Wayne M. Trott

Semiconductor bridge: A plasma generator for the ignition of explosives

Small metal bridgewires are commonly used to ignite energetic powders such as pyrotechnics, propellants, and primary or secondary explosives. In this paper we describe a new means for igniting explosive materials using a semiconductor bridge (SCB). When driven with a short (20 μs), low‐energy pulse (less than 3.5 mJ), the SCB produces a hot plasma that ignites explosives. The SCB, a heavily n‐doped silicon film, typically 100 μm long by 380 μm wide by 2 μm thick, is 30 times smaller in volume than a conventional bridgewire. SCB devices produce a usable explosive output in a few tens of microseconds and operate at one‐tenth the input energy of metal bridgewires. In spite of the low energies for ignition, SCB devices are explosively safe. We describe SCB processing and experiments evaluating SCB operation. Also discussed are the SCB vaporization process, plasma formation, optical spectra from the discharge, heat transfer mechanisms from the SCB to the explosive powders, and SCB device applications.

High-power Nd:glass laser transmission through optical fibers and its use in acceleration of thin foil targets

Measurements on commercially available fused silica fibers and fiber tapers have shown that Nd:glass laser pulses of irradiance >3.5 GW cm−2 can be transmitted without bulk or surface damage. Optical energy at the fiber output can be used for efficient acceleration of thin metal foils. For 0.025‐mm‐thick aluminum coupled to a 0.2‐mm‐diam fiber, target foil velocities up to 2.3 km s−1 have been achieved with very modest laser energies (<25 mJ).

CO2‐laser‐induced deflagration of fuel/oxygen mixtures

Weakly focused pulsed‐CO2‐laser radiation has been examined as an ignition source for low‐pressure (85–100 Torr), near‐stoichiometric fuel/oxygen mixtures containing the following fuels: ethylene, methyl fluoride, methanol, ethanol, dimethyl ether, p‐dioxane, n‐propyl nitrate, and iso‐propyl nitrate. The data analysis includes characterization of the spatially dependent ir absorption in the test cell, calculation of appropriate medium temperatures from the absorbed laser energy, and correlation of observed ignition delay times tig with the calculated temperatures. Effects of hydrodynamic motion on the pressure, density, and temperature profiles of the irradiated samples were modeled using a computer code for two‐dimensional wave propagation. Code predictions are in accordance with experimental pressure‐time histories obtained using a piezoelectric transducer. Minimum ignition temperatures ranged from 590 K for the iso‐propyl nitrate/O2 system to 1645 K for CH3F/O2. The observed functional relationship bet...

Theoretical analysis of a pulsed-laser-driven hypervelocity flyer launcher☆

Abstract High-power but low-total-energy pulsed lasers can be used to accelerate small-diameter, thin flyers to velocities in excess of several kilometers per second. The geometry under consideration involves placing the flyer on the end of an optical fiber through which the laser pulse is delivered. The blowoff products driving the flyer are thus fully tamped. a model, based on the Gurney theory for explosively driven plates, is derived for predicting the final velocity of these flyers. All but two of the required input parameters are readily available; those two can be extracted from one limited set of experimental measurements. Data on aluminum flyers illustrate that once the input parameters have been determined, the model predicts changes resulting from variations of laser fluence and pulse duration as well as flyer thickness and diameter. Additional data on copper and magnesium indicate that the energy-coupling efficiency can vary by at least 50%, depending on the flyer material.

Investigation of the mesoscopic scale response of low-density pressings of granular sugar under impact

The mesoscopic scale response of low-density pressings of granular sugar (sucrose) to shock loading has been examined in gas-gun impact experiments using both VISAR and a line-imaging, optically recording velocity interferometer system in combination with large-volume-element, high-resolution, three-dimensional numerical simulations of these tests. Time-resolved and spatially resolved measurements of material motion in waves transmitted by these pressings have been made as a function of impact velocity, sample thickness, and sample particle size distribution. Observed wave profiles exhibit a precursor regime arising from elastic stress wave propagation and a dispersive compaction wave with superimposed localized particle velocity fluctuations of varying amplitude. Material motion associated with dynamic stress bridging leads compaction wave arrival by ∼2μs at the lowest impact velocity (0.25kms−1) employed in this study and <200ns at the higher values (0.7–0.8kms−1). Over the same range, the compaction wa...

