We present a new integrated experimental and modeling effort that assesses the intrinsic sensitivity of energetic materials based on their reaction rates. The High Explosive Initiation Time (HEIT) experiment has been developed to provide a rapid assessment of the high-temperature reaction kinetics for the chemical decomposition of explosive materials. This effort is supported theoretically by quantum molecular dynamics (QMD) simulations that depict how different explosives can have vastly different adiabatic induction times at the same temperature. In this work, the ranking of explosive initiation properties between the HEIT experiment and QMD simulations is identical for six different energetic materials, even though they contain a variety of functional groups. We have also determined that the Arrhenius kinetics obtained by QMD simulations for homogeneous explosions connect remarkably well with those obtained from much longer duration one-dimensional time-to-explosion (ODTX) measurements. Kinetic Monte Carlo simulations have been developed to model the coupled heat transport and chemistry of the HEIT experiment to explicitly connect the experimental results with the Arrhenius rates for homogeneous explosions. These results confirm that ignition in the HEIT experiment is heterogeneous, where reactions start at the needle wall and propagate inward at a rate controlled by the thermal diffusivity and energy release. Overall, this work provides the first cohesive experimental and first-principles modeling effort to assess reaction kinetics of explosive chemical decomposition in the subshock regime and will be useful in predictive models needed for safety assessments.

There are few techniques available for chemists to obtain time-to-explosion data with known temperature inputs at the early stages of the design and synthesis of new explosives. In the 1960s, a technique was developed to rapidly heat milligram-quantities of confined explosives to ∼1000 K on microsecond timescales. Wenograd [Trans. Faraday Soc. 57, 1612 (1961)] loaded explosives inside stainless steel hypodermic needles, connected them to a fireset and rapidly discharged a capacitor through the steel. He obtained the temperature by measuring the needle resistance in a Wheatstone bridge arrangement and the time to explosion from a needle rupture. However, owing to the narrow-gauge needles used in the original research, the experiment was only possible with melt-castable explosives; it was never replicated, and modern diagnostics are now available with advances beyond the 1960s. Here, we report the development of the High Explosives Initiation Time (HEIT) test, which utilizes a 250 J pulsed power system to heat the needles. This work extends the Wenograd approach by using optical diagnostics, computational modeling, and advanced techniques to measure needle resistance and needle rupture. Preliminary rate information for pentaerythritol tetranitrate (PETN) will be presented.

Two concentric-sphere apparatuses were used to measure the energies deposited by electric sparks through air to calibrated loads. Both sets of spheres had clean, uninsulated metal surfaces. The 127-mm diameter pair of spheres had a gap of 10 mm and a capacitance of 52 pF. The maximum energy stored on these spheres was 12 mJ and the maximum energy delivered to the 50-mΩ load was 11 µJ, with or without inductance. This load energy amounted to 0.09% of the available input energy; the remainder of the input energy was expended in forming the spark. The second set of spheres was 330 mm in diameter with a gap of 14 mm and a capacitance of 240 pF. The maximum energy stored on these spheres was 71 mJ and the maximum energy delivered to the 50-mΩ load was 150 µJ without added inductance. This load energy was 0.27% of the input energy; again, the remainder of the input energy was expended in forming the spark. The inclusion of 250 nH and 500 nH inductances in the discharge path of the smaller spheres caused the current discharges to resonate, but had little effect on the maximum energies deposited in the loads. At the time of writing, inductances had not yet been added to the larger spheres. The magnitudes of the observed spark discharges were stochastic, so more than 1200 tests were required to develop the statistical bounds of the spark behavior, i.e., to estimate the worst case (highest likely) energy depositions in the loads. The action integrals of the current discharges $∫ i^2dt$ were found to be approximately independent of the load resistances for loads of less than ~1 Ω. Hence the energy deposition scaled with resistance as 207 µJ/Ω and 2.93 mJ/Ω for the two apparatuses.

