V A Maiorov

Modelling of the homogeneous barrier discharge in helium at atmospheric pressure

Numerical calculations of spatio-temporal characteristics of the homogeneous barrier discharge in helium are performed by means of a one-dimensional fluid model. The influence of the elementary processes on the discharge behaviour is studied by variation of the corresponding rate constants. The simulation and the analytical interpretation are carried out for two basic modes of the homogeneous barrier discharge, i.e. the Townsend and glow modes. The Townsend discharge is characterized by the absence of quasineutral plasma; several current peaks may occur during the half-cycle. The oscillations of the current are caused by a lag between the ion production nearby the anode and the subsequent ion–electron emission on the cathode. The specificity of the glow discharge is the development of a cathode region and a positive column during the breakdown, as well as the presence of quasineutral plasma in subsequent phases. The positive column occurs because the shielding of the external field by the plasma is not instantaneous. The dependence of the discharge behaviour on the external parameters, such as the amplitude and frequency of the applied voltage, discharge gap width, and thickness of dielectric barriers, is analysed. The mode of the discharge is governed mostly by the gap width and barrier thickness and depends weakly on the amplitude and frequency of the applied voltage. As the barriers are thin and the discharge gap is sufficiently wide, the glow mode occurs; otherwise, the discharge is Townsend.

Influence of interaction between charged particles and dielectric surface over a homogeneous barrier discharge in nitrogen

A fluid model of the homogeneous barrier discharge is constructed for nitrogen at atmospheric pressure. The primary excitation and ionization processes specific for this discharge are pointed out. The calculations show that, in a wide range of external conditions, the homogeneous barrier discharge in nitrogen has a form of Townsend discharge which is easy to study. The influence of different mechanisms of electron emission from dielectric barriers and surface recombination over the electrical characteristics of a barrier discharge is studied. Introduction of a finite lifetime at the surface for adsorbed electrons allows us to obtain the results qualitatively corresponding to the experimental data.

Diffuse barrier discharges in nitrogen with small admixtures of oxygen: discharge mechanism and transition to the filamentary regime

Diffuse barrier discharges (BDs) are characterized by the periodicity of their discharge current and by the uniform coverage of the entire electrode surface by the plasma. Up to now the discharge development, their appearance and dynamics cannot be adequately explained by elementary processes. Different processes are discussed in the literature controversially, in particular the importance of volume and surface processes on the pre-ionization (Penning-ionization, secondary (?-) processes, role of surface charges). Diffuse BDs in nitrogen with small admixtures of oxygen are investigated by plasma diagnostics (current/voltage-oscillography, optical emission spectroscopy) and numerical modelling. Special attention is paid to the transition to the usual filamentary mode, characterized by the presence of micro-discharges and caused by the admixture of oxygen in the range of 0?1200?ppm (parts-per-million). This transition starts at low values of O2 (about 450?ppm) and is introduced by an oscillative multi-peak mode. At higher admixtures (about 1000?ppm) the micro-discharges are generated. According to the results of numerical modelling, secondary electron emission by N2(A?3?u) metastable states plays a major role in discharge maintenance. Due to the much more effective quenching of these states by O2 and NO than by N2 the subsequent delivery of electrons will be decreased when the oxygen amount is increased.

Effect of the barrier material in a Townsend barrier discharge in nitrogen at atmospheric pressure

A Townsend-like barrier discharge in nitrogen at 7 kHz frequency is studied experimentally and theoretically. The discharge is homogeneous under a certain range of parameters, which depends on the material of the barriers. The higher the dielectric permittivity of barriers is, the narrower is this range. It is shown that the discharge properties do not only depend on the total capacitance of barriers but they also explicitly depend on the permittivity of a dielectric sheath near the surface. Measured ranges of existence of a Townsend discharge agree with the calculations based on a self-consistent model. Also, the two-dimensional simulations of the barrier discharge show that the stability of the discharge relative to radial fluctuations may depend on the permittivity of barriers. The effect of barrier material is interpreted as the influence of dielectric permittivity on the electric field induced by surface charges.

On the stability of a homogeneous barrier discharge in nitrogen relative to radial perturbations

The influence of small radial perturbations of the cathode current on the characteristics of a homogeneous barrier discharge in nitrogen is investigated on the basis of a two-dimensional fluid model. In a Townsend discharge, radial fluctuations are substantially suppressed, which is the evidence of its stability. The oscillative mode of the Townsend discharge is also stable with regard to radial perturbations. As the discharge turns into a form controlled by spatial charge (a streamer is developed), disturbances of all radii grow in time. Such a behaviour testifies the instability of a streamer front and may cause the discharge filamentation. Since only the Townsend discharge is stable, it is possible to use a one-dimensional model to determine the domain of existence for a homogeneous discharge. The study of homogeneity domains by means of the one-dimensional model shows that at relatively large values of the voltage growth rate, discharge gap width, or capacitance of dielectric barriers the discharge tends to be filamentary. Calculation of the domain of the discharge homogeneity agrees satisfactory with the experimental data available as the account is taken for the gas heating.

