W. Hartmann

Pseudospark switches for high repetition rates and high current applications

In this paper the state of the art of pseudospark switch development is reported. In addition to the replacement of thyratrons for high power applications in TE gas discharge lasers, pseudospark switches have been tested at high repetition rates up to 2 kHz. In order to minimize the erosion rate and to reduce total switch in ductance, multichannel pseudospark switches with various different geometrical configurations have been investigated: a linear, a coaxial, and a radial arrangement of the parallel discharge channels. All three configurations possess distinct advantages in pulsed power technology, i.e. linear systems fit well into striplines. Beyond this pseudospark switches gain increasing importance in high current devices like fast pinches, plasma focii, or powerful modulators, i.e. for magnetoforming. A single-channel switch for hold-off voltage up to 30 kV at peak currents of -- 100 kA has been tested for a damped sinusoidal pulse of 5 /spl mu/sec duration at a repetition rate up to 0.1 Hz, without and with electrode cooling. The expected lifetime for tungsten or related alloys as electrode material is greater 10/sup 5/ shots under these conditions corresponding to a charge transfer of 2 x 10/sup 5/ Cb.

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. >

A 40 Kv/20 Ka Pseudo-Spark Switch For Laser Applications

This article reports measurements of the characteristic parameters of a pseudo-spark switch designed for operation voltages up to 40 kV and peak currents of 20 kA for a pulse length of 90 ns. In all experiments the switch was operated with permanent external gas flow. There was no gas reservoir. Part of the investigations were performed on a water-filled matched pulse line. The repetition rate was limited to 40 Hz at full power. The switch parameters are comparable or superior to thyratrons, with respect to rate of current rise or current reversal. The tests with the .(Ω.-pulse line delivered dI/dt values between 8 and 9 x 1011 A/sec. The prototype switches were triggered by a pulsed low current D.C. discharge. Two switch configurations were used, one partly the other totally derived from metal-ceramic technology. The electrode degradation seems to be small and lifetimes of more than 108 discharges can be expected. Aside from this, some more fundamental investigations of the switch plasma are presented. The overall inductance is about 10 nH, the resistance during the con-ductive phase is less than 10 mdΩ.

High Repetition Rate Pseudo-Spark Switches For Laser Applications

A new switch for laser applications is described. Experiments are reported wherein it is shown that according to voltage operation range, switching precision, trigger efficiency, voltage reversal, and recovery time the pseudo-spark switch is superior to high pressure spark gap switches and comparable to thyratrons. Several pseudo-spark trigger methods are presented. The discharge can be initiated by a dielectric surface breakdown trigger. A dif ferent trigger method is based on a pulseslilL low-current gas discharge, which has practically unlimited lifetime and allows repitition rates up to 100 kHz. Several modifications of switches triggered in this manner have been tested in different types of gas lasers (copper va pour, N2-laser), which are normally driven by thyratrons or high-pressure spark gap switches, respectively.

Cathode-Related Processes in High-Current Density, Low Pressure Glow Discharges

In transient high current, low pressure gas discharges like the pseudospark /1/ and other hollow cathode switching devices /2/, a diffuse glow discharge mode has been reported with current densities of the order of 104 A/cm2 at cathode surface - plasma interaction cross-sectional areas of the order of 1 cm2 /3–7/. This extraordinary high current density from an initially cold metal cathode surface cannot be explained in terms of secondary electron emission yield due to particle impact or the photo effect. To find an explanation for this high cathode emission, a much more detailed investigation of the plasma - cathode interaction is necessary, taking into account non-ideal cathode surface properties which are usually neglected in gas discharge modelling /8/. In this contribution, an attempt is made to explain the experimentally observed diffuse, nonarcing glow mode of very high current density in terms of field-enhanced thermionic electron emission from the initially cold cathode surface. Heating of a thin layer at the cathode surface is thought to be due to ion bombardment by ions which are accelerated in the cathode fall voltage drop. Although a strong ion pumping effect is expected to occur at the plasma boundary facing the cathode, long pulse duration current pulses can be transfered due to the thermal desorption of adsorbed gas layers from the cathode surface; this leads to an effective ion replenishment through electron impact ionization of the resulting dense gas layer.

