A model for streamer propagation

A model is presented to describe the propagation of positive corona streamers in the low field region of a non-uniform field gap in atmospheric air. It has been assumed that the growth is a property solely of the streamer tip, uninfluenced by the channel conductivity. Calculations from the model indicate that the criterion for propagation of a streamer in zero external field is that the number of ions in the tip be 10⁸ and the radius about 3×10^?3 cm. It is proposed that the streamer ceases to propagate as a result of the loss of energy of the tip due to the formation of ion pairs in the channel. The results of previous experimental observations of streamers are compared with calculations derived from the model, and a prediction from the model of the lifetime of streamers after voltage removal is discussed.     

Computer-generated images of streamer propagation in nitrogen

We present the computer-generated images of streamer propagation in an overvolted parallel plate gap at atmospheric pressure. Two-dimensional colored images of cross sections of streamers and electric field are presented. The results show the filamentary nature of the streamers and the strong field distortions caused by the space charge     

Improved model of streamer propagation in liquid dielectrics

Breakdown and pre‐breakdown cause extensive damage in dielectric oils used in power systems subjected to high voltages. Consequently, several research works are devoted to these phenomena. Related investigations are made very difficult by the numerous influencing parameters and the large number of characteristics of these phenomena. The main goal of the present work was to establish a new electrical model that describes the propagation of streamers in liquid dielectrics for a point‐plane configuration subjected to high‐voltage fields. The novelty consists in the presence of an inductance in the equivalent electrical circuit, which represents the dielectric during the partial discharges characterized by fast variations of current. Criticisms were first made concerning previous electrical models that show some disadvantages. The developed model consists of a series RLC circuit whose element values change during propagation. In particular, the value of the resistance decreases in anticipation of breakdown. The simulation of the developed model gave satisfactory results, similar to experimental ones. For instance, the influence of electrical and geometrical parameters on the current pulses amplitude as well as on the streamers propagation velocity and the quantity of charge and the energy injected were studied. Good agreement was also obtained between the obtained results and experimental ones, which reinforces the validity of the model.     

Experimental investigation of nonlocality effects in the electron energy spectrum in an electronegative gas

The nonlocality of the electron energy distribution function (EEDF) in a dc discharge in oxygen is observed experimentally. A method is developed for measuring the isotropic part of the EEDF in a low-temperature plasma of electronegative gases. The radial dependence of the EEDF and the radial distributions of the electron density, the average electron energy, and the potential are determined. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 511–516 (10 April 1996)     

Single bubble of an electronegative gas in transformer oil in the presence of an electric field

The influence of the electric field on a single air bubble in transformer oil has been studied. It has been shown that, depending on its size, the bubble may initiate breakdown. The sizes of air and sulfur hexafluoride bubbles at which breakdown will not be observed have been estimated based on the condition for the avalanche-to-streamer transition.     

Threshold for a cumulative resonant microwave streamer discharge in a high-pressure gas

Results are reported from experiments aimed at investigating how the structure of a microwave streamer discharge depends on the gas pressure. The formation of a bright core in the streamer channel is shown to be of a threshold nature: in discharges initiated in the field of a standing electromagnetic wave of an open two-mirror cavity, a bright core forms in air and hydrogen in the pressure ranges p ₀≥540±50 torr and p ₀≥740±70 torr, respectively. Estimates are presented, according to which the appearance of a bright core can be attributed to the onset of a local microwave pinch effect.     

Two-dimensional modeling of positive streamer propagation in fluegases 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     

The townsend coefficient and runaway of electrons in electronegative gas

We have studied the character of variation of the number of electrons formed in an electronegative gas (SF₆) under the action of an external electric field. At any value of the electric field strength E, the number of generated electrons exponentially increases with the distance from the cathode, while the average velocity and energy of electrons attain constant values. At small values of the reduced field strength, E/p<94 V/(cm Torr) (p is the gas pressure), the regime of electron attachment prevails that is characterized by negative values of the exponent (negative Townsend coefficients). For E/p>94 V/(cm Torr), the electron multiplication proceeds in the usual Townsend regime with positive exponents. In the intermediate region of E/p=40–160 V/(cm Torr), the electron multiplication coefficient exhibits a linear dependence on E/p. Numerical calculations based on a simple model show that the Townsend multiplication regime takes place even in very strong fields where the drag caused by ionization can be ignored. A universal function describing the electron runaway in SF₆ is obtained.     

Study of plasma—solid interaction in electronegative gas mixtures

Low-temperature reactive plasmas employing electronegative gases are often used in modern technologies. Negative ions in such plasmas affect the transport of charged species and in this way influence the processes in the boundary layer between plasma and surface of metal substrate or probe. The contribution presents results of the computer experiment describing the interaction of electronegative plasma with immersed substrates. The method of solution was the particle simulation technique and several approximations were used; the most important was the simplified geometry of substrates. The simulation is based on experimental data obtained in a dc glow discharge in mixtures of oxygen with rare gases. This work is a part of the research plan MSM0021620834 that is financed by the Ministry of Education of Czech Republic.     

Study of streamer regime for a narrow-gap detector with simple gas inflation

This paper presents the results of a study of the process of gas amplification in a wireless flat cell with a homogeneous electric field with a 1-mm gap between the cathode and anode. It is shown that the transition from proportional to streamer mode of operation of this design occurs when the Reter threshold is reached in a simple two-component gas mixture of the type Ar + 20% CO₂ without freon or hydrocarbon additives. This fact is a decisive argument when choosing a working gas while planning long-term experiments in a closed environment from the ecological point of view.     

