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.
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 (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 (γ-) 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 O 2 (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 N 2 (A 3 u) metastable states plays a major role in discharge maintenance. Due to the much more effective quenching of these states by O 2 and NO than by N 2 the subsequent delivery of electrons will be decreased when the oxygen amount is increased.
Experimental investigation of the electrical and optical discharge characteristics is performed in the barrier discharge in nitrogen at 50 Hz. The time dependences of the current and the applied voltage are measured. Short-time photographs of the light intensity distribution in the discharge are obtained. When the frequency of the external voltage is low and dielectric barriers made of ‘Mylar’ are used, it is possible to obtain a homogeneous form of the discharge. During the pulse, the discharge current reaches the value of some amperes, and the maximum power is of the order of some tens of kilowatts. The appearance of a homogeneous discharge is interpreted as the widening of a single-electron avalanche due to photoemission. On the basis of a two-dimensional fluid model, the possibility of the initiation of the discharge by a narrow avalanche is demonstrated. It is shown that the observed discharge is a glow discharge, where the space charge plays a crucial role. The discharge passes successively through the Townsend phase, the streamer phase, the phase of radial expansion and, finally, the afterglow phase. The theory predicts that the distribution of the light intensity in the phase of discharge expansion must have the form of a ring widening in time. The properties of the power supply play an important role in the model. A comparison between the theoretical and experimental results shows good agreement.
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.
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