Mechanism for propagation of a positive leader

A model of leader breakdown in air is considered. The channel is formed due to heating of the streamer trace in the field of the streamer zone. A previous model of a streamer is generalized with allowance for recombination of charged particles. A mathematical model of heating of the streamer trace is developed. It is demonstrated that, at a given potential, the ignition of the channel is provided by streamers that possess a certain charge and the corresponding propagation velocity. This velocity determines the propagation velocity of a steady leader. The dependence of the leader velocity on the cloud potential is found. The results obtained are compared with the data from in-situ observations and laboratory studies.     

The lifetime of positive streamers in a pulsed point-to-plane gap in atmospheric air

A determination has been made of the lifetime of positive streamers produced in atmospheric air in a positive point-to-plane gap to which was applied a 40 nsec, 40 kV voltage pulse. The streamer tips have been detected at a given plane in the gap by means of a photomultiplier and high speed oscilloscope. It was found that the streamer tips continued to propagate in the gap, long (up to 35 nsec) after the voltage pulse was removed, in accordance with the predictions of a recently presented model of streamer propagation. Association of this long lifetime with the properties of an isolated tip was made possible by the detection of a new phenomenon at the anode. At the end of the voltage pulse a second luminosity was observed to leave the anode and to extend along the paths of the primary streamers for about one third of their length. This is shown to be the maximum extent of a more highly conducting trunk, behind and separated from the streamer tip, affecting both the propagation of the tip and the eventual transition of a streamer to a breakdown spark.     

Die ionisierenden Potentialwellen beim Funkendurchschlag

Extending earlier studies on spark breakdown, the development of the impulse prebreakdown streamers leading to complete breakdown were investigated. By taking advantage of the proportionality between point potential and length of the positive Lichtenberg figure produced, the potential of the streamer tips progressing from a point anode towards a plane cathode were determined. It was found that the streamer tip potential decreases as streamer advances. At voltages much lower than sparking potential this decrease is rapid. By approaching the sparking potential the loss of tip potential diminishes considerably near the anode. The decrease of potential remains very rapid near the cathode until sparking potential is reached. At the cathode the tip potential increases with the applied potential. At a certain value electrons are emitted from the cathode under the influence of high tip fields. These electrons are accelerated towards the tips forming avalanches by collision. These avalanches appear on the photographic film as minute dots whose number and density depend on the intensity of tip fields. If the applied potential exceeds the sparking voltage, tip potentials are recorded on the cathode between 15 and 25 kV. The avalanches formed in the high intensity field region reach the critical size and form negative, anode — directed streamers. These streamers appear on the film as negative Lichtenberg figures. This ionising potential wave, known as return stroke, travels along the channel of the positive primary streamer. It increases its ionisation to such a degree that the main stroke can take place.     

Study of streamer development in high-pressure electric discharge: Applications to excimer lasers

This work represents a study of the streamer formation in plasma for XeCl excimer laser at high pressure. It is based on a longitudinal mono-dimensional model of the cathodic zone. In this model, we show the possibility of the streamer development in the cathodic sheath and its propagation during the phase of plasma formation. The model gives the space and time evolution of the electron density and the discharge electric field in the presence of the streamer. The obtained results clearly indicate that, for conditions close to experiments for 50–100 ns laser pulse durations and electron power deposition in the MW/cm³ range in a 300 cm³ chamber, the streamer instability, related to the sheath evolution, patently appears. The drift velocity reaches a typical value of about 10⁸ cm/s.     

Vorstadium des Funkens,untersucht mit dem Bildverstärker

Streamer development (near cathode, midgap, and near anode streamers) was investigated by means of image intensification and image deflection as to conditions for streamer onset and streamer velocities during the various stages of the avalanche-to-plasma channel transition in several gases, e.g. N₂, H₂, A, CH₄ as well as air, and some more mixtures of gases, in the pressure range 100 to 500 Torr. Quantitative results were obtained, and the influence of the discharge parameters upon streamer development is discussed. — The enhanced and accelerated streamer stage (II) was now observed in either direction; the significantly fast and steep ionization waves (stage III) were found to start at the cathode as well as at the anode on arrival of the respective streamers. Velocities up to 10⁹ cm/sec were measured.     

