Trigger Electrode Research Articles

Sub-kilohertz flash X-ray generator utilising a glass-enclosed cold-cathode triode.

The construction and fundamental studies are described for a sub-kilohertz X-ray generator for producing low-dose rate flash X-rays. The X-ray tube was a glass-enclosed cold-cathode triode, composed of a tungsten plate target, a rod-shaped graphite cathode, a mesh-type trigger electrode made of tungsten wires, and a glass tube body. The coaxial condenser was charged up to 60 kV by a power supply, and the electric charges in the condenser were discharged to the X-ray tube repetitively when a negative high-voltage pulse was applied to the trigger electrode. The maximum tube voltage before the discharging was equivalent to the initial charged voltage of the condenser, and the maximum tube current was about 0.3 kA with a charged voltage of 60 kV. The X-ray durations were about 1 microsecond, and the X-ray intensity was about 0.47 microC kg-1 at 0.5 m per pulse with a charged voltage of 60 kV. The maximum repetition rate of the X-rays was about 0.4 kHz, and high-speed radiography was performed.

Development of MEVVA ion source

Research results of the metal vapor vacuum arc (MEVVA) ion source are reported in this paper. As cathode materials, for example, Al, Ti, Fe, and Cu are used, respectively. A triggering pulse with a pulse height of 12 kV, pulse length of 90 μs, and repetition rate of 1–5 pulses/s is applied between the trigger electrode and the cathode. The pulse duration of the extracted ion beam is 800 μs. When the extraction voltage is 28 kV, the extracted beam current intensity for Al, Ti, Fe, and Cu is 255, 290, 240, and 300 mA, respectively.

A trigger power supply for vacuum arc ion sources

Arc initiation plays a key role in the operation of pulsed metal vapor vacuum arc (MEVVA) ion sources. The favored triggering method in most sources is a surface discharge across an insulator inserted between an auxiliary trigger electrode and the main gap cathode. The trigger power supply must be capable of meeting specific requirements. As part of an ongoing program to develop such a power supply the authors have designed, constructed, and tested several simple units. The most successful of these is a design employing a resonant 1/2-cycle capacitor discharge which generates peak voltages to 10 kV. The maximum current and duration of the output pulse in the present configuration are 40 A and 50 mu s, respectively. The trigger unit has been tested on a MEVVA II ion source equipped with a tungsten cathode. For the trigger pulse parameters given, a misfire rate of >

Switching characteristics of a gas trigatron operated in single and double trigger mode

The delay time, risetime, jitter, and operating range have been studied in detail for single- as well as double-trigger mode. The best switching performance is obtained in a double-trigger mode for a trigger polarity configuration that produces high field intensification between the opposed trigger electrodes. For a trigatron with a gap spacing of 9.7 mm, using air at atmospheric pressure, an operating range of 12 to 28 kV with low delay time ( >

Single- and double-mode gas trigatron: main-gap and trigger-gap interactions

In the past, it has been presumed that the main-gap breakdown in a trigatron is caused by a precursor spark channel occurring either at the trigger gap or between a trigger electrode and oppositely located main electrode. Based on the detailed analysis of voltage and current waveforms at the trigger gaps of single- and double-mode gas trigatrons, the authors have proposed an interaction between the main gap and the trigger gap under favourable conditions. This interaction gives rise to two or more auxiliary spark channels which develop between different over-volted electrode pairs in a predictable manner. The enhanced UV illumination and electric field distortion as a result of these multiple auxiliary arcs lead to the development of a streamer breakdown between the main electrodes.

Analysis of absorptive bistable characteristics of multisegment lasers

The paper presents a new model of the static I-L characteristics of inhomogeneously pumped laser devices, such as SLEDs and multisegment lasers. These devices are known to exhibit hysteresis, and the paper considers both electrically triggered switching of multisegment lasers and optically triggered switching of SLED-like devices. Features of the model are; the inclusion of the wavelength dependence of the stimulated gain and the spontaneous emission characteristics, and the variation of the carrier density and photon flux in the longitudinal direction of the laser. The paper shows how the topology of the device, namely the relative lengths of the pumped and passive segments of the device, affect its threshold current and hysteresis. The effect on the threshold current and hysteresis of the injection of small currents into the triggering electrode of a split-stripe laser is also shown. It is seen that there are limits on both the relative length of the passive segment and also on the value of current extracted from the trigger electrodes if maximum hysteresis is to be achieved. The effect of the wavelength of externally injected light on its ability to initiate switching is also considered.

Construction techniques of a molecular hydrogen laser

Simple techniques are described for construction of a discharge channel, and spark gap trigger electrode for a traveling-wave excited molecular hydrogen laser.

