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A streamer discharge, also known as ''filamentary discharge'', is a type of transient
electrical discharge An electric discharge is the release and transmission of electricity in an applied electric field through a medium such as a gas (ie., an outgoing flow of electric current through a non-metal medium).American Geophysical Union, National Research C ...
which forms at the surface of a conductive
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...
carrying a high
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...
in an insulating medium such as air. Streamers are luminous writhing branching sparks, plasma channels composed of
ionized Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...
air molecules, which repeatedly strike out from the electrode into the air. Like the related
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 a ...
s and
brush discharge A brush discharge is an electrical disruptive discharge similar to a corona discharge that takes place at an electrode with a high voltage applied to it, embedded in a nonconducting fluid, usually air. It is characterized by multiple luminous w ...
s, a streamer discharge represents a region around a high voltage conductor where the air has suffered
electrical 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 ...
and become conductive (
ionized Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...
), so
electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respecti ...
is leaking off the electrode into the air. It occurs when 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 ...
at the surface of a conductor exceeds the
dielectric strength In physics, the term dielectric strength has the following meanings: *for a pure electrically insulating material, the maximum electric field that the material can withstand under ideal conditions without undergoing electrical breakdown and becom ...
of air, around 30 kilovolts per centimeter. When the electric field created by the applied voltage reaches this threshold, accelerated
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...
s strike air
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
s with enough energy to knock other electrons off them,
ionizing Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...
them, and the freed electrons go on to strike more molecules in a chain reaction. These
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 ionizat ...
s (Townsend discharges) create ionized, electrically conductive regions in the air near the electrode. The
space charge Space charge is an interpretation of a collection of electric charges in which excess electric charge is treated as a continuum of charge distributed over a region of space (either a volume or an area) rather than distinct point-like charges. Thi ...
created by the electron avalanches gives rise to an additional electric field, causing the ionized region to grow at its ends, forming a finger-like discharge called a ''streamer''. Streamers are transient (exist only for a short time) and filamentary, which makes them different from
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 a ...
s. They are used in applications such as ozone production, air purification or plasma medicine. If a streamer reaches the opposite polarity conductor it creates an ionized conductive path through which a large current can flow, releasing a large amount of heat, resulting in 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. ...
; this is the process through which
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 ...
leader Leadership, both as a research area and as a practical skill, encompasses the ability of an individual, group or organization to "lead", influence or guide other individuals, teams, or entire organizations. The word "leadership" often gets view ...
s create a path for lightning bolts. Streamers can also be observed as sprites in the upper atmosphere. Due to the low pressure, sprites are much larger than streamers at ground pressure, see the similarity laws below.


History

The theory of streamer discharges was preceded by John Sealy Townsend's discharge theory from around 1900. However, it became clear that this theory was sometimes inconsistent with observations. This was especially true for discharges that were longer or at higher pressure. In 1939, Loeb and Raether independently described a new type of discharge, based on their experimental observations. Shortly thereafter, in 1940, Meek presented the ''theory of spark discharge'', which quantitatively explained the formation of a self-propagating streamer. This new theory of streamer discharges successfully explained the experimental observations.


Applications

Streamers are used in applications such as ozone generation, air purification and plasma-assisted combustion. An important property is that the plasma they generate is strongly non-equilibrium: the electrons have much higher energies than the ions. Therefore, chemical reactions can be triggered in a gas without heating it. This is important for plasma medicine, where "plasma bullets", or guided streamers, can be used for wound treatment, although this is still experimental.


Streamer physics

Streamers can emerge when a strong electric field is applied to an insulating material, typically a gas. Streamers can only form in areas where the electric field exceeds the
dielectric strength In physics, the term dielectric strength has the following meanings: *for a pure electrically insulating material, the maximum electric field that the material can withstand under ideal conditions without undergoing electrical breakdown and becom ...
(breakdown field, disruptive field) of the medium. For air at atmospheric pressure, this is roughly 30 kV per centimeter. The electric field accelerates the few
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...
s and
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conv ...
s that are always present in air, due to natural processes such as
cosmic rays Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
,
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
, or
photoionization Photoionization is the physical process in which an ion is formed from the interaction of a photon with an atom or molecule. Cross section Not every interaction between a photon and an atom, or molecule, will result in photoionization. The prob ...
. Ions are much heavier, so they move very slowly compared to electrons. As the electrons move through the medium, they collide with the neutral molecules or atoms. Important collisions are: *
Elastic collision In physics, an elastic collision is an encounter ( collision) between two bodies in which the total kinetic energy of the two bodies remains the same. In an ideal, perfectly elastic collision, there is no net conversion of kinetic energy int ...
s, which change the direction of motion of the electrons. * Excitations, where the neutral particle is excited, and the electron loses the corresponding energy. *
Impact ionization Impact ionization is the process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers. For example, in semiconductors, an electron (or hole) with enough kinetic energy can knock a bound elec ...
, where the neutral particle becomes ionized, with the incident electron losing the energy. *
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, where the electron attaches to the neutral to form a negative ion. When the electric field approaches the breakdown field, the electrons gain enough energy between collisions to ionize the gas atoms, knocking an electron off the atom. At the breakdown field, there is a balance between the production of new electrons (due to impact ionization) and the loss of electrons (due to attachment). Above the breakdown field, the number of electrons starts to grow exponentially, and an
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 ionizat ...
(
Townsend avalanche The Townsend discharge or Townsend avalanche is a gas ionisation process where free electrons are accelerated by an electric field, collide with gas molecules, and consequently free additional electrons. Those electrons are in turn accelerated and ...
) forms. The electron avalanches leave behind positive ions, so in time more and more
space charge Space charge is an interpretation of a collection of electric charges in which excess electric charge is treated as a continuum of charge distributed over a region of space (either a volume or an area) rather than distinct point-like charges. Thi ...
is building up. (Of course, the ions move away in time, but this a relatively slow process compared to the avalanche generation as ions are much heavier than electrons). Eventually, the electric field from all the space charge becomes comparable to the background electric field. This is sometimes referred to as the "avalanche to streamer transition". In some regions the total electric field will be smaller than before, but in other regions it will get larger, which is called electric field enhancement. New avalanches predominantly grow in the high-field regions, so a self-propagating structure can emerge: a streamer.


