V. A. Shklyaev

Initial stage of gas discharge in an inhomogeneous electric field

A model of the initial stage of gas discharge has been developed within the framework of the particle in cell (PIC) method, with allowance for the space charge and particle collisions described using the Monte Carlo (MC) numerical simulation technique. The PIC/MC simulations of the initial stage of discharge under conditions of the electric field strength to gas pressure ratio E/P > 1 kV/(cm Torr) showed that a beam of runaway electrons is formed within ∼10 ps near the cathode, which consists of both emitted electrons and those generated as a result of the gas ionization. The duration of the beam pulse is determined primarily by plasma screening of the external electric field near the cathode and amounts to 10–20 ps.

Simulation of the formation of a runaway electron beam in an overvolted gas gap breakdown

The paper reports on numerical simulation to inquire into the breakdown of a gas-filled diode in a highly inhomogeneous electric field. It is shown that early in the breakdown a runaway electron beam (RAEB) is formed in the diode and this strongly affects the rate of breakdown development. The energy gained by RAEB electrons corresponds to the electron energy gained under the same conditions in vacuum. The properties of the emission surface of the cathode determine the instant at which the beam is formed during subnanosecond voltage pulse rise time and hence the beam current and the energy spectrum of runaway electrons.

Effect of cathode emissivity on runaway electron beam formation in gas-filled diode with inhomogeneous electric field

Mechanisms of the formation of runaway electron beams at the initial stage of breakdown in a gasfilled diode with sharply inhomogeneous electric field have been studied by numerical simulations using a specially developed PIC/MC code. It is established that a beam of runaway electrons can be generated either immediately at the cathode or at a discharge plasma boundary. The obtained dependence of the runaway electron beam current on the state of the cathode surface (emissivity) has the characteristic shape.

On the dynamics of a subnanosecond breakdown in nitrogen below atmospheric pressures

The dynamics of a breakdown in a gas-filled diode with a highly inhomogeneous electric field was studied in experiments at a time resolution of ∼100 ps and in numerical simulation by the 2D axisymmetric particle-in-cell (PIC) code XOOPIC. The diode was filled with nitrogen at pressures of up to 100 Torr. The dynamics of the electric field distribution in the diode during the breakdown was analyzed, and the factors that limit the pulse duration of the runaway electron beam current at different pressures were determined.

Numerical investigation of the parameters of a runaway electron beam generated in a gas-filled atmospheric-pressure hot-channel diode

The generation of runaway electrons in an inhomogeneous medium under normal conditions is investigated numerically. The medium is represented by a hot channel (spark channel, laser flame, etc.) surrounded by air. A model is suggested that makes it possible to consistently calculate the initiation of a subnanosecond glow discharge and the generation of runaway electrons under such conditions. The possibility of generating 100-ps-wide runaway electron current pulses with an amplitude of several hundred amperes is shown. The influence of an air gap and an external magnetic field on the generated beam’s parameters is studied.

Experimental and numerical investigation of two mechanisms underlying runaway electron beam formation

The electrical breakdown of a gas-filled diode with a highly nonuniform electric field is studied in the case when a 25-kV voltage pulse generates runaway electron beams with time-separated maxima of different duration behind anode foil. Experimental data are analyzed and numerically simulated using the PIC/MC code OOPIC-Pro. It is shown that, in terms of the model used, both beams arise at the cathode but their formation mechanisms differ. The first runaway electron beam no longer than 500 ps is attributed to the ionization mechanism; the second one, which may last several nanoseconds, is due to emission.

Stability of runaway electron beam currents in nitrogen at 100 Torr. Experiments and numerical simulation

This paper reports on a series of identical experiments and on a respective particle-in-cell and Monte Carlo simulation to investigate the stability of runaway electron beam currents produced in nitrogen at a highly inhomogeneous electric field. The experiments demonstrate a strong effect of cathode emissivity on the beam current. The numerical simulation shows that the cathode emissivity influences not only the current of the beam but also the mechanisms of its formation.

Generation of accelerated electrons in a gas diode with hot channel

Generation of fast electrons in an inhomogeneous medium composed of a hot channel (spark channel, laser plume, etc.) surrounded by air under normal conditions has been numerically analyzed. The model used makes it possible to carry out consistent calculation of the formation of subnanosecond gas discharge and generation of accelerated electrons under these conditions. The fast-electron current is found to consist of two pulses. One of them has an amplitude of 50 A, width of 30 ps, and electron energy of more than 100 keV. These electrons are generated in the hot channel. The other pulse has an amplitude of 170 A, width of 20 ps, and electron energy in the range of 8–50 keV. These electrons are generated in cold air. Since these pulses pass successively and barely overlap, the total width of fast-electron pulse is almost 50 ps.

Simulating gas-discharge processes at a single cold microscopic point

The development of ionization avalanches in nitrogen at atmospheric pressure near a single cold microscopic point on a cathode surface has been simulated under the conditions of E/P ≫ 1 kV/(cm Torr), where E is the electric field strength and P is the gas pressure. It is established that a layer of dense gas-discharge plasma with a density of ∼1016 cm−3 is formed within a period of ∼1 ps as a result of the gas ionization by electrons emitted from the cathode. The current of fast electrons, which appears due to gas ionization is more than ten times greater than the field emission current and can reach I ∼ 1 A for one microscopic point.

Current in a pulsed gas breakdown at a highly inhomogeneous electric field

This paper reports on a particle-in-cell and Monte Carlo simulation of the evolution of a pulsed breakdown in a gas-filled diode at a highly inhomogeneous electric field. The simulation shows that even during the formation of a conducting plasma channel, the diode can experience a current flow capable of greatly decreasing the diode voltage compared to its value in idle mode. This current is almost independent of the gap width and is due to fast plasma motion from the cathode to the anode.