A. V. Kozyrev

Optical studies of plasma inhomogeneities in a high-current pulsed magnetron discharge

Results are presented for experimental studies of the plasma glow in a high-current pulsed magnetron discharge by using a high-speed optical frame camera. It is found that the discharge plasma is inhomogeneous in the azimuthal direction. The plasma bunches rotate with a linear velocity of ∼1 cm/μs in the direction of electron Hall drift, and their number is proportional to the discharge current. Plasma inhomogeneities in the form of plasma jets propagate in the form of plasma jets from the cathode region toward the anode. It is shown analytically that the formation of inhomogeneities is caused by the necessity to transfer high-density electron current across the magnetic field.

Soft X-ray generation and its role in breakdown of air gap at elevated pressures

Fast runaway electrons, which appear during the electric breakdown in an air-filled discharge gap under the action of a short-front voltage pulse, generate 525-eV photons from the K-shell of oxygen. This soft X-ray emission effectively generates new electrons in the region of enhanced field. The proposed mechanism may explain the fast streamer propagation and the formation of a diffuse discharge at atmospheric pressure.

Spectrum of fast electrons in a subnanosecond breakdown of air-filled diodes at atmospheric pressure

In this work, the spectra of electron beams produced in air-filled diodes at atmospheric pressure were studied for different cathode designs. The feasibility of correct reconstruction of the electron beam spectrum from an experimental dependence of its attenuation factor in foils of different thicknesses was demonstrated. The electron energy distributions were calculated on minimum a priori assumptions by regularization of an ill-posed problem—a Fredholm integral equation. The spectra of a subnanosecond electron beam generated in the gas gap during the voltage pulse rise time were reconstructed and analysed. A time-of-flight spectrometer study and reconstruction of the spectrum from the data on e-beam attenuation confirmed the fact that groups of electrons with two-three characteristic energies can be generated in gas-filled diodes. In experiments, electrons of energy greater than that corresponding to the nominal voltage amplitude across the gap were detected.

A physical model of the low-current-density vacuum arc

A one-dimensional (1-D) physical model of the low-current-density steady-state vacuum arc is proposed. The model is based on the continuity equations for ions and electrons and the energy balance for the discharge system; the electric potential distribution in the discharge gap is assumed to be nonmonotonic. It is supposed that the ion current at the cathode is generated within the cathode potential fall region due to the ionization of the evaporated atoms by the plasma thermal electrons having Boltzmanns energy distribution. The model offers a satisfactory explanation for the principal regularities of a hot-cathode vacuum arc with diffuse attachment of the current. The applicability of the model proposed to the explanation of some processes occurring in a vacuum arc, such as the flow of fast ions toward the anode, the current cutoffs and voltage bursts, and the backward motion of a cathode spot in a transverse magnetic field is discussed.

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.

Runaway electrons and x-rays from a corona discharge in atmospheric pressure air

The characteristics of a corona discharge in atmospheric pressure air are studied using pulsed power generators that produce voltage pulses of different durations, polarities and shapes. The characteristics are measured in the single pulse, batch, and repetitively pulsed modes. It is shown that no matter what the voltage pulse polarity is, a corona discharge starts developing as a conical diffuse discharge near the electrode tip with a voltage rate of increase of 10 15 Vs 1 across an electrode of small curvature radius. With lower voltage rate of increase ( 10 13 Vs 1 or lower), one or several diffuse jets develop from this electrode. The diameter of the jets at their front is less than 1mm and depends on many factors (voltage pulse amplitude and increase, inter- electrode gap width, pulse repetition rate, etc). It is found that at long voltage pulse durations, the radiation spectrum of the corona discharge changes, and the bands and lines of the material of the electrode appear in the UV region at 200-300nm. It is demonstrated that a runaway electron beam in a corona discharge is generated and detected at a distance several times greater than the brightly glowing plasma region of the corona discharge. It is shown that x-rays are generated from a corona discharge at high pulse repetition rates of up to 1kHz.

Reconstruction of electron beam energy spectra for vacuum and gas diodes

In this paper, the spectra of electron beams produced in vacuum and gas diodes were analyzed to study the capabilities and limitations of their reconstruction from beam attenuation in foils of different thickness. The electron energy distributions were calculated using the Tikhonov regularization for Fredholm integral equations on minimum a priori assumptions. The spectra reconstructed in the study were those of electron beams, including a supershort avalanche electron beam, produced in experiments on a DUET plasma-cathode electron accelerator and SLEP-150M accelerator.

Spectra of electrons and X-ray photons in a diffusive nanosecond discharge in air under atmospheric pressure

The spectra of electrons and X-ray photons generated in nanosecond discharges in air under atmospheric pressure are investigated theoretically and experimentally. Data for the discharge formation dynamics in a nonuniform electric field are gathered. It is confirmed that voltage pulses with an amplitude of more than 100 kV and a rise time of 1 ns or less causing breakdown of an electrode gap with a small-radius cathode generate runaway electrons, which can be divided into three groups in energy (their energy varies from several kiloelectronvolts to several hundreds of kiloelectronvolts). It is also borne out that the formation of the space charge is due to electrons appearing in the gap at the cathode and a major contribution to the electron beam behind the foil comes from electrons of the second group, the maximal energy of which roughly corresponds to the voltage across the gap during electron beam generation. X-ray radiation from the gas-filled diode results from beam electron slowdown both in the anode and in the gap. It is shown that the amount of group-3 electrons with an energy above the energy gained by runaway electrons (in the absence of losses) at a maximal voltage across the gap is much smaller than the amount of group-2 electrons.

Two regimes of heat exchange between a metal particle and a nonequilibrium plasma

A theoretical model is developed for the process of energy exchange between a spherical drop and a nonequilibrium plasma. It is demonstrated that, in a certain range of the plasma parameters, a quasistationary temperature of the drop can be maintained in two different heat exchange regimes. In one of these, the energy flux from plasma to drop is compensated by cooling due to the thermal emission of electrons; in the other, the energy supply is equilibrated by intensive evaporation of the drop material. The two regimes, characterized by certain quasistationary temperatures, are separated by a temperature interval featuring unstable states of the system. The particular realization of one or another quasistationary regime is determined by the initial temperature of the drop.

Theoretical Simulation of a Gas Breakdown Initiated by External Plasma Source in the Gap With Combined Metal–Dielectric Electrodes

This paper is devoted to the theoretical investigation of the breakdown in short discharge gaps of different geometries under the influence of the plasma stream from an external source. The structural feature is the presence of dielectric elements in the electric discharge gap area, which have high emission activity and the ability to accumulate a surface charge. The simulation was performed for a 2-D planar geometry of the discharge gap between two metal electrodes with dielectric coating surrounded by the gaseous medium. Charged particles generation and dynamics have been described by a system of partial differential equations in the diffusion-drift approximation within the two-fluid hydrodynamics plasma model. The basic advantages of the proposed model (such as a wide variety of boundary conditions and geometries of the discharge gap, the scalability of the critical parameters of the environment, and a simple representation of surface reactions) are demonstrated successfully. During computations, the range of gas pressure and the external preionization level at which the probability of low-voltage self-sustained discharge is high enough were identified.