Positive and negative streamers

In
direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or e ...
(DC) circuits, the streamers that form at electrodes with positive and negative voltages are different in appearance and form by different physics mechanisms. Negative streamers propagate against the direction of the electric field, that is, in the same direction as the electrons
drift velocity In physics, a drift velocity is the average velocity attained by charged particles, such as electrons, in a material due to an electric field. In general, an electron in a conductor will propagate randomly at the Fermi velocity, resulting in an ...
. Positive streamers propagate in the opposite direction. In both cases, the streamer channel is electrically neutral, and it is shielded by a thin space charge layer. This leads to an enhanced electric field at the end of the channel, the "head" of the streamer. Both positive and negative streamers grow by impact ionization in this high-field region, but the source of electrons is very different. For negative streamers, free electrons are accelerated from the channel to the head region. However, for positive streamers, these free electrons have to come from farther away, as they accelerate into the streamer channel. Therefore, negative streamers grow in a more diffuse way than positive streamers. Because a diffuse streamer has less field enhancement, negative streamers require higher electric fields than positive streamers. In nature and in applications, positive streamers are therefore much more common. As noted above, an important difference is also that positive streamers need a source of free electrons for their propagation. In many cases
photoionization Photoionization is the physical process in which an ion is formed from the interaction of a photon with an atom or molecule. Cross section Not every interaction between a photon and an atom, or molecule, will result in photoionization. The prob ...
is believed to be this source. In nitrogen-oxygen gas mixtures with high oxygen concentrations, excited nitrogen emits UV photons which subsequently ionize oxygen. In pure nitrogen or in nitrogen with small oxygen admixtures, the dominant production mechanism of photons, however, is the
Bremsstrahlung ''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...
process.


Similarity laws

Most processes in a streamer discharge are two-body processes, where an electron collides with a neutral molecule. An important example is
impact ionization Impact ionization is the process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers. For example, in semiconductors, an electron (or hole) with enough kinetic energy can knock a bound elec ...
, where an electron ionizes a neutral molecule. Therefore, the
mean free path In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...
is inversely proportional to the gas
number density The number density (symbol: ''n'' or ''ρ''N) is an intensive quantity used to describe the degree of concentration of countable objects ( particles, molecules, phonons, cells, galaxies, etc.) in physical space: three-dimensional volumetric number ...
. If the electric field is changed linearly with the gas number density, then electrons gain on average the same energy between collisions. In other words, if the ratio between electric field E and number density N is constant, we expect similar dynamics. Typical lengths scale as 1/N, as they are related to the mean free path. This also motivates the Townsend unit, which is a physical unit of the E/N ratio.


Emission of run-away electrons and high-energy photons

It has been observed that discharges in laboratory experiments emit X-rays and that lightning discharges emit X-rays and
terrestrial gamma-ray flash A terrestrial gamma-ray flash (TGF), also known as dark lightning, is a burst of gamma rays produced in Earth's atmosphere. TGFs have been recorded to last 0.2 to 3.5 milliseconds, and have energies of up to 20 million electronvolts. It is spe ...
es, bursts of photons with energies of up to 40 MeV. These photons are produced by runaway electrons, electrons which have overcome the
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...
force, through the
Bremsstrahlung ''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...
process. However, it has not been fully understood how electrons can gain such high energies in the first place since they constantly collide with air molecules and lose energy. A possible explanation is the acceleration of electrons in the enhanced electric fields of the streamer tips. However, it is uncertain whether this process can really explain a sufficiently high production rate. Recently, it has been proposed that ambient air is perturbed in the vicinity of streamer discharges and that this perturbation facilitates the acceleration of electrons into the run-away regime


The relation between pressure waves and the production of X-rays in air discharges

Pressure and shock waves released by electric discharges are capable of perturbing the air in their vicinity up to 80%. This, however, has immediate consequences on the motion and properties of secondary streamer discharges in perturbed air: Depending on the direction (relative to the ambient electric field), air perturbations change the discharge velocities, facilitate branching or trigger the spontaneous initiation of a counter discharge. Recent simulations have shown that such perturbations are even capable to facilitate the production of
X-rays An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10  picometers to 10 nanometers, corresponding to frequencies in the range 30  ...
(with energies of several tens of keV) from such streamer discharges, which are produced by run-away electrons through the
Bremsstrahlung ''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...
process.


See also

*
electrical discharge An electric discharge is the release and transmission of electricity in an applied electric field through a medium such as a gas (ie., an outgoing flow of electric current through a non-metal medium).American Geophysical Union, National Research C ...
*
sprite (lightning) Sprites or red sprites are large-scale electric discharges that occur high above thunderstorm clouds, or cumulonimbus, giving rise to a varied range of visual shapes flickering in the night sky. They are usually triggered by the discharges of p ...
*
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 a ...
*
Townsend discharge The Townsend discharge or Townsend avalanche is a gas ionisation process where free electrons are accelerated by an electric field, collide with gas molecules, and consequently free additional electrons. Those electrons are in turn accelerated and ...
* avalanche breakdown


References

{{Reflist Electrical phenomena