On the non-local electron kinetics in spatially periodic striation-like fields

The fundamental questions of the formation of the electron distribution function are investigated by numerically solving the Boltzmann kinetic equation in sinusoidal modulated fields for two limiting cases; domination of either elastic or inelastic energy loss in the electron energy balance. The role of non-local effects is demonstrated in the formation of both the distribution function and macroscopic properties, such as electron density and average energy, excitation and ionization rates. The phase shifts of these parameters are considered, which are responsible for the propagation of the ionization wave. Stratification mechanisms differ considerably for these two cases and are determined by the peculiarities of the electron non-local kinetics, whereas the fluid models of striations are inapplicable. Precise calculations are carried out for a discharge in neon for two cases observed in real experiments at low and intermediate pressures and small currents.

On the density of metastable and resonance atoms in a stratified positive column in neon

The densities of metastable and resonance states 33P2 (S5) and 31P1 (S2) are measured by a method of laser absorption in S and P striations in neon at low (1.4 Torr) and intermediate (12.8 Torr) pressures. The theoretical description of excited atoms in striations requires the solution of the nonlocal kinetic equation in a spatially periodic field to obtain the excitation rates, as well as the analysis of both the resonance radiation transport and diffusion equations for excited atoms. The solution of the radiation transport equation is obtained for the case of both the radial and axial (in particular, periodic) inhomogeneities. The densities of excited atoms are calculated in different striation phases taking into account the transport processes, direct intermixing by atomic and electron impact, and intermixing by excitation and radiation of the system 2p53p. Comparison of the theoretical and experimental results is performed for weakly nonlinear striations near a lower bound of existence by pressure and striations at low and intermediate pressures.

On the formation of electron velocity distribution functions in striation-like fields

Peculiarities of the formation of electron distribution functions in spatially periodic striation-like fields in a neon dc glow discharge are analysed on the basis of an accurate solution of the Boltzmann kinetic equation, taking into account elastic and inelastic collisions, as well as spatial gradients. Mean axial electric fields and striation lengths are measured in a wide range of pressures. The transition from the local formation of electron distribution function through the weakly nonlocal case at advanced pressures to the distinctly nonlocal formation at low pressures is demonstrated. The phase shifts between the electron density and rates of excitation and ionization due to nonlocal effects and finiteness of the lifetime of excited atoms are illustrated. Comparison between numerical and approximate analytical solutions is performed for the limiting cases of low and advanced pressures, when the kinetic equation can be solved analytically by means of the small parameter expansion.

Modelling of atmospheric pressure dielectric barrier discharges with emphasis on stability issues

The properties of a barrier discharge in nitrogen near the transition from the Townsend mode to the filamentary mode are studied on the basis of a two-dimensional fluid model. The formation of an intermediate mode (multipeak Townsend discharge) is discussed. The surface processes (ion–electron emission and photoemission) and the positive volume charge are proposed as the mechanisms of formation of a multipeak mode. It is shown that the Townsend mode is stable relative to radial fluctuations, whereas the glow mode is unstable and turns into a filamentary mode. The development of a radial fluctuation into a filament is demonstrated. The homogeneity of the barrier discharge depends on the barrier material. In particular, the widening of the stability region for the discharge with low-permittivity barriers is proved.

Dynamics of gas heating in a pulsed microwave nitrogen discharge at intermediate pressures

The time-resolved measurements of the rotational temperature in a pulsed microwave discharge in nitrogen in the range of pressures from 1 to 100?mbar are performed by means of the spectroscopic study of the rotational structure of the second positive system. The rotational temperature at the steady state ranges from 1000 to 3000?K depending upon the gas pressure. The equality between the rotational and gas temperatures is discussed. The time dependence of the temperature during the pulse is characterized by a fast (100??s) heating time followed by saturation. The steady-state temperature at 100?mbar is lower than at 40?mbar. The results are analysed with the help of the heat balance equation, and the anomalous behaviour of the gas temperature at higher pressures is discussed. As the BaTiO3 pellets are added inside the discharge, the gas temperature is reduced by half, which is caused by more effective gas cooling. Moreover, the pellets make it possible to obtain the discharge at pressures up to the atmospheric pressure.