An Experimental Assembly for Precise Measurement of Thermal Accommodation Coefficients.

An experimental apparatus has been developed to determine thermal accommodation coefficients for a variety of gas-surface combinations. Results are obtained primarily through measurement of the pressure dependence of the conductive heat flux between parallel plates separated by a gas-filled gap. Measured heat-flux data are used in a formula based on Direct Simulation Monte Carlo (DSMC) simulations to determine the coefficients. The assembly also features a complementary capability for measuring the variation in gas density between the plates using electron-beam fluorescence. Surface materials examined include 304 stainless steel, gold, aluminum, platinum, silicon, silicon nitride, and polysilicon. Effects of gas composition, surface roughness, and surface contamination have been investigated with this system; the behavior of gas mixtures has also been explored. Without special cleaning procedures, thermal accommodation coefficients for most materials and surface finishes were determined to be near 0.95, 0.85, and 0.45 for argon, nitrogen, and helium, respectively. Surface cleaning by in situ argon-plasma treatment reduced coefficient values by up to 0.10 for helium and by ∼0.05 for nitrogen and argon. Results for both single-species and gas-mixture experiments compare favorably to DSMC simulations.

Measurements of spatially resolved velocity variations in shock compressed heterogeneous materials using a line-imaging velocity interferometer

Relatively straightforward changes in the design of a conventional optically recording velocity interferometer system (ORVIS) can be used to produce a line-imaging instrument that allows adjustment of spatial resolution over a wide range. As a result, line-imaging ORVIS can be tailored to various specific applications involving dynamic deformation of heterogeneous materials as required by their characteristic length scales (ranging from a few μm for ferroelectric ceramics to a few mm for concrete). A line-imaging system has been successfully interfaced to a compressed gas gun driver and fielded on numerous tests in combination with simultaneous dual delay-leg, “push-pull” VISAR measurements. These tests include shock loading of glass-reinforced polyester composites, foam reverberation experiments (measurements at the free surface of a thin aluminum plate impacted by foam), and measurements of dispersive velocity in a shock-loaded explosive simulant (sugar). Results are presented that illustrate the capabi...

ArF Laser-induced decomposition of simple energetic niolecules

Chemiluminescent products formed from ArF laser-induced decomposition of nitromethane, n -propyl nitrate and nitrobenzene have been studied. Implications to explosive chemistry are briefly, discussed.

Measurements of thermal accommodation coefficients.

A previously-developed experimental facility has been used to determine gas-surface thermal accommodation coefficients from the pressure dependence of the heat flux between parallel plates of similar material but different surface finish. Heat flux between the plates is inferred from measurements of temperature drop between the plate surface and an adjacent temperature-controlled water bath. Thermal accommodation measurements were determined from the pressure dependence of the heat flux for a fixed plate separation. Measurements of argon and nitrogen in contact with standard machined (lathed) or polished 304 stainless steel plates are indistinguishable within experimental uncertainty. Thus, the accommodation coefficient of 304 stainless steel with nitrogen and argon is estimated to be 0.80 {+-} 0.02 and 0.87 {+-} 0.02, respectively, independent of the surface roughness within the range likely to be encountered in engineering practice. Measurements of the accommodation of helium showed a slight variation with 304 stainless steel surface roughness: 0.36 {+-} 0.02 for a standard machine finish and 0.40 {+-} 0.02 for a polished finish. Planned tests with carbon-nanotube-coated plates will be performed when 304 stainless-steel blanks have been successfully coated.

Use of Z-pinch sources for high-pressure equation-of-state studies

In this paper, we describe a new technique for using a pulsed power source (Z pinch) to produce planar shock waves for high-pressure equation of state (EOS) studies. Initial EOS experiments indicate that these sources are effective for shock wave studies in samples with diameters of a few millimeters and thicknesses of a fraction of one millimeter, and thus provide the possibility for achieving accuracy in shock and particle velocity measurements of a few percent. We have used the Z pinch source to produce the first in-situ time-resolve particle velocity profiles obtained with pulsed radiation sources in the Mbar regime. Particle velocity profiles obtained with a VISAR interferometer are compared with 1-D numerical simulations performed with a radiation-hydrodynamics code, ALEGRA. Good agreement with experimental results was achieved in the simulations and suggests that the Z pinch source should be a valuable tool for high-pressure EOS studies in thermodynamic regimes important to hypervelocity impact.