A new design concept for field distortion trigger spark gaps

A common field distortion triggered spark gap utilizing geometric field enhancement at sharp edges usually operates in a cascade mode via the trigger electrode. A new trigger concept is proposed allowing strong field enhancement and direct breakdown between the two main electrodes. A test setup was designed to prove the feasibility of this concept. Experimental results on delay and jitter depending on percent breakdown voltage are presented. Best results achieved are a delay of 9 ns and a jitter of 2 ns at a self-breakdown voltage of 15 kV.

Effect of Trigger Electrode on the Channel Behavior of Surface Discharge Switch

The effects of trigger electrodes on the behavior of multichannel gas breakdown in a gas-filled surface discharge switch was investigated experimentally. A straight wire or a metal plate was placed under dielectric surfaces at various distances from the main electrodes. These served as the trigger electrode. Various geometric configurations and polarities of the trigger electrode were studied. The experiments were mainly carried out in a N2 environment of 0.1 to 0.2 MPa pressure, and with voltage pulses of 40 to 90 kV peak and 150 to 500 ns risetimes. The location and number of the plasma channels formed during breakdown was observed to be strongly dependent of the geometric configuration and polarities of the trigger electrode. For example, when a wire trigger electrode was placed near the cathode, almost all the channels were found to be concentrated in a region directly above the wire, whereas if the wire electrode was placed near the anode, no such concentration was noted. The observations were qualitatively analyzed. It was concluded that the results were mainly due to the presence of the third electrode and space charges in a non-uniform filed. Based on this analysis, an effective plate trigger electrode for surface discharge switches was successfully developed.

Low-jitter, low-inductance, electrically triggered spark gap

This investigation is concerned with the development of a low-inductance, low-jitter, electrically triggered, high-voltage annular rail spark gap. Three trigger electrode geometries—blade, dull, and sawtooth edge—are tested for the effects of different field stresses on the spark gap switching performance. All possible polarity combinations are applied to the high-voltage and trigger electrodes of the rail gap, and the switching inductance and jitter is measured for each case. Finally, the effects of photopreionization from bare sparks on the spark gap switching performance is explored. A switch inductance of 4 nH and jitter of 0.6 ns have been achieved. A practical application of the rail gap is also described.

Reproducible triggering of a vacuum spark

The reproducibility of a vacuum spark discharge has been improved by plasma injection triggering, in which the discharge is initiated by the injection of plasmas formed from a triggering electrode. With this triggering method, the first Z pinch of the discharge occurs reproducibly with a jitter of only 50 ns.

A study of the effects localised discharge in dielectric liquids

The sequence of events following the triggering of a trigatron gap in dielectric liquids has been studied. An initial shock wave travelling at sonic velocity is followed by an event which may be either an expanding bubble or a heat wave obeying the heat diffusion equation. This region of reduced density subsequently collapses in a manner predicted by Rayleigh's theory of cavitation. Finally a jet of bubbles is ejected from the trigger electrode. These observations are all of importance in relation to the electrical breakdown of dielectric liquids.

Breakdown and Recovery of a 250-kV Low-Pressure Spark Gap

Tests of an experimental low-pressure spark gap Blumlein switch are presented. Gas flows from a pulsed valve into the gap at a steady rate through holes in the cathode and results in a steady pressure of about 0.1 torr. The pressure outside the gap over the Blumlein insulator is much lower. Trigger electrodes are mounted in the grounded cathode. Initially the current rises exponentially with time with rate constant proportional to gas density. Measurements of rise time, jitter, and voltage holding recovery time are presented for charge voltages up to 250 kV for variations of charge time, gas density, gas type, and triggering method.

High-voltage pulser development

This paper highlights the development of a pulse generator that utilizes a single, midplane, triggered spark gap (TSG) to discharge a p ulse-forming network (PFN) into a CS 2 laser cavity load. The PFN is a four-section line pulser. When charged to 116 kV, it can discharge 300 J in 700 ns, full width half maximum (FWHM). The system operates at 0 to 50 pps. In designing the PFN inductors, the pulse rise time was kept long enough to avoid excessive ringing in the output cables. Such ringing can cause arcing in the cavity or damage to the cables themselves. The PFN capacitors were potted into a coaxial configuration to minimize inductance. As the pulse repetition rate increases or the charge duration decreases, special consideration must be given to design of the midplane TSG voltage grading network to ensure the trigger electrode always stays at some set fraction of the PFN voltage during charge. This paper describes a circuit that makes operation of the grading network independent of frequency and charge duration, within certain limits. Tests were run in synthetic air and in nitrogen to evaluate operating voltage and the amount of jitter and misfire as a function of pressure.

