O. B. Frants

Sealed-off pseudospark switches for pulsed power applications (current status and prospects)

The paper describes the sealed-off pseudospark switches that have recently been developed for various pulsed power applications. The switches with the trigger unit based on a flashover are intended for a use in fast electric circuits at a pulse current of the order of 100 kA in microsecond range of the pulse duration. The switches with the trigger unit based on an auxiliary glow discharge have been designed for fast current circuits with a moderate range of current (of the order of 10 kA) and high pulse repetition rate.

Auxiliary glow discharge in the trigger unit of a hollow-cathode thyratron

Results from studies of a low-current glow discharge with a hollow cathode are presented. A specific feature of the discharge conditions was that a highly emissive tablet containing cesium carbonate was placed in the cathode cavity. In the absence of a tablet, the discharge ignition voltage was typically ≥3.5 kV, while the burning voltage was in the range of 500–600 V. The use of the tablet made it possible to decrease the ignition voltage to 280 V and maintain the discharge burning voltage at a level of about 130 V. A model of the current sustainment in a hollow-cathode discharge is proposed. Instead of the conventional secondary emission yield, the model uses a generalized emission yield that takes into account not only ion bombardment of the cathode, but also the emission current from an external source. The model is used to interpret the observed current−voltage characteristics. The results of calculations agree well with the experimental data. It is shown that, in some discharge modes, the external emission current from the cathode can reach 25% of the total discharge current.

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.

Simulation of nonstationary phenomena in atmospheric-pressure glow discharge

Nonstationary processes in atmospheric-pressure glow discharge manifest themselves in spontaneous transitions from the normal glow discharge into a spark. In the experiments, both so-called completed transitions in which a highly conductive constricted channel arises and incomplete transitions accompanied by the formation of a diffuse channel are observed. A model of the positive column of a discharge in air is elaborated that allows one to interpret specific features of the discharge both in the stationary stage and during its transition into a spark and makes it possible to calculate the characteristic oscillatory current waveforms for completed transitions into a spark and aperiodic ones for incomplete transitions. The calculated parameters of the positive column in the glow discharge mode agree well with experiment. Data on the densities of the most abundant species generated in the discharge (such as atomic oxygen, metastable nitrogen molecules, ozone, nitrogen oxides, and negative oxygen ions) are presented.

Pseudospark switches for the power supply circuits of pulsed lasers

A family of sealed-off low-pressure spark gap switches developed at the Institute of High Current Electronics (Tomsk) and at the Plasma Scientific Research Institute (Ryazan) with participation of researchers from the Physical Institute of the University of Erlangen (Germany) is described. The devices are of cermet construction, are capable of switching pulsed currents over wide ranges of amplitudes and durations, and have a number of unique characteristics. The principle of operation of the spark gap switches is described and their designs are given. The triggering circuits of the switches are considered.

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.

Operating stability of a self-breakdown spark-gap frequency switch rated at a voltage of 300 kV and a switched power of up to 450 J

A test bench for studying two-electrode spark gaps rated at a voltage of 300 kV and a pulse repetition rate of up to 10 Hz and operating in air at elevated pressure. The typical time of pulse charging of a capacitive storage in the bench equals about 100 μs. The object of investigation is a spark gap the operating stability of which at a level of 10% of the rate voltage is achieved by initiating a corona discharge at the prebreakdown stage. It is shown that unstable operation is due to the accumulation of nitrogen oxides in the gap. To maintain the oxide content at an acceptable level, continuous gas purging is applied and necessary gas flow rates are estimated.

Specifics of operation of a cold-cathode thyratron with a backward voltage half-wave

The specifics of operating a metal-ceramic TPI1-10k/50 thyratron in electric circuits with capacitance, inductance, and active resistance have been examined under circuit parameters that establish oscillatory current. Experiments have been performed at an anode voltage as high as 30 kV, a forward current of up to 7.6 kA, and a length of the first current half-period that varies from 0.38 to 1.9 μs. The data on operating modes in which this thyratron may handle a backward current wave and when current interruption is observed in the second half-period have been obtained. It has been demonstrated that a certain current flows through the thyratron in the backward direction during the interruption process. The amplitude of this current and the maximum backward voltage at the thyratron define whether the current is interrupted or repeat back-voltage device breakdown occurs. If the maximum backward current is on the level of several hundred amperes, complete current interruption occurs at backward voltages of up to 12 kV. The physical mechanisms of current interruption have been discussed.

Sealed-off pseudospark switches for pulsed power applications

The paper describes the sealed-off pseudospark switches that have been developed for various pulsed power applications. The switches with the trigger unit based on a flashover are intended for a use in fast electric circuits at a pulse current of the order of 100 kA in the microsecond range of pulse duration. The switches with the trigger unit based on an auxiliary glow discharge have been designed for fast current circuits with a moderate range of current (of the order of 10 kA) and high pulse repetition rate.

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.