Isentropic Compression Experiments Research Articles

Circuit Model for Driving Three-Dimensional Resistive MHD Wire Array $Z$-Pinch Calculations

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Compact tungsten wire array <formula formulatype="inline"><tex Notation="TeX">$Z$</tex></formula>-pinches imploded on the Z generator at Sandia National Laboratories have proven to be a powerful reproducible X-ray source. Wire arrays have also been used in dynamic hohlraum radiation flow experiments and as an intense K-shell source, while the generator has been used extensively for isentropic compression experiments. A problem shared by all these applications is current loss, preventing the <formula formulatype="inline"><tex Notation="TeX">$\sim$</tex></formula>20-MA drive current from being reliably coupled to the load. This potentially degrades performance, while uncertainties in how this loss is described limit our predictive capability. We present details of a transmission line equivalent circuit model of the Z generator for use in driving 3-D resistive MHD simulations of wire array loads. We describe how power delivery to these loads is affected by multiple current losses and demonstrate how these may be calculated or reconstructed from available electrical data for inclusion in the circuit model. We then demonstrate how the circuit model and MHD load calculation may be combined to infer an additional current loss that has not been directly diagnosed for wire arrays. </para>

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. Wave analysis of in situ particle 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.

40 GW Linear Transformer Driver stage for pulse generators of Mega-ampere range

AbstractLinear transformer driver (LTD) technology is actively developed at the Institute of High Current Electronics in Tomsk, Russia. This technology is being examined for use in high current high voltage pulsed accelerators. Recent development of high voltage low inductance capacitors and low inductance switches enabled to achieve ~100 ns rise time of the LTD output pulse. This technique allows one to eliminate intermediate pulse forming sections, used in the present accelerator technology, which would keep the footprint of an LTD accelerator small. LTD based drivers are currently considered for many applications, including future very high current Z-pinch drivers for inertial confinement fusion, medium current drivers with adjustable pulse length for isentropic compression experiments, and finally relatively low current accelerators for radiography and X-pinches. In this article, we present the design and test results for a new LTD stage, that operates at 100 kV charging voltage. Current amplitude up to 850 kA with ~140 ns rise time was obtained on a 0.05 Ω load. Stack of the LTD stages can be easily assembled in series or in parallel, thus providing voltage or current multiplication, respectively. Design of multi-mega-volt and multi-mega-ampere generators becomes straightforward with the LTD technology.

Dislocation mechanics of copper and iron in high rate deformation tests

Different dislocation processes are shown to be operative under high rate loading by impact-induced shock tests as compared with shockless isentropic compression experiments (ICEs). Under shock loading, the plastic deformation rate dependence of the flow stress of copper is attributed to dislocation generation at the propagating shock front, while in shockless ICEs, the rate dependence is attributed to drag-controlled mobile dislocation movement from within the originally resident dislocation density. In contrast with shock loading, shockless isentropic compression can lead to flow stress levels approaching the theoretical yield stress and dislocation velocities approaching the speed of sound. In iron, extensive shock measurements reported for plate impact tests are explained in terms of plasticity-control via the nucleation of deformation twins at the propagating shock front.

Veloce: A Compact Pulser for Magnetically Driven Isentropic Compression Experiments

The use of magnetic fields to isentropically compress materials for equation-of-state studies has been first demonstrated on the Z machine at Sandia National Laboratories. Sharing similarities with the GEPI pulser, a compact pulser has been designed and built, focusing on isentropic compression experiments. In order to achieve high compacity and fast turn around, the design is built around a solid dielectric transmission line to couple current from eight low-inductance capacitors that are switched with ultra-low-inductance multichannel gas switches operating in dry air at atmospheric pressure. A peaking stage made of 72 capacitors enhanced by a low-inductance multichannel sharpening switch brings the fundamental rise time of the pulser down to 350 ns (10%-90%). A set of inductances in parallel with the sharpening switch as well as using various gases into this switch allow us to modify the current wave shape. The pulser delivers a peak current of 4 MA at a charge voltage of 80 kV on a short circuit. The rise time can be lengthened to around 650 ns for a current of 4.2 MA. The use of postholes convoluting in a solid dielectric insulation design makes that pulser unique, as well as its compact size, ease of use, and ease of access to the load.

