James P. O'Loughlin

JOLT: a highly directive, very intensive, impulse-like radiator

Ultrawideband (UWB) systems that radiate very high-level transient waveforms and exhibit operating bandwidths of over two decades are now in demand for a number of applications. Such systems are known to radiate impulse-like waveforms with rise times around 100 ps and peak electric field values of tens of kilovolts per meter. Such waveforms, if properly radiated, will exhibit an operating spectrum of over two decades, making them ideal for applications such as concealed object detection, countermine, transient radar, and communications. In this paper, we describe a large, high-voltage transient system built at the Air Force Research Laboratory, Kirtland AFB, NM, from 1997 to 1999. The pulsed power system centers around a very compact resonant transformer capable of generating over 1 MV at a pulse-repetition frequency of /spl sim/ 600 Hz. This is switched, via an integrated transfer capacitor and an oil peaking switch onto an 85-/spl Omega/ half-impulse radiating antenna. This unique system will deliver a far radiated field with a full-width at half-maximum on the order of 100 ps, and a field-range product (rE/sub far/) of /spl sim/ 5.3 MV, exceeding all previously reported results by a factor of several.

High repetition rate charging a Marx type generator

Resistive ladder networks are commonly used as the charging and isolation means for Marx type generators. The efficiency is limited to 50% and the charging time is long or equivalently the PRR (Pulse Repetition Rate) is low. The efficiency can be considerably improved by replacing the resistive ladder with inductor elements and the PRR is also improved. In this paper is it shown that by introducing mutual coupling, k, between the two parallel inductors in each stage of the ladder network, the effective inductance during the charging mode is decreased b/sup 1/y a factor of (1-k)/(1+k). Since it is feasible to achieve a coupling, k, on the order of 0.99, this speeds up the charging time by about an order of magnitude compared to uncoupled inductive charging. During the erected or discharge mode the inductors must provide isolation between stages and must not excessively rob energy from the energy store. The mutual coupling is beneficial in two ways. During the erected or discharge mode, it is shown that the effective inductance of the ladder elements are actually increased by a factor (1+k). The Marx switches cause a re-arrangement of the coupled inductors from parallel during the charging to series during the discharge modes. This results in a much faster charging time, by reducing the effective inductance by (1-k)/(1+k); while providing an effective isolation inductance that is (1+k) greater than the uncoupled value. A practical design of the coupled inductor implementation and modeled simulations of the performance are compared to uncoupled and resistive charging.

Ultra-wideband sources and antennas

Ultra-Wideband (UWB) sources and antennas are of interest for a variety of potential applications that range from transient radar systems to communications systems. In this paper, we discuss the research issues and progress being made in gas, oil and solid state switching, UWB sources, and antennas.

Solid dielectric transmission lines for pulsed power

This paper documents recent work developing solid dielectric transmission lines for sub-microsecond, 100 kV class compact pulsed power systems. Polymer-ceramic nanocomposite materials have demonstrated sub-microsecond discharge capability in parallel plate capacitors and transmission lines [1, 2]. With a dielectric constant of approximately 50, the propagation velocity is 2.5 cm/ns, necessitating lines of several meters length to achieve > 100 ns pulse lengths. By folding the line in a fashion analogous to ceramic multilayer capacitors, the physical length of the line can be significantly shorter than the electrical length. We present the results of an experimental effort to develop a folded transmission line using a polymer-ceramic nanocomposite dielectric. The pulse length was somewhat shorter than expected based on a simple calculation using the geometry and the dielectric constant. Fully 3-D electromagnetic calculations were used to examine the role of the edges in curtailing the pulse length. Dielectric breakdown in this device occurred below the electric field threshold demonstrated in the prior work [1]. Improvements in the large scale fabrication of TiO2 beginning with nanoscale grains have opened the possibility for producing single layer high voltage devices. Given a dielectric constant approaching 140, transmission lines using nano-TiO2 can be considerably shorter than with other materials. Relatively thick, flat sheets of TiO2 have been fabricated for testing up to 50 kV. Several transmission lines, employing a serpentine electrode geometry, have been manufactured and tested. Testing up to several 10s of kV has confirmed the operation of the lines according to the design. As expected, the triple point between the TiO2, electrode, and insulating medium has proven difficult to manage for high voltage operation. Several techniques to mitigate the effects of the triple point, including resistive grading at the edges of the electrodes, are discussed. Fully 3-D electromagnetic modeling is used to examine the effects of electrode geometry and composition on the performance of the lines.

Improved inverter transformer winding design for high-current low-voltage rectifier applicationsa

High-frequency fast-recovery rectifiers have limited maximum current ratings; this requires parallel circuits for high-current high-frequency applications. To ensure good current sharing, it is advantageous to use separate transformer windings for each rectifier set. Each section should be as closely matched as possible. An improved winding design consists of a simplified coil with multiple secondaries for inverter transformers used in low-voltage high-current rectifier applications. Since the forward drop of a rectifier is significant in relation to outputs of only a few volts, the preferred configuration, for efficiency reasons, is a center-tapped winding. This configuration imposes only one forward drop of the rectifier compared to two forward drops in a bridge circuit. The coil design uses foil conductors and provides multiple parallel secondary windings for the purpose of controlling skin-effect losses, improving the winding space factor, improving the heat transfer, and providing coil designs that are simple and economic to manufacture. In addition, the configuration provides matched and balanced coupling and leakage inductance for each secondary section

Marxed transmission lines for compact pulsed power

The Marxed Blumlein line is a concept that has been touted as a highly compact pulsed power system because it combines the functions of energy storage, voltage scaling, and pulse shaping into a single sub-system. Inherent voltage reversal is compounded in a Marxed arrangement. The voltage reversal and transients induced by the switch jitter in a Marxed configuration require significant derating of the dielectric strength. Consequently, the system must be operated much below the intrinsic dielectric energy density, compromising efforts to design a compact pulsed power system. This paper presents the conclusion that for all practical configurations, the Marxed transmission line is the more compact architecture.

Electromagnetic field coupled to filaments in air for plasma characterization studies

We present new insights into laser filament coupling to a high AC electric field strength of 100kV/m. Previous studies showed a short delay time for filament coupling. Our observations indicate a large time delay between the plasma and the discharge of the high field strength.