I. A. Shemyakin

Processes in the prebreakdown stage of a low‐pressure discharge and the mechanism of discharge initiation in pseudospark switches

A time‐dependent model for the rapid current rise during initiation of the discharge in a pseudospark switch is presented. The following assumptions are made: (1) the gas is ionized only by electrons that originate at the cathode; (2) the cathode emissivity is infinite; (3) the average ionization cross section is constant; (4) the ion charge density is independent of gap position. The results are in qualitative agreement with a previous experimental study of a nitrogen‐filled switch operating between 6 and 20 kV for a range of gas pressures. The model provides a straightforward approach for estimating pseudospark breakdown times.

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.

Processes in the cathode region of a low-pressure arc discharge

In this paper, we outline the physical aspects of the operation of an arc discharge in a low-pressure gas, when the particle path length is comparable to or greater than the dimensions of the discharge chamber. We analyze the processes occurring in the discharge gap after initiating plasma formation in the near-cathode region. We demonstrate a mechanism for a nonmonotonic dependence of the potential within the discharge volume. We note the determining role of oscillating electrons both during formation and during burning of a quasisteady-state arc discharge. Based on our proposed model, we calculate the formation time for a low-pressure discharge. We focus especially on description of the burning conditions for an arc with diffuse clamping of the current at a vaporizing cathode, which may be used in efficient sources of metal ions. We calculate the basic characteristics of such an arc, which are found to be in good agreement with experimental data.

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.

Implosion of Multilayer Liners

Experiments on the implosion of multilayer cascade light liners were carried out on the IMRI‐3 (0.3 MA, T/4 = 700 ns) and IMRI‐4 (0.24 MA, 900 ns) installations. It has been demonstrated that when using three sequantial cascades, owing to the suppression of the Rayleigh‐Taylor instability, the ratio r10/rf ∼ 70 (where r10 is the initial radius of the outer cascade) can be achieved. The input energy density and the X‐ray output therein h are observed to increase (respectively, up to 1 kJ and 200 J for Kr). Measurements performed with the use of a grating spectrograph for different gases have shown that the temperature in the plasma column formed in liner implosion increases with atomic number.

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.

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.