Development of a 5.4 MV laser triggered gas switch for multimodule, multimegampere pulsed power drivers

Laser triggered, megavolt, megampere gas switches are frequently utilized to synchronize multiple pulsed power driver modules for inertial-confinement fusion, isentropic compression, and radiation physics experiments. The device developed to synchronize the 36 modules of the refurbished Z accelerator at Sandia National Laboratories is a 5.4 MV, 700 kA, sulfur-hexafluoride (SF6) filled, laser triggered gas switch. At this operating level, switch jitter is 5 ns, the prefire rate is less than 0.1%, the average optic lifetime is greater than 200 shots, and the flashover rate is less than 1%. Over 1000 shots on a single-module test facility were conducted while iterating several potential design improvements, including utilizing low-erosion electrode material, varying SF6 pressure, and modifying internal switch geometry all while keeping the basic switch architecture and footprint constant. Results of this development effort are presented herein.

The compact capacitor bank CQ-1.5 employed in magnetically driven isentropic compression and high velocity flyer plate experiments

Based on the low inductance capacitor, the parallel-plate transmission line, and the explosive network closing switch, a compact pulsed power generator CQ-1.5 has been developed at the Institute of Fluid Physics and is capable to deliver a current of peak of 1.5 MA within rise time of 500-570 ns into a 2-3 nH inductive load. The work is motivated to do isentropic compression experiments (ICEs) on metals up to 30-50 GPa and to launch flyer plates at velocities over 8 kms. The experiments were conducted with the diagnostics of both Doppler pin system and velocity interferometer system for any reflectors, and the measured free surface velocity histories of ICE samples were treated with a backward integration code. The results show that the isentropes of Cu and Al samples under 35 GPa are close to their Hugoniots within a deviation of 3%. The LY12 aluminum flyer plates were accelerated to a velocity over 8.96 kms.

High strain rate properties of metals and alloys

The high strain rate dependence of the flow stress of metals and alloys is described from a dislocation mechanics viewpoint over a range beginning from conventional tension/compression testing through split Hopkinson pressure bar (SHPB) measurements to Charpy pendulum and Taylor solid cylinder impact tests and shock loading or isentropic compression experiment (ICE) results. Single crystal and polycrystal measurements are referenced in relation to influences of the crystal lattice structures and nanopolycrystal material behaviours. For body centred cubic (bcc) metals, the strain rate sensitivity (SRS) is in the yield stress dependence as compared with the face centred cubic (fcc) case of being in the strain hardening property. An important consequence is that an opposite ductility influence occurs for the tensile maximum load point strain that decreases with strain rate for the bcc case and increases with strain rate for the fcc case. Different hexagonal close packed (hcp) metals are shown to follow either the bcc or fcc case. A higher SRS for certain fcc and hcp nanopolycrystals is explained by extrapolation from conventional grain sizes of an inverse square root of grain size dependence of the reciprocal activation volume determined on a thermal activation strain rate analysis (TASRA) basis. At the highest strain rates, additional deformation features enter, such as deformation twinning, adiabatic shear banding and very importantly, for shock induced plasticity, transition from plastic flow that is controlled by the mobility of the resident dislocation density to plasticity that is controlled by dislocation or twin generations at the shock front. The shock description is compared with the very different high rate shockless ICE type loading that occurs over nanoseconds and leads to higher compressive strength levels because of dislocation drag resistance coming into play for the originally resident mobile dislocation density. Among the high strain rate property, concerns are the evaluation of ductile to brittle transition behaviours for bcc and related metals and also, projectile/target performances in ballistic impact tests, including punching. Very complete metallographic and electron microscope observations have been reported in a number of the high rate deformation investigations.

A compact strip-line pulsed power generator for isentropic compression experiments

Veloce is a medium-voltage, high-current, compact pulsed power generator developed for isentropic and shock compression experiments. Because of its increased availability and ease of operation, Veloce is well suited for studying isentropic compression experiments (ICE) in much greater detail than previously allowed with larger pulsed power machines such as the Z accelerator. Since the compact pulsed power technology used for dynamic material experiments has not been previously used, it is necessary to examine several key issues to ensure that accurate results are obtained. In the present experiments, issues such as panel and sample preparation, uniformity of loading, and edge effects were extensively examined. In addition, magnetohydrodynamic simulations using the ALEGRA code were performed to interpret the experimental results and to design improved sample/panel configurations. Examples of recent ICE studies on aluminum are presented.

Analysis and modeling of phase contrast radiography of gradient density laser targets

Laser experiments, such as those planned at the National Ignition Facility (NIF) and the Omega facility, use small targets with the goal of studying high-energy density physics and inertial confinement fusion. One particular application is a target with layers whose density changes in a carefully designed gradient (from 0.2 to 1.2 g/cm 3) for use in isentropic compression experiments (ICE). We are nondestructively determining the density of these layers using two X-ray microscopes. Because of the many interfaces that comprise the layers, a plethora of X-ray phase contrast fringes appear in the images, leading to many radiographic and tomographic artifacts which compromise the ability to infer the density of the layer. In this paper, we describe how we are attacking this problem with a variety of radiographic standards and through radiographic simulation using the HADES radiographic simulation code.

