Randy J. Hickman

Design of a Thermal Imaging Diagnostic Using 90-Degree, Off-Axis, Parabolic Mirrors

Thermal imaging is an important, though challenging, diagnostic for shockwave experiments. Shock-compressed materials undergo transient temperature changes that cannot be recorded with standard (greater than ms response time) infrared detectors. A further complication arises when optical elements near the experiment are destroyed. We have designed a thermal-imaging system for studying shock temperatures produced inside a gas gun at Sandia National Laboratories. Inexpensive, diamond-turned, parabolic mirrors relay an image of the shocked target to the exterior of the gas gun chamber through a sapphire vacuum port. The 3000-5000-nm portion of this image is directed to an infrared camera which acquires a snapshot of the target with a minimum exposure time of 150 ns. A special mask is inserted at the last intermediate image plane, to provide dynamic thermal background recording during the event. Other wavelength bands of this image are split into high-speed detectors operating at 900-1700 nm and at 1700-3000 nm, for time-resolved pyrometry measurements. This system incorporates 90-degree, off-axis parabolic mirrors, which can collect low f/# light over a broad spectral range, for high-speed imaging. Matched mirror pairs must be used so that aberrations cancel. To eliminate image plane tilt, proper tip-to-tip orientation of the parabolic mirrors is required. If one parabolic mirror is rotated 180 degrees about the optical axis connecting the pair of parabolic mirrors, the resulting image is tilted by 60 degrees. Different focal-length mirrors cannot be used to magnify the image without substantially sacrificing image quality. This paper analyzes performance and aberrations of this imaging diagnostic.

The Isentrope of Unreacted LX‐04 to 170 kbar

We present new data on the unreacted approximate isentrope of the HMX-based explosive LX-04, measured to 170 kbar, using newly developed long pulse isentropic compression techniques at the Sandia National Laboratories Z Machine facility. This study extends in pressure by 70% the previous state of the art on unreacted LX-04 using this technique. This isentrope will give the unreacted Hugoniot from thermodynamic relations using a Gruneisen equation of state model. The unreacted Hugoniot of LX-04 is important in understanding the structure of the reaction front in the detonating explosive. We find that a Hugoniot given by U{sub s}= 2950 m/s + 1.69 u{sub p} yields for an isentrope a curve which fits our LX-04 ICE data well.

Superfast assembly and synthesis of gold nanostructures using nanosecond low-temperature compression via magnetic pulsed power

Gold nanostructured materials exhibit important size- and shape-dependent properties that enable a wide variety of applications in photocatalysis, nanoelectronics and phototherapy. Here we show the use of superfast dynamic compression to synthesize extended gold nanostructures, such as nanorods, nanowires and nanosheets, with nanosecond coalescence times. Using a pulsed power generator, we ramp compress spherical gold nanoparticle arrays to pressures of tens of GPa, demonstrating pressure-driven assembly beyond the quasi-static regime of the diamond anvil cell. Our dynamic magnetic ramp compression approach produces smooth, shockless (that is, isentropic) one-dimensional loading with low-temperature states suitable for nanostructure synthesis. Transmission electron microscopy clearly establishes that various gold architectures are formed through compressive mesoscale coalescences of spherical gold nanoparticles, which is further confirmed by in-situ synchrotron X-ray studies and large-scale simulation. This nanofabrication approach applies magnetically driven uniaxial ramp compression to mimic established embossing and imprinting processes, but at ultra-short (nanosecond) timescales.

High precision Hugoniot measurements on statically pre-compressed fluid helium

The capability for statically pre-compressing fluid targets for Hugoniot measurements utilizing gas gun driven flyer plates has been developed. Pre-compression expands the capability for initial condition control, allowing access to thermodynamic states off the principal Hugoniot. Absolute Hugoniot measurements with an uncertainty less than 3% on density and pressure were obtained on statically pre-compressed fluid helium utilizing a two stage light gas gun. Helium is highly compressible; the locus of shock states resulting from dynamic loading of an initially compressed sample at room temperature is significantly denser than the cryogenic fluid Hugoniot even for relatively modest (0.27–0.38 GPa) initial pressures. The dynamic response of pre-compressed helium in the initial density range of 0.21–0.25 g/cm3 at ambient temperature may be described by a linear shock velocity (us) and particle velocity (up) relationship: us = C0 + sup, with C0 = 1.44 ± 0.14 km/s and s = 1.344 ± 0.025.

