Igor' V Kochetov

Negative corona, glow and spark discharges in ambient air and transitions between them

Obtaining new information about different forms of self-sustained dc discharges that can be realized in pin-to-plane electrode geometry in ambient air is the goal of this paper. Experimental and numerical calculation data uncovering the physics of the temporal and spatial evolution of the negative corona and glow discharge (GD), with increase in current up to the transition to the spark, are presented. Special attention is paid to the properties of diffusive GD at atmospheric pressure, which is a necessary stage (steady-state or transient) preceding the spark and determining the threshold conditions of sparking.

A numerical simulation of Trichel-pulse formation in a negative corona

A new quasi-one-dimensional model for a negative corona in air is formulated allowing for the quantitative description of the mechanism of Trichel-pulse formation. Detailed analysis of the processes controlling pulse dynamics is made. Comparison with experiment for a short-gap corona demonstrates a reasonable agreement with the shape of the pulse and in the average characteristics of the negative corona.

The ionization kinetics and electric field in the leader channel in long air gaps

A kinetic model for the ionization processes in high-temperature air in a strong electric field is developed. The model is used to study numerically the problem of the relaxation of the plasma properties in atmospheric-pressure air to a new steady state after an instantaneous change in the gas temperature between 300 and 1000 - 6000 K. Our simulation shows that the electrical properties of the plasma under the conditions of the leader being a long spark nearly follow the local value of the gas temperature. The reduced field E/N in the leader channel is found to decrease considerably as the gas temperature T increases from 1000 to 6000 K. The decrease in E/N is caused by the formation of NO molecules with low ionization energy at T 4500 K. As a result, the value of E/N falls to 10 Td at T = 5000 K and to 1.5 Td at T = 6000 K for a current of 1 A and a channel radius of 0.1 cm. This agrees well with the experimental evidence showing that the leader bridges very long air gaps (about 200 m) at a moderate applied voltage (< 5 MV).

Non-thermal plasma instabilities induced by deformation of the electron energy distribution function

Non-thermal plasma is a key component in gas lasers, microelectronics, medical applications, waste gas cleaners, ozone generators, plasma igniters, flame holders, flow control in high-speed aerodynamics and others. A specific feature of non-thermal plasma is its high sensitivity to variations in governing parameters (gas composition, pressure, pulse duration, E/N parameter). This sensitivity is due to complex deformations of the electron energy distribution function (EEDF) shape induced by variations in electric field strength, electron and ion number densities and gas excitation degree. Particular attention in this article is paid to mechanisms of instabilities based on non-linearity of plasma properties for specific conditions: gas composition, steady-state and decaying plasma produced by the electron beam, or by an electric current pulse. The following effects are analyzed: the negative differential electron conductivity; the absolute negative electron mobility; the stepwise changes of plasma properties induced by the EEDF bi-stability; thermo-current instability and the constriction of the glow discharge column in rare gases. Some of these effects were observed experimentally and some of them were theoretically predicted and still wait for experimental confirmation.

Studies on cold plasma-polymer surface interaction by example of PP- and PET-films

This paper presents the results of experimental and theoretical studies aimed at developing mathematical models for plasma–polymer interactions. Experiments on plasma treatment of polypropylene (PP) and poly(ethylene terephthalate) foils were carried out with the use of atmospheric pressure plasma sources operating with ambient air and purified nitrogen as plasma forming gases. Water contact angle was measured, and x-ray photoelectron spectroscopy characterization of the treated samples was done. A theoretical model based on a mechanism proposed by Kushner for plasma processing of PP foil is described. A comparison between experimental and theoretical results for oxygen enrichment of PP-polymer samples is presented.

Electron energy distribution function in decaying nitrogen plasmas

The electron energy distribution function (EEDF) in decaying nitrogen plasmas is investigated numerically. The comparison of the calculated and experimentally measured EEDF allows a correction to be made to the cross sectional magnitude for electron scattering from vibrationally excited molecules. The influence of electron-electron (e-e) collisions on the EEDF is investigated. It is shown that e-e collisions may have a strong influence on the EEDF in decaying plasmas even at very low degrees of ionization (2*10-6).

Transition of a Multipin Negative Corona in Atmospheric Air to a Glow Discharge

It is commonly accepted that, as the current increases, a diffuse negative corona inevitably goes over to a strongly nonuniform and nonsteady spark discharge. In this paper, a new effect—the transition of a negative corona to a diffuse glow discharge at atmospheric pressure—is studied experimentally and numerically. The evolution of the corona parameters during the transition to the regime of a glow discharge is traced.

Decomposition of toluene in a steady-state atmospheric-pressure glow discharge

Results are presented from experimental studies of decomposition of toluene (C6H5CH3) in a polluted air flow by means of a steady-state atmospheric pressure glow discharge at different water vapor contents in the working gas. The experimental results on the degree of C6H5CH3 removal are compared with the results of computer simulations conducted in the framework of the developed kinetic model of plasma chemical decomposition of toluene in the N2: O2: H2O gas mixture. A substantial influence of the gas flow humidity on toluene decomposition in the atmospheric pressure glow discharge is demonstrated. The main mechanisms of the influence of humidity on C6H5CH3 decomposition are determined. The existence of two stages in the process of toluene removal, which differ in their duration and the intensity of plasma chemical decomposition of C6H5CH3 is established. Based on the results of computer simulations, the composition of the products of plasma chemical reactions at the output of the reactor is analyzed as a function of the specific energy deposition and gas flow humidity. The existence of a catalytic cycle in which hydroxyl radical OH acts a catalyst and which substantially accelerates the recombination of oxygen atoms and suppression of ozone generation when the plasma-forming gas contains water vapor is established.

Kinetics of low-temperature plasmas for plasma-assisted combustion and aerodynamics

Kinetic processes in a weakly ionized non-equilibrium plasma are considered under conditions that are typical for plasma-assisted ignition/combustion and flow control. The focus is on the simulation of active species production that leads to ignition delay reduction, flame stabilization and expansion of the flammability limit of combustible mixtures. We discuss the lack of information on electron cross sections for hydrocarbons and the accuracy of widely used approaches to simulate kinetics of active species production in air and combustible mixtures. Fast gas heating after a high-voltage nanosecond discharge is studied for various gas mixtures and reduced electric fields. We analyze the effect of negative ions generated in the afterglow of a high-voltage discharge with regard to plasma-assisted ignition and plasma aerodynamics application.

Numerical simulations of Trichel pulses in a negative corona in air

Numerical simulations of a negative corona in air demonstrate that the experimentally observed regime of self-oscillations, known as Trichel pulses, is well described by a three-dimensional axisymmetric model that is based on the standard transport equations and in which only the ion-induced secondary electron emission at the cathode is taken into account. The quantitative difference between the measured and calculated values of the mean current and the pulse repetition rate most likely stems from the insufficiently large dimensions of the computation region and from the fact that the point shape adopted in simulations somewhat inexactly conforms to that used in experiments. It was found that the transverse discharge structure near the cathode radically changes during the pulse. Specifically, as the current grows, a cathode sheath forms at the discharge axis and expands over the cathode surface. When the current falls off, the cathode sheath is rapidly destroyed; as a result, the characteristic field structure is well defined only near the discharge axis and becomes virtually indistinguishable as the current decreases by an order of magnitude.