A long-standing but fundamental question in lightning studies concerns how lightning is initiated inside storms, given the absence of physical conductors. The issue has revolved around the question of whether the discharges are initiated solely by conventional dielectric breakdown or involve relativistic runaway electron processes. Here we report observations of a relatively unknown type of discharge, called fast positive breakdown, that is the cause of high-power discharges known as narrow bipolar events. The breakdown is found to have a wide range of strengths and is the initiating event of numerous lightning discharges. It appears to be purely dielectric in nature and to consist of a system of positive streamers in a locally intense electric field region. It initiates negative breakdown at the starting location of the streamers, which leads to the ensuing flash. The observations show that many or possibly all lightning flashes are initiated by fast positive breakdown.

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Observations of narrow bipolar events reveal how lightning is initiated in thunderstorms.

A critical analysis of photoionization as the major process for seed electron production ahead of the cathode-directed streamer has been made. The accumulation of ions between pulses and fast electron detachment in an electric field could be an effective source of seed electrons for repetitively pulsed discharges in electronegative gases. Measurements and two-dimensional calculations in the hydrodynamic approximation of streamer parameters in air (anode current, propagation velocity and radiative channel diameter) can only agree if one assumes uniform background pre-ionization and not by taking into account only the photoionization process generally adopted for pulsed discharges in air. The results of analytical estimates and full three-dimensional simulations confirm that the seed charge distribution in the undisturbed gap has an effect on the streamer head formation and leads to the streamer branching phenomena.

Role of electronegative gas admixtures in streamer start, propagation and branching phenomena

The results of two-dimensional modelling of positive streamer dynamics in 2 cm point-to-plane gap are presented, including the process of photoionization in a gas volume. The model of photoionization in air, developed by Zhelezniak, Mnatsakanian and Sizykh is considered in detail and a comparison with the experimental data of Penney and Hummert is presented. The model of photoionization in nitrogen, based on the data of Penney and Hummert is described. The simulations of streamers were performed for air and nitrogen-like gas. It is shown, that fundamental space scale of streamer - the width of space charge layer lρ (ionization domain) around its head is defined by the length of absorption of photoionizing radiation la. This length defines all other parameters: electron density, radius of streamer channel etc, so one may speak about standard streamer in air, whose properties depend only on pressure. Based on simulation results one may assume that streamer branching occurs in high field, when lρ exceeds la. The process of branching may be interpreted as an instability which transforms the non-standard streamer into a number of standard ones.

https://iopscience.iop.org/article/10.1088/0022-3727/33/12/314/pdf

The role of photoionization in positive streamer dynamics

In this review we describe a transient type of gas discharge which is commonly called a streamer discharge, as well as a few related phenomena in pulsed discharges. Streamers are propagating ionization fronts with self-organized field enhancement at their tips that can appear in gases at (or close to) atmospheric pressure. They are the precursors of other discharges like sparks and lightning, but they also occur in for example corona reactors or plasma jets which are used for a variety of plasma chemical purposes. When enough space is available, streamers can also form at much lower pressures, like in the case of sprite discharges high up in the atmosphere.
We explain the structure and basic underlying physics of streamer discharges, and how they scale with gas density. We discuss the chemistry and applications of streamers, and describe their two main stages in detail: inception and propagation. We also look at some other topics, like interaction with flow and heat, related pulsed discharges, and electron runaway and high energy radiation. Finally, we discuss streamer simulations and diagnostics in quite some detail.
This review is written with two purposes in mind: First, we describe recent results on the physics of streamer discharges, with a focus on the work performed in our groups. We also describe recent developments in diagnostics and simulations of streamers. Second, we provide background information on the above-mentioned aspects of streamers. This review can therefore be used as a tutorial by researchers starting to work in the field of streamer physics.

/pdf/the-physics-of-streamer-discharge-phenomena-2ttm9yemgu.pdf

The physics of streamer discharge phenomena

The influence of long-term corona on the degradation of non-ceramic composite insulator (NCI) housing materials was studied. Polymer samples were obtained from full-scale NCI, employing three different housing materials, namely silicone rubber, ethylene propylene diene monomer rubber and a blend of EPDM with silicone rubber. The effect of relative humidity and externally applied mechanical stress as acceleration factors was assessed. Several modes of degradation were observed, namely, cracking, roughening, and discoloration. In general the severity of degradation increased with the relative humidity level and application of mechanical stress. Periodic monitoring of corona discharge magnitude and pulse repetition rate allowed for a simplistic computation of the energy needed for initiation of surface changes. A comparison of these calculated values with expected conditions in the field was made, It is postulated that even under severe operating conditions over a period of many years, degradation of the housing materials evaluated due to water droplet corona is highly unlikely.

