Gas-discharge closing switches are still the main option for pulsed-power systems where high hold-off voltage and high-power handling capabilities are required. One property of the switch that often is of great importance is the precision in time when the switch is to be closed. This review is a survey of existing gas-discharge closing switches, and in particular of their switching time jitter.

Gas-Discharge Closing Switches and Their Time Jitter

Summary form only given. LTD stages are designed to be used as primary storage of pulsed power generators. Their main difference compared to the Marx scheme is ground potential on the stage housing during the shot. This reduces the dimensions of the storage, makes it flexible for reconstruction, simplifies the assembling and maintenance. Other advantage of the grounded housing is the possibility to trigger each stage from the external trigger circuit. This allows to distribute the current flow inside the stage in multiple parallel channels, reducing the equivalent inductance and the current rise time. Previously LTD stages providing 1000 ns and 400 ns current rise time were reported (Bastrikov et al., 1997, Kim et al., 1999). This report presents the design and test results of a fast LTD stage with 100 ns current rise time.

100 ns current rise time LTD stage

A long-pulse generator TPG700L based on a Tesla transformer and a series pulse forming network (PFN) is constructed to generate intense electron beams for the purpose of high power microwave (HPM) generation. The TPG700L mainly consists of a 12-stage PFN, a built-in Tesla transformer in a pulse forming line, a three-electrode gas switch, a transmission line with a trigger, and a load. The Tesla transformer and the compact PFN are the key technologies for the development of the TPG700L. This generator can output electrical pulses with a width as long as 200 ns at a level of 8 GW and a repetition rate of 50 Hz. When used to drive a relative backward wave oscillator for HPM generation, the electrical pulse width is about 100 ns on a voltage level of 520 kV. Factors affecting the pulse waveform of the TPG700L are also discussed. At present, the TPG700L performs well for long-pulse HPM generation in our laboratory.

An 8-GW long-pulse generator based on Tesla transformer and pulse forming network.

In this paper, a multigap multichannel gas spark closing switch (MMCS) has been modified to improve the synchronous closing characteristics of ten switches designed for linear transformer driver (LTD). Convexo-convex discal electrodes have been used instead of the plane toroidal electrodes to improve the electric field across the gaps. A new endo-grooved plane bracing structure has been designed for fixing the electrodes instead of the small ball anchoring method to ensure the length of the gap uniformity. In order to observe the discharge process of the MMCS, the transparent lucite insulation chambers instead of the nylon ones have been used. A test circuit comprising a single discharge circuit called LTD brick has been established to investigate the self-breakdown and triggered breakdown characteristics of ten switches. The results show that when switches are filled with pure nitrogen of 0.14 MPa and charged to plusmn60 kV, the synchronous discharge of ten MMCSs has output a current pulse with an amplitude of about 100 kA and a rise time of 100 ns.

Experimental Study on Multigap Multichannel Gas Spark Closing Switch for LTD

The synchronous characteristics of ten multigap multichannel gas switches have been investigated on one stage of linear transformer driver. An equivalent circuit has been used to simulate the closures of ten switches with different delay times and jitters. The simulated result indicates that the synchronous closing of ten switches can be well performed when the switch jitter is less than 10 ns. The influence of the switch jitter on the rise time is much greater than that on the amplitude of the output current when the switch jitter is about 20-30 ns. The experimental result indicates that a 100-ns 100-kA output current can be achieved with about six switches closing synchronously at plusmn 65-kV charge voltage. To improve the performance of the switches, two methods have been proposed for future work.

Experimental Study on Synchronous Discharge of Ten Multigap Multichannel Gas Switches

The main advantage of the primary storage based on linear transformer scheme is the ground potential on the capacitor bodies during the shot, allowing exclusion of the total output voltage insulation of the highest stages, and to trigger all the stages simultaneously by using an external trigger pulse. The problem was to build a fast linear transformer driver (LTD), providing both high efficiency of energy transfer into the secondary turn and the current rise time below /spl sim/1 CLs, because of relatively high inductance of the caps and the switches. The key elements of the LTD described below are the HAEFELY capacitors (75 kV, 5.65 /spl mu/F, 13 nH, 40 mOhm, or 90 kV, 3.95 /spl mu/F, 10 nH, 13 mOhm) and the multi gap, multi channel spark switches developed at HCEI for SYRINX project (90 kV, 0.7 MA, 8.5 nH). This LTD was designed as an alternative to the Marx generator in order to improve the performance of the IES technology in the direct drive approach where the only unit providing power multiplication on the load is the POS. The LTD was designed as a single stage with two HAEFELY caps in parallel. The authors describe a 75 kV LTD stage with oil insulation and a 90 kV LTD stage with SF/sub 6/ insulation.