Schottky-contact coupling between Schottky-electrode-triggered Gunn elements

Signal transmissions between Schottky-electrode-triggered Gunn elements are investigated when an output electrode, also of Schottky type, is directly coupled to a trigger electrode of the next stage. Expressions are derived that describe the voltages across the two Schottky barriers when the barriers behave as pure capacitances. Also cases are studied where a conductive current flows through either of the barriers. In those cases, it is pointed out that excess electrons accumulate on the metal sides of the barriers after a cycle of potential changes at the semiconductor sides Experimental results agree well with those predicted by the present theory, and it is shown that the electron-accumulation phenomenon really takes place. Also a Gunn-effect memory device based on the above electron-accumulation phenomenon is proposed and demonstrated.

Plasma Buildup and Breakdown Delay in a Triggered Vacuum Gap

The phenomena leading to the surface flashover across solid insulators in vacuum and the subsequent spread of the trigger plasma thus formed to bridge the main gap in a triggered vacuum switch are investigated experimentally. The results show that the breakdown proceeds in two stages. In the first stage a plasma is formed by electrons releasing and ionizing absorbed gases. The electrons are field emitted at the insulator-electrode junction and the breakdown delay is primarily affected by the trigger voltage. In the second stage the trigger plasma expands into the main gap with a speed depending on the trigger current and trigger electrode geometry, in accordance with a simple model.

Experimental study of the control characteristics of a Schottky-trigger electrode on a Gunn device

To clarify the condition for the generation of a high-field domain in a Gunn device equipped with a Schottky-trigger electrode, a detailed experimental study was carried out. The difficulties in such experiments had so far rested in the accurate evaluation of the potential difference between the trigger electrode and the semiconductor thereunder, which we solved by an equivalent circuit representation of the device. Moreover, the accuracy of the evaluation of the potential difference was further increased by making use of the critical condition for the forward current through the Schottky barrier of the trigger electrode. From the experimental results it was made clear that a high-field domain is not necessarily generated when the field in the channel under the trigger electrode reaches the threshold field, but some form of the over-threshold-field state is established before a domain is generated. This shows that the device could be biased to higher anode voltages, leading to higher output voltages and wider operational margines than had earlier been expected.

2.13 Behaviour of ceramic insulator materials during initiation of trigger discharge in TVG

An effort has been made by the author to design a simple triggered vacuum gap (TVG) in which a plasma discharge over the surface of a ceramic insulator initiates an arc in high vacuum ( p ≤ 10 −5 torr). This design is reported elsewhere. A comprehensive study on the performance of different insulator materials is not reported so far. Thermally stable insulator materials like barium titanate (LCC HTD), silicon carbide, lead titanate, rutile, supramica, alumina, sillimanite and fused quartz in the order of decreasing permittivities, were investigated. The plasma discharge is triggered with a positive voltage pulse to the trigger electrode under low voltage conditions ≤220 V, 100 A for the main electrodes. Samples like barium titanate, rutile, sillimanite showed very consistent performance even up to 1000 trials indicating that a low voltage switching device operating in high vacuum is feasible. During these experimental investigations, both external triggering and self triggering modes have been tried, the latter successfully solving the arc-initiation problem in a much simpler way. This paper presents the results obtained with the above samples at 1–5 × 10 −6 torr.

Triggering mechanisms in triggered vacuum gaps

Experiments were conducted on a vacuum gap with a trigger electrode mounted in one of the main electrodes. Waveforms under various circuit conditions, photographic and visual observations, probe measurements, and various trigger materials were studied to investigate the triggering mechanisms. Different mechanisms are proposed according to the polarity of the remote main electrode. For positive polarity, the trigger arc establishes its own cathode spots, which then supply current to the main discharge, dividing to form new spots as needed. For negative polarity, the plasma produced by the trigger produces a concentration of the main electrode potential difference across a thin sheath adjacent to the surface of the remote main electrode.

Vacuum arc anode plasma. I. Spectroscopic investigation

A spectroscopic investigation was made of the anode plasma of a pulsed vacuum arc with an aluminum anode and a molybdenum cathode. The arc was triggered by a third trigger electrode and was driven by a 150-A 10-μs current pulse. The average current density at the anode was sufficiently high that anode spots were formed; these spots are believed to be the source of the aluminum in the plasma investigated in this experiment. By simultaneously measuring spectral emission lines of Al I, Al II, and Al III, the plasma electron temperature was shown to decrease sequentially through the norm temperatures of Al III, Al II, and Al I as the arc was extinguished. The Boltzmann distribution temperature TD of four Al III excited levels was shown to be kTD/e=2.0±0.5 V, and the peak Al III 4D excited state density was shown to be about 5×1017 m−3. These data suggest a non-local-thermodynamic-equilibrium (non-LTE) model of the anode plasma when compared with the Al3+ production in the plasma. The plasma was theoretically shown to be optically thin to the observed Al III spectral lines.