Douglas G. Tasker

RANCHERO explosive pulsed power experiments

The authors are developing the RANCHERO high explosive pulsed power (HEPP) system to power cylindrically imploding solid-density liners for hydrodynamics experiments. Their near-term goal is to conduct experiments in the regime pertinent to the Atlas capacitor bank. That is, they will attempt to implode liners of /spl sim/50 g mass at velocities approaching 15 km/sec. The basic building block of the HEPP system is a coaxial generator with a 304.8 mm diameter stator, and an initial armature diameter of 152 mm. The armature is expanded by a high explosive (HE) charge detonated simultaneously along its axis. The authors have reported a variety of experiments conducted with generator modules 43 cm long and have presented an initial design for hydrodynamic liner experiments. In this paper, they give a synopsis of their first system test, and a status report on the development of a generator module that is 1.4 m long.

Dynamic measurements of electrical conductivity in metastable intermolecular composites

Metastable intermolecular composite (MIC) materials are comprised of a mixture of oxidizer and fuel with particle sizes in the nanometer range. Dynamic electrical conductivity measurements have been performed on a reacting MIC material. Simultaneous optical measurements of the wavefront position have shown that the reaction and conduction fronts are coincident within 160μm. It has been observed that MICs, like high explosives, are insulators before reaction is initiated. Once reaction is induced, there is a conduction zone that corresponds with the reaction zone behind the reaction front. Unlike detonating high explosives (HEs) where the conductivity profile is represented by an initial peak followed by an exponential decay of conductivity, the MIC conductivity profile is a gradual, irregular ramp which increases from zero over many microseconds. This supports other studies that show the MIC reaction process to be significantly different from detonating HEs. Static measurements of conductivity of pressed ...

High Current, Low Jitter, Explosive Closing Switches

Isentropic compression experiments (ICE) using a high explosive pulsed power (HEPP) system have been developed to obtain isentropic equation of state data for metals at megabar pressures [1][2]. The HEPP system comprises a magnetic flux compressor, an explosively-driven opening switch and a series of closing switches; fast rising current pulses are produced, with rise times of ~500 ns at current densities exceeding many MA/cm. These currents create continuous magnetic loading of the metals under study. The success of these experiments depends on the precise control of the current profile, and that in turn depends on the precise timing of the closing switches to within 50 ns at currents of the order 10 MA and voltages of ~150 kV. We first used Procyon closing switches [3] but found their timing to be unacceptably imprecise for ICE with a jitter of typically 600 ns. We suspected that the switch timing was sensitive to applied voltage; this was subsequently confirmed by experiment, as we will show. A simple shock model was developed to explain the voltage sensitivity of closure time, dt/dV, and from the model we designed a low jitter switch that uses the shock-induced electrical conduction of polyimide. The predicted dt/dV was exactly equal to the measured value, thus confirming the model. This new switch design proved successful and met the 50 ns criterion; it is now used routinely in HEPP-ICE experiments.

Advances in isentropic compression experiments (ICE) using high explosive pulsed power

We are developing a prototype high explosive pulsed power (HEPP) system to obtain isentropic Equation of State (EOS) data with the Asay technique. Asay, JR (1999). Our prototype system comprises a flat-plate explosive driven magnetic flux compression generator (FCG), an explosively formed fuse (EFF) opening switch, and a series of explosively-actuated closing switches. The FCG is capable of producing /spl sim/10 MA into suitable loads, and, at a length of 216 mm, the EFF will sustain voltages in excess of 200 kV. The load has an inductance of /spl sim/3 to 10 nH, allowing up to /spl sim/7 MA to be delivered in times of /spl sim/0.5 /spl mu/s. This prototype will produce isentropic compression profiles in excess of 2 Mbar in a material such as tungsten. Our immediate plan is to obtain isentropic EOS data for copper at pressures up to /spl sim/1.5 Mbar with the prototype system; eventually we hope to reach several tens of Mbar with more advanced systems.

Analysis of explosively formed fuse experiments

Explosively formed fuse (EFF) opening switches have been used in a variety of applications to divert current in high explosive pulsed power (HEPP) experiments. Typically, EFFs operate at 0.1-0.2 MA/(cm switch width), and have an /spl sim/2 /spl mu/s risetime to a resistance of 10s-100s m/spl Omega/. We have demonstrated voltage standoff of /spl sim/7 KV/(die pattern) in some configurations, and typical switches have up to 100 die patterns. In these operating regimes, we can divert large currents (10-20 MA) to low impedance loads, and produce voltage waveforms with risetime and shape determined by the shape of the resistance curve and amount of magnetic flux in the circuit. Progress in quantitatively modeling EFF performance with magnetohydrodynamic (MHD) codes has been slow, and much of our understanding regarding the operating principles of EFF switches still comes from small-scale experiments coupled with hydrodynamic (hydro) calculations. These experiments are typically conducted at currents of /spl sim/0.5 MA in a conductor 6.4 cm wide. A plane-wave detonation system is used to drive the EFF conductor into the forming die, and current and voltage are recorded. The resulting resistance profiles are compared to the hydro calculations to get insight into the operating mechanisms. Our original goals for EFF development were limited in scope, and in pursuing specific large systems, we have left behind a valuable body of small-scale test data that has been largely unused. We now have a charter to achieve a complete understanding of EFF devices, and our first step has been to review existing data. In this paper, we present some of the results of these investigations.

