Thomas Holt

A Fabrication Method for a Mid-Sized, High-Energy-Density, Flux Compression Generator

Performance reproducibility is a necessity when considering sources for single-shot, high-voltage applications. Helical flux compression generators (HFCGs) are attractive for a variety of single-shot applications and are capable of high energy amplification that can be used in conjunction with other pulse-shaping techniques such as an exploding wire fuse for achieving high output voltages [1,2]. Small scale HFCGs (with active volumes on the order of ~100-200 cm3), however, are known to perform unreliably from shot to shot [3] and can lose as much as 80% of the flux available in the system based on previous experience with small to mid-sized HFCGs [4]. The performance variation is often attributed to erratic armature expansion behavior and/or fabrication methods and tolerances [3, 4]. As the compressible volume increases, HFCGs are known to conserve more flux and perform more reliably [2]. A fabrication method is presented for a midsized (with active volumes on the order of ~300-400 cm3) dual-stage HFCG that aims to improve the reproducibility in shot to shot performance with the goal of increasing the appeal for use of HFCGs in single-shot pulsed-power applications. Results of experiments with inductive loads of ~3 muH are discussed.

A Compact, Self-Contained High Power Microwave Source Based on a Reflex-Triode Vircator and Explosively Driven Pulsed Power

Single-shot high power microwave (HPM) systems are of particular interest in the defense industry for applications such as electronic warfare. Virtual cathode oscillators (vircators) are manufactured from relatively simple and inexpensive components, which make them ideal candidates in single-shot systems. The flux compression generator (FCG) is an attractive driver for these systems due to its potential for high energy amplification and inherent single-shot nature. A self-contained (battery operated prime power), compact (0.038 m3), FCG-based power delivery system has been developed that is capable of delivering gigawatts of power to a vircator. Experiments were conducted with the delivery system connected to a resistive dummy load and then to a reflex-triode vircator. In order to optimize the performance of the vircator when driven by the power delivery system, a second experimental setup was constructed using a Marx-generator based system operating at similar voltages and rise-times. Performance measures of the delivery system when discharged into a resistive load will be presented, as well as vircator output power levels and waveforms from both experimental setups.

Thermodynamic state of the magnetic flux compression generator volume

The thermodynamic state of the gas trapped in the volume of helical magnetic flux compression generators was measured using optical emission spectroscopy and fast pressure probes. Three main stages of operation are discussed: (1) the initial stage, which can be represented by a freely expanding armature, that shows fairly low gas temperatures, as low as 2000 K; (2) the intermediate stage during 14-4 /spl mu/s before generator burnout that exhibits mainly an atomic copper line transition at about 0.8 eV; (3) the last few /spl mu/s that reveal a highly compressed gas with temperatures of about 5000 K and pressures of about 1500 bar. Most experiments were conducted in air, initially at STP, some results are given for argon and sulfur hexafluoride initially at one atmosphere. Additionally, the thermodynamic state is linked to the electrical volume breakdown threshold via simple resistance measurements that were conducted in current-free flux compression generators.

A Marx generator driven impulse radiating antenna

APELC has developed an Impulse Radiating Antenna (IRA) that consists of a TEM-horn-fed parabolic reflector that is directly driven by a 22-J, 400-kV Marx generator. The system is based on standard Marx generator designs offered by APELC. The Marx generator output couples directly to the TEM horn via a transition from a coaxial geometry that approximates a standard coaxial-to-parallel plate transition. Primary design considerations that facilitate achievement of high instantaneous radiated power include appropriate Marx generator rise time, transition design, and TEM horn focal point positioning. Data collected over the course of the system design is presented.

Marx generators for high-power RF and microwave applications

Recent technological advancements in the field of directed energy have led to increased demand for sources capable of driving high-power RF and high-power microwave (HPM) radiators. APELCs line of Marx generators are uniquely qualified for use in various directed energy applications. Extensive testing performed on a 33-J Marx generator, which has been used as a source to drive various RF loads, will be summarized. Testing included characterizing the thermal behavior of the Marx generator during operation at various pulse repetition frequencies as well as monitoring output pulse characteristics and reproducibility. Pulse characteristics for nine other Marx generators varying from 10 mJ to 1.8 kJ in output energy will also be provided. In addition, measured RF and HPM data from various radiators sourced by APELCs Marx generators will be presented.

