Jeffrey W. Burger

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.

Ultra-wideband transmitter research

The generation of ultra-wideband (UWB) pulses is a challenging problem that involves generating pulses with fast rise times on the order of 100 ps and voltages of more than 500 kV. Pulsewidths from 130 ps to a few nanoseconds (ns) are possible. A critical step involves switching high voltages with precision. The use of both gas and oil for the switching medium has been accomplished with varying results. The Air Force Research Laboratory (AFRL) is pursuing both media in the gas-switched H-series of pulsers and in studies of oil switches that promise good performance in compact packages. We are also pursuing solid-state switched systems that have demonstrated the potential for use in compact systems and in transient antenna arrays with steerable beams. The paper reviews recent progress in fast, high voltage switching and UWB transmitter development. These UWB pulsers and antennas have the potential for use in transient radar, target identification, and communications.

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.

Ultra-wideband source 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, USA. 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.

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.

Design and development of a 1 MV, compact, self break switch for high repetition rate operation

The design of compact, high voltage switches is generally plagued by the tradeoff between reduced size and the probabilities of bulk and surface breakdown. Moreover, typical breakdown field values for wide classes of materials with desirable electrical and mechanical properties are unavailable. In the development of a compact gas switch, capable of holding off voltages of up to 1 MV at repetition rates of 200 Hz, a criterion to insure the inhibition of surface flashover in a high pressure atmosphere has been developed. The switch is composed of copper tungsten electrodes with a coaxial Torlon pressure containment housing of length 12.7 cm and a diameter of 15.3 cm. Pulse repetition rates in the 100s of Hz are achieved by using moderate hydrogen pressures as the insulating medium. The electrodes are shaped to produce a uniform field distribution in the gap with an adjustable spacing of 0.5, 0.75 and 1.0 cm. Additional stress is put on the switch by charging with a dual-resonant pulse transformer.

Progress in gallium arsenide photoconductive switch research for high power applications

Gallium arsenide (GaAs) photoconductive semiconductor switches (PCSS) have been studied as an enabling technology for a variety of applications at both the Air Force Research Laboratory and Sandia National Laboratories. High gain PCSS can be triggered with small laser diodes or laser diode arrays. The requirements of these applications require the switching of high voltage in sub-nanosecond time with low temporal jitter of the switches relative to the trigger laser. There have been several configurations and sizes of these switches studied by the Air Force Research Laboratory over the last several years. The most recent designs are with small structures where the electrical contacts are placed on opposite sides of the bulk material. This configuration allows for different electrical conditions on either side depending on the nature of the semiconductor structure; i.e., p-i-n or n-i-n. In addition to the type of structure used and geometry of the contacts, the performance of these switches (switch time, voltage, and jitter) is dependent on the thickness of the GaAs. Several thicknesses have been studied during the past year. This paper reports on the results of several studies to investigate the ultra-fast switching properties of these structures.

Progress toward a compact, high power ultra-wideband array using gallium arsenide photoconductive semiconductor switches

There is considerable demand within the technical applications community for sources which produce high power transient radiation. Lateral geometry high power photoconductive semiconductor switches (PCSS) are being developed for use in phased array, ultra-wideband sources. The present source configuration is a stacked Blumlein configuration commuted with a PCSS. A new liquid insulating media, HFE-7100, has been demonstrated to have an equivalent voltage hold off capability as sulphur hexafluoride, but with greater compatibility to gallium arsenide. Independent low jitter PCSS operation is demonstrated by coupling two independent Blumleins into a common load. Measurements of both the system jitter and the isolated jitter of the PCSS switch has been performed.

Recent Developments in Ultra-Wideband Sources and Antennas

Ultra-wideband (UWB) sources and antennas are of interest for applications such as the location and identification of hidden objects, that is, land mines and unexploded ordnance (UXO). Much of the current research is being performed at the Directed Energy Directorate of the Air Force Research Laboratory (AFRL) at Kirtland AFB, NM. Although significant progress has been made in UWB technology since 1980s, new challenges continue to arise.1, 2, 3, 4, 5, 6 Recent requirements have included antennas that are lighter weight, have wider bandwidths, and operate at lower frequencies. Noteworthy progress is also evident in the development of fast switching technology. In this paper, we describe recent progress in sources, antennas, and lens technology and new developments in gas and solid-state switching.

Design and Development of a High Voltage Coaxial Hydrogen Switch

The high power microwave program at the Air Force Research Lab (AFRL) includes high power source development in narrow band and wideband technologies. The H2 source is an existing wideband source that was developed at the AFRL. A recent AFRL requirement for a wideband impulse generator to use in materials tests has provided the need to update the H2 source for the current test requirements. The H2 source is composed of a dual resonant transformer that charges a short length of coaxial transmission line. The transmission line is then discharged into an output coaxial transmission line with a self-break Hydrogen switch. The dual resonant transformer is driven by a low inductance primary capacitor bank operating through a sel-break gas switch. The upgrade of the coaxial Hydrogen output switch is the focus of this paper. The Hydrogen output switch was developed through extensive electrical and mechanical simulations. The switch insulator is made of Ultem 2300 and is designed to operate with a mechanical factor of safety equal to 4.0 at 1,000 psi. The design criteria, design data and operational data will be presented.