Fast solid state high voltage pulse generator

Abstract A fast solid state pulse generator is described which is used to trigger high voltage, high current switches. It consists of a 7-stage marx generator bank switched by avalanche transistors and delivers a negative pulse with a rise time of less than 2 ns and an amplitude of 2.4 kV into a load of 200 ω. The delay between the trigger pulse of TTL level and the output pulse is 16 ns. The jitter is well below 100 ps.

Comparison of Electrode Effects in High-Pressure and Low-Pressure Gas Discharges Like Spark-Gap and Pseudo-Spark Switch

The mechanisms of the interaction between plasma and electrode material in a pseudo-spark-discharge seem to be of fundamental interest. Therefore, molybdenum electrodes, stressed by long term operation in a pseudo-spark switch (PSS) as well as in a spark gap switch, were studied with a scanning electron microscope. In addition, electrodes with a 10µm molybdenum coating deposited by a PVD-process, were also applied, both in the PSS and in a spark-gap configuration. In this contribution observations about the influence of the pseudo-spark discharge plasma on the electrode surface are reported. By comparing the results with the effects caused by the high pressure spark discharge, conclusions concerning the discharge mechanism can be drawn.

An investigation of the temporal development of the pseudospark discharge

A study of the different discharge phases of the pseudospark discharge is presented. During the temporal development several modes of the pseudospark discharge are observed leading to a transient, high current, low pressure gas discharge with current densities of the order of 10/sup 4/A/cm/sup 2/ and a forward voltage drop of about 10/sup 2/ volts. Therefore measurements of the discharge current, the total charge transported, the forward voltage drop and the influence of the gas pressure and LRC circuit, from initial phase of discharge until the transient phase to the superdense glow, are reported. Through comparison of experimental results with literature the different types of discharges are characterized. Optical studies of the radially expanding plasma column enable the temporal and spatial location of the discharge plasma. In the high current phase estimations of the temperature on the cathode surface and the solution of the Schottky-Equation show that a constriction of the ion current at microscopic surface irregularities is necessary to sustain the discharge. >

Low Pressure Glow Discharge Switches For High Power Excimer Lasers

Pseudo-spark switches are promising condidates to overcome the limitations of pulsed power networks, imposed by the commercially available thyratrons in use. The pseudo-spark switch uses a high power density, low pressure glow discharge, and is capable of withstanding up to 100 % current reversal. Metal-ceramic prototypes have switched up to 40 kV/30 kA at a rate of current rise of 8 x 1011 A/sec. At 32 kV/20 kA and a pulse duration of 90 nsec (FWHM), repetition rates of over 100 pps and a lifetime of over 5 x 106 discharges have been achieved in a discharge circuit with a short circuit load. Streak camera measurements confirm that the discharge in the switch remains a homogeneous glow. The switches seem to be scalable to higher power levels; in addition, paralleling of separate discharge channels in a common housing has been shown to be possible.

Fast rise time, high sensitivity MCP ion detector for low-energy ion spectroscopy

A two-stage MCP ion detector for steady ion currents of the order of some pA and ion pulses of a pulse width between 10 and 500 ns is described. Detector current gain for Ne+ and H2+ ions at an energy of 0.1 to 4 keV was found to be dependent on ion energy and species. In pulse current mode, linearity is maintained up to an output charge density of 1.9 nC cm-2; the detector rise time is below 3.5 ns. For the first time an MCP ion detector is used at an ambient gas pressure of 10-3 Pa to investigate ions emitted by a transient gas discharge. With an appropriate operating voltage the detector operated without degradation of performance for at least 500 hours. First measurements of ion species, ion-energy distribution function, ion current density and pulse shape of the axial ion pulse of a pseudospark discharge were performed.