A compact high power wideband system

A number of recent efforts have been made to develop high power wideband sources for test and evaluation and general electronic disruption. Applied Physical Electronics, L.C. has developed technology covering 100 MHz to 400 MHz, and is continually working to broaden this area of coverage. The system is based on a single compact pulse power source, capable of delivering 1.7 GW peak power with repetition rates up to 200 Hz. Interchangeable dipole antennas are connected to the pulse power source via high voltage cabling, and are capable of radiating electric field strengths of several hundred kV/m. This paper presents the basic characteristics of the system, followed by experimental data.

Compact Marx generators modified for fast risetime

Traditionally, the 1.6-MV Marx generator offered by APELC operates at a charge voltage of 40 kV, an erected voltage of 1.6 MV, a stored energy of 260 J, and an output pulse rise time between 6-8 ns. APELC has developed a pulse conditioning system (PCS) that can be retrofitted into the existing MV Marx generator housing to improve output pulse rise time at a minimal cost of stored energy. The performance characteristics of the newly developed PCS driven by a slightly modified version of APELCs MV Marx generator will be provided. APELC has also retrofitted its staple 15-stage, 33-J, Marx generator with a scaled version of the same PCS. Preliminary results of the scaled version of the PCS are presented as well.

Compact 200-Hz pulse repetition GW Marx generator system

The compact, wave-erection, GW-class Marx generator has been previously reported for use in 5 ns to sub-ns risetime pulsed power applications. This generator topology has recently been adapted for high Pulse Repetition Frequency (PRF) applications and is the basis for a new high-PRF pulsed power system. The 33-J generator is capable of delivering a 300-kV pulse into a matched 50-Ohm load, or 600 kV into an open circuit. The high-PRF system includes an 8 kJ/sec TDK-Lambda high-voltage power supply and an APELC trigger and control unit. The APELC trigger unit contains a 150-mJ thyratron-based pulser and facilitates the synchronous pulse charging of the Marx generator. Additionally, the trigger unit provides analog output signals of the thyratron and Marx charging signals and features LED diagnostics and fault indicators on the front panel. Applications of the high-PRF system include sourcing of High Power Antennas. Design considerations, system architecture, and experimental results of the high-PRF pulsed power system are presented in this paper.

Fuse and Load Testing with Mid-Sized, High Energy Density Flux Compression Generators

Compact Pulsed Power Systems (CPPSs) require power sources that are small in size yet can produce the necessary electrical energy required to drive a given load. Helical Flux Compression Generators (HFCGs) are attractive for single shot applications due to their rapid conversion of chemical energy to electrical energy. Mid-sized generators occupy little total volume (∼4,000-cm3 total with a compressible volume of ∼300-cm3 in the present generator design), while the high explosives used in an HFCG provide an energy density of ∼8,000 MJ/m3. Consistent output current and energy gain from shot to shot are key variables in the ability of an HFCG to drive CPPSs effectively. An investigation into the practicality of using mid-sized HFCGs as the driver for single shot CPPSs is presented. Data and waveforms from generators fired into 3 μH inductive loads are shown, with results measuring the generator’s performance as a driver for an inductive energy storage (IES) system. Results are also shown from adding a power conditioning system to the output of the HFCG, where the measurements demonstrate the ability of an HFCG to drive high impedance loads. The effectiveness of a mid-sized HFCG as drivers for these systems will be evaluated.

Physical efficiency limits of inch-sized helical MFCG's

Helical magnetic flux compression generators (MFCG) are attractive energy sources with respect to their specific energy output. A variety of one-time use applications would benefit from small inch-sized helical generators with high specific energy output. However, it is widely accepted that the generator performance deteriorates with decreasing size. Previous experimental data have shown that the increase of the ohmic resistance of the MFCG with a reduction in size is the primary cause for the observed behavior when the initial generator inductance is held constant. We will analyze the situation in more depth and quantify how much the efficiency is determined by ohmic losses and intrinsic flux losses (flux that is left behind in the conductors and lost for compression) for different generator sizes and geometries. Our simple constant diameter MFCGs exhibit more intrinsic than ohmic losses (69% compared to 16%), while our MFCGs with tapered armatures display less intrinsic and more ohmic flux losses (13% compared to 66%), however, at increased overall efficiency. We will show experimental and calculated data and discuss the physical efficiency limits and scaling of generator performance at small sizes.