William D. Prather

Survey of worldwide high-power wideband capabilities

Wideband and ultrawideband (UWB) technologies have achieved notable progress in recent years, aided immeasurably by improvements in solid-state manufacturing, computers, and digitizers. As a result, wideband systems that were difficult or impossible to implement ten years ago are now being built for an increasingly wide variety of applications including transient radar, concealed object detection, mine clearing, pipeline inspections, archeology, geology, electronic effects testing, and communications. In this paper, we discuss current wideband source technology around the world as well as laboratory and test facilities. We also will briefly touch on frequency limitations of anechoic chambers, design of transmission line simulators, frequency regulation requirements for outdoor ranges, and personnel exposure limits.

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.

Design, fabrication, and testing of a paraboloidal reflector antenna and pulser system for impulse-like waveforms

In this paper, we describe an antenna and pulser system designed to radiate an impulse-like waveform. Specifically, we have analyzed, designed, fabricated, and tested a reflector antenna fed by a pair of conical transmission lines. The voltage waveform that feeds the antenna is a fast-rising (10-90% rise of /spl sim/100 ps), slowly decaying (e-fold decay of 20 ns) pulse with a differential amplitude of /spl plusmn/60 kV. The calculated and measured bore-sight-radiated field, at a distance of 304 m, is impulse-like with a peak amplitude of 4.2 kV/m.

Measurement of the electric breakdown strength of transformer oil in the sub-nanosecond regime

The dielectric strength of highly purified insulating transformer oil has been measured in the sub-nanosecond regime under single pulse and repetitive burst conditions. Single pulse breakdown fields have been measured to be 11 MV/cm. Repetitive bursts to 1 kHz reduce the threshold field value by a factor of two, with lower breakdown fields recorded at a 1.2 kHz repetition rate. The high-pressure hydrogen source provides a 130 ps risetime and a 1 MV peak amplitude at repetition frequencies to 1.2 Hz. An experimental setup was used which permits the breakdown of the oil spark gap while protecting the high power source in case of total wave reflection, at the cost of excitation source fidelity. The breakdown electric fields are measured with self-integrating electric field sensors and an advanced diagnostic system which uses Fourier compensation to measure the fast risetime of the ultra-wideband pulse accurately. The anomalously high breakdown voltages measured with high power ultra-wideband sources compare favorably with Zheltovs prediction of the breakdown strength for sub-nanosecond pulse duration. The anomalously high field strengths permit the design of ultra wide band (UWB) high power microwave (HPM) sources with a reduced geometrical inductance which can result in significantly faster HPM UWB sources.

Ultra-wideband source and antenna research

Ultra-wideband (UWB) microwave sources and antennas are of interest for a variety of applications, such as transient radar, mine detection, and unexploded ordnance (UXO) location and identification. Much of the current research is being performed at the Air Force Research Laboratory (AFRL) at Kirtland AFB, NM. The approach to high power source development has included high pressure gas switching, oil switching, and solid-state-switched arrays. Recent advances in triggered gas switch technology and solid-state-switched shockline technology have opened up new possibilities for the development of much higher power systems and have thus opened the door to many new applications. The research into UWB transient antennas has also made significant contributions to the development and improvement of wideband continuous wave (CW) antenna designs and has brought new knowledge about the complex behavior of ferrites, dielectrics, and resistive materials in short pulse, very high voltage environments. This has in turn led to advances in the technology of transformers, transmission lines, insulators, and UWB optics. This paper reviews the progress to date along these lines and discusses new areas of research into UWB technology development.

Interaction Between Geometric Parameters and Output Waveforms in High-Power Quarter-Wave Oscillators

Quarter-wave switched oscillators (SWOs) are an important technology for the generation of high-power mesoband waveforms. The operation of these SWOs has been discussed for the past several years, but a detailed discussion of the design of these sources for particular waveforms has been lacking. In this paper, we relate several important parameters such as gap spacing, oscillator shape, and antenna to the properties of the radiated waveform.

