Recoil leader formation and development

The existing interpretation in the lightning literature, based on field measurements, defines recoil leaders as negative leaders. However recoil leaders are floating conductors, and, based on this physical assumption, they should be defined as bipolar and bidirectional leaders. This physics-based assumption has never previously been verified experimentally. Such verification, reported in this paper, has been obtained from observations of branched upward positive leaders from a tall tower using a high-speed video system synchronized with electric and magnetic field change and luminosity measurements on the ground. The analysis of these observations clearly reveals the nature of recoil and dart leaders as bidirectional and bipolar electrodeless discharges that develop from a small region along a path of the decayed channels of a previous positive leader, or a positively charged return stroke of negative CG flashes.     

Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse. II. Modeling

We have used the Bondio-Gallimberti model of positive leader propagation to simulate laboratory experiments of laser triggering and guiding of upward leaders initiated from a ground rod. The model proves to be capable of reproducing all the important features of laser-guided leader propagation that have been observed experimentally. The leader guiding effect of the laser-created plasma channel is taken into account in the model by adjusting the value of the charge per unit length of the leader, which has been measured in the laboratory to be lower for a laser-guided leader than for an ordinary one. The charge per unit length of the leader is related in the model to the critical temperature at which the air in the transition region at the leader tip must be heated to be conductive enough to become a new leader portion. For an ordinary leader, this critical temperature is 1500 K, at which the electrons all detach from the negative ions in the leader corona, increasing the air conductivity. We give the interpretation that in the case of the laser-guided leaders, because of the relatively high density of negative ions per unit length in the laser-ionized channel, the right conditions of conductivity can be met in the transition region without the electrons being all detached from the ions, allowing a reduction of the critical temperature and of the charge per unit length.     

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.     

On the nature of bipolar flashes that share the same channel to ground

The paper examines the relationship between sequential events in bipolar flashes of two groups sharing the same channel to ground: one initiated by an upward positive leader, the other by a natural flash. Two quite different mechanisms are involved in the occurrence of the bipolar flashes of each group: The bipolar events that start as a natural flash are results of recoil leaders sharing parts of the initial branching structure; the bipolar events that start as upward positive leaders are the result of the interaction between these leaders' initial branching structures and the branches of the intracloud flashes they triggered.     

Observations and modeling of lightning leaders

The development of atmospheric lightning is initiated by a ‘leader’ phase during which ionized channels appear in virgin air. The use of rapid cameras, the measure of fields and currents associated with the discharge allow one to compare the propagation of laboratory leaders with those of natural or artificially triggered lightning. The corresponding physical processes can be analyzed with the help of models developed for laboratory leaders provided that the non linear effects due to the intense current circulation leading to lightning leader thermalization are taken into account. A self-coherent simulation of triggered lightning leaders for both polarities is presented is this paper. Furthermore, these models make it possible to define the ‘stabilization field’ concept, equal to the minimum ambient field allowing the stable progress of a leader from a ground structure, expressed as a height and curvature function of this structure. This concept can be validated through triggered lightning tests. Finally, the stabilization field analysis is completed by a simplified analytical model based upon an electrostatic approach of propagation equilibrium. To cite this article: P. Lalande et al., C. R. Physique 3 (2002) 1375–1392.     

Identification of lightning vulnerability points on complex grounded structures

The identification of the most vulnerable points on a given structure to be struck by lightning is an important issue on the design of a reliable lightning protection system. Traditionally, these lightning strike points are identified using the rolling sphere method, through an empirical correlation with the prospective peak return stroke current. However, field observations in Kuala Lumpur and Singapore have shown that the points where lightning flashes strike buildings also depend on the height and geometry of the structure. Since a lightning strike point is believed to be the place on a grounded structure where a propagating upward leader is first initiated, a physical leader inception model is used here to estimate the background electric field required to initiate a stable upward leader from the corners of some complex buildings. The computed location of the points from where leaders are incepted are compared with the damaged points on buildings struck by lightning. The observed lightning strike points coincide rather well with the corners of the buildings which are characterized by lower leader inception electric fields. Furthermore, it is found that the geometry of the buildings significantly influences the conditions necessary to initiate upward leaders and, therefore, the location of the most likely strike points.     

The electromagnetic mechanism of plasmon decay to a neutrino pair in a strongly magnetized electron gas

An expression for the neutrino luminosity of a degenerate electron gas in a strong magnetic field via plasmon decay to a neutrino pair due to electromagnetic neutrino moments is derived. The neutrino luminosity of the medium in an electromagnetic reaction channel is shown to be comparable with the luminosity in a weak channel. The relative upper bounds for the effective magnetic neutrino moment are obtained.     