Effect of long-term corona on non-ceramic outdoor insulator housing materials

Results concerning the propagation of streamers along insulating surfaces under uniform electric field are presented. The basic properties of streamers, namely the electric field required for a stable propagation and the propagation velocity, have been measured and compared with propagation in air alone as a reference. The results have shown that in the experimental arrangement used significant space charges due to streamer branching are absent, therefore the properties observed are considered as characteristic of a single streamer. Streamers propagate stably with an intrinsic propagation field and a characteristic velocity that depend on the nature of the insulating material. For electric fields higher than the minimum field required for a stable propagation, a streamer system propagates with a 'surface' and an 'air' component.

Streamer propagation along insulating surfaces

The uses and limitations of the electrostatic probe for the measurement of charge densities on insulating surfaces are discussed. A development of the technique is described in which two important limitations have together been overcome: (i) The effects on the probe signal of charges on all points of the surface have been taken into account by means of a matrix inversion procedure; (ii) A robotic control system has been developed which enables the probe to follow and scan a wide range of axi-symmetric insulator profiles. The degree of resolution achieved enables the probe system to display and measure charge densities in individual streamer channels of a corona discharge on a polytetrafluoroethylene (PTFE) surface. An example is given and comparison made with a dust figure of the same event.

/pdf/high-resolution-measurements-of-surface-charge-densities-on-8zu7bpa2zd.pdf

High resolution measurements of surface charge densities on insulator surfaces

Results concerning the dynamics of streamer propagation in air under a uniform electric field are presented and discussed. Experiments were performed in a plane-parallel electrode arrangement with positive streamers initiated at a sharp point in the earthed anode. The basic properties of streamers are described in terms of the electric field required for a stable propagation and the associated propagation velocity. Critical parameters are the ambient electric field, the voltage used for streamer initiation and the distance of traverse. The present experiments permit the separation between the effects of the above parameters upon streamer advancement and propagation over the whole path up to the cathode. It is shown that an intrinsic propagation field with an associated velocity can be defined, which determine the propagation of streamers of a limiting, minimum energy. The propagation velocity is a power function of the electric field and, with the aid of an empirical equation, values can be expressed accurately in terms of these intrinsic streamer properties.

https://iopscience.iop.org/article/10.1088/0022-3727/32/8/012/pdf

Dynamics of streamer propagation in air

The electric field required to sustain streamer propagation is important to the interpretation of breakdown in nonuniform field geometries. Basic measurements of the minimum field for propagation have been carried out in uniform fields. These have shown that above a ‘threshold’ value of field, there is an increasing probability of propagation up to a ‘stability’ field at which propagation always occurs. Comparison is made (a) with earlier work in both uniform and nonuniform fields, (b) with a survey of the mean stresses required for breakdown under lightning impulse and steady voltages. This leads to the conclusions that the stability field is a better parameter to use than the threshold field in making comparisons with lightning impulse breakdown and that the threshold field corresponds more closely with the stress required for steady voltage breakdown. Humidity effects are discussed.

Study of the electric fields required for streamer propagation in humid air

A description is given of some properties of the corona discharge when propagating over a cylindrical polytetrafluoroethylene insulator surface placed along the axis of a rod-plane electrode arrangement. A scanning electrostatic probe has been used to measure the density of charge deposited on the surface; it is shown that the total net deposited charge is small compared with the total charge injected into the gap. The velocity of propagation has been measured. Effects of preceding coronas on succeeding ones and the nature of deposited charge after breakdown are described. Discussion is given in terms of electron lifetimes and attachment and photoemission processes.

/pdf/corona-propagation-and-charge-deposition-on-a-ptfe-surface-39fngt4npf.pdf

N.L. Allen

Papers

Streamer propagation along insulating surfaces

High resolution measurements of surface charge densities on insulator surfaces

Dynamics of streamer propagation in air

Study of the electric fields required for streamer propagation in humid air

Corona propagation and charge deposition on a PTFE surface