Using a Voitenko accelerator [1-3], a series of experiments were performed with the goal of attaining shock velocities in gases approaching 90 km/s. Typically, the basic apparatus comprises a hemispherical bowl filled with a gas at atmospheric pressure; a metal piston across its diameter; and a small bore evacuated shock tube at its apex, Fig. 1. The evacuated shock tube is separated from the gas bowl by a thin diaphragm. A combination of a plane wave explosive lens and a high explosive pad accelerates the piston to a velocity of the order of 4 km/s and subsequently compresses the gas in the bowl. The thin diaphragm at the other end of the bowl then ruptures and the high pressure (shock compressed) gas escapes into the shock tube.

Detonators are fundamental to most explosive systems, yet the detonators themselves are rarely diagnosed to determine how well they function or why they fail. To remedy this, various electrical diagnostics were explored, including: Rogowski coils for high fidelity detection of changes in electrical current in explosive bridgewires and how they relate to detonator performance; and radio frequency (RF) antennas to detect any electromagnetic emissions associated with reaction and growth in the explosives. A new type of Rogowski coil is described and some interesting phenomena were detected with the radio frequency antenna.

A strong dependence of the dynamic resistivity of Explosively-Formed Fuses (EFF) on metal temper is reported. The explosively driven switch has been extremely effective in a wide range of large-scale experiments but good, predictive, computational models of its performance have proven elusive, possibly because the temper of the metal conductor has not been previously considered. Small-scale experiments were employed to study the effects of metal temper. It was found that the dynamic resistivity of the switch increased significantly with metal hardness; the dynamic resistivity of the hardest foils being approximately three times that of the dead-soft material. These results suggest that metal fracture is an important aspect of the EFF behavior.

A novel combination of diagnostics is being used to research the physics of detonator initiation. The explosive PETN (Pentaerythritol tetranitrate) commonly used in detonators, is also a piezo-electric material that, when sufficiently shocked, emits an electromagnetic field in the radio frequency (RF) range, along crystal fracture planes. In an effort to capture this RF signal, a new diagnostic was created. A copper foil, used as an RF antenna, was wrapped around a foam fixture encompassing a PETN pellet. Rogowski coils were used to obtain the change in current with respect to time (di/dt) the detonator circuit, in and polyvinylidene difluoride (PVDF) stress sensors were used to capture shockwave arrival time. The goal of these experiments is to use these diagnostics to study the reaction response of a PETN pellet of known particle size to shock loading with various diagnostics including an antenna to capture RF emissions. Our hypothesis is that RF feedback may signify the rate of deflagration to detonation transition (DDT) or lack thereof. The new diagnostics and methods will be used to determine the timing of start of current, bridge burst, detonator breakout timing and RF generated from detonation. These data will be compared to those of currently used diagnostics in order to validate the accuracy of these new methods. Future experiments will incorporate other methods of validation including dynamic radiography, optical initiation and use of magnetic field sensors.

Experiments were performed using a copper antenna to observe transient electric fields generated by shocked Pentaerythritol Tetranitrate (PETN), which behaves as a piezoelectric explosive. PETN pellets were shocked using a detonator and polyvinyl chloride (PVC) as shock attenuation. The amount of attenuation was varied until an initiation threshold was found to increase the probability of a deflagration-to-detonation (DDT) transition occurring. These experiments showed distinct transient shifts in frequency over the duration of the experiments. Correlation was observed between certain shifts in frequency and if the PETN pellet detonated.