闪电双向先导理论及观测:极性不对称、不稳定及间歇性

闪电双向先导原理的提出及观测验证是闪电物理研究近几十年最重要的进展之一,而正、负先导及流光的极性不对称性及传输的持续、间歇性,是理解闪电各种过程物理机制的关键.本文对闪电双向先导的概念及进展进行了总结和讨论,重点强调了正、负先导流光传输机制特别是门限电场的不对称性,阐述了正先导传输的连续性及其在闪电始发、负先导的空间先导形成、不稳定先导通道中的反冲先导建立等过程中独特的启动作用.     

Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse. I. Experimental results

Using a plasma channel produced by an ultrashort laser pulse, we have studied the laser triggering and guiding of a positive leader from the tip of a 2-m vertical rod standing on the bottom plane of a 7-m plane-plane gap. The purpose of this setup was to reproduce in the laboratory the electric field conditions leading to the onset of a positive upward leader from a ground rod as a downward negative leader is approaching during a thunderstorm, in order to demonstrate the working principle of a possible future laser lightning rod. The leader triggering properties of the laser-created plasma channel have been studied as a function of the synchronization of the laser pulse with the voltage impulse applied to the gap. We show that the laser pulse reduces the inception voltage of the leader compared to its normal value and that the laser plasma channel guides the propagation of the upward leader at a velocity ten times higher than that of an ordinary leader, with a significantly lower charge per unit length. We show that laser guiding of the leader significantly reduces the breakdown voltage of the gap and that the effect of the laser channel at the end of a lightning rod can be compared quite favorably with the effect of an additional metal rod of the same length.     

Weak and electromagnetic mechanisms of neutrino-pair photoproduction in a strongly magnetized electron gas

Expressions for the power of neutrino radiation from a degenerate electron gas in a strong magnetic field are derived for the case of neutrino-pair photoproduction via the weak and electromagnetic interaction mechanisms (it is assumed that the neutrino possesses electromagnetic form factors). It is shown that the neutrino luminosity of a medium in the electromagnetic reaction channel may exceed substantially the luminosity in the weak channel. Relative upper bounds on the effective neutrino magnetic moment are obtained.     

Main stage of the lightning discharge: Mechanism and output characteristics

Physical processes responsible for the lightning discharge are considered. An expression relating the basic physical quantities describing the evolution of the lightning discharge to the magnitude and waveform of the lightning current is found. It is shown that the peak lightning current is proportional to the lightning channel length, the steepness of the current pulse depends on the physical constants of the spark discharge, and the current pulse duration depends on the physical constants of the spark discharge and also on the leader’s space charge density.     

Identification of striking distance considering the velocity ratio and competition among the leaders

The striking distance is an important parameter to design a reliable lightning protection system. To protect an object against direct strikes, an improved leader progression model combined fractal geometry and the leader initiation criteria proposed by M. Becerra is introduced to determine the striking distance on a given structure. Both the deterministic and stochastic features of the lightning leader are considered and more detailed three-dimensional numerical simulation of the leader discharge is achieved due to the application of extended grid. Some of the basic methods and strategies are outlined and the influence on protection zone caused by leader velocity ratio is analyzed. Moreover, a comparative analysis of the results obtained through the improved model, the rolling sphere method and the Cooray model is performed. The results indicate that the presented method is closer to the natural lightning and the selection of the breakdown point is actually a complex competitive process among numerous upward leaders.     

Shielding failure rate calculation by means of downward and upward lightning leader movement models: Effect of environmental conditions

In this paper the effects of environmental conditions on shielding failure rate (SFR) of transmission lines are investigated. The study utilizes a previously published work in which leader progression model for lightning upward and downward leaders are used to calculate the SFR. Taking into account the effects of reduced air density and humidity on the parameters of upward leader model and wind pressure on the movement of lightning leaders and wires, SFR and maximum lightning stroke current causing shielding failure are computed. The electric field in all simulations is calculated by means of charge simulation method. The results of simulation show that the effects of relative air density and height of installation are quite higher than that of the wind pressure and humidity while the humidity has the lowest impact on the SFR of investigated transmission line.     

