M. Schlaug

Pseudospark switches-technological aspects and application

We report results of the development of fast closing switches, so-called pseudospark switches, at Erlangen University. Two different parameter regimes are under investigation: medium power switches (32 kV anode voltage, 30 kA anode current and 0.02 C charge transfer per shot) for pulsed gas discharge lasers and high power switches (30 kV anode voltage, 400 kA anode current and 3.4 C charge transfer per shot) for high current applications. The lifetime of these switches is determined by erosion of the cathode. The total charge transfer of devices with one discharge channel is about 220 kC for the medium and 27 kC for the high power switch. At currents exceeding 45 kA a sudden increase in erosion rate was observed. Multichannel devices are suited to increase lifetime as the current per channel can be reduced. Successful experiments with radial and coaxial arrangements of the discharge channels were performed. In these systems the discharge channels move due to magnetic forces. A skilful use of this phenomena will result in a considerably increase of switch lifetime. Multigap devices enable an increase of anode voltage. A three gap switch has run reliably at an anode voltage of 70 kV. >

Trigger devices for pseudospark switches

Effective triggering of pseudospark switches with long lifetime and low jitter remains an important problem. This paper presents results of investigations of trigger methods for pseudospark switches. based upon pulsed glow discharges in planar and hollow electrode geometry for charge injection. The influence of different wiring and geometries of the electrodes for preionization is investigated. The effect of additional blocking potentials in the hollow cathode to improve different trigger systems was measured. Calculations of the static potential in the hollow cathode with or without blocking potential are compared with parameters of the discharge. >

Scientific and technological progress of pseudospark devices

This paper presents an overview on the state-of-the-art of research and development with pseudospark devices. There is an ongoing interest worldwide in this novel low pressure gas discharge device. This is proven by the several papers recently published. Careful studies of breakdown characteristics with two-electrode pseudospark devices show that the simple relation of the old Paschen law is modified for this geometry. Especially for operating the pseudospark reliably at low gas pressure, it is necessary to superimpose external magnetic fields to initiate the discharge. At low pressure intense beam formation is enhanced but in parallel is hampered by less efficient space-charge-neutralization. Based on the original pseudospark geometry several modified beam configurations were developed like the channel spark and the preionization-controlled open-ended hollow cathode system. In pulsed electrical circuits for discharge currents below 10 kA, distinct discharge phenomena appear which have to be suppressed for any application. One of these is transient impedance transition, correlated with steps in forward voltage drop. By geometry and choice of electrode material the irregular transitions in impedance can be controlled over a wide parameter range. Another annoying effect is quenching obvious by sudden and irregular interruption of the discharge current. Quenching is observed as a random effect, which is influenced by a manifold of parameters. Results from the experiment indicate that quenching is strongly dependent on the number density of gas atoms in the discharge volume. Since silicon carbide (SiC) as part of the switch electrode downsizes the quenching current to negligible values (<1 kA) optical spectroscopy was used to investigate the influence of this semiconducting material on the temporal development of the discharge, by looking for emission lines of the released silicon and/or carbon atoms. The technological aspects of pseudospark devices are naturally to achieve higher lifetime and improved overall reliability. Multichannel configurations and two-gap systems are under development to reduce erosion rate and to increase hold-off capability, respectively. Under clean conditions a hold-off voltage of 65 kV was realized by a two-gap system.

Interaction of a Vacuum Arc With an SF6 Arc in a Hybrid Circuit Breaker During High-Current Interruption

A hybrid circuit breaker consists of a series-connected vacuum- and SF6 interrupter. The vacuum interrupter has the function to withstand very steep-rising transient recovery voltages, whereas the SF6 interrupter is stressed with the peak of it. Full scale tests are described of a 145-kV prototype, which is subjected to short-line faults based on 63 kA. During these tests, current through the breaker and voltages across both interrupters were monitored with a high-resolution measurement system. It is shown that the principal idea works correctly. From measured results, it is demonstrated that the interaction between the two arcs of completely different physical nature has a positive effect on the interruption: Immediately before current zero, the SF6 arc assists the vacuum arc to interrupt, whereas after current zero, the vacuum arc assists the SF6 arc in the recovery against the recovery voltage. From the analysis of the measurements, it becomes clear that high values of post arc current (up to 30 A) have (at least initially) a positive effect on the breaker performance. Detailed modeling of the interaction of arc-arc and arc-circuit is described, showing a good agreement with the measured results. The model is used to predict the voltage distribution across both interrupters when other (capacitive) voltage grading is applied in the design than present during the tests.

