Yu B Golubovskii

Transport mechanisms of metastable and resonance atoms in a gas discharge plasma

Atoms in electronically excited states are of significant importance in a large number of different gas discharges. The spatio-temporal distribution particularly of the lower excited states, the metastable and resonance ones, influences the overall behavior of the plasma because of their role in the ionization and energy budget. This article is a review of the theoretical and experimental studies on the spatial formation and temporal evolution of metastable and resonance atoms in weakly ionized low-temperature plasmas. Therefore, the transport mechanisms due to collisional diffusion and resonance radiation are compared step by step. The differences in formation of spatio-temporal structures of metastable and resonance atoms in plasmas are attributed to these different transport mechanisms. The analysis is performed by obtaining solutions of the diffusion and radiation transport equations. Solutions of stationary and non-stationary problems by decomposition over the eigenfunctions of the corresponding operators showed that there is, on the one hand, an effective suppression of the highest diffusion modes and, on the other hand, a survival of the highest radiation modes. The role of the highest modes is illustrated by examples. In addition, the differences in the Green functions for the diffusion and radiation transport operators are discussed. Numerical methods for the simultaneous solution of the balance equations for metastable and resonance atoms are proposed. The radiation transport calculations consider large absorption coefficients according to the Lorentz contour of a spectral line. Measurements of the distributions of metastable and resonance atoms are reviewed for a larger number of discharge conditions, i.e. in the positive column plasma, afterglow plasma, constricted pulsed discharge, stratified discharge, magnetron discharge, and in a discharge with a cathode spot.

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 NONLOCAL ELECTRON KINETICS IN S- AND P-STRIATIONS OF DC GLOW DISCHARGE PLASMAS: I. ELECTRON ESTABLISHMENT IN STRIATION-LIKE FIELDS

The nonlocal behavior of the electrons in strongly modulated and period-averaged electric fields typical of s- and p-striations in neon glow discharge plasmas is investigated by numerically solving the axially inhomogeneous electron Boltzmann equation. A good agreement between the period lengths measured in the striations and those obtained from the spatially periodic electron relaxation in the period-averaged field of the striations is found confirming the close relation of both phenomena. The s- and p-striations represent the fundamental and first harmonics of the inherent periodic electron relaxation. Furthermore, starting from different boundary conditions the establishment of the velocity distribution function and of selected macroscopic quantities of the electrons into unique periodic states under the action of strongly modulated striation-like fields is investigated. It is shown that the same damping processes that cause in homogeneous fields a relaxation into homogeneous states lead to unique periodic states in strongly modulated fields.

About the EDF formation in a capacitively coupled argon plasma

The formation of the electron distribution function (EDF) in the bulk plasma of a capacitively coupled radio-frequency (rf) discharge in argon generated in the plasma-chemical reactor PULVA-INP is investigated experimentally and theoretically. Measurements of the EDF and internal plasma parameters were performed by means of a Langmuir probe at pressures of 0.5–100 Pa and discharge powers of 5–100 W. The observed EDFs have revealed a two-temperature behaviour at low pressures and evolved into a Maxwellian distribution at high gas pressures and large discharge powers. Theoretical determination of the EDF is based on the numerical solution of the Boltzmann kinetic equation in the local and non-local approaches under experimental conditions. The model includes elastic and inelastic electron–atom collisions and electron–electron interactions. Low electron temperatures and relatively high ionization degrees are the features of the PULVA-INP rf discharge. This leads to significant influence of the electron–electron collisions on the EDF formation. The modelled and measured distributions show good agreement in a wide range of discharge parameters, except for a range of low gas pressures, where the stochastic electron heating is intense. Additionally, mechanisms of the EDF formation in the dc and rf discharge were compared under similar discharge conditions.

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.

Radial behaviour of the electron energy distribution function in the cylindrical magnetron discharge in argon

The cylindrical magnetron consists of a coaxial inner cathode and an outer anode. The magnetic field is applied in the axial direction and is almost homogeneous in the whole magnetron volume. The electric field has radial direction and therefore the charged particles in the cylindrical magnetron discharge move under the influence of the × field. Due to its comparatively simple geometry, the cylindrical magnetron represents a suitable experimental tool that can be used to confirm theoretical results of modelling and theoretical studies of magnetrons in general. We studied experimentally the radial behaviour of the electron velocity distribution function (EVDF) in a cylindrical magnetron discharge in argon. We checked experimentally the anisotropy of the EVDF due to the influence of the magnetic field. For the assessment of the anisotropy of the EVDF we used a planar probe, whose collecting surface was adjustable at different angles to the direction of the magnetic field in the plane perpendicular to the electric field, as well as being movable in the radial direction. We found that in the measurable range of electron energies (energies greater than approximately 2 eV) and at magnetic fields up to 40 mT the anisotropy of the EVDF is not detectable within the experimental error limits. Therefore, for the study of the radial behaviour of the EVDF we used the thin (42 µm in diameter) tungsten cylindrical probe that was movable in the radial direction by a precise screw. For the theoretical determination of the EVDF in the cylindrical magnetron discharge we solved numerically the Boltzmann equation in a crossed × field, assuming the usual simplifications. The results of the calculation and the experiment in argon are compared and discussed.

Contraction of the positive column of discharges in noble gases

This review describes the experimental studies of contraction in neon, argon and helium, discussing the basic regularities of the phenomenon. These studies, extended over a long time, are still urgent. For pressures that are not too high a noticeable contraction of the plasma glow and a smooth non-monotonic dependence of the degree of contraction on the current are observed. Above a critical pressure the plasma immediately contracts into a bright thin cord, if the current reaches a critical value. A hysteresis phenomenon is observed during the transition from the diffuse state to the contracted state and vice versa. Experiments that show the secondary role of non-homogeneous gas heating for contraction in neon and argon, and the main role for contraction in helium, are described. Studies of the ionization waves (the strata), which propagate as pulses of the current cord area, are reviewed showing the close relationship between contraction and stratification. The roles of various mechanisms leading to the contraction and describing the general picture of the observed phenomena are analysed. For heavy noble gases the main role is played by ionization non-linearity as a function of electron concentration, which is related to the competition of electron–atom and electron–electron collisions. This non-linearity leads to plasma shrinkage and the development of ionization instability in the radial (contraction) and longitudinal (stratification) directions. For helium such non-linearity does not play a leading role, since the frequency of the elastic electron–atom collisions is considered to be constant over a large energy range, and this yields a Maxwellian distribution function. The contraction in helium is defined by thermal effects. In addition, recent studies on the numerical modelling of the contraction are discussed.