M. Iberler

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.

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.

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.

Progress in enhanced triggering and increasing of hold-off voltage capability with pseudospark plasma switches

This paper reports on two novel trigger techniques for pseudosparks switches. One is a trigger unit, based upon a modified surface flashover; the other one works by electron emission from high-dielectric materials such as PLZT perovskite ceramics or related materials of similar high dielectric constant. With this trigger scheme pseudospark plasma switches have been operated at pulse repetition rates (PRR) up to 20 Hz and more. A PRR of 1 kHz in burst mode is possible. The cumulative number of trigger discharges is above 10/sup 7/ without any significant performance degradation. The surface flashover unit consists of a cylindrical ceramic rod with two contact electrodes. Depending on trigger voltage polarity one electrode turns positive to the grounded hollow cavity. In other words, the cavity starts playing a part of a hollow-anode with respect to this electrode. As a consequence a manifold of mini-arc discharges directed towards the hollow-anode appear inside the cavity. The hollow-anode plasma itself serves now as an electron source for the main gap. The main advantage of both triggering schemes lie in their simultaneous simplicity and reliability allowing high repetition rates, long lifetime, and the complete absence of keep-alive electrodes and stand-by power consumption. In order to improve hold-off voltage capability two-stage pseudospark switches with an intermediate electrode are under test. The apertures of this electrode are not aligned with the boreholes of the main electrodes. By this slight modification the hold-off voltage reliability could be extended beyond 35 kV compared to 22 kV for a one-stage device.

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.

Spectroscopic investigations in the dense discharge plasma of pseudospark switches

The pseudospark discharge can be subdivided in up to five different phases. In chronological order the first two phases, the low current predischarge and the hollow cathode phase, are well understood. In the following borehole phase the responsible mechanism of electron emission from the cathode material is controversially discussed in the literature. Due to the presented measurements the secondary electron emission caused by impact of discharge gas ions seems to be the favourite candidate. The cathode material seems to be important to start the next discharge phase, the high current phase with the appearance of macroscopic cathode spots. Also higher ionised states of cathode material appear. The last phase in the pseudospark phase, the metal vapour arc, can be compared with typical metal vapour arcs in a vacuum. This arc appears only at very high currents with more than 20 kA and is responsible for the extreme electrode erosion. This strong increase of the erosion at high currents mainly limits the lifetime of the switch. The use of new electrode materials like SiC could reduce this erosion and increase the lifetime. In laser spectroscopic measurements like tomographic two wavelength interferometry a diffuse discharge can be observed with the use of SiC as electrode material, in opposition to a constricted discharge as typical for metal electrodes.

Hollow-cathode auxiliary discharge in a trigger unit of pseudospark switch

A special design of pseudospark switch in which a high-emissivity insert is placed in the cathode cavity is described. The insert allows the forward voltage drop of the switch to be decreased Just the same, the insert influences the burning modes of the auxiliary glow discharge in the trigger unit. The characteristic features of the auxiliary discharge are discussed in the paper.

Performance characteristics of pseudospark plasma switches

Precise triggering is one of the most important issues with high power pseudospark switches. The hollow-cathode geometry offers a variety of trigger systems. For sealed-off devices used to carry high coulombic charge, a trigger unit based upon flashover was developed. Delay and jitter are about 100 ns and 10 ns, respectively. Medium power switches for high repetition rates are usually equipped with a trigger unit based upon an auxiliary glow discharge. This trigger methods provides delay numbers of 40 ns. One problem of great concern is still the appearance of current quenching at relatively low discharge currents. Recent experiments point out that quenching is a inevitable process with the original pseudospark geometry. This is observed over a wide range of parameters.

Increase of the breakdown voltage of a pseudospark discharge by applying a blocking potential

Investigations of the static breakdown voltage in dependence of the gas pressure for a pseudospark (PS) discharge with an additional blocking electrode are presented. Experiments by variation of the position and geometry of the blocking electrode in the hollow cathode of PS and the applied blocking potential were performed. With respect to the potential use of a hollow-cathode-based gas discharge as source of extreme-ultraviolet radiation for future lithography, xenon as working gas has been used.