Vladimir S. Syssoev

Observation of a new class of electric discharges within artificial clouds of charged water droplets and its implication for lightning initiation within thunderclouds

We have observed unusual plasma formations (UPFs) in artificial clouds of charged water droplets using a high-speed infrared camera operating in conjunction with a high-speed visible-range camera. Inferred plasma parameters were close to those of long-spark leaders observed in the same experiments, while the channel morphology was distinctly different from that of leaders, so that UPFs can be viewed as a new type of in-cloud discharge. These formations can occur in the absence of spark leaders and appear to be manifestations of collective processes building, essentially from scratch, a complex hierarchical network of interacting channels at different stages of development (some of which are hot and live for milliseconds). We believe that the phenomenon should commonly occur in thunderclouds and might give insights on the missing link in the still poorly understood lightning initiation process.

High-speed optical studies of the long sparks in very transient stages

High-speed optical studies of different stages of the long sparks have been carried out at the High Voltage Research Center (HVRC), Istra, Russia, using up to three devices operated synchronously: two novel universal streak and framing cameras K004M and K008 developed by the BIFO Company and an analog image converter streak camera of FER14- type instrumented with a CCD readout system of SU04-type. Special attention was paid to the improved spatiotemporal resolution when recording the pictures of large sizes. The measurements were conducted in rod-to-plane gap of 5.5÷6 m in length under both polarities HV pulses of 2÷3 MV in altitude with waveform of 20/7500 and 100/7500 μs. Under negative polarity, an additional thin metallic rod about of 1 m in length was placed on the grounded plane to provide an origin of the positive upward leader and its reliable observation. The velocity of approaching the downward negative streamer-leader system and upward positive leader toward each other near the junction point has been found to be around 5÷10 m/μs. Some supporting exemplary pictures have been recorded to additionally clarify the phenomenology of the stepped or/and attempted leaders, which can be useful for analysis of long spark and lightning treeing and development.

High speed optical studies of long spark (Istra, Russia) and triggered lightning (Camp Blanding, Florida): initial results

Experimental results on classical long spark development optically studied in July-August, 2002 at the High Voltage Research Center (HVRC), Istra, Russia have been presented. Main goal of these studies was to test universal streak and framing cameras K004M and K008 developed by the BIFO Company, Russia for the University of Florida, USA to be used at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida. Both cameras were tested at the conditions (solid angles, spatial and time resolution, sensitivity etc.) being like as much as possible to those expected in actual triggered-lightning experiments at Camp Blanding. Basic results on testing have been found to give hope well enough for the triggered-lightning research. Additionally to the testing, some features of long spark development in different stages have been studied using up to three devices operated synchronously. In combination with two cameras pointed out above, an old analog image converter streak camera of FER14-type (designed in 1980 by the VNIIOFI, Moscow as requested by the All-Union Electrotechnical Institute, Istra) instrumented with a novel CCD readout system SU04-type (designed by the BIFO Company) was being used. This combined system FER14+SU04 has been found to operate well enough too. Some experimental results with pictures presented expand available data and current knowledge on electrical discharge in large air gaps.

Infrared images of bidirectional leaders produced by the cloud of charged water droplets

Detailed infrared (2.7–5.5 µm) images of bidirectional leaders produced by the cloud of small (typical radius of 0.5 µm), positively charged water droplets are presented. The leader was composed of the downward extending positive part and the upward extending negative part, these two parts (both branched, although in different ways) being connected by the single-channel middle part. The downward extending part included the tortuous positive leader channel (similar to its upward extending counterpart observed when the cloud polarity was negative) that was often accompanied by much less tortuous but often equally bright downward extending plasma formations of unknown nature. Very faint positive streamer zone was also observed. Either the positive leader channel or the unusual plasma formation (UPF) can come in contact with the grounded plane. The upward extending part is associated with a large network of faint channels, mostly fanning out of the upper part of the usually much brighter leader channel and apparently pervading the entire upper part of the cloud. Some of those faint channels could be unusually long and bright negative streamers, while others could be similar to UPFs. The IR luminosity along the brightest part of the bidirectional leader channel is often nonuniform. Some variations in channel brightness are localized and suggest the involvement of space leader-type processes at multiple positions along the channel, changes in channel orientation, or variations in channel radius.

