J.M. Lehr

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 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.

Analysis of polarity effects in the electrical breakdown of liquids

Electrical breakdown simulations are carried out for liquids in response to a sub-microsecond ({approx}100-200 ns) voltage pulse. This model builds on our previous analysis and focuses particularly on the polarity effect seen experimentally in point-plane geometries. The flux-corrected transport approach is used for the numerical implementation. Our model adequately explains experimental observations of pre-breakdown current fluctuations, streamer propagation and branching as well as disparities in hold-off voltage and breakdown initiation times between the anode and cathode polarities. It is demonstrated that polarity effects basically arise from the large mobility difference between electrons and ions. The higher electron mobility leads to greater charge smearing and diffusion that impacts the local electric field distributions. Non-linear couplings between the number density, electric field and charge generation rates then collectively affect the formation of ionized channels and their temporal dynamics.

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.

Optical and pressure diagnostics of 4-MV water switches in the Z-20 test Facility

We are studying the behavior of self-breaking, high-voltage water switches for the Z refurbishment project. In Z-20, three or four water switches in parallel are charged to 4 MV in /spl sim/220 ns. The water gap between switch electrodes is 13-15 cm, and the enhancement of the positive and negative electrodes is varied to study time-evolution of the breakdown arcs, current sharing, and switch simultaneity. In addition to the standard electrical diagnostics (V,I), we are looking at one or more of the switches during the breakdown phase with two optical diagnostics: a streak camera and a fast framing camera. The streak camera has /spl sim/1-ns resolution, and the framing camera provides seven frames with >5 ns exposure times. For identical electric fields, the streamers originating on the positive electrode form earlier and move more rapidly than the streamers originating on the negative electrode. We observe four distinct phases in the closure of the water switches that depend on the macroscopic electric fields in the water: 1) No streamers propagate at E-fields below /spl sim/100 kV/cm from positive electrodes or voltages below /spl sim/140 kV/cm for negative electrodes; 2) streamers propagate with constant velocity between 100 and /spl sim/300 kV/cm; 3) above 300 kV/cm, the streamer velocities become linearly proportional to the electric field; 4) above 600 kV/cm, the velocity of streamers from the negative electrodes appears to saturate at /spl sim/100 cm//spl mu/s. The velocity of the streamers from the positive electrode continues to increase with E-field, reaching /spl sim/1% of the speed of light when the switch reaches closure.

Surface flashover of oil-immersed dielectric materials in uniform and non-uniform fields

The applied electrical fields required to initiate surface flashover of different types of dielectric material immersed in insulating oil have been investigated, by applying impulses of increasing peak voltage until surface flashover occurred. The behavior of the materials in repeatedly over-volted gaps was also analyzed in terms of breakdown mode (some bulk sample breakdown behaviour was witnessed in this regime), time to breakdown, and breakdown voltage. Cylindrical samples of polypropylene, low-density polyethylene, ultra-high molecular weight polyethylene, and Rexolite, were held between two electrodes immersed in insulating oil, and subjected to average applied electrical fields up to 870 kV/cm. Tests were performed in both uniform- and nonuniform- fields, and with different sample topologies. In applied field measurements, polypropylene required the highest levels of average applied field to initiate flashover in all electrode configurations tested, settling at ~600 kV/cm in uniform fields, and ~325 kV/cm in non-uniform fields. In over-volted point-plane gaps, ultra-high molecular weight polyethylene exhibited the longest pre-breakdown delay times. The results will provide comparative data for system designers for the appropriate choice of dielectric materials to act as insulators for high-voltage, pulsed-power machines.