The borehole phase of the pseudospark discharge-a transition between hollow cathode and high current phase

The borehole phase is one of the five phases in the development of a pseudospark discharge. In chronological order, the borehole phase follows the low current predischarge and the hollow cathode phase with currents up to some 100 A. This discharge phase makes the transition between the hollow cathode phase and the high current phase which is connected to the appearance of cathode spots. The transition is fast and is characterised by a sudden decrease of the switch impedance. One problem in understanding the borehole phase is the cause of the high current density of more than 10/sup 4/ A/cm/sup 2/ and the mechanism responsible for the emission of such a high density of electrons. Self sustained self sputtering of cathode material, thermionic field emission and the emission caused by impact of discharge gas ions are discussed as possible processes. Different optical and spectroscopic measurements show that secondary emission by gas ion bombardment is the main reason for the high current density. During the borehole phase, only neutral atoms and single ionised ions from the cathode material can be detected, which seem not to have enough energy to extract electrons from the surface. However, bulk ions (i.e. hydrogen) have enough energy to generate secondary emission of electrons.

Triggering of radial multichannel pseudospark switches by a pulsed hollow cathode discharge

We report on a special trigger discharge for pulsed high-power pseudospark switches. The switch used is a radial three-channel pseudospark switch. For triggering, a cylindrical trigger electrode is inserted into the hollow cathode of the main gap. This electrode acts as a hollow cathode for the dc preionization, while the hollow cathode of the main gap is the anode. A negative high-voltage pulse supplied to the trigger electrode ignites the main discharge. We report the temporal evolution of the trigger discharge observed with a fast camera. This trigger method gives an excellent current distribution among the discharge channels, as can be proven by fast photography. The switch has a delay of 220 ns and a jitter of 15 ns.

Sealed-off pseudospark switches for pulsed power

There has been a considerable progress in understanding the physics and technology of high power pseudospark switches, which allows their implementation in pulsed power applications. These switches have very wide dynamic range of breakdown voltage operation: from 1% to 99%. Single-gap systems still have the disadvantage of small hold-off voltage, which is lower than 25 kV and which requires the construction of two-stage systems. In this way, a more reliable hold-off voltage as high as 30 kV would be achieved. Based upon a better understanding of fundamental discharge processes, it is possible to suppress the quenching phenomenon. This might prove helpful in some pulsed power applications. Semiconductor electrode materials are the best choice for devices with considerably reduced electrode erosion and prevention from statistically distributed drop of impedance. The feasibility of a durable composite of semiconductor and metallic bath electrode will facilitate tremendously the construction of sealed-off pseudospark switches for pulsed power applications, especially to replace ignitron and spark gaps. Up to now, the tested prototypes operate with hold-off voltages up to 20 kV, peak currents up to 150 kA and pulse length up to 20 /spl mu/s. Tests with longer pulse length are planned in order to extend the commercial applicability of these devices.

Pseudospark switches for high-power applications

Longterm tests with several electrode materials and discharge channel arrangements were performed. Two possible geometries were tested, a radial and a coaxial one. Optical fast shutter technique, and spectroscopic and interferometric methods were used to get more information about the discharge-character, the pinching of the discharge channels due to magnetic forces, and the erosion mechanism. The homogeneous ignition and the equal distribution of the current into the individual discharge channels was proved by fast photography and current measurements. At currents exceeding some kA the pseudospark transforms from a specific hollow cathode discharge into a metal-vapor-arc like behavior. Measurements of the forward voltage drop provide values of less than 100 V which are typical for metal-vapor-arcs. Also cathode spots and their traces on the electrode surface were observable. The performance of these switches in pulsed power devices will be reported.

Optical diagnostics of high current pseudospark discharges

The pseudospark is a high voltage, low pressure transient discharge of high discharge current and current rise. Several optical diagnostics were used to study the discharge development and to study the different plasma parameters. Laser induced fluorescence, laser absorption spectroscopy, laser tomography measurements on pseudospark switches and fast shutter and streak measurements on the electron beam pseudospark devices are summarized.