Material characterization with ramp wave experiments

Ramp wave experiments on the Sandia Z accelerator provide a different approach to study the rapid compression response of materials at pressures, temperatures, and stress or strain rates not attainable in conventional shock experiments. Due to its shockless nature, the ramp wave experiment is often termed as an isentropic (or quasi-isentropic) compression experiment (ICE) and the analysis of ICE has been focused on determination of the isentropes. One objective of the current study is to show that ramp wave experiment can be used as a much more general material characterization tool for studying material behavior under high strain rates and pressures. The second objective is to suggest practical methodology to design the experiment and analyze experimental data. Numerical simulations were used to achieve these objectives. It is demonstrated that the ramp wave experiment is essentially a controlled-strain-rate material test. The strain rate can be varied through the rise time and shape of the ramp wave. The resultant stress-strain relation is a specific relation for a specific strain-rate history. The isentrope, which is a limiting case of such relations, may be approximated through a very low strain-rate loading path. Because of the rate dependence of the material behavior, each material point experiences different strain-rate loading paths. Lagrangian analysis requires information pertinent to a local Lagrangian material point. This information may be obtained through a pair of wave profiles measured at two very close, but essentially the same, Lagrangian points.

Characteristics analysis of Isentropic Compression Experiments (ICE)

Several new techniques for high-pressure (quasi-) isentropic compression of materials are becoming available: pulsed-power-driven magnetic pressure, graded-density impactors on gas-guns, and laser drive. These experiments are analysed to give sound speed as a function of particle velocity, which may then be integrated to pressure and volume, giving data for a complete isentrope with one shot. However, to do this the experimentally-measured free-surface - or window-interface - velocities must be corrected to give the in-situ material velocities in the absence of the surface or interface. This may be done by either backward integration (integrate the equations of motion in space rather than time using surface velocity vs time as the boundary conditions) or by a characteristics method. This presentation describes the characteristics method and its application to AWE experiments on the isentropic compression of lead. The experiment has produced results consistent with pre-existing experimental data within errors in sound speed and pressure of ∼4%. Iterative corrections to in-situ velocity, using the EOS calculated during the previous step, reduce - or do away with - any dependence of the results on an assumed EOS. Simultaneous characteristics calculations of the input pressure-time are used to estimate the errors.

Dynamic material characterization under ramp wave compression with GEPI device

GEPI is a pulsed power generator developed by ITHPP for Centre d'Etudes de Gramat (CEG), devoted to Isentropic Compression Experiments in the 1 GPa to 100 GPa range, and to non shocked high velocity flyer plates in the 0.1 km/s to 10 km/s range. The main idea is to generate a high magnetic pressure in a strip line where the samples are located. The whole design is based on low inductance technologies. Depending on the load, the current reaches between 3 and 4 MA in 600 ns. The entire design has been done in a cost effective way and in order to achieve an easy-to-use capability. A description of the generator is shown and typical results of the studies led by CEG are presented. The matters of concern are equations of state, phase transitions, sample recovery and impact of high velocity flyer plates.

Thermal and mechanical analysis of material response to non-steady ramp and steady shock wave loading

Ramp wave experiments on the Sandia Z accelerator provide a new approach to study the rapid compression response of materials at pressures, temperatures and stress or strain rates not attainable in conventional shock experiments. Due to its shockless nature, the ramp wave experiment is often termed as an isentropic (or quasi-isentropic) compression experiment (ICE). However, in reality there is always some entropy produced when materials are subjected to large amplitude compression even under shockless loading. The entropy production mechanisms that cause deformation to deviate from the isentropic process can be attributed to mechanical and thermal dissipations. The former is due to inelasticity associated with various deformation mechanisms and the rate effect that is inherent in all the deformation processes and the latter is due to irreversible heat conduction. The main purpose of the current study is to gain insights into the effects of ramp and shock loading on the entropy production and thermomechanical responses of materials. Another purpose is to investigate the role of heat conduction in the material response to both the non-steady ramp wave and steady shock. Numerical simulations are used to address the aforementioned research objectives. The thermomechanical response associated with a steady shock wave is investigated first by solving a set of nonlinear ordinary differential equations. Using the steady wave solutions as the reference, the material responses under non-steady ramp waves are then studied with numerical wave propagation simulation. It is demonstrated that the material response to ramp and shock loading is essentially a manifestation of the interaction between the time scale associated with the loading and the intrinsic time scales associated with mechanical deformation and heat transfer. At lower loading rates as encountered in ramp loading, the loading path is closer to an isentrope and results in lower entropy production. The reasonable ramp rate to obtain a quasi-isentropic state depends on the intrinsic time scales of the dissipation mechanisms which are strongly material dependent. Thus shockless loading does not necessarily produce an isentropic response. Between two equilibrium states, heat conduction was shown to have significant effect on the temperature history but it contributes little to the overall temperature change if the specific heat remains constant. It also affects the history of entropy, but only the irreversible part of heat conduction contributes to the net entropy change. The various types of thermomechanical responses of materials would manifest themselves more significantly in terms of the thermal history than the mechanical history. Thus temperature measurement appears to be an important experimental tool in distinguishing the various mechanisms for the thermomechancial responses of the materials.