VELOCE: A COMPACT PULSER FOR DYNAMIC MATERIAL CHARACTERIZATION AND HYPERVELOCITY IMPACT OF FLYER PLATES

Sharing similarities with the GEPI pulser which is dedicated to Isentropic Compression Experiments (ICE), VELOCE, an even more compact electrical pulser, has been designed and built in duplicate for SNL and WSU. This type of machine complements gun and laser facilities in the study of material response. In order to achieve a broad loading capability and fast turn around, the design is built around a solid dielectric transmission line to couple current from low inductance capacitors and electrically triggered switches. Peaking capacitors enhanced by a low inductance, multi‐channel sharpening switch reduce the quarter period of the pulser to about 470 ns (0–100%). Gas mixtures in the switch cavity and inductances in parallel allow modification of the shape of the induced pressure wave. At 80 kV of charge voltage, the peak current can reach 3.5 MA. Design of the pulser, range of pressures and velocities, as well as potential applications are presented. A consistent numerical tool developed for pulsers design...

The Veloce Pulsed Power Generator for Isentropic Compression Experiments

Summary form only given. Veloce is a medium-voltage, high-current, compact pulsed power generator developed for cost effective isentropic and shock compression experiments at Sandia National Laboratories. It is based on a strip line design where no oil, water, and vacuum are used for insulation, thus making it easy to operate and maintain. The machine delivers up to 3 MA of current rapidly (~ 440-530 ns) into an inductive load where significant magnetic pressures are produced. The resulting magnetic pressure can be used either to drive ramp pressure waxes (peak pressures up to 12 GPa) into material samples or to launch a flyer plates to high velocities (1-4 km/s). Because of its lower cost per shot and greater availability, Veloce is well suited for studying isentropic compression experiments (ICE) in much greater detail than previously allowed in larger pulsed power machines. Several key issues in ICE, such as panel and sample preparation, uniformity of loading, and edge effects, have been examined extensively on Veloce. In addition, magnetic hydrodynamic simulations using the Alegra code have been performed to interpret the experimental results.

LINE‐IMAGING ORVIS MEASUREMENTS OF INTERFEROMETRIC WINDOWS UNDER QUASI‐ISENTROPIC COMPRESSION

A line‐imaging optically recording velocity interferometer system (ORVIS) has been implemented on the Veloce pulsed power generator to enable measurement of spatially resolved velocity histories of materials under dynamic compression. Interferometric windows are regularly used to maintain the high‐pressure state of shock and ramp (quasi‐isentropic) loaded materials. Although imaging through a shock or fast ramp (⩽10 ns) loaded window material has been reasonably successful, for slower ramp loading (∼440 ns) experiments, the elastic‐plastic yielding of the window has an adverse effect on return light to the line‐imaging ORVIS. The results of quasi‐isentropic loading experiments with various interferometric windows such as LiF, PMMA, NaCl, quartz and sapphire are presented.

Effects of Annealing and Preheating on the Impact Response of Selected Braze Materials

A series of gas‐gun experiments has probed the planar (uniaxial strain) impact response of six different commercial brazing alloys that were subjected to a peak shock stress in the range of 5.5 – 7.2 GPa. The alloys studied were copper + gold (65/35 wt%), copper + gold (50/50 wt%), Cusil®, Nicusil®‐3, Nicoro® + titanium (98/2 wt%), and silver zirconate. Both as‐received and annealed samples of each alloy were tested under ambient (room temperature) and preheated (100 C) initial conditions. Velocity interferometer data acquired during this investigation have been evaluated to determine the dynamic yield strength (i.e., Hugoniot Elastic Limit), shock Hugoniot state, and spall strength for baseline alloy samples (i.e., unheated, as‐received material) and for samples whose initial condition involved annealing and/or preheating.