Jürgen Urban

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.

Switching characteristics of microplasmas in a planar electrode gap

Microplasmas at high pressure have been the authors’ special interest for its practical relevance to the development of a switch. They concentrated on repetitive switching with a possibility to exceed the up to now known values for plasma closing switches and simultaneously maintaining a subnanosecond rise time of the switched pulses at a load. They examined several parameters for this purpose such as the electrode gap spacing, the electrode geometry, the gas type, the gas pressure, and including the applied voltage and current rating to operate these plasmas.

Low-voltage triggering for a pseudospark switch with an auxiliary glow discharge

Different electric circuits for triggering the switch with a trigger unit based on an auxiliary glow discharge are discussed. Most attention is concentrated on the recent experimental results on low voltage triggering of the switch and on the mechanisms for the main discharge initiation under the action of the trigger pulse. Due to modifications in the trigger electric circuit, the switch is triggered starting from a voltage of 50 V at a current level in the trigger circuit of 10 mA. With a voltage of 200-250 V and a trigger current of about 0.25 A, the delay time to triggering does not exceed 200 ns.

Fundamental investigation in two flashover-based trigger methods for low-pressure gas discharge switches

Modern switches for pulse-power technology have special requirements such as long lifetime, reliability in a wide pressure and voltage range, as well as small delay time. In order to meet these requirements, two trigger methods were developed and examined. These two different trigger methods based on a flashover were tested for the emission behavior by variation of different parameters. The first configuration is a semiconductor surface flashover trigger, where electron emission is based on a surface flashover between the contact area of a copper spring and a carbide cylinder. The second trigger concept is the high-dielectric trigger, where electrons are released by the field emission effect at the transition between metal-vacuum and dielectric. For this system, high dielectric materials with dielectric constants in the order of 2000 are available. The electrical and optical measurements of both trigger systems were done in a modular structured vacuum chamber. For lower pressure, the high-dielectric trigger shows better performances and higher emitted charge of the electron emission within all adjusted parameters like gas pressure, applied voltage, and different wirings. In addition to the higher emitted charge, the emitted electrons from the high-dielectric material have higher energies. For the lifetime characteristic, the high-dielectric trigger shows lifetimes much higher than 100 million discharges.

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.

Role of trigger to avoid current quenching in pseudospark switch

In this paper, experiments have been reported on finding a possible solution to the current quenching problem in the pseudospark switch (PSS). The charge injection trigger system has been used to investigate this aspect. Trigger current pulses with varying duration and varying charge content have been used for injection of charge from a single dielectric trigger unit into the hollow cathode geometry of the PSS. Quenching has been suppressed in the PSS by optimizing the triggered charge content. This approach has the advantage of using existing switch technologies as well as assemblies for solving the current quenching problem. Possibilities for further work on triggered charge in hollow cathode geometry have been outlined.

Minimization of impedance fluctuations in cold-cathode pseudospark switches (PSS)

The pseudospark discharge is characterized as a low-pressure gas discharge located on the left branch of the Paschen curve. Based on this discharge, a family of fast gas discharge closing switches for pulsed-power applications have been developed at Erlangen for more than 10 years. Due to the similarity to thyratrons, however, without having a hot cathode, the pseudospark devices are often described in literature as cold-cathode thyratron. The main features of a pseudospark switch are a high current rise closed to 10/sup 12/ A/s, the ability to carry reverse currents up to 100% and a high lifetime. The cold cathode also reduces the required standby power one magnitude lower than in thyratrons. On the other hand, especially at low-peak currents, undesired phenomena appear by the physics of the cold cathode. Current quenching, chopping, and impedance fluctuations are problems which occur at currents in the range of a few kiloamperes. Especially, the impedance fluctuations cause pulse-to-pulse fluctuations of the energy transferred to the load which could influence, i.e., the output energy of a laser system. For reducing this described problem of impedance fluctuations the dependencies of the geometry and material are investigated. With use of tungsten compound electrodes, this behavior of a cold cathode could be reduced significantly. This material shows also an improved reignition behavior after current zero.

Mechanism of the pseudospark initiation for the switches with a trigger unit based on flashover

One of the methods for triggering a pseudospark switch implies a use of a surface discharge (flashover) in a trigger unit, which is normally placed inside the main cathode cavity. In various designs of the switches the surface discharge is ignited over a dielectric insert with low E, a dielectric insert with high /spl epsiv/, a semiconductor insert and so on. The present paper demonstrates that the best results on triggering are achieved when, during the flashover development, a potential difference appears between the flashover plasma and the main cathode cavity. Mechanism for the main discharge initiation is general for most of the trigger systems. It is associated with the current interception from the trigger plasma to the main cathode cavity and the succeeding development of discharge in the main gap. The features of development of the main discharge at the initial stages of formation and burning are also discussed.

Time-resolved spectroscopic characterization of the pseudospark-discharge plasma

The high-current discharge in a pseudospark geometry can be subdivided in several phases. While the first and the last ones are well understood, the plasma parameters as well as the current carrying mechanisms of the phases in between are still subject of investigations. Different temporally resolved spectroscopic measurements of the localized copper vapor plasma were performed. The electron densities, measured by Stark broadening of the H-/spl beta/ line, are between 14/sup 14/ and 10/sup 16/ cm/sup -3/. The electron temperature was determined by the intensity ratio of atomic to ionic copper lines. It was taken into account that the plasma was not in local thermal equilibrium. Estimations of the main equilibrium and nonequilibrium processes allowed to correct the measured temperature values. During the bore-hole phase, the corrected temperatures are between 1.5 and 2.5 eV. During later phases, the discharge plasma is self absorbing and less reproducible for the plasma parameters to be determined. The plasma parameters indicate that the current between the localized copper plasma sheet and the cathode is mainly carried by positive ions. Electron emission is mainly caused by ion impact and is only of inferior significance for the current balance.

Mechanism of the current quenching phenomenon in pseudospark discharge

Experimental data on the current quenching phenomenon in the pseudospark discharge in typical conditions of EUV source operation (extremely short pulse duration and high current) are presented. The mechanism of the phenomenon is proposed.