脉冲放电产生螺旋流注的等离子体特性研究

通过脉冲放电方式产生三维螺旋形的等离子体放电通道,在高速摄像机拍摄下观察到放电通道中的发光电离体以流注形式沿螺旋轨迹快速传播.建立电磁模型解释螺旋放电的形成机制,对造成对称性破缺及影响其手性性质的极向电场进行分析.研究表明,螺旋放电产生的放电通道存在两种不同的手性特征,而脉冲重复频率等放电参数及边界条件对螺旋流注的传播特性存在影响.脉冲电源驱动的电磁场在介质管内所形成的波模是极向电场形成的一个重要来源,当极向电场与轴向电场强度相近时则形成螺旋流注放电.     

Guided ionization waves: Theory and experiments

This review focuses on one of the fundamental phenomena that occur upon application of sufficiently strong electric fields to gases, namely the formation and propagation of ionization waves–streamers. The dynamics of streamers is controlled by strongly nonlinear coupling, in localized streamer tip regions, between enhanced (due to charge separation) electric field and ionization and transport of charged species in the enhanced field. Streamers appear in nature (as initial stages of sparks and lightning, as huge structures—sprites above thunderclouds), and are also found in numerous technological applications of electrical discharges. Here we discuss the fundamental physics of the guided streamer-like structures—plasma bullets which are produced in cold atmospheric-pressure plasma jets. Plasma bullets are guided ionization waves moving in a thin column of a jet of plasma forming gases (e.g., He or Ar) expanding into ambient air. In contrast to streamers in a free (unbounded) space that propagate in a stochastic manner and often branch, guided ionization waves are repetitive and highly-reproducible and propagate along the same path—the jet axis. This property of guided streamers, in comparison with streamers in a free space, enables many advanced time-resolved experimental studies of ionization waves with nanosecond precision. In particular, experimental studies on manipulation of streamers by external electric fields and streamer interactions are critically examined. This review also introduces the basic theories and recent advances on the experimental and computational studies of guided streamers, in particular related to the propagation dynamics of ionization waves and the various parameters of relevance to plasma streamers. This knowledge is very useful to optimize the efficacy of applications of plasma streamer discharges in various fields ranging from health care and medicine to materials science and nanotechnology.     

Probe Measurements of Parameters of Streamers of Nanosecond Frequency Crown Discharge

Investigations of the parameters of single streamers of nanosecond frequency corona discharge, creating a voluminous low-temperature plasma in extended coaxial electrode systems, are performed. Measurements of the parameters of streamers were made by an isolated probe situated on the outer grounded electrode. Streamers were generated under the action of voltage pulses with a front of 50–300 ns, duration of 100–600 ns, and amplitude up to 100 kV at the frequency of 50–1000 Hz. The pulse voltage, the total current of the corona, current per probe, and glow in the discharge gap were recorded in the experiments. It was established that, at these parameters of pulse voltage, streamers propagate at an average strength of the electric field of 4–10 kV/cm. Increasing the pulse amplitude leads to an increase in the number of streamers hitting the probe, an increase in the average charge of the head of a streamer, and, as a consequence, an increase in the total streamer current and the energy introduced into the gas. In the intervals up to 3 cm, streamer breakdown at an average field strength of 5–10 kV/cm is possible. In longer intervals, during the buildup of voltage after generation of the main pulse, RF breakdown is observed at Е_av ≈ 4 kV/cm.     

Pulsed corona discharge driven by Marx generator: Diagnostics and optimization for NOx treatment

A compact, repetitive Marx generator with an external trigger is constructed and coupled with a wire-to-plate corona reactor for a positive pulsed corona discharge studies. The reactor resistance and capacitance behavior during the pulse was observed. It was found that the reactor's capacitance increases three times during the pulse due to the streamer propagation from anode to grounded electrode. Using the time development of the capacitance and resistance during the pulse and the reactor inter-electrode distance, the streamer velocity has been calculated to be 1 × 10⁶ m/s, for system arrangement presented in this work. As an indicator of chemical activity of pulsed corona, ozone production was measured. Emission spectroscopy measurements in the UV region were performed to detect species that appear in the discharge and to determine vibrational and rotational temperatures, which are found to be 3200 K and 340 K respectively. As a measure of pollution control potential of the constructed pulsed corona system, NO oxidation efficiency was investigated and compared with results presented in literature. It was shown that pulsed corona systems with significantly longer pulse durations are competitive with several times shorter pulse duration systems, which implies that chemical efficiency of secondary streamers is comparable with efficiency of primary streamers.     

