more accurate predictive capability. In this paper we describe the full-machine circuit model of Z, and indicate how machine parameters are derived. Many were determined with commercially-available field calculation software, but parameters for switches and other non-linear elements were determined empirically. We show comparisons of circuit code predictions with machine performance. Finally, we show where improvements to the model can yet be made.