Initiation and Development of Lightning Discharge: Physical Mechanism and Basic Problems

This paper considers three interconnected problems: (i) initiation of a downward lightning in a thundercloud and that of an upward lightning near tall grounded structures; (ii) conditions for lightning development in the cloud-to-ground gap and the effects of lightning trajectory and branching on discharge parameters; and (iii) physical mechanism of the lightning return stroke and peculiarities of the propagation of current and voltage waves along the plasma channel with non-linear parameters. The focus is on the estimation of lightning discharge parameters required to solve applied problems including the simulation of lightning electromagnetic fields and frequency of lightning strokes to grounded and flying objects. Key problems of lightning physics which hinder the progress in practical lightning protection are posed. INTRODUCTION Lightning is a spark discharge with extreme parameters. Many details of its physical mechanism have never been quantified or, sometimes, even understood. A knowledge of lightning mechanism is required to predict lightning hazards, to calculate stroke frequency for protected structures, and to develop reliable systems of lightning protection. Lightning theory has not kept pace with demands of practical lightning protection. Conventional protection systems fail to provide a high reliability level necessary for high-rise buildings, largedimension aircrafts, microelectronics devices and inflammable and explosive stores. This leads to a desire to use ‘non-conventional’ methods to act upon lightning. Unfortunately, an insufficient development of the theory clears the way to doubtful engineering proposals. This paper makes an attempt to generalize available understanding (i) about lightning initiation, (ii) its development in a thundercloud electric field and (iii) impulse current forming after bridging the cloud-to-ground gap. LIGHTNING INITIATION Our consideration is based on observations of a long spark under laboratory conditions. We use a semiempirical model proposed by Bazelyan and Raizer [1997, 2000] which can give the conditions for the leader formation and its steady development in the gap of a given length. Under normal conditions, a leader is initiated in a long air gap only once the voltage drop across the length ∆r ≈ 1 m near the electrode exceeds the critical voltage ∆Ucr ≈ 400 kV. Then, the energy deposited into the leader channel is sufficient to heat and maintain plasma in it. The leader current iL and its velocity vL are governed by the difference ∆U(x) = Utip(x) U0(x) between the leader tip potential Utip(x) and the potential U0(x) of an undisturbed external electric field at the tip point x: U a vL ∆ = a = 15 m sV iL = τLvL = C1∆UvL, ) / ln( cov R L C 0 1 2πε = (1) where С1 is the capacitance per unit length of the leader, L is its length and Rcov is the effective radius of the space charge cover around the leader channel. Similar to an arc discharge, the channel has a falling voltagecurrent characteristic L L i b E = b = 3×10 V mA (2) In the atmosphere free of space charge, the condition for the initiation of a viable leader from the top of a vertical grounded conductor of the height h in the external electric field E0 is written as [Bazelyan and Raizer, 2000]