Simple analytic relations for estimating parameters of the positive column of an electronegative glow discharge in a halogen-containing gas mixture

Asymptotic analysis of the continuity equations for the positive charge plasma of a low- and moderate-pressure dc electronegative discharge is performed under the simple assumption of independence of the rates of plasma-chemical processes from coordinates. As a novelty, the ion diffusion is taken into account for the case when the ion-ion recombination dominates among the perishing mechanism of negative ions. The resultant simple analytic expressions make it possible to estimate (with an error not exceeding 15–25%) the geometrical parameters of spatial distributions of charged particle concentrations in this plasma, as well as the energy parameters, without resorting to complicated numerical simulation.     

Field reversal in an electronegative plasma containing moving striations

Evidence is presented to show that under some conditions the radial ambipolar field, in a diffuse positive column containing moving striations, reverses near the discharge tube axis.     

斜磁场作用下碰撞电负性等离子体鞘层的玻姆判据

采用流体模型研究了外加斜磁场作用下的碰撞电负性等离子体鞘层的玻姆判据, 讨论了负离子和磁场对鞘层玻姆判据的影响. 研究结果表明: 在电负性鞘层中, 正离子更容易进入鞘层, 负离子的存在只能影响离子马赫数取值的下限; 而外加磁场可以使离子马赫数的取值范围整体发生移动.     

电负性等离子体磁鞘的玻姆判据

采用流体模型研究了电负性等离子体磁鞘的玻姆判据,讨论了负离子以及磁场对鞘层玻姆判据的 影响.研究结果表明:负离子的存在促进离子进入鞘层;而磁场强度、角度以及离子进入鞘层的状态将共同决定 离子马赫数的取值.     

Evolution of the density profiles and flows of charged particles during the diffusive decay of an electronegative gas plasma

Different scenarios of the spatiotemporal evolution of the parameters of the diffusive decay of a pulsed electronegative gas plasma in the absence of plasma chemical processes are studied. It is shown that nonlinear diffusion in a plasma with negative ions occurs in several stages. The rate of electron density decay increases with time and, in the beginning of the second stage, almost all the electrons escape from the discharge volume. On the other hand, the ion density profile is smoothed out due to ion-ion ambipolar diffusion and the flow of negative ions toward the wall is absent in the first stage of decay. In the second stage, the main diffusion mode is first established and then the ion-ion (electronless) plasma decays exponentially with a characteristic time determined by ion-ion ambipolar diffusion.     

Breakdown threshold in the microwave field at low and high pressures in electronegative gas mixtures

The dependence of the critical value of the microwave field amplitude in electronegative mixtures, which corresponds to the local equality of the ionization frequency and the dissociation attachment frequency, is investigated theoretically in a wide range of pressures with allowance for electron temperature pulsations. The resultant values are compared with the critical value for the static field. The range of pressures in which electron temperature pulsations become significant is determined, and the excess of the critical amplitude over the value in the static field is calculated in the high-pressure limit. Simple formulas are recommended for estimating the critical value of the microwave field amplitude.     

Resonant propagation of coherent pulses in a gas with Doppler broadening

A perturbation treatment is given for resonant propagation of optical pulses in a gas with Doppler broadening. We develop the dipole moment into infinite series and, by coupling this expression to Maxwell's equations, the results for losses, delay and reshaping of the pulses are derived. The perturbation series converges, in the broad line case, for intensities which fall between the region of Beer's law and somewhat beyond the intensity region which gives maximum delay. The theoretical results are compared with experimental data and good agreement is obtained.     

General solution of a system of equations for acoustic wave propagation in a partially ionized gas in an external electric field

In this paper a system of equations describing the propagation of acoustic waves in multicomponent partially ionized plasma in an external electric field is solved generally, when these waves are generated by an external neutral sound wave in plasma. The equations are solved by means of Laplace transformation. It has been found that a sound wave generates a group of waves with different frequencies and wave vectors. For the sake of comparison, the solution without an external field and the linear solution in the form of waves having the same frequencies as the frequency of the excitation wave are given.     

水中流光放电流光丝的再发光和暂停行为

水中流光放电是研究水中放电基本物理、化学过程的主要研究对象.本文利用四分幅超高速相机、采用针-板电极结构、在20—800μS/cm水电导率范围内研究了水中微秒脉冲流光放电流光丝的再发光和暂停行为,探讨了高水电导率下观测不到流光丝的再发光的原因.结果发现:再发光在不同的流光丝之间交替发生并存在两种模式:一种为整根丝熄灭后再发光;一种为只有先端部分发光熄灭随后恢复发光.随着水电导率的增大,观测到流光丝的再发光现象的频度急剧减小, 540μS/cm水电导率时降到零;在20—800μS/cm水电导率条件下都可观测到流光丝伴生冲击波串分段现象,冲击波串分段现象的出现频度在65%以上,表明在20—800μS/cm水电导率条件下流光丝的暂停是一种普遍行为.通过测量两段冲击波的半径差得到流光的暂停时间平均为157 ns,几乎不受水电导率的影响;随着水电导率的增大,流光丝的发光强度显著增大,水电导率大于350μS/cm时,流光丝暂停期间内流光丝的光强度无法衰减到相机分辨水平以下,在相机获得的发光图像上看上去是持续发光的,难以分辨出流光丝熄灭-再发光过程.