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 (&amp;gt;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 &amp;gt;1keV at 0.1% of solid density. These plasmas produce x-ray energies approaching 2MJ at powers &amp;gt;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 &amp;gt;3Mbar and accelerates flyer plates to &amp;gt;30km∕s for equation of state (EOS) experiments at pressures up to 10Mbar in aluminum. Development of multidimensional radiation-magnetohydrodynamic codes, coupled with more accurate material models (e.g., quantum molecular dynamics calculations with density functional theory), has produced synergy between validating the simulations and guiding the experiments. Z is now routinely used to drive ICF capsule implosions (focusing on implosion symmetry and neutron production) and to perform HEDP experiments (including radiation-driven hydrodynamic jets, EOS, phase transitions, strength of materials, and detailed behavior of z-pinch wire-array initiation and implosion). This research is performed in collaboration with many other groups from around the world. A five year project to enhance the capability and precision of Z, to be completed in 2007, will result in x-ray energies of nearly 3MJ at x-ray powers &amp;gt;300TW.

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.

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.

Continuous index of refraction measurements to 20 GPa in Z-cut sapphire

Isentropic compression experiments (ICE) on Sandia National Laboratories’ Z Accelerator are used to make continuous measurements of the index of refraction in single crystal Z-cut sapphire as the longitudinal stress is gradually increased to 20 GPa (uniaxial strain of about ∼3.9%). A planar, ramp compression wave, generated by a large magnetic field, propagates through a planar copper base plate. Part of the rear surface of the base plate is a free surface and part is covered with a sapphire window. Comparisons are made of velocity histories measured simultaneously at the free surface and at the copper/sapphire interface using a velocity interferometer system for any reflector (VISAR). This is sufficient to determine the index of refraction of the sapphire continuously as a function of density. Perturbations to the compression waves caused by interactions with the free surface and with the sapphire are accounted for by backward or forward solutions to the equations of motion. The effect of the unsteady nature of the compression wave in sapphire on the interferometry measurements is also taken into account. Results validate the interpretation of VISAR measurements on continuous loading experiments performed with optical windows. The measured index of refraction of sapphire agrees with previous studies validating this experimental technique and showing that it is useful for studying other materials for which index of refraction data do not exist. In addition, measured compression isentropes for copper and sapphire compare favorably with existing theoretical models.

Isentropic Compression Experiment on Aluminum Using the Z-Accelerator

The capability of using the Z-Machine at Sandia to perform isentropic compression experiments has been discussed by Hall previously. Pressures exceeding 1.5 Mbar have been launched into materials and the pressure wave can be shaped by varying the load current in Z. In this paper, theoretical results will be presented for an aluminum sample in which we obtain isentropic equations of state (EOS) information.Obtaining the isentropic EOS is necessary in many scientific and technological fields for computer simulations. We will follow the procedure outlined by Reisman to determine the EOS. From these steps, we will determine the theoretical EOS of aluminum using data obtained from BUCKY. We will discuss any variances we have in our results due to the use of two different sets of EOS opacity data.The results presented here were obtained using BUCKY, a 1-D MHD code developed at the University of Wisconsin-Madison. BUCKY is a code that simulates highenergy density plasmas and target yields for Inertial Confinement Fusion (ICF). BUCKY was originally designed to study target physics and target chamber designs for ICF reactors but can be used to study Isentropic Compression Experiments.We will describe the procedure used to determine the velocity wave profile measurements that leads to determining EOS. From the velocity wave profile we will be able to determine the isentropic compression equations of state of the aluminum sample modeled.

Isentropic compression experiments to 1 Mbar using magnetic pressure

Isentropic compression experiments that utilize intense magnetic fields tocompress samples have been designed, developed and performed. The techniquehas been shown to work to pressures of more than 1 Mbar on Sandia NationalLaboratory’s Z pulsed power machine. We are extending the technique to usehigh-explosive pulsed power.