Isentropic compression of metals, at multi-megabar pressures, using high explosive pulsed power

Accurate, ultra-high pressure isentropic equation of state (EOS) data, are required for a variety of applications and materials. Asay (1999) reported a new method to obtain these data using pulsed magnetic loading on the Sandia Z-machine. Fast rising current pulses (risetimes from 100 to 300 ns) at current densities exceeding many MA/cm, create continuous magnetic loading up to a few Mbar. As part of a collaborative effort between the Los Alamos and Lawrence Livermore National Laboratories, the authors are adapting their high explosive pulsed power (HEPP) methods to obtain isentropic EOS data with the Asay technique. This year, they plan to obtain isentropic EOS data for copper and tantalum at pressures up to /spl sim/2 Mbar; eventually we hope to reach several tens of Mbar. They describe the design of the HEPP systems and show their attempts to obtain EOS data to date.

CHANGES IN BLOW-OFF VELOCITY OBSERVED IN TWO EXPLOSIVES AT THE THRESHOLD FOR SUSTAINED IGNITION USING THE MODIFIED GAP TEST

The Modified Gap Test was used to quantify different levels of partial reaction for various input stresses. This test configuration has been historically useful in highlighting thresholds for first reaction, sustained ignition, and detonation. Two different HMX based compositions were studied; a cast‐cured composition with 87% HMX and a pressed composition with 92% HMX. Each explosive was prepared from large industrially produced batches consisting of different unreactive polymeric binder systems. Short samples (50.8 mm in diameter and 12.7 mm thick) were shock loaded using the standard large‐scale gap test donor system. Product‐cloud blow‐off velocities at the opposite end of the sample were measured using a high‐speed digital‐camera. Velocity versus input pres sure plots provided changes in reactivity that had developed by the 12.7 mm run distance. Results appear consistent for the lower input stresses. In contrast, the results varied widely in a range of input stresses around the transition to detonati...

Electromagnetic field effects in explosives

Present and previous research on the effects of electromagnetic fields on the initiation and detonation of explosives and the electromagnetic properties of explosives are reviewed. Among the topics related to detonating explosives are: enhancement of performance; and control of initiation and growth of reaction. Two series of experiments were performed to determine the effects of 1‐T magnetic fields on explosive initiation and growth in the modified gap test and on the propagation of explosively generated plasma into air. The results have implications for the control of reactions in explosives and for the use of electromagnetic particle velocity gauges.

Summary of Isentropic Compression Experimentsperformed with High Explosive Pulsed Power

One-dimensional isentropic compression experiments (ICE) have been performed over the last few years at the Los Alamos National Laboratory (LANL) using a High Explosive Pulsed Power (HEPP) system. Accurate, high pressure, isentropic Equations of State (EOS) data have been obtained for copper and tungsten. A number of important issues have been identified, such as: magnetic field (B-field) uniformity; sample-to-sample B-field uniformity; sample size constraints; the maximum stress before shock-up; and accuracy. The results for tungsten show non-ideal elastic to plastic transition features, but an experimental isentrope that is close to theoretical values.

Overview of Nanoscale Energetic Materials Research at Los Alamos

Metastable Intermolecular Composite (MIC) materials are comprised of a mixture of oxidizer and fuel with particle sizes in the nanometer range. Characterizing their ignition and combustion is an ongoing effort at Los Alamos. In this paper we will present some recent studies at Los Alamos aimed at developing a better understanding of ignition and combustion of MIC materials. Ignition by impact has been studied using a laboratory gas gun using nano-aluminum (Al) and nano-tantalum (Ta) as the reducing agent and bismuth (III) oxide (Bi 2 O 3 ) as the oxidant. As expected from the chemical potential, the Al containing composites gave higher peak pressures. It was found, for the Al/Bi 2 O 3 system, that impact velocity under observed conditions plays no role in the pressure output until approximately 100 m/s, below which speed, impact energy is insufficient to ignite the reaction. This makes the experiment more useful in evaluating the reactive performance. Replacing the atmosphere on impact with an inert gas reduced both the amount of light produced and the realized peak pressure. The combustion of low-density MIC powders has also been studied. To better understand the reaction mechanisms of burning MIC materials, dynamic electrical conductivity measurements have been performed on a MIC material for the first time. Simultaneous optical measurements of the wave front position have shown that the reaction and conduction fronts are coincident within 160 μm.