Michael D. Abdalla

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.

Ultra-wideband source using gallium arsenide photoconductive semiconductor switches

An ultrawide-band (UWB) pulse generator based on high-gain (lock-on mode) gallium arsenide (GaAs) photoconductive semiconductor switches (PCSSs) is presented. Revised PCSS contact design shows improved performance in hold-off field, on-state switch potential, and switching jitter, while reducing the switch volume by 75% compared to previous designs. A compact laser diode module operates at 904 nm and triggers the PCSS at pulse repetition rates (PRR) of up to 2 kHz. The 625 W laser diode output power is found to be sufficient to produce switching jitter of 65 ps rms at a switched field of 80 kV/cm. The PCSS switching jitter is found to have a strong dependence upon the switched field when triggered with the laser diode module. The revised PCSS geometry is easily integrated into a compact, parallel-plate source used to drive a TEM horn impulse-radiating antenna. The radiated field has a rise time of 330 ps and an adjustable low-frequency spectrum.

Switched Oscillators and Their Integration Into Helical Antennas

In this paper, the problem of designing switched oscillators at four different frequencies (200, 300, 400, and 500 MHz) has been addressed. These oscillators are quarter-wavelength long coaxial transmission lines with a nitrogen spark gap switch at one end. Two of these switched oscillators at 200 and 500 MHz with a charge voltage of 30 kV have also been fabricated. These two oscillators are modeled using PSpice and their output into a 100 Ω load is estimated and tested by fabricating a 100 Ω transmission line. Use is made of a modified commercial helical antenna with a bandwidth of 400-600 MHz and a switched oscillator has been integrated into this helical antenna. Measurements have been made of the S11 , the voltage into the antenna, and also the transient fields at two distances. Indeed, starting from electrical power from a 12 V battery, electrical field strengths in excess of 10 kV/m with damped sinusoidal waveforms at 500 MHz (for example) have been demonstrated.

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.

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.

An Efficient, Electrically Small, Three-Dimensional Magnetic EZ Antenna for HPM Applications

Metamaterial (MTM)-inspired antennas leverage techniques that have been developed over the past decade for designing artificial materials whose electromagnetic properties can be tailored to specific applications. One of the key features of the MTM-inspired antennas is their ability to motivate electrically small antenna designs through planar and volumetric loadings of space with resonant parasitic capacitive and inductive structures. In a previous work, we developed the magnetic EZ antenna as a resonant antenna that operates below ka = 0.5. In this paper, we adapt the magnetic EZ antenna concept for use with high-power mesoband quarter-wave oscillator microwave sources that can operate with hundreds of megawatts of peak power and charge voltages in excess of 100 kV in the ultrahigh frequency (500-650 MHz) and demonstrate their performance with charge voltages up to 10 kV. The principal challenges that were overcome in this effort include field management to prevent undesired breakdown and capacitive isolation to decouple the EZ antenna from the source during the charge phase. Antenna design, modeling, and experimental verification are presented here, demonstrating an operating EZ antenna/source system at 510 MHz with antenna ka = 0.436 . The results demonstrate that the EZ antenna is a viable antenna to consider when traditional high-power microwave antennas are too large to be integrated into a given platform.

Development of a hermetically sealed, high-energy trigatron switch for high repetition rate applications

Triggered gas switches increase the reliability and repeatability of pulsed power systems. In particular, the performance of high-power, repetitively rated impulse generators is greatly enhanced by including a triggered switch. Impulse generators generally consist of cascaded energy storage elements, and the pulse-to-pulse voltage variations depend, to the first order, on the consistency of the voltage in the first energy storage stage. Thus, a repetitively operated trigatron has been developed to discharge a low-inductance, 70-J primary capacitor bank with very repeatable voltage at a pulse repetition frequency of 600 Hz. In order to simplify the pulsed power for mobile field-testing, it is desirable to seal the spark gaps to eliminate the need for a gas reservoir. Although high-pressure spark gaps have been sealed previously, long-term containment has not been pursued. To operate at high-pulse repetition frequencies, hydrogen, which is notoriously hard to seal at high pressures, must be used for its superior recovery properties. The hermetic seal was achieved by the careful selection of materials and fabrication techniques and designed for at least one year of use without refreshing.

A small size resonant antenna for high power applications

One way of designing high power mesoband sources uses a wideband source to drive a self-resonant antenna. A self-breakdown switch is used to discharge the antenna after charging it to the breakdown voltage of the switch. The antenna resonates at its quarter wavelength. In many high applications it is advantageous to: reduce the size of such source for a given resonance frequency, and increase the number of the radiated cycles after each switch closure. In the paper, we propose a new design for a conical helix antenna that help in achieving these two goals. Numerical simulations are used to compare the radiated far-field from our new design to those from conical helix and biconical designs.

Frequency and bandwidth agile pulser for use in wideband applications

We have developed an architecture to produce a wideband pulser that is tunable in the important parameters of pulsewidth, bandwidth, and center frequency. The pulser is based on a parallel plate Blumlein pulse-forming network (PFN) with a movable center (charge) conductor. When the center conductor is displaced from the ideal Blumlein position, the balance in the PFN is lost, resulting in a slightly ringing waveform that can be tuned. By selecting the amount of ringing, the output bandwidth of the PFN can be adjusted. Furthermore, by simply sliding the center conductor out of the PFN, the effective electrical length can be adjusted, allowing the fundamental period of the ringing waveform to be altered, thereby changing the center frequency of the output waveform. Our pulser is designed to operate in the range of 300 MHz to 2 GHz, but the architecture is scalable outside this range. In this paper, we present design and simulation results, low-voltage tests, and preliminary high-voltage (15-kV) data that is obtained with a pressurized gas trigatron designed specifically for this test fixture. The pulser is being designed for laboratory use in testing the effects of interaction of high-intensity wideband electromagnetic fields with small-scale electrical and biological systems.

Temporal switching jitter in photoconductive switches

Gallium arsenide photoconductive semiconductor switches (PCSS) are being studied as enabling technologies for a variety of applications. High grain PCSS can be triggered with small laser diodes or laser diode arrays. Some of the applications require low temporal jitter of the switches relative to the trigger laser. The purpose of this study was to compare the temporal switch jitter times for different systems: we varied the type of trigger laser and its risetime, the type of pulse charger and transmission line that was discharged through the PCSS, and the geometry of PCSS used. One of the PCSS was an opposed contact PCSS geometry used by the Air Force Research Laboratory. The other was a coplanar geometry switch made by Sandia National Laboratories. It is found that the optical trigger laser characteristics are dominant in determining the PCSS jitter while the nature of the contact geometry (opposed or coplanar) is not as important.