A new static calculation of the streamer region for long spark gaps

Different electrostatic approximations have been proposed to calculate the streamer region without going in deep details of the behavior of density of particles under the effect of high electric fields; this kind of approximations have been used in numerical calculations of long spark gaps and lightning attachment. The simplifications of the streamer region are achieved by considering it to be a geometrical region with a constant geometrical shape. Different geometrical shapes have been used, such as cones or several parallel filaments. Afterward, to simplify the procedures, the streamer region was approximated by two constants, one denoted K_Q, called the geometrical constant and in other cases K named as geometrical factor.However, when a voltage that varies with time is applied to an arrangement of electrodes (high voltage and grounded electrodes), the background electric field will change with time. Thus, if the background electric field is modified, the streamer zone could cover a larger or smaller area.With the aim of reducing the number of assumptions required in the calculation of long gap discharges, a new electrostatic model to calculate the streamer region is presented. This model considers a variable streamer zone that changes with the electric field variations. The three-dimensional region that fulfills the minimum electric field to sustain a streamer is identified for each time step, and the charge accumulated in that region is then calculated. The only parameter that is being used in the calculation is the minimum electric field necessary for the propagation of streamers.     

Die Entwicklung der Elektronenlawine in den Plasmakanal,untersucht mit Bildverstärker und Wischverschluß

The application of high gain image intensifier and image converter streak shutter techniques to investigations of faintly luminous early stages of gaseous discharges discloses in some detail the development of electron avalanches into anode- and cathode-directed streamers as soon as the critical amplification (n≈10⁸) is attained in the gap. The velocities of both of the streamer branches were measured quantitatively as to the dependence on pressure (p), field (E/p) and gas (Nitrogen and various mixtures), typical values being 0.5 ... 1 · 10⁸ cm/sec. We find that streamer propagation is suitably described in a coordinate-system drifting with the electrons (driftvelocityv _? ≈ 1 · 10⁷ cm/sec) and that the way it depends on the discharge parameters suggests that ionization by electron impact (in the eigen-spacecharge distorted field) and photoionization are the determining effects. — The cathode-directed streamers show a considerable intensification and acceleration as soon as they are encountered by cathodic secondary avalanches, which however are obviously not essential for the initiation of the process in itself. — The streak images give also an impression of persisting excitation and ionization processes in the region of the streamer-paths and of the transition into the beginning of the bright spark, correlated with several ionizing fronts observed moving towards anode or cathode with velocities even >10⁸ cm/sec. (The paper includes also details of the gaseous near UV-radiation which was utilized for registration with image intensifier or converter, especially with respect to the decaytime of the excited states.) The results obtained are essentially a verification and to some extend completion of predictions made on the basis of early cloud chamber investigations.     

Impulsive-discharge formation in water

This paper reports on research on an impulsive electrical discharge in deionized water (=2 × 10⁴ · m) in the voltage range 17–50 kV. The research showed that the discharge proceeds through the development of subsonic (U=17–22 kV) or supersonic (U=25–50 kV) streamers, depending on the electric-field strength at the surface of the tip. When voltage impulses having an amplitude of 20–25 kV are unleashed, the development of both types of streamers is observed in the discharge gap. The experiments carried out by the authors do not confirm the hydrodynamic origin of the disturbances at the initiating electrode that give rise to the discharge's development. The authors hypothesize that streamer development is due to the ionization of water molecules. The energy expenditures to ionize the molecules in the streamer channel are estimated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 88–92, January, 1989.     

Die weitere Entwicklung der Elektronenlawine,untersucht mit Bildverstärker und Wischverschluß

By simultaneously applying image intensifier and image converter streak shutter techniques it is possible to investigate the local and temporal development of electron avalanches in a homogeneous field into the weakly ionised streamer channels. Velocities of the anode-directed and cathode-directed streamers are measured (order of cm magnitude: several 10⁷ and approximately 10⁸sec/cm respectively). Persisting processes in the plasmachannels are revealed and it is shown that once the electrodes are bridged by the streamer-channels the high intensity spark starts rapidly.     

Step propagation of a streamer in an electronegative gas

It is shown on the basis of numerical simulation and an experimental investigation that a streamer can propagate in a step manner in an electronegative gas. The experiments and most calculations were performed for air under close to normal conditions. The step motion is associated with the appearance of a secondary ionization wave near the electrode and propagation of this wave along the channel of the streamer; this wave maintains the channel in a conducting state and allows for the propagation of the streamer in a nonuniform external field over distances which are inaccessible under ordinary conditions of a streamer discharge. Simulation in heated air, oxygen, and SF₆ demonstrated that the phenomenon studied is common for various gases and that the special features of its manifestation remain in a wide range of decay rates of the streamer channel.     

