Ch. Bickes

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.

Interaction of heavy ions with plasmas

Abstract Intense heavy ion beams have been used in recent years to study the properties of matter under extreme conditions. The intense heavy ion beams of the radio frequency accelerator MAXILAC at GSI were used at ion energies of 45 keV/u to produce the first heavy ion driven plasma and to study the hydrodynamic reactions of this plasma. Similar experiments at 300 MeV/u ion energy using the heavy ion synchrotron SIS have now proven to be successful. The understanding and modelling of ion beam driven plasmas requires a precise knowledge of the stopping power in a plasma. Thus, in parallel to the ion beam driven plasmas, the stopping power of a fully ionized hydrogen plasma has been investigated experimentally testing an ion energy range of 45 keV/u to 10 MeV/u at plasma densities from 1016 to 1019 cm−3. The stoppping power experiments revealed the principle of new stopping behavior of ions in a plasma. In comparison to cold, non-ionized matter an increased stopping in hydrogen plasma up to a factor 35 has been observed.

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.

Design criteria for high performance, high power pseudospark switches

The main development objective is to replace commercial switching devices with pseudospark switches. The principle is similar to that of a cold-cathode thyratron. The basic mechanisms facilitate a simple construction. On the other hand one gets undesired phenomena by the physics of cold cathode emission at low peak currents (less than 2 kA), like quenching, chopping and impedance fluctuations. At very high peak currents (more than 50 kA) transition to a metal vapor arc occurs with correlated high cathode erosion by cathode spot formation. With molybdenum as electrode material beyond 45 kA additionally anode spot formation contributes considerably to electrode erosion. Below 45 kA erosion rate amounts typically to 100 /spl mu/g/C and jumps by one magnitude up to 100 /spl mu/g/C and more with anode spot formation. A quite early used method to avoid this was to change from one-channel to multichannel configurations; another possibility discovered, is to use silicon-carbide (SiC) as durable electrode material. Experiments over a wide range of discharge currents demonstrate that arc formation is suppressed. As a consequence the power input per square unit is reduced. Strong emphasis was put on the solution of technological problems by optimization of design parameters. In parallel, two-gap switch configurations are under development to achieve reliable hold-off voltage capability up to 40 kV. This paper summarizes the most important results of fundamental research as well as of technological progress.

Experimental study to accumulate, accelerate and focus a massive plasma beam onto a target

Abstract Experimental investigations are presented, where a device similar to a plasma thruster using j × B forces has been used, to accumulate and accelerate a plasma with a mass of about 0.2 mg to beam velocities up to 110 km/s. A pulse line with a capacity of 108 μF was used to provide discharge currents up to 450 kA for a pulse length of 6.5 μs. The discharge vessel consists of a set of coaxial electrodes with a diameter of 5 cm at a length of 50 cm. The total energy efficiency of the plasma acceleration from this process is estimated to be better than 10%. An estimate of the mass of the plasma beam was achieved by measuring the momentum transfer from plasma beam to flyer plates. Using conus-shaped electrodes at the exit of the plasma accelerator a focusing of the plasma to a diameter below 1 cm was achieved.

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.

Ceramic-metal sealed-off pseudospark switch with a trigger unit based on flashover

Considerable interest has recently been generated in a new type of high-voltage switching device that depends on a low-pressure gas discharge with cold cathode, often called the pseudospark switch. This switch is considered as an alternative to thyratrons in facilities that require high di/dt and small jitter. The electrode system of the switch consists of an anode and a hollow cathode whose cavity communicates with the main gap through a central bore hole or several holes. The trigger unit is typically placed inside the cathode cavity. The unit is intended to provide an electron flux into the main gap and so to trigger the switch. In order that the switch be triggered in nanosecond time scale, its design should ensure easy communication between the cavity and the main gap. However, such a design causes a decrease in the static breakdown voltage of the switch. A construction with a flashover trigger unit may eliminate the above mentioned contradiction that is demonstrated in the present paper.

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.