It has been shown that the temperature of explosively generated plasma (EGP) is of the order of 1 eV and plasma ejecta can be focused to achieve velocities as high as 25 km/s. Proof-of-principle tests were performed to determine if EGP could be used for explosive ordnance demolition and other applications. The goals were: to benignly disable ordnance containing relatively sensitive high performance explosives (PBX-9501); and to investigate the possibility of interrupting an ongoing detonation in a powerful high explosive (again PBX-9501) with EGP. Experiments were performed to establish the optimum sizes of plasma generators for the benign deactivation of high explosives, i.e., the destruction of the ordnance without initiating a detonation or comparable violent event. These experiments were followed by attempts to interrupt an ongoing detonation by the benign disruption of the unreacted explosive in its path. The results were encouraging. First, it was demonstrated that high explosives could be destroyed without the initiation of a detonation or high order reaction. Second, ongoing detonations were successfully interrupted with EGP. [LA-UR-15-25350]

This paper describes the design of a novel MOSFET pulsed constant current supplies for low impedance Manganin stress gauges. The design emphasis has been on high accuracy, low noise, simple, low cost, disposable supplies that can be used to energize multiple gauges in explosive or shock experiments. The Manganin gauges used to measure 50 A. Conventional pulsed, constant current supplies for these gauges are high voltage devices with outputs as high as 500 V. Common problems with the use of high voltage supplies at explosive firing sites are: erroneous signals caused by ground loops; overdrive of oscilloscopes on gauge failure; gauge signal crosstalk; cost; and errors due to changing load impedances. The new circuit corrects these issues. It is an 18-V circuit, powered by 9-V alkaline batteries, and features an optically isolated trigger, and single-point grounding. These circuits have been successfully tested at the Los Alamos National Laboratory in explosive experiments. [LA-UR-15-24819]

A single-turn magnet pulsed power system, at the Los Alamos National Laboratory (LANL) National High Magnetic Field Laboratory (NHMFL), was originally designed to measure actinide samples in extremes of high magnetic field (to 300 Tesla) [1, 2]. A simple modification to the system has converted it to a fast turnaround, inexpensive, magnetic system for Isentropic Compression Experiments (ICE). For the design and predictions of performance of the NHMFL-ICE experiment a circuit code simulation was chosen to model all aspects of the experiment, electrical and physical. This paper describes the potential performance of the system, recent experiments, and plans for a portable system. The 2.2 mu s rise time of the system allows sample thicknesses up to similar to 5 mm. With the present design the maximum stresses are similar to 50GPa (0.5 Mbar) at the maximum bank voltage of 60 kV.

We demonstrate that the established pulsed power infrastructure at the National High Magnetic Field Laboratory - Pulsed Field Facility (NHMFL-PFF) at the Los Alamos National Laboratory can be adapted to obtain high quality isentropic compression experiment (ICE) data on materials in extreme conditions of dynamic high pressure. Experiments utilized a single-turn magnet pulsed power system at the NHMFL-PFF that was originally designed to measure actinide samples in extremes of high magnetic field (to 300 Tesla). A simple modification to the single-turn magnet has converted it to a fast turnaround dynamic high pressure measurement system. This paper details the work done including important background details that indicate that much more can be accomplished with optimization of the load characteristics in terms of ultimate peak pressures. To match the rise time of the NHMFL capacitor bank ({approx}2 {mu}s versus {approx}0.5 {mu}s for the Sandia Z-machine) the sample dimensions can be relatively large, i.e., up to 5 mm thickness. The maximum stresses are {approx}50GPa (0.5 Mbar) at the maximum bank voltage (60 kV) and higher pressures may be possible if the sample is tamped. For the design and predictions of performance of the NHMFL-ICE experiment it is important to have good predictive models.more » A SPICE code simulation was chosen to model all aspects of the experiment, electrical and physical. To this end, accurate dynamic load models were developed to simulate the compression and expansion of the dynamic load at high pressures using shock physics principles. A series experiments have been performed which demonstrated the feasibility of the NHMFL-ICE technique. The results will be shown and discussed. The NHMFL-ICE technique is an excellent method for measuring equations of state (EOS) at megabar pressures. Because a complete EOS can be obtained in one experiment from zero to the peak pressure, and because many shots can be fired in one day, the technique promises to provide high quality EOS data at relatively low cost.« less