Timothy Pierce

Orientation dependent far-infrared terahertz absorptions in single crystal pentaerythritol tetranitrate (PETN) using terahertz time-domain spectroscopy.

Terahertz time-domain spectroscopy (THZ-TDS) has been used to measure the absorption spectra in the range 7-100 cm(-1) (0.2-3 THz) of single crystal pentaerythritol tetranitrate (PETN). Absorption was measured in transmission mode as a function of incident polarization with the incident and transmitted wave vectors oriented along the crystallographic directions [100], <10(a/c)(2)>, and <110>. Samples were rotated with respect to the incident polarization while absorption was measured at both 300 and 20 K. Comparatively minor differences were observed among the three orientations. Two broad absorptions at 72 and >90 cm(-1), and several weaker absorptions at 36, 55, 80, and 82 cm(-1), have been observed at cryogenic temperatures.

High-density polyethylene damage at extreme tensile conditions

In-situ and postmortem observations of the dynamic tensile failure and damage evolution of high-density polyethylene (HDPE) are made during Dynamic-Tensile-Extrusion (Dyn-Ten-Ext) loading. The Dyn-Ten-Ext technique probes the tensile response of materials at large strains (>1) and high strain-rates (>105 s−1) by firing projectiles through a conical die. Postmortem sectioning elucidates a mechanism of internal damage inception and progression. X-ray computed tomography corroborates shear damage with cracks nearly aligned with the extrusion axis but separated by unfailed internal bridges of material. In-situ measurements of damage are made with the impact system for ultrafast synchrotron experiments (IMPULSE) using the advanced imaging X-ray methods available at the Advanced Photon Source. Multiple frame phase-contrast imaging (PCI) elucidates the evolution of damage features in HDPE during Dyn-Ten-Ext loading that is observed in postmortem sectioning and X-ray tomography.

Temperature of shocked plastic bonded explosive PBX 9502 measured with spontaneous Stokes/anti-Stokes Raman

Raman spectra and velocimetry of shocked PBX 9502 (plastic bonded explosive composed of 95% triaminotrinitrobenzene (TATB) and 5% 3M Company Kel F-800 polymer binder) are reported with the Stokes/anti-Stokes ratio used to determine temperature after the shock reflects from a lithium fluoride window. Final pressures up to 14.5 GPa were tested, but the pressure induced absorption of TATB caused the Raman signal to decrease exponentially with pressure. The reflected shock temperature could be determined to 7 GPa, with an average increase of 14.9 K/GPa. Suggestions to adapt the technique to permit thermometry at higher temperatures are discussed, as are comparisons to a recently proposed equation of state for PBX 9502.

Chemical Compatibility of Silica Aerogel Processes with ICF Hohlraums

Abstract Inertial Confinement Fusion (ICF) energy hohlraums are composed of a high-Z material filled with foam. Because of the small pore size and transparency, silica aerogels are used in some ICF targets. The traditional synthesis of silica aerogels require sol-gel polymerization of silicon alkoxide followed by supercritical drying. Some constituents in sol-gel polymerization have been found to contribute to leaching of certain metals at the silica/metal interface. Since the hohlraums are composed of metals, possible chemical reactivity at the silica aerogel and metal hohlraum interface was investigated. The hohlraums studied are aluminum lined with either copper or copper/chromium. Upon initial inspection, the aerogel appeared transparent and uniform, however, closer inspection of the copper wall suggested possible leaching. Alternatively the quality of the aerogel in the copper-chromium hohlraum was very poor with the chromium layer of the hohlraum and some copper completely etched. Control experiments were used to determine the cause of the leaching. When copper is in the presence of sol-gel constituents, Cu2+ ion formed, thus leaching copper from the hohlraum walls. In the presence of chromium, Cr2O72- or CrO42- was identified in solution with the Cu2+, these anions are believed to form copper chromite under the aerogel synthesis procedures utilized.

Production and Metrology of Cylindrical Inertial Confinement Fusion Targets with Sinusoidal Perturbations

Abstract Shock waves generated during inertial confinement fusion implosions propagate toward the center of the capsule encountering interfaces between materials with different densities, such as between the ablator and the DT fuel. These interactions are hydrodynamically unstable and the resulting instability causes mixing of the materials at the interface, which is predicted to have detrimental effects on fusion burn. In this experiment, the growth of a single-mode perturbation machined into a radiographically opaque marker layer, driven by a strong shock, is measured during a cylindrically symmetric implosion. These measurements are used to validate simulations and theories of the complex hydrodynamics. Since any perturbation on the marker layer surface will lead to instability growth, precise knowledge of the initial conditions is critical. The targets used in this experiment have up to a 3.0-μm-amplitude, mode 28 (θ = 98 μm) sinusoidal perturbation machined into a 438-μm-outerradius aluminum band with a nominal thickness of 8 μm. The perturbations were machined using a fast-tool servo [B. JARED and T. A. DOW, Precision Engineering Center Annual Report, North Carolina State University, Raleigh NC, p. 123 (1996)] and were metrologized using a linear variable differential transformer [FRANK J. OLIVER, Practical Instrumentation Tranducers, p. 42-45, Hayden Book Company (1971)]. In this paper, the importance of metrology is discussed and is shown to be critical to the interpretation of experimental results.