Multifunction impulse radiating antennas: theory and experiment

A Multifunction IRA is an extension of a standard Impulse Radiating Antenna that has the additional flexibility of an adjustable beamwidth. This adjustability is implemented by defocusing the feed, in order to select between a narrow or broad beam. We provide here the theory of operation of the antenna, for both in-focus and out-of-focus situations. Furthermore, we built and tested a design with a 46 cm diameter. We found good agreement of the experiment with theory.

Fundamental physical considerations for ultrafast spark gap switching

For future applications, the limit of spark gap technology for ultrafast switching is explored. Specifically, an estimate of the fastest risetime achievable with a single channel spark gap has been investigated using three approaches. The first examines the growth of the electron avalanche in gases to estimate its growth rate. The avalanche growth rate determines the fastest possible risetime of the resultant pulse. The second approach uses the components of the velocity of electromagnetic propagation to estimate the achievable risetime. The third uses an equivalent circuit of a single channel spark gap to calculate the maximum achievable rate of voltage rise. The first two estimates indicate that risetimes on the order of 1-10 ps are achievable. The last treatment, however, illustrates the dependence of the pulse risetime on the peak voltage and calculates the maximum rate of voltage rise to be on the order of 10/sup 16/ V/s. To reduce the effect of the intrinsic inductance of the channel, a simple geometrical alteration to the spark gap geometry has been devised which effectively reduces the inductance per unit length of the spark gap to that of its transmission line feed. This simple change alleviates the constraint imposed by the maximum rate of voltage rise and is anticipated to permit the realization of picosecond risetime high power electromagnetic sources.

Electrostatic field management and electrodynamic modeling of switched quarter-wave oscillators

Quarter-wave switched oscillators (SWOs), sometimes referred to as MATRIX oscillators, are an important technology for the generation of high-power, moderate bandwidth (mesoband) waveforms. The use of SWOs in high power microwave sources has been discussed for the past 10 years but a detailed discussion of the design of this type of oscillators for particular waveforms has been lacking. In this work a design methodology for a realization of SWOs is developed. A key element in the design of SWOs is the self-breakdown switch, which is created by a large electric field. In order for the switch to close as expected from the design, it is essential to manage the electrostatic field distribution inside the oscillator during the charging time. This enforces geometric constraints on the shape of the conductors inside the oscillator. At the same time, the electrodynamic operation of the system is dependent on the geometry of the structure. In order to generate a geometry that satisfies both the electrostatic and electrodynamic constraints, a new approach is developed to generate this geometry using iterative solutions to the 2-D static Laplace equation, subject to a particular set of boundary conditions. These boundary conditions are manipulated to generate equipotential lines with specific dimensions that satisfy the electrodynamic constraints. Meanwhile, these equipotential lines naturally support an electrostatic field distribution that meets the requirements for the field enhancement. To study the electrodynamic aspects of SWOs, three different (but inter-related) numerical models are built. Depending on the assumptions made in each model, different information about the electrodynamic properties of the designed SWO are obtained. In addition, the agreement and consistency between the different models, validate and give confidence in the calculated results.

Aspects of ultrafast spark gap switching UWB HPM generation

The Air Force is interested in compact ultra-wideband systems which utilize a minimum volume of high pressure gas. These desires lead us to look closely at single channel spark gaps, because both the size and volume of gas, for example, hydrogen, under pressure can be much less than needed for sources containing ring gap switches. White single channel spark gap switches are desirable, the intrinsic inductance of the spark gap is prohibitively high to achieve large rates of voltage rise. For future applications, the limit of spark gap technology for ultrafast switching is explored. Of primary interest is the fastest possible risetime achievable with a single channel spark gap. Thus far we have calculated the limit on the achievable risetime with spark gap technology, using three different approaches, which are all in good agreement. The first examines the excitation rates in gases to determine its limitations. The second assumes a streamer mechanism and uses the velocity of propagation to estimate the achievable risetime. The third utilizes an equivalent circuit model. It is commonly believed that the impedance mismatch in the spark region, caused by the additional spark gap inductance, is unavoidable. To reduce the effect of the intrinsic inductance of the channel, the High Energy Source Division has devised a simple geometrical alteration to the spark gap geometry which reduced the inductance per unit length of the spark gap to that of its transmission line feed. This is anticipated to permit the realization of picosecond risetime UWB HPM sources.