Jeff A. Alexander

Low-Inductance Gas Switches for Linear Transformer Drivers

Summary form only given. We are investigating several alternate gas-switch designs for use in linear transformer drivers. To meet linear-transformer-driver requirements, these switches must be DC charged to 200 kV, be triggerable with a jitter of 5 ns or less, have very low pre-fire and no-fire rates, and have a lifetime of at least several thousand shots. Since the switch inductance plays a significant role in limiting the rise time and peak current of the LTD circuit, the inductance needs to be as low as possible. The switches will be required to conduct current pulses with ~100-ns rise times and 20-80 kA peak currents, depending on the application. Our baseline switch, designed by the High Current Electronics Institute in Tomsk, Russia is a robust six- stage switch with an inductance on the order of 120 nH that is insulated with 30-40 PSIG of air. We are also testing two smaller two-stage switches that have inductances on the order of 60 - 70 nH. The smaller switches are insulated with 100-200 PSIG of air. We will discuss details of the operation of each of the switches, including voltage hold-off, triggerability, and lifetime testing.

Performance of the hermes-III laser-triggered gas switches

This paper reports the performance of the SF/sub 6/ insulated, multistage, laser-triggered gas switch used in the Hermes-III accelerator.sup 1/ In this accelerator, 20 of these switches are used to transfer energy from intermediate energy storage water dielectric capacitors to the pulse forming lines (PFLs). Approximately 8,000 laser-triggered switch shots have been taken with seven prefires. Nearly 70% of these shots have been at nominal operating parameters. The average first-to last spread in firing times for 20 switches is approximately 8 ns. Removal of systematic differences reduces this spread to /spl tilde/6 ns. This spread implies a one-sigma jitter for a single switch of <2 ns at 70-75% of self-breakdown voltage. Results show that the jitter does not change significantly over an operating range of 70-90% of self-breakdown. In addition, the jitter is insensitive to the factor of two variation in the laser energy delivered to the various switches by the optical system. A detailed summary on the performance, reliability, and maintenance of the switches and optical system is presented. Initial results of a study to investigate the performance of these switches under varying laser trigger conditions is also presented.

Plastic Laminate Pulsed Power Development

The desire to move high-energy Pulsed Power systems from the laboratory to practical field systems requires the development of compact lightweight drivers. This paper concerns an effort to develop such a system based on a plastic laminate strip Blumlein as the final pulseshaping stage for a 600 kV, 50ns, 5-ohm driver. A lifetime and breakdown study conducted with small-area samples identified Kapton sheet impregnated with Propylene Carbonate as the best material combination of those evaluated. The program has successfully demonstrated techniques for folding large area systems into compact geometrys and vacuum impregnating the laminate in the folded systems. The major operational challenges encountered revolve around edge grading and low inductance, low impedance switching. The design iterations and lessons learned are discussed. A multistage prototype testing program has demonstrated 600kV operation on a short 6ns line. Full-scale prototypes are currently undergoing development and testing.

Ceramic-Polymer Composite for High Energy Density Capacitors

The U.S. Department of Defense vision for future directed energy weapons systems (e.g., high power microwave systems) requires the development of electrical pulsers that exceed current state-of-the-art in energy storage density by an order of magnitude or more. Capacitors made from composite dielectric materials consisting of ceramic nanoparticles embedded in a polymer matrix show promise for attaining these goals. Sandia National Laboratories (SNL) and TPL Inc. (TPL) have teamed to investigate the limits of these new materials for use in high energy density and high power capacitor designs. The major challenges encountered thus far are quality control in the processing of the materials as well as mechanical stresses resulting from the thermal curing process while forming prototype capacitor devices. This paper reports on the current status and results achieved by this investigation.

Performance of the hermes-III pulse forming lines

Hermes III is a new 20-MV, 730-kA, 40-ns pulsed power accelerator. In Hermes III, eighty pulse forming lines (PFLs) generate 1.1-MV, 220-kA pulses, which are then added in a series/parallel configuration to produce the desired output pulse. Under normal Hermes-III operation, the PFLs produce a /spl tilde/40-ns FWHM pulse with a /spl tilde/15-ns, 10-90% rise time, and a one-sigma jitter of /spl tilde/4 ns. The Hermes-III water-dielectric PFLs have undergone 18 months of testing. During this period, over 59,000 PFL-shots have been accumulated, most of these at or near peak power. The PFLs have met design specifications, and they have proven to be highly reliable. This paper presents a brief overview of the PFL design, together with a detailed discussion of the system performance and limitations during this extensive testing period. A modification to the PFLs to produce a ramped output pulse was designed and tested. Results indicate that a ramped output pulse can be produced using a stepped impedance PFL.

Low inductance switching studies for Linear Transformer Drivers

We are developing new low-inductance gas switches for Linear Transformer Drivers (LTDs). Linear Transformer drivers are a new pulsed-power architecture that may dramatically reduce the size and cost of future pulsed-power drivers, but which place stringent requirements on gas switches.1 ,2 A typical large LTD may have 10,000 or more gas switches that are DC-charged to 200 kV and that must be triggered with a jitter of 5-ns or less with a very low prefire rate3. We are studying new air-insulated gas switches with total inductances of 68–90 nH. These switches transfer 100 J of stored energy for thousands of shots. Typical 10%–90% current rise-times are less than 50 ns and peak currents are on the order of 40 kA into a matched load. One of the switches has a 1-sigma jitter less than 800 picoseconds. Since triggering 10,000 or more switches is a significant challenge, we are also making a detailed study triggering requirements for these switches. We have previously reported tests of four competing switches for LTDs4. In this article we report new tests on improved versions of two of the four switches described previously.

Characterization of a synchronous wave nonlinear transmission line

Many aspects of Non Linear Transmission Lines (NLTLs) make them attractive sources for High Power Microwave (HPM) systems. This paper describes a test bed that is being used to investigate synchronous wave NLTL operations.

Ceramic-polymer composite for high energy density capacitors

he U.S. Department of Defense vision for future weapons systems requires the development of electrical pulsers that exceed current state-of-the-art in energy storage density by an order of magnitude or more. Capacitors made from composite dielectric materials consisting of ceramic nanoparticles in a polymer matrix show promise for attaining these goals. TPL Inc. and Sandia National Laboratories have teamed to investigate the limits of these new materials for use in high energy density and high power capacitor designs. The major challenges encountered thus far are quality control in the processing of the materials as well as mechanical stresses resulting from the thermal curing process while forming prototype capacitor devices. This paper reports on the development progress of the composite dielectric for high energy density capacitors.

Comparison of trigger requirements for gas switches for linear transformer drivers

Linear Transformer Driver (LTD) technology is being developed for short pulse electron beam applications as well as high current Z-pinch drivers. Designs for both applications require low inductance spark gap switches which hold off 200 kV and trigger with low jitter. LTD cells or cavities typically contain many parallel switches which must close with low jitter to insure efficient operation of the system. The switch jitter must be much less than the risetime of the output pulse to prevent switches from firing after the peak in output power. Experiments with a 10-brick Ursa Minor cavity indicate that the switch jitter must be less than 2 ns to limit the late switch rate to less than 2%. Three swith designs have been tested in a single switch platform to evaluate switch jitter as a function of the peak trigger voltage, trigger pulse risetime, and switch pressure. Operating parameters were determined for each switch to meet the 2 ns jitter requirement.