G V Naidis

Modelling of plasma bullet propagation along a helium jet in ambient air

The results of simulation of positive streamer propagation along a helium jet in ambient air are presented. A two-dimensional axially symmetric streamer model, accounting for variation of helium–air mixture composition in the jet, is used. The obtained distributions of plasma parameters have a ring-shaped structure, typical for plasma bullets. The calculated radial profiles of emitting nitrogen molecules agree with experimental data.

Two-dimensional modelling of positive streamer dynamics in non-uniform electric fields in air

The results of two-dimensional (2D) numerical simulation of single positive streamer development in atmospheric air in sphere - plane electrode configuration are presented. Correlation of streamer parameters with sphere radius and applied voltage U is revealed. The values of the electric field at the streamer head and in the channel are shown to be almost independent of and U and equal to 150 - 180 and correspondingly. Streamer velocity and radial dimensions linearly increase with U, the coefficient in these linear relations being independent of . Comparison of the results obtained by 2D modelling and in the frame of the commonly used 1D (or quasi-2D) models is made.

Modelling of low-current discharges in atmospheric-pressure air taking account of non-equilibrium effects

A model of stationary direct current discharges in atmospheric-pressure air is developed that accounts for deviation of plasma state from the local thermodynamic equilibrium. It is shown that at the current range 0.01–0.1 A a change in the dominant ionization mechanism takes place, from ionization by electron impact at low currents to associative ionization in atomic collisions at high currents. Results of the calculation of discharge parameters over a wide current range are presented. Calculated plasma parameters agree with the available experimental data.

On photoionization produced by discharges in air

Recent data on ionizing radiation in discharges in dry and moist air are analysed. It is shown that an account of both quenching of radiating states and absorption of ionizing radiation by water molecules allows one to explain these data in the framework of a commonly accepted model that is widely used for streamer simulations.

Modelling of plasma chemical processes in pulsed corona discharges

By a two-dimensional numerical simulation of positive streamers in atmospheric air in sphere - plane gaps, the G-values (numbers of radicals per 100 eV of input energy) for the production of oxygen and nitrogen atoms are obtained. Calculated G-values weakly depend on the applied voltage and parameters of the gap and are about 3 - 4 and 0.3 - 0.4 respectively. In the frame of the qualitative theory of streamer propagation in nonuniform electric fields, approximate formulae for G-values in streamer corona discharges are derived. The G-values are determined by the magnitudes of the reaction rate coefficients at electron energies corresponding to the maximum of the electric field in the streamer head. The formulae obtained describe well the relations between the G-values for different plasma chemical reactions and can be used for estimations of the absolute G-values with an accuracy up to a factor of two.

Unified modelling of near-cathode plasma layers in high-pressure arc discharges

A model of a near-cathode region in high-pressure arc discharges is developed in the framework of the hydrodynamic (diffusion) approximation. Governing equations are solved numerically in 1D without any further simplifications, in particular, without explicitly dividing the near-cathode region into a space-charge sheath and a quasi-neutral plasma. Results of numerical simulation are reported for a very high-pressure mercury arc and an atmospheric-pressure argon arc. Physical mechanisms dominating different sections of the near-cathode region are identified. It is shown that the near-cathode space-charge sheath is of primary importance under conditions of practical interest. Physical bases of simplified models of the near-cathode region in high-pressure arc discharges are analysed. A comparison of results given by the present model with those given by a simplified model has revealed qualitative agreement; the agreement is not only qualitative but also quantitative in the case of an atmospheric-pressure argon plasma at moderate values of the near-cathode voltage drop. The modelling data are compared with results of spectroscopic measurements of the electron temperature and density in the near-cathode region.

Conditions for inception of positive corona discharges in air

Conditions for inception of dc corona discharges in air near spherical and cylindrical anodes are considered. The parameter K (ionization integral) in the standard discharge inception criterion is evaluated for a wide range of electrode radii and air densities.

The effects of an external electric field on the dynamics of cold plasma jets—experimental and computational studies

Atmospheric pressure plasma jets provide a convenient and stable means to transport highly reactive plasma species beyond the confines of the plasma generating electrodes and into the ambient air; such characteristics make them an ideal tool for many emerging plasma processing applications. As the guided streamer exits the jet capillary, the application of an external electric field can significantly influence the dynamics of propagation, potentially providing a means to manipulate the transport of plasma species to a downstream substrate. In this paper the influence of positive and negative voltages pulses applied to an external electrode situated along the axis of streamer propagation is examined experimentally and computationally using a simplified 1.5D model. It is shown that application of a positive voltage pulse to the external electrode reduces the velocity of propagation of the cathode-directed streamer and the application of a negative voltage pulse increases the velocity of propagation. Further to this, the application of high positive voltages to the external electrodes effectively inhibits propagation and results in a significant decrease in the emission intensity from excited states populated by energetic electrons. The results obtained experimentally are compared and contrasted with those from the computational model to uncover the underlying physical mechanisms at play.

On streamer interaction in a pulsed positive corona discharge

An approximate method is presented to take account of interaction of positive streamers simultaneously propagating between a wire and a plate. The method is based on a quasi-2D positive streamer model modified by inclusion in the expression for electric field of an additional term describing the field of charges of all streamer channels in the gap. All streamers are assumed to be identical and to move parallel to each other to the plate from points equidistantly distributed along the wire surface. The model is used for calculation of streamer dynamics in atmospheric air for conditions under which simultaneously propagating streamers have been observed. It is shown that taking account of streamer interaction improves the agreement of the calculated dependence of streamer velocity on its length with the experimental one.

Two-dimensional modeling of positive streamer propagation in flue gases in sphere-plane gaps

The results of two-dimensional numerical simulation of positive streamer dynamics in flue gases in sphere-plane electrode configuration are presented. The G-values (numbers of radicals produced per 100 eV of input electrical energy) for the production of chemically active particles participating in flue gas cleaning from toxic components are calculated. Obtained G-values are almost independent on the discharge conditions. Simple estimates for G-values, based on analytical streamer theory, are shown to agree with the results of numerical simulation.