Dmitry Levko

Two-dimensional model of orificed micro-hollow cathode discharge for space application

In this paper, we describe results of self-consistent two-dimensional (x-z) particle-in-cell simulations, with a Monte Carlo collision model, of an orificed micro-hollow cathode operating in a planar diode geometry. The model includes thermionic electron emission with Schottky effect, secondary electron emission due to cathode bombardment by the plasma ions, several different collision processes, and a non-uniform xenon background gas density in the cathode-anode gap. Simulated results showing behavior of the plasma density, potential distribution, and energy flux towards the hollow cathode and orifice walls, are discussed. In addition, results of simulations showing the effect of different Xe gas pressures, orifice size, and cathode voltage, on operation of the micro-hollow cathode are presented.

Electron kinetics in a microdischarge in nitrogen at atmospheric pressure

Electron kinetics during a microdischarge in nitrogen at atmospheric pressure is studied using the one-dimensional Particle-in-Cell/Monte Carlo Collisions model. It is obtained that the electron energy distribution function can be divided into three parts, namely, the non-equilibrium low-energy part, the Maxwellian function at moderate energies, and the high-energy tail. Simulation results showed that the role of the high-energy tail of electron energy distribution increases, when the distance between electrodes increases.

Temporal and spatial structure of a runaway electron beam in air at atmospheric pressure

The time- and spatial structure of a runaway electron beam generated in air at atmospheric pressure by a high-voltage pulse with a rise time of ∼300 ps is studied experimentally and numerically. It is obtained that the duration of the runaway electron current is a few tens of picoseconds, and it can consist of two or many peaks. It is shown that the many-peak temporal structure of the beam is caused by the non-simultaneous appearance of several emission centers on the cathode edge.

Early stage time evolution of a dense nanosecond microdischarge used in fast optical switching applications

The mechanism of high-voltage nanosecond microdischarges is studied by the self-consistent two-dimensional Particle-in-Cell/Monte Carlo Collisions model. These microdischarges were recently proposed for use as fast switches of visible light in Bataller et al. [Appl. Phys. Lett. 105, 223501 (2014)]. The microdischarge is found to develop in two stages. The first stage is associated with the electrons initially seeded in the cathode-anode gap. These electrons lead to the formation of a cathode-directed streamer. The second stage starts when the secondary electron emission from the cathode begins. In this stage, a rather dense plasma (∼1016 cm−3) is generated which results in the narrow cathode sheath. The electric field in this sheath exceeds the critical electric field which is necessary for the runaway electrons generation. We have found that the presence of these energetic electrons is crucial for the discharge maintenance.

Conductivity of nanosecond discharges in nitrogen and sulfur hexafluoride studied by particle-in-cell simulations

The conductivity of the discharge gap during the nanosecond high-voltage pulsed discharge in nitrogen and sulfur hexafluoride is studied using particle-in-cell numerical simulations. It is shown that the conductivity in different locations of the cathode-anode gap is not uniform and that the conductivity is determined by both the runaway and the plasma electrons. In addition, it is shown that runaway electrons generated prior to the virtual cathode formation pre-ionize the discharge gap, which makes it conductive.

The physical phenomena accompanying the sub-nanosecond high-voltage pulsed discharge in nitrogen

Results of one-dimensional Particle-in-Cell numerical simulations of mechanism of sub-nanosecond high-voltage pulsed discharge in nitrogen are presented. It is shown that the decrease of the cathode-anode gap changes drastically both the discharge dynamics and mechanism of runaway electrons generation responsible for the discharge initiation. It is obtained that the virtual cathode exists only during tens of picoseconds for short gaps. The conditions when the virtual cathode is not formed are found. Also, the comparison between the experimental [Rybka et al., Tech. Phys. Lett. 38, 653 (2012)] and simulation results indicates the dominant role of the virtual cathode in termination of runaway electrons generation and on separate nature of emission sources from the cathode surface.

Fluid modeling of a high-voltage nanosecond pulsed xenon microdischarge

A computational modeling study of high-voltage nanosecond pulsed microdischarge in xenon gas at 10 atm is presented. The discharge is observed to develop as two streamers originating from the cathode and the anode, and propagating toward each other until they merge to form a single continuous discharge channel. The peak plasma density obtained in the simulations is ∼1024 m−3, i.e., the ionization degree of plasma does not exceed 1%. The influence of the initial gas pre-ionization is established. It is seen that an increase in the seeded plasma density results in an increase in the streamer propagation velocity and an increase in the plasma density obtained after the merging of two streamers.

Numerical simulation of runaway electrons generation in sulfur hexafluoride

The results of the numerical simulation of the generation of runaway electrons in sulfur hexafluoride are presented. It is shown that the emission of the runaway electrons occurs from the cathode and its vicinity. The generation of these electrons is terminated because of the formation primarily by negative ions of a virtual cathode which causes the field emission to be screened. The simulations showed that the virtual cathode consists mainly of negative ions and cannot be an effective source of runaway electrons. In addition, the results of the simulations showed that the parameters of the runaway electrons depend on the amplitude and rise-time high-voltage pulse and gas pressure.

Influence of field emission on the propagation of cylindrical fast ionization wave in atmospheric-pressure nitrogen

The influence of field emission of electrons from surfaces on the fast ionization wave (FIW) propagation in high-voltage nanosecond pulse discharge in the atmospheric-pressure nitrogen is studied by a one-dimensional Particle-in-Cell Monte Carlo Collisions model. A strong influence of field emission on the FIW dynamics and plasma parameters is obtained. Namely, the accounting for the field emission makes possible the bridging of the cathode–anode gap by rather dense plasma (∼1013 cm−3) in less than 1 ns. This is explained by the generation of runaway electrons from the field emitted electrons. These electrons are able to cross the entire gap pre-ionizing it and promoting the ionization wave propagation. We have found that the propagation of runaway electrons through the gap cannot be accompanied by the streamer propagation, because the runaway electrons align the plasma density gradients. In addition, we have obtained that the field enhancement factor allows controlling the speed of ionization wave propag...

Particle-in-cell simulations of the runaway breakdown of nitrogen

The runaway breakdown initiated by a mono-energetic beam of runaway electrons propagating through a cathode-anode gap filled with nitrogen at atmospheric pressure is studied using the one-dimensional particle-in-cell numerical model. It is shown that the breakdown is strongly influenced by the amplitude of the beam, its duration, and the electric field in the vicinity of the cathode. In addition, the simulation results showed that, in spite of the formation of rather dense plasma inside the cathode-anode gap by runaway electrons, the electric field is not screened because of frequent electron–neutral collisions.