Yu. D. Korolev

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.

Drift model of the cathode region of a glow discharge

A one-dimensional drift model of the cathode region of a glow discharge with allowance for both electron-impact ionization and charged particle loss is proposed. An exact solution to the model equations is obtained for the case of similar power-law dependences of the ion and electron drift velocities on the electric field strength. It is shown that, even in the drift approximation, a relatively wide transition layer in which the ion-to-electron current ratio approaches a constant value typical of the positive column of a glow discharge should occur between the thin space-charge sheath and the quasineutral plasma, the voltage drop across the space-charge sheath being comparable to that across the transition layer. The calculated parameters of the normal and anomalous glow discharges are in good agreement with available experimental data.

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.

Low-current discharge plasma jets in a gas flow. Application of plasma jets

The paper describes the results of investigations of low-current discharges in a gas flow at atmospheric pressure. The primary focus is on glow discharges in coaxial plasmatrons and the so-called gliding arc. Such discharges are typically used for obtaining a plasma jet at the exit of the electrode system. The jet contains active chemical species playing an important role in various applications of the discharge. Plasma-assisted combustion and oxidation of hydrocarbon fuels is also considered. Besides, the applications of plasma jet for modification of surfaces and for use in biology and medicine are discussed.

Momentary interruption of current passing through zero in subnanosecond high-pressure gas-discharge switches

Data are reported for the performance of subnanosecond high-pressure gas-discharge switches aimed at generating high-voltage bipolar voltage pulses with a width of less than 1 ns, an amplitude of up to 200 kV, and a repetition rate of 100 Hz. Pulse formers, in which the polarity of the current passing through the switch does not change in the course of switching, seem to be the most appropriate. When the current changes sign, passing through zero, the effect of its momentary interruption is observed. The explanation of this effect is that, when the voltage across the gap reverses, a certain time interval is necessary for a cathode layer to form, which can transmit a high discharge current at a low voltage drop across the layer.

Subnanosecond processes in the stage of breakdown formation in gas at a high pressure

Results are presented from experimental studies of the prebreakdown stage of a discharge in nitrogen at pressures of a few tens of atmospheres, gap voltages higher than 140 kV, and a voltage rise time of about 1 ns. Breakdown occurs at the front of the voltage pulse; i.e., the time of breakdown formation is shorter than the front duration. It is shown that, in gaps with a nonuniform electric field, the breakdown formation time is mainly determined by the time of avalanche development to the critical number of charge carriers. The subsequent stages of breakdown (the development of the ionization wave and the buildup of the conductivity in the weakly conducting channel bridging the gap) turn out to be shorter than this time or comparable to it.

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.

Operating features of a high-voltage spark gap switch with gas blow normal to the breakdown path in a repetitively pulsed mode

The results of studies of a high-voltage two-electrode spark gap switch (SGS) with forced gas blow at an operation voltage of up to 1.2 MV are presented. An SGS filled with nitrogen as the working gas at a pressure of up to 16 atm operated as the high-voltage switch of a high-current nanosecond electron accelerator. The gas flow was directed normally to the breakdown path. The SGS switched a 50-Ω forming line with an electrical length of 10 ns to a matched load. The voltage rise time across the electrodes before breakdown was ∼25µs. A stable repetitively pulsed mode is realized at operating voltages of 100–680 kV and pulse repetition rates of up to 270 Hz with a standard deviation of the pulse breakdown voltage of ≤1%. The physical mechanisms that determine unstable operation of the device during self-breakdown are analyzed.

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.