Fast primary energy storage based on linear transformer scheme

The switches that are designed for application in high pulsed power drivers should meet some hard requirements, the main of them being small inductance and big charge flowing through the switch. In order to reduce the inductance and to increase the charge it is preferable to divide the switched current into multiple parallel channels. Such parallel current channels could be obtained in railgap three-electrode switches, or in surface switches, due to electrical field disturbance which occurs when a trigger pulse rising as -10/sup 13/ V/s is applied to the switch. Multi channel discharge could also be realized in multi gap spark switches. We report the results of the multi gap, multi channel switch development performed at the High Current Electronics Institute in order to reach the requirements of the SYRINX project (France). These requirements are: operating voltage 90 kV, switch current >500 kA, current rise time <1/spl mu/s, and switch inductance <10 nH. Different switches are described developed in consecutive steps of our collaboration. In these switches, the multi channel discharge is realized in multi gap spark switch configuration, where the voltage during charging is uniformly distributed between the few gaps connected in series. The triggering occurs due to disturbance of voltage distribution between the gaps when a trigger pulse is applied. The number of parallel current channels is controlled by the number of parallel multi gap chains.

Multi gap, multi channel spark switches

A very low inductance pressurized surface discharge switch has been developed for an inductive energy storage generator. This switch was developed as an integral part of a strip-line Marx assembly. This technology performed reliably up to 50 kV-700 kA with a switch inductance as low as the inductance of the equivalent strip-line. The measured inductance was less than 4.5 nH. Part of this development was done to improve the reliability of the switch and its electrical characteristics including the effects of the trigger pulse on the current distribution along the width of the strip-line. We will show that jitter and current distribution can be controlled by the trigger pulse voltage and rise time. For the first quarter period of the discharge we correlated the number of channels during operation with the trigger pulse parameters. We carried out experiments to estimate the main characteristics (self-breakdown voltage, number of channels, self inductance, peak current discharge value) of such a switch for different gases (synthetic air, SF/sub 6/, SF/sub 6//N/sub 2/O/sub 2/ mixture). Optimized parameters are deduced. These studies give us a model of switch operation which enables this kind of design for very low inductance multi-megajoule systems.

Low inductance triggered multichannel surface switch for inductive energy storage generator

The future of PRS programmes is oriented towards very high pulsed power generators of few tens of MJ. At the present moment SATURN and mainly PBFA2-Z are at the edge of this range. It may happen that their technologies, even if they produce excellent results, are not easily extrapolable to higher power, in terms of cost, volume and efficiency. From this investigation, one may conclude that meeting requirements of future powerful plasma radiation sources needs to develop new technological concepts, in order to deliver the appropriate power to a Z-pinch load at affordable cost. This is the main motivation of the SYRINX program which was initiated by the French Defense Department in 1993. The Centre d'Etude de Gramat was asked to realize this research and development program in order to prepare the construction of future large pulsed power facilities for X-rays production which could be used by several customers. The technical specifications and program strategy for SYRINX are outlined as are the main components (HAEFELY-TRENCH capacitors, multigap multichannel coaxial switch and linear transformer driver prototype). The GSI generator prototype and plasma opening switch approach are discussed, as are diagnostics and numerical simulations.

http://ieeexplore.ieee.org/iel4/5524/14895/00679276.pdf

SYRINX technological program, developments and recent results

The research module GSI (Generator a Stockage Inductif), a 640 kJ inductive energy storage generator, is one of the two prototype candidate to bring the future MAG3 z-pinch load driver into operation. The GSI is made of 4 capacitor banks, a water transmission line, a vacuum storage line and a plasma opening switch. Results of parametrical circuit simulations that have been performed on SABER and SCREAMER to choose its electrical features are given. Also, design details of each element and schedules are presented.

J.F. Leon

Papers

Fast primary energy storage based on linear transformer scheme

Multi gap, multi channel spark switches

Low inductance triggered multichannel surface switch for inductive energy storage generator

SYRINX technological program, developments and recent results

SYRINX project: design of the GSI, a 640 kJ inductive energy storage generator