BEST PRACTICE PROCEDURES FOR MAKING DIRECT DRIVE CYLINDRICAL TARGETS FOR STUDIES OF CONVERGENT HYDRODYNAMICS

Abstract The production of cylindrical targets involves numerous steps. These steps are shared in common with many other types of Inertial Confinement Fusion (ICF) targets but no other single target encompasses such a wide range of fabrication techniques. These targets consist of a large number of individual parts, virtually all fabricated from commercially purchased raw material. As an example, the polystyrene used is synthesized in house from purchased monomer material. This material must be polymerized, purified, characterized and put into solution before it is even first used in the making of a target. Because virtually every manufacturing and assembly process we currently use is involved in the production of these targets, this paper is written as a way documenting the methods used.

In Situ and Postmortem Measures of Damage in Polymers at High Strain-Rates

Significant progress has been made in the in situ and postmortem observations of the dynamic tensile failure and damage evolution of a range of polymers employing Taylor anvil and Dynamic-Tensile-Extrusion (Dyn-Ten-Ext) loading. The classic Taylor anvil technique involves impacting a right-cylinder against a semi-infinite rigid anvil to access large compressive strains (>1) and high strain-rates (>1,000/s). The Dyn-Ten-Ext technique probes the tensile response of materials at large strains (>1) and high strain-rates (>1,000/s) by firing projectiles through a conical die. Depending on the extrusion ratios and velocities damage varies from bulk deformation with substantial internal damage, to a stable jet with finite particulation, to catastrophic fragmentation. Postmortem sectioning and X-ray computed tomography access mechanisms of internal damage inception and progression. In situ measurements of damage are made with the impact system for ultrafast synchrotron experiments (IMPULSE) using the advanced imaging and X-ray diffraction methods available at the Advanced Photon Source. The time resolved phase-contrast imaging elucidates the evolution of damage features during dynamic loading that is observed in post mortem sectioning and tomography.

Analysis of the mini-deflagration cylinder test: Inference of internal conditions from wall motion

Following G.I. Taylor and our own previous work based upon it, we continue to explore HE confiner motion as test diagnostic. A handful of authors (including us) have adapted Taylors analysis to infer equation-of-state information from detonation cylinder tests. We have also successfully applied this class of analysis to deflagration cylinder tests (DFCTs). Here, we analyze a miniature DFCT diagnosed by multiple PDV probes. We spatially interpolate between PDV records to obtain a smooth function for lateral expansion vs. axial distance, z, and time, t. From this we may construct snapshots of the tube at any time; hence we may create animations of the tube motion and compute the product gas volume V vs. t. Combining Newtons law with simple stress theory, we estimate the gas pressure P vs. z and t. We then estimate the burned HE mass from the wall kinetic energy. Finally, we discuss some important test scaling issues.

Recent Developments in Fabrication of Direct Drive Cylinder Targets for Hydrodynamics Experiments at the OMEGA Laser

Abstract Experimental campaigns are being conducted at the 60 beam OMEGA laser at the University of Rochester’s Laboratory for Laser Energetics to acquire data to validate hydrodynamic models in the high energy-density regime. This paper describes targets that have been developed and constructed for these experimental campaigns. Targets are 860 μm inner diameter by 2.2 mm length cylinders with 70 μm thick polymer ablator. On the ablator inner surface and located halfway along the axis of the cylinder is a 500 μm wide Al marker band. Band thicknesses in the range 8-16 microns are used. CH foam with densities in the range 30-90 mg/cc fills the inside of the cylinder. While these targets have been fabricated for years, several new improvements and features have recently been developed. Improvements include the use of epoxy instead of polystyrene for the ablator, and the use of electrodeposited Al for the marker band. A critical feature of the target is the surface feature that is placed on the marker band. Experiments are aimed at understanding the hydrodynamic behavior of imploding cylinders as a function of this surface feature. Recent development work has focused on production of engineered surface features on the target marker band. Using a fast tool servo on a diamond turning lathe, a wide range of specified surface features have been produced. This paper will address improvements to the cylinder targets as well as current development efforts.