Klaus Frank

Pseudospark produced pulsed electron beam for material processing

An intense pulsed electron beam produced by a pseudospark discharge is used for material processing. The electron beam propagates in a self-focused manner in the background gas. Hardly 12 ns after the beginning of the discharge the fraction of space charge neutralization is about 96%. To sustain the neutralization effect high energy electrons (E >

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. >

Investigation of the different discharge mechanisms in pseudospark discharges

The intention of this paper is to give an overview of recent experiments explaining the development and transition of the discharge phases in a pseudospark. The reported experiments include single gap pseudospark discharges in ultra-high-vacuum systems with hydrogen as working gas, as well as multigap pseudospark discharges in argon. Temporally and spatially resolved framing photography, spectrometry, raster electron microscopy and time resolved electrical measurements are presented. The experiments comprise a current range of some hundreds of amps to 60 kA. The results are used to specify the four characteristic phases of the pseudospark: Townsend-, hollow cathode-, high current- and metal vapor arc phase. >

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. >

Pulsed intense electron beams generated in transient hollow cathode discharges: fundamentals and applications

For the commercial application of pulsed power, material processing with intense pulsed particle beams is a very interesting subject. Recently, high-voltage (1-70 kV), low-pressure (1-100 Pa) transient hollow-cathode discharges turned out to be sources for pulsed intense electron beam generation suitable for this application. The remarkable parameters of these electron beams-beam currents of 50-1000 A (10-30% of the maximum discharge current) with a high energy component (mean energy of about 0.25-0.75 of maximum applied voltage) of 20-70% of the maximum beam current, power density up to 10 W/cm/sup 2/, beam diameters of 0.1-3 mm, beam charge efficiency of 3-5%-captured the attention not only of the scientific community in the last decade. The electron beam is emitted during the early phases of the discharge, and only weak dependence of the high energetic peak of the beam current was found on the external capacity, which determine the development of the later high-current phases. However, the beam parameters depend on the breakdown voltage, gas pressure, and discharge geometry (including self-capacity). In this paper, the characteristics of the pulsed intense electron beams generated in two configurations-multigap pseudosparks and preionization-controlled open-ended hollow-cathode transient discharges (PCOHC)-are described. Such electron beams already were used successfully in a variety of pulsed power applications in material processing, deposition of superconducting (YBaCuO) and diamond-like thin films, microlithography, electron sources for accelerators, and intense point-like X-ray sources, and some preliminary experiments revealed new potential applications such as pumping of short-wavelength laser active media. These pulsed electron beams could be used further in any kind of pulsed power applications that require high-power density, small or high electron energy, and small-beam diameters.

High-repetition rate sealed-off pseudospark switches for pulsed modulators

Experimental results of high-repetition-rate pseudospark switch testing are reported. Typical test parameters are hold-off voltage of about 20 kV, peak currents on the order of 10 kA, and pulse durations between 50 and 100 ns. The discharge circuit, with discrete ceramic capacitors up to 10.8 nF total, produced a ringing discharge with peak currents on the order of 10 kA. Current reversal under these conditions was up to 80% of the peak current. A sealed-off, ceramic-metal pseudospark switch with integral deuterium reservoir tested in this setup achieved repetition rates of up to 1.8 kHz without latching or triggering failures. Some 10/sup 7/ shots have been performed without degradation of the switch performance. >

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.

Development of a high current pseudospark switch and measurement of electron density

The pseudospark, a low-pressure gas discharge in a special geometry, is suitable for high-current switching. A single-channel prototype is tested with a 3.3- mu F capacitor bank, voltages up to 30 kV, and peak currents up to 120 kA. The electrical circuit, not comprising any load resistor causes weakly damped sinusoidal pulses of 5- mu s duration at 90% current reversal. For lifetime tests, a switch with an alumina insulator and copper seals is used. Hydrogen is the working gas. Several electrode materials like molybdenum, tungsten, graphite and chromium-copper are tested. Optical investigations of the discharge and of plasma parameters are done with an O-ring sealed pseudospark switch. The light of the discharge is observed spectrally integrated with a streak camera. Spectral resolution is obtained by using a high-speed shutter in combination with a monochromator. The radial electron density is determined by measuring the Stark broadening of the Balmer H/sub beta /-line. At 60 kA a maximum electron density of about 2*10/sup 17/cm/sup -3/ is calculated. >

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.

System for time resolved spectral studies of pulsed atmospheric discharges in the visible to vacuum ultraviolet range

Vacuum ultraviolet (VUV) emission is believed to play a major role in the development of plasma streamers in pulsed atmospheric discharges, but detection of VUV light is difficult in pulsed experiments at atmospheric pressures. Since VUV light is absorbed in most standard optical materials as well, careful attention must be given to the selection of the lens and mirror optics used in these studies. Of highest interest is the VUV emission during the initial stage of pulsed atmospheric discharges, which has a typical duration in the nanosecond regime. An experiment was designed to study this fast initial stage of VUV emission coupled with fast optical imaging of streamer propagation, both with temporal resolution on the order of nanoseconds. A repetitive solid-state high voltage pulser was constructed which produces triggered flashover discharges with low jitter and consistent pulse amplitude. VUV emission is captured utilizing both photomultiplier and intensified charge-coupled device detectors during the fast stage of streamer propagation. These results are discussed in context with the streamer formation photographed in the visible wavelength regime with 3 ns exposure time.