Development/tests of 6-MV triggered gas switches at SNL

Gas switch development for application in Z refurbishment (ZR) has continued since PPPS Conference 2001. Several iterations have been tested both in oil and water dielectrics. The first switch tested was an evolved version of the Sandia designed HERMES III switch [G. J. Denison et al., 1985]. The 6 MV ZR baseline switch consists of a self-breakdown (cascade) section in which the discharge current flows in several parallel channels, and a trigger section where the current flows through a single spark channel. The second switch tested is a cantilevered switch. The entire cascade section is cantilevered on a plastic support rod allowing for a single continuous insulator housing but having the same characteristics as the baseline. The third switch tested is a Sandia hybrid switch. The laser triggered section of the hybrid switch includes 5 cascade like electrodes connected in parallel to a triggered gap via a /spl sim/2 /spl mu/H series isolation inductor. The discharge current in the hybrid switch trigger section flows in several parallel channels eliminating the single channel flow as in the baseline switch trigger section. The design iterations of these switches and results of these tests are presented.

Tapered transmission-line transformers for fast high-voltage transients

One technique for raising the voltage in a fast pulse involves the use of a tapered transmission line transformer, whose transit time is large compared to the risetime of the pulse. For continuous variation of the characteristic impedance, the high-frequency/early-time voltage gain is given by the square root of the impedance ratio of each end of tapered line. An undesirable product of the tapered or transmission line transformer is the droop of the pulse after the initial step rise at the transmission line output. A formulation based on the voltage/current variables and renormalized wave variables has been used to examine the pulse droop with the idea of minimizing it. In the analysis, when the dominant term in expansion is optimized, the resulting geometry is that of the exponentially tapered line. The case of the exponentially tapered transmission line is discussed in detail and is shown to have an optimal transfer function in terms of early time voltage gain and improved droop characteristics.

Optically activated switches for low jitter pulsed power applications

Optically activated high voltage switches are commonly used in pulsed power systems for reliable low jitter, multichannel and multiswitch (low inductance) applications. In addition to low jitter switching, optical activation provides a high degree of electrical isolation between the triggering and switching power systems simplifying pulsed power design. The disadvantages of optical triggering for large-gap gas switches are the optical energy, line-of-sight optics, and system maintenance required to obtain reliable operation. This paper describes two technologies which can reduce or eliminate these disadvantages and provide more flexible optically activated switches for pulsed power systems. One approach is to reduce the optical trigger energy requirement for gas gap switches. Shorter optical pulses require less energy to initiate a plasma discharge. An experiment is being assembled to trigger a 50-100 kV gas gap switch with 120 fs wide optical pulses. Lower trigger energy has also been demonstrated by the introduction of metallic aerosols into a gas gap W. Frey (1997). The apparatus will be added to this experiment to reproduce these results and determine the optical energy and power density requirements over a range of wavelengths and pulse widths. The status of this experiment will be discussed. A second approach uses solid state switching in two configurations: (1) as the main switch and (2) as a trigger. High-gain photoconductive semiconductor switches (PCSS) are practical for some direct pulsed power switching applications. We have demonstrated switching up to 220 kV and 8 kA. Higher power optically activated switching can be obtained by combining solid state and gas gap switching technologies. Multimegavolt (MMV) switches can be triggered with fiber-optically triggered, remotely located PCSS. By placing the compact PCSS trigger extremely close to the trigger point, reliable, low-jitter, high power switching is achieved with low energy fiber-optic trigger systems that can easily be controlled and adjusted from a remote control center. Power for the trigger system can be derived from the electrical fields near the trigger, so all electrical cables to the trigger system are eliminated and replaced with 100 micron diameter fibers that trigger and monitor the operation of the system. Results from experiments with PCSS triggered gas gap switches and the design for a PCSS triggered multimegavolt switch will be reported. PCSS switching properties including new picosecond pulse results and fabrication procedures for improved longevity will also be described. A 120 fs wide 780 nm laser pulse was used to radiate THz bandwidth pulses with a GaAs PCSS operating in the linear mode. New approaches for PCSS contact fabrication are being developed and tested to simplify the growth procedure, increase the current per filament capability, and improve device longevity. Progress continues to make PCSS a more useful component for pulsed power applications.

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.