Production of chemically active species in the air by a singlepositive streamer in a nonuniform field

We report the results of a two-dimensional (2-D) simulation of a cylindrically symmetric positive streamer in atmospheric pressure air between a positively stressed point and a grounded plate. A hydrodynamic diffusion drift model was used. It is shown that a streamer propagates with the constant velocity and constant field at the tip (≃140 kV/cm) and constant electron density in the head (≃10¹⁴ cm^-3). Special attention has been paid to the generation of excited molecules, radicals, and atoms [chemically active species (CAS)] by the streamer, 2-D distributions of CAS are obtained. The total number of CAS linearly increases with time (except excited oxygen molecules) in accordance with the constant velocity of streamer propagation. The mean energy cost of each active specie is 35 eV     

一次人工触发闪电上行正先导的传输特征

利用山东一次人工触发闪电的高速摄像和30 m, 60 m和480 m的同步电场测量, 清晰显示了人工触发闪电上行正先导的传输特征.上行正先导头部光强相对较强, 二维发展速度变化波动较大并呈现明显不规则性,表明上行正先导发展具有明显的梯级特征. 在先导开始阶段从340 m到705 m高度之间上行正先导平均发展速度为9.8×10⁴ m/s,起始速度是 3.8×10⁴ m/s,局部速度总体上随高度呈现增加趋势.电场变化在近距离产生有规律的梯级状 变化,记录到的28个梯级相邻梯级间隔变化从14 μs 到 39 μs,几何平均值为25.1 μs. 估计的先导梯级长度分布在0.9 m到3.7 m,几何平均值为1.7 m.先导电场变化由慢的正向梯级状变化和 脉冲变化组成,结合光学和电场变化测量结果,得出正先导头部通道发生弯曲可使其电场变化的梯级特征减弱 或消失;正负先导梯级形成机制可能类似,均由其先导头部前端的双向流光发展而来.     

基于VHF辐射源短基线定位系统对闪电放电过程的研究

利用闪电VHF辐射源短基线定位系统2009年东北大兴安岭地区的观测资料,对正、负地闪和云闪放电过程中的VHF辐射源进行了定位研究.根据闪电VHF辐射源的二维位置(方位角和仰角)随时间的演变特征,结合同步观测的快、慢电场变化资料分析发现,持续时间较长的地闪预击穿过程在云中的放电通道呈双层结构,预击穿过程结束阶段的云内放电表现为反冲流光,预击穿过程的平均速度在10⁴m/s量级.预击穿过程为梯级先导的发展提供了必要的条件,梯级先导从预击穿起始位置开始并向下发展,产生较强的辐射,平均速度在10⁵m/s量级.K过程主要是流光沿之前已电离通道的传输.正、负地闪回击阶段前后的放电特征有明显不同,正地闪回击之后,连续电流期间的较长时间的云内放电产生较强的VHF辐射.对闪电在125---200MHz频段范围的VHF辐射频谱特征的统计分析发现,辐射能量呈现出幅值随频率增加而减小的趋势,在通带范围上基本遵循f^-2.9的衰减率递减.     

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     

Streamer radius model and its assessment using two-dimensionalmodels

The variable radius method is proposed to approximate the radius of the ionization channel in the one and a half-dimensional (1.5-D) fluid models for studying streamer development, in which the unreasonable constant radius in the traditional 1.5-D fluid models is corrected. The streamer development and propagation between the 1.5-D fluid model with the variable radius method and the two-dimensional (2-D) fluid model using the same initial and external conditions are compared. The radius in each stage of streamer development from the 1.5-D fluid model with the variable radius method shows agreement to a certain degree with that of the 2-D fluid model. The purpose of this paper is not to negate the role of the 2-D fluid models, but to explore the potential of the 1.5-D fluid models and make them more useful and accurate as well as to understand the evolution of streamer radius. The streamer development from the 1.5-D fluid model with the variable radius method not only maintains simplicity of the 1.5-D fluid models, but also presents agreement with the 2-D fluid models for streamers     

The velocity of streamer tips in impulse point-to-plane corona in air,using Lichtenberg figure techniques

Positive and negative streamer lengths were measured in point-to-plane impulse corona in air at atmospheric pressure using Lichtenberg figure technique. Applying short time pulses from 9 to 40 nsec duration time-distance plots were obtained, which allowed one to determine the streamer tip velocity. For a 4 cm gap, a 1 mm diameter point, and a 30 kV pulse an average tip velocity of 1.8×10⁸ cm/sec was found. Comparing these data withHudson's photomultiplier measurement, we may identify the Lichtenberg figure with the “primary” streamer ofHudson. The negative streamer tip velocity in the cathode part of the gap was approximately an order of magnitude smaller than the positive streamer velocity.