V. V. Ryzhov

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.

Effect of electronegative impurities on the generation of ozone in air

A self-consistent numerical model is used to investigate the effect of electronegative impurities on the ozone yield in a dielectric barrier discharge with a pulsed voltage supply, and the range of impurity concentrations giving a substantial (two-or threefold) increase in the ozone yield is established. Sulfur hexafluoride is considered as a representative component having strong electronegative properties. It is shown that a tiny admixture [SF6]<0.1% can have an appreciable effect on the characteristics of an ozonator. The calculations are compared with published experimental data and given an interpretation.

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.

Kinetics of free radicals in the plasma of a spark discharge in methane

The kinetics of reactions in the plasma of a spark discharge in methane at atmospheric pressure is analyzed using numerical simulation, taking account of the gas-dynamic expansion of the channel. The processes leading to the formation and annihilation of CHx radicals are investigated and the conditions under which their maximum concentration is reached are determined. The results obtained are of interest for plasma chemical technologies for processing natural gas.

Experimental investigation of electron beam in the squeezed state

The electron beam transported in a two-section drift tube of a SINUS-7 setup has been studied. A high-density electron beam in the “squeezed” state has been obtained with a relativistic factor γb below that corresponding to the limiting current (γ > Γ1/3).

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.

Measuring the virtual cathode velocity

An electron beam with a virtual cathode (VC) transported in a two-section drift tube of a SINUS-7 high-current electron accelerator has been studied. The dependence of the VC velocity on the injected current has been experimentally determined for the first time. It is established that the VC motion in the drift tube is accompanied by microwave oscillations, which are caused by transient processes involved in the formation of a compressed electron beam.

The virtual cathode velocity during electron beam transport in a drift tube

A solution corresponding to a thin annular electron beam in a homogeneous drift tube featuring a uniformly moving virtual cathode (VC) has been obtained. The dependence of the VC velocity on the injected and transmitted currents is determined and it is established that this velocity has a limiting value. Theoretical results agree with the results of numerical calculations performed by means of the electromagnetic PIC code KARAT.

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.