A leader is a hot, highly conductive channel of
plasma that plays a critical part during
dielectric breakdown
Electrical breakdown or dielectric breakdown is a process that occurs when an electrical insulating material, subjected to a high enough voltage, suddenly becomes an electrical conductor and electric current flows through it. All insulating mate ...
within a long
electric spark
An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an ionized, electrically conductive channel through a normally-insulating medium, often air or other gases or gas mixtures. Michael F ...
.
Mechanism
When a gas is subjected to
high voltage
High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, ''high voltage'' refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant sp ...
stress, the
electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field ...
is often quite non-uniform near one, or both, of the high voltage electrodes making up a
spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conductor ...
. Breakdown initially begins with the formation of
corona discharge
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone electrical breakdown ...
s near the electrode with the highest electrical stress. If the electrical field is further increased, longer length ''cold'' discharges (called ''
streamers'' or ''burst corona'') sporadically form near the stressed electrode. Streamers attract multiple
electron avalanche
An electron avalanche is a process in which a number of free electrons in a transmission medium are subjected to strong acceleration by an electric field and subsequently collide with other atoms of the medium, thereby ionizing them (impact ioniza ...
s into a single channel, propagating forward quickly via photon emission which leads to photoelectrons producing new avalanches. Streamers redistribute charge within the surrounding gas, temporarily forming regions of excess charge (
space charges) in the regions surrounding the discharges.
If the electrical field is sufficiently high, the individual currents from multitudes of streamers combine to create a hot, highly conductive path that projects from the electrode, going some distance into the gap. The projecting channel of hot plasma is called a leader, and it can have an electrical conductivity approaching that of an
electric arc
An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma; the plasma may produce visible light. ...
. The leader effectively ''projects'' the electrical field from the nearby electrode further into the gap, similar to introducing a short length of wire into the gap. The tip of the conductive leader now forms a new region from which streamers can entend even further into the gap. As new streamer discharges feed the tip of the leader, the streamer currents help to keep the leader hot and conductive. Under sufficiently high voltages, the leader will continue to extend itself further into the gap, doing so in a series of jumps until the entire gap has been bridged. Although leaders are most often associated with the initial formative stages of a
lightning
Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions, both in the atmosphere or with one on the ground, temporarily neutralize themselves, causing the instantaneous release of an average ...
stroke, they are characteristic of the development of all long sparks. In the case of a lightning leader, each extension (called a
step leader) is typically 10 – 50 meters in length. The branching, visible blue-white discharges from a
Tesla Coil
A Tesla coil is an electrical resonant transformer circuit designed by inventor Nikola Tesla in 1891. It is used to produce high-voltage, low- current, high-frequency alternating-current electricity. Tesla experimented with a number of differen ...
are also examples of leaders.
See also
*
Lightning
Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions, both in the atmosphere or with one on the ground, temporarily neutralize themselves, causing the instantaneous release of an average ...
*
Breakdown voltage
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to experience electrical breakdown and become electrically conductive.
For diodes, the breakdown voltage is the minimum reverse voltage that mak ...
*
Electron avalanche
An electron avalanche is a process in which a number of free electrons in a transmission medium are subjected to strong acceleration by an electric field and subsequently collide with other atoms of the medium, thereby ionizing them (impact ioniza ...
Lightning
Electrical breakdown
External links
*Kurt Feser, Hermann Singer
"From the glow corona into the breakdown" originally published on haefely.com, archived 9 March 2012, translated from Elektrotechnische Zeitschrift A, vol. 93 (1972), pp. 36–39.
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