Features of application of image converter cameras for research on lightning and discharges in long air gaps

The present report generalizes materials of publication /1-3/. In doing so /1/ and /3/ were presented at appropriate symposiums only as poster reports and were not widely discussed. Creation of reliable physical and engineering models of sequence of leader-return stroke of lightning (L-RS) and an attachment process is hampered by lack of actual information on the optical picture of low-luminous streamer structures of lightning. Cameras based on an image converter tubes (ICT) /4/ serve as an alternative of traditional optical-mechanical means for recording a lightning image. Such cameras allowed to obtain new reults when investigating streamer processes of a long spark what made it possible to formulate a set of hypothesizes relating to a leader process of lightning /5-7/. Here there are given the characteristics of the image converter instrumentation complex adapted to the work with lightning and a long spark and there are presented the results of its tests in the All-Russian Electro-technical Institute (VEI) named after V.I. Lenin when recording a long spark on the open high-voltage stand in Istra (near Moscow).

Observations of the connection of positive and negative leaders in meter‐scale electric discharges generated by clouds of negatively charged water droplets

Detailed observations of the connection between positive and negative leaders in meter-scale electric discharges generated by clouds of negatively-charged water droplets are presented and their possible implications for the attachment process in lightning are discussed. Optical images obtained with three different high-speed cameras (visible-range with image enhancement, visible-range regular, and infrared) and corresponding current recordings were used. Two snapshots of the break-through phase of the leader connection, showing significant leader branching inside the common streamer zone, are presented for the first time. Positive and negative leader speeds inside the common streamer zone for two events were found to be similar. Higher leader speeds were generally associated with higher leader currents. In the case of head-to-head leader connection, the infrared brightness of the junction region (probably representing the gas temperature and, hence, the energy input) was typically a factor of 5 or so higher than for channel sections either below or above that region. In 16% of cases, the downward negative leader connected to the upward positive leader below its tip (attached to the lateral surface of the positive leader), with the connection being accomplished via a channel segment that appeared to be perpendicular to one or both of the leader channels.

An upward connecting leader at tests of large-scale lightning-rod models

Estimations of connecting leader length for grounded objects of various geometries have been performed. The data of experimental studies of protection zones for large-scale lightning-rod models of different types are presented. In experiments the connecting leaders developing towards a spark discharge from a high-voltage electrode were observed. Experimental results on protection zones are compared to calculations under Russian standards of lightning protection

Electrical Strength of Ultra-Long Air Gaps

Some experimental results on discharge development in ultra-long air gaps (breakdown voltages, parameters of the leader stage) are presented. The obtained data are compared to available data on breakdown of long air gaps under positive switching pulses with critical time-tocrest. Existing empirical and theoretical formulae for calculation of sparkover voltage of rod-to-plane air gaps are reviewed. Electrical strength of ultra-long air gaps is considered based on general analysis of physical processes in leader stage of spark discharge. The electrical strength has been thus expressed in the given paper via some characteristic stabilization electric field for each spatial scale. As predicted here, the electrical strength along the lightning development path may decrease down to 3÷10 kV/m.

A new class of electric discharges in clouds of negatively charged water droplets

We have observed unusual plasma formations (UPFs) in artificial clouds of charged water droplets using a high-speed infrared camera operating in conjunction with a high-speed visible-range camera. Inferred plasma parameters were close to those of long-spark leaders observed in the same experiments, while the channel morphology was distinctly different from that of leaders, so that UPFs can be viewed as a new type of in-cloud discharge. These formations can occur in the absence of spark leaders and appear to be manifestations of collective processes building, essentially from scratch, a complex hierarchical network of interacting channels at different stages of development (some of which are hot and live for milliseconds). We believe that the phenomenon should commonly occur in thunderclouds and might give insights on the missing link in the still poorly understood lightning initiation process.

Electric discharges produced by artificially charged clouds: Influence of rapidly moving conductive object

The possibility of initiation of electric discharges by a crossbow bolt (projectile) moving in the electric field of a cloud of negatively charged water droplets has been demonstrated for the first time [1]. Over one hundred of discharges have been produced. For each event, a high-speed video camera recorded the images of upward positive leaders developing from both the nearby grounded sphere and the projectile, followed by the return-stroke-like process. Corresponding currents were measured and integrated photos of the events were obtained. The results can help to improve our understanding of lightning initiation by airborne vehicles and by a vertical conductor rapidly extended below the thundercloud in order to trigger lightning with the rocket-and-wire technique.