闪电的人工触发和研究

介绍了人工引发闪电技术的发展历史和原理,以及近几年来我们利用这一技术在闪电物理研究方面取得进展。给出了人工引发闪电民流和高速摄像同步测量结果,并分析了各电流分量及其涉及的物理过程。在对比人工引发闪电与自然闪电异同的基础上,批出它的各方面都与从高建筑物始发的上行雷电十分相似。人工引发闪电在研究雷击机理、雷击效应、雷电防护以及电波传播、植物诱变育种等方面也具有广泛的应用前景。     

Inflections of spark leaders in elevated-pressure nanosecond gas discharges

The formation of spark leaders and a diffuse discharge due to the breakdown of gaps with a non-uniform electric field is studied in different elevated-pressure gases under the conditions of runaway electron beam and X-ray generation. Negative voltage pulses are applied to an electrode with a small radius of curvature. For pulse rise times of 0.5 and 1.5 ns, inflections on the spark leaders were observed. In a number of cases, two inflections appear making a right angle to the longitudinal axes of the leaders. It is shown that the formation of spark leaders is preceded by the appearance of diffuse tracks, which bridge the gap for no longer than 1 ns.     

Fast simulation of the forward tracking detector of HPLUS

The necessity of installing a forward tracking detector stack is discussed for the Hadron Physics LanzhoU Spectrometer(HPLUS).A local tracker is developed to solve the multi-track finding problem.The track candidates are searched iteratively via Hough Transform.The fake tracks are removed by a least square fitting process.With this tracker we have studied the feasibility of pp→pp + φ(→K+K-),a typical physical channel proposed on HPLUS.The single track momentum resolution due to the uncertainty of the positioning in FTD is 1.3%.The multiple scattering effect contributes about 20% to the momentum resolution in the FTD coverage.The width and the signal-to-background ratio of the reconstructed φ are 1.51 MeV and 4.36,respectively,taking into account the direct Kaon channel pp→pp+K+K- as background.The geometry coverage of FTD for φ events is about 85.4%.Based on the current fast simulation and estimation,the geometrical configuration of FTD meets the physical requirement of HPLUS under the current luminosity and multiplicity conditions.The tracker is applicable in the full simulation coming next and is extendable to other tracking component of HPLUS.     

Flocking in Multi-Agent Systems with Multiple Virtual Leaders Based Only on Position Measurements

Most existing flocking algorithms assume one single virtual leader and rely on information on both relative positions and relative velocities among neighboring agents. In this paper, the problem of controlling a flock of mobile autonomous agents to follow multiple virtual leaders is investigated by using only position information in the sense that agents with the same virtual leader asymptotically attain the same velocity and track the corresponding virtual leader based on only position measurements. A flocking algorithm is proposed under which every agent asymptotically attains its desired velocity, collision between agents can be avoided, and the final tight formation minimizes all agents' global potentials. A simulation example is presented to verify and illustrate the theoretical results.     

Study of the leader of a spark discharge over a water surface

This paper discusses an experimental study of the leaders of incomplete spark discharges with a capacitance of 0.1 and 1 μF over a water surface when the initial voltage is 3–6 kV in discharge gaps 8 and 22 cm long, having side branches and without branches. The distributions of the field, the current, the current density, the conductivity, and the electron concentration along the leader have been determined, as well as the changes in the velocity and length of the leader as it evolves. It has been established that the evolution of the leader has a self-maintained character, and that the product of the storage capacitance and the initial potential difference between the head of the leader and the water surface is an invariant of its spatial evolution. Zh. Tekh. Fiz. 68, 44–50 (July 1998)     

Steady-state leader breakdown. Nitrogen atmosphere

Physical arguments about the possible mechanism of formation of a leader channel are presented. A mathematical model describing steady-state leader breakdown is constructed. An algorithm for determining the propagation velocity, dimensions, and electric field in the streamer zone is developed. A numerical simulation of the channel formation stage in the plasma of a streamer zone a nitrogen atmosphere is performed. The dependence of the leader velocity on the potential is obtained. The tentative model proposed here can be used to describe the leader breakdown of a long gap at high positive potentials. Zh. Tekh. Fiz. 68, 37–44 (June 1998)     

Observation of the Streamer–Leader Propagation Processes of Long Air-Gap Positive Discharges

A novel observation method is proposed to study the streamer–leader propagation processes of long air-gap positive discharges. Experiments of 3–12 m air-gap discharges have been conducted in this paper. The observed images could clearly and particularly describe the characteristics of the positive streamer–leader propagation processes. Some important phenomena of the propagation processes are indicated and discussed, such as the leader's velocity that varied with the time, the different lengths of inception leaders, the leader's branching phenomena, the continuous streamer–leader propagation processes simultaneously with the voltage's descent, and the upward leader's propagation processes.