Keith A. Thomas

A Robust One-Shot Switch for High-Power Pulse Applications

High-voltage switches capable of operating at high speeds and over a wide range of voltages and energies are used in a wide variety of applications in material science and plasma physics. This paper discusses the fabrication and characterization of a novel high-voltage shock switch. The structure has been designed to operate as a fast-turn-on, low-impedance device. The switch is a planar structure that allows for direct integration into the stripline geometries used in a conventional capacitive discharge unit.

A high-voltage single-shot switch implemented with a MOSFET current source and avalanche diode

The description of a novel circuit which utilizes a field-effect transistor (FET) current source and a high-voltage diode to realize a simple inexpensive single-shot high-voltage switch is presented. The switch was specifically designed for use with fast low-impedance pulse-power discharge circuits such as those commonly used for plasma physics and high-pressure research. This switch can also be utilized in any application where speed, low cost and small size are important concerns. The design readily lends itself to implementation as a discrete component or hybrid circuit. The circuit has been simulated and the design parameters of the configuration have been numerically investigated utilizing PSPICE. These simulations and experimental data are presented.

Measurements of the DDT Process in Exploding Bridgewire Detonators

The deflagration‐to‐detonation transition (DDT) of low density (0.88 g/cc) PETN during exploding bridgewire (EBW) initiation has been studied using laser interferometry and streak photography. Cutback experiments using VISAR have confirmed a 1.0 mm run‐distance to detonation in this low density PETN powder. In a detonation system using a combination of low and high density powders, an apparent center of initiation (COI) analysis of streak data has yielded a surprisingly similar result. This data suggested that a compaction of low density powder to near theoretical maximum density (TMD) may occur before the onset of detonation, which is consistent with work done previously. These experiments show this is not the case and COI analysis reveals a non‐ideal initial propagation front. Additionally, data show that although function time increases significantly with decreasing firing voltage, the apparent COI changes very little. This indicates that the detonation criterion is not dependent upon the rate of deflagration, but on a volume of material that must be burned in a confined space to create the critical pressure needed at the compaction front.

On the Initiation Mechanism in Exploding Bridgewire and Laser Detonators

Since its invention by Los Alamos during the Manhattan Project era the exploding bridgewire detonator (EBW) has seen tremendous use and study. Recent development of a laser‐powered device with detonation properties similar to an EBW is reviving interest in the basic physics of the deflagration‐to‐detonation (DDT) process in both of these devices. Cutback experiments using both laser interferometry and streak camera observations are providing new insight into the initiation mechanism in EBWs. These measurements are being correlated to a DDT model of compaction to detonation and shock to detonation developed previously by Xu and Stewart. The DDT model is incorporated into a high‐resolution, multi‐material model code for simulating the complete process. Model formulation and the modeling issues required to describe the test data will be discussed.

On the development of a laser detonator

The initiation of explosives by laser illumination has been known for many years. In this paper we will discuss the development of a working detonator design that reduces the energy required for detonation in a low-density secondary explosive by vaporizing a thin metal coating. We present data on the development of the design for a workhorse laser detonator that provides enhanced safety over existing exploding bridgewire detonators (EBWs). Comparison of this laser initiated data to an exploding-bridgewire (EBW) provides insight into the mechanism of initiation of detonation in low-density PETN by the plasma source. A novel diagnostic technique to determine the run-distance to detonation also known as the apparent Center-of-Initiation (COI) will also be discussed.

Instrumented Floret Tests of Detonation Spreading

The floret test was originally devised to permit comparison of detonation‐spreading performance of various insensitive explosive materials, using only the dent in a copper witness plate as a metric. Dent depth in the copper plate is directly related to the fraction of a thin acceptor pellet that was detonated by impact of a small explosive‐driven flyer plate. We have now added instrumentation to quantitatively measure the detonation corner‐turning behavior of IHEs. Results of multi‐fiber optical probe measurements are shown for LLM‐105 and UF‐TATB explosive materials. Results are interpreted and compared with predictions from one reaction‐rate model used to describe detonation spreading, and may be advantageous for comparison with other reactive‐flow wave‐code models. Detonation spreading in UF‐TATB occurred with formation of a non‐detonating region surrounding a detonating core, and re‐establishment of detonation in a “lateral” direction beyond that region.

MULTI‐SCALE STATISTICAL DESIGN OF HIGH ENERGY DENSITY MATERIALS

High energy density materials [HEDM] find wide ranging application in commercial and defense applications. The engineering design of these materials is represented by a hierarchy of specifications on materials and processes. The specifications range in scale from molecular by specifying polymorphic crystal structure to macroscopic specifying geometry and global density. These specifications are used to control the configuration of the production HEDM component in the system design. A formalism analogous to that used in statistical mechanics is presented to aid in the interpretation of physical variability of the design based on specification.

USING SCHLIEREN VISUALIZATION TO TRACK DETONATOR PERFORMANCE

Several experiments will be presented that are part of a phased plan to understand the evolution of detonation in a detonator from initiation shock through run to detonation, to full detonation, to transition, to booster and booster detonation. High‐speed multiframe schlieren imagery has been used to study several explosive initiation events, such as exploding bridgewires (EBWs), exploding foil initiators (EFIs or “slappers”), direct optical initiation (DOI), and electrostatic discharge. Additionally, a series of tests has been performed on “cut‐back” detonators with varying initial pressing heights. We have also used this diagnostic to visualize a range of EBW, EFI, and DOI full‐up detonators. Future applications to other explosive events, such as boosters and insensitive high explosives booster evaluation, will be discussed. The EPIC finite element code has been used to analyze the shock fronts from the schlieren images to solve iteratively for consistent boundary or initial conditions to determine the ...

Optical initiation spot size effects in low-density PETN

Los Alamos National Laboratory is currently designing a series of direct optically initiated (DOI) detonators. The primary purpose of this series of detonators is to achieve a level of safety in the face of unintentional initiation from an electrical source. The purpose of these experiments is to determine the minimum spotsize that will initiate the low density initial pressing in these laser detonators. With this information it is expected that a more robust optically initiated detonator can be designed and manufactured. Results from a series of experiments will be discussed. First a range of small core diameter fiber optics with varying energy injection levels will be tested to find the minimum energy level necessary to achieve reliable initiation. Second, a range of apertures will be employed to trim the spotsize down to a minimum size that will still maintain reliable initiation. This information will help to understand whether the initiation criteria for the DOI Laser Detonator are dominated by energy density, total energy or a combination of these criteria.

Damage Progression in Explosively Loaded Polycrystals

One of the current challenges facing researchers in the field of dynamic properties of materials is the need for a predictive modeling capability for damage and fragmentation. A series of small‐scale, explosively‐driven experiments were designed and executed in order to gain a better understanding of the nucleation and growth of damage under explosive loading. The interaction of varying obliquity detonation waves with the test articles was of particular interest. The material in these tests experiences a combination of hydrostatic and deviatoric stresses that is spatially and temporally varying. Variations in shot geometries and explosive load causes direct variations in the nature of the resulting damage fields in the recovered samples. The characterization of a number of samples from a test series of tantalum discs will be presented and compared to numerical analyses of the experiments. Insights gained from the post‐mortem examination of the discs and the accompanying simulations will be presented.