TheInfoList

Plasma ()πλάσμα
, Henry George Liddell, Robert Scott, ''A Greek English Lexicon'', on Perseus
is one of the
four fundamental states of matter 4 (four) is a number A number is a mathematical object A mathematical object is an abstract concept arising in mathematics. In the usual language of mathematics, an ''object'' is anything that has been (or could be) formally defined, and with ...
. It consists of a gas of
ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
satoms or molecules which have at least one orbital
electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are general ...

stripped (or an extra electron attached) and, thus, an electric charge. It is the most abundant form of
ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic particl ...
in the
universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

, being mostly associated with
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

s, including the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

. It extends to the rarefied
intracluster medium In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathe ...
and possibly to intergalactic regions. Plasma was first systematically studied by
Irving Langmuir Irving Langmuir (; January 31, 1881 – August 16, 1957) was an American chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin Medieval Latin was the form of Latin Latin (, or , ) is a class ...

in the 1920s. It can be artificially generated by heating a neutral gas or subjecting it to a strong
electromagnetic field An electromagnetic field (also EM field or EMF) is a classical (i.e. non-quantum) field Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the in ...
. The presence of charged particles makes plasma
electrically conductive Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that quantifies how strongly it resists electric current An electric current is a stream of charged particles, ...
, with the dynamics of individual particles and macroscopic plasma motion governed by collective electromagnetic fields and very sensitive to externally applied fields. The response of plasma to electromagnetic fields is used in many modern technological devices, such as or
plasma etching Plasma etching is a form of plasma processing used to fabricate integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuit File:PExdcr01CJC.jp ...
. Depending on temperature and density, a certain amount of neutral particles may also be present, in which case plasma is called partially ionized.
Neon sign Neon is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

s and
lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden flow of electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan ...

are examples of partially ionized plasmas. Unlike the
phase transition In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in ...
s between the other three states of matter, the transition to plasma is not well defined and is a matter of interpretation and context. Whether a given degree of ionization suffices to call a substance 'plasma' depends on the specific phenomenon being considered.

# Early history

Plasma was first identified in laboratory by
Sir William Crookes Sir William Crookes (; 17 June 1832 – 4 April 1919) was a British chemist and physicist who attended the Royal College of Chemistry in London, and worked on spectroscopy Spectroscopy is the study of the interaction between matter In ...

. Crookes presented a
lecture A lecture (from the Greek ''lecture'', meaning reading) is an oral presentation intended to present information or teach people about a particular subject, for example by a university or college teacher. Lectures are used to convey critical in ...

on what he called "radiant matter" to the
British Association for the Advancement of Science The British Science Association (BSA) is a charity Charity may refer to: Giving * Charitable organization or charity, a non-profit organization whose primary objectives are philanthropy and social well-being * Charity (practice), the practice ...
, in Sheffield, on Friday, 22 August 1879. Systematic studies of plasma began with the research of
Irving Langmuir Irving Langmuir (; January 31, 1881 – August 16, 1957) was an American chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin Medieval Latin was the form of Latin Latin (, or , ) is a class ...

and his colleagues in the 1920s. Langmuir also introduced the term "plasma" as a description of ionized gas in 1928:
Lewi TonksLewi Tonks (1897–1971) was an United States, American quantum mechanics, quantum physicist noted for his discovery (with Marvin D. Girardeau) of the Tonks-Girardeau gas. Tonks was employed by General Electric for most of his working life, research ...
and Harold Mott-Smith, both of whom worked with Langmuir in the 1920s, recall that Langmuir first used the term by analogy with the
blood plasma Blood plasma is a yellowish liquid component of blood Blood is a body fluid in humans and other animals that delivers necessary substances such as nutrients and oxygen to the Cell (biology), cells and transports metabolic waste products ...
. Mott-Smith recalls, in particular, that the transport of electrons from thermionic filaments reminded Langmuir of "the way blood plasma carries red and white corpuscles and germs."

# Definitions

## The fourth state of matter

Plasma is called the ''fourth
state of matter In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...
'' after
solid Solid is one of the four fundamental states of matter 4 (four) is a number A number is a mathematical object A mathematical object is an abstract concept arising in mathematics. In the usual language of mathematics, an ''object'' is an ...

,
liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical objects, ...

, and
gas Gas is one of the four fundamental states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

. It is a state of matter in which an ionized substance becomes highly
electrically conductive Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that quantifies how strongly it resists electric current An electric current is a stream of charged particles, ...
to the point that long-range dominate its behaviour. Plasma is typically an electrically quasineutral medium of unbound positive and negative particles (i.e. the overall charge of a plasma is roughly zero). Although these particles are unbound, they are not "free" in the sense of not experiencing forces. Moving charged particles generate electric currents, and any movement of a charged plasma particle affects and is affected by the fields created by the other charges. In turn this governs collective behaviour with many degrees of variation. Plasma is distinct from the other states of matter. In particular, describing a low-density plasma as merely an "ionized gas" is wrong and misleading, even though it is similar to the gas phase in that both assume no definite shape or volume. The following table summarizes some principal differences:

## Ideal plasma

Three factors define an ideal plasma: *The plasma approximation: The plasma approximation applies when the plasma parameter Λ, representing the number of charge carriers within the
Debye sphere In plasmas and electrolyte An electrolyte is a substance that produces an electrically conducting solution when dissolved in a polar solvent, such as water. The dissolved electrolyte separates into cations and anions, which disperse uniformly th ...
is much higher than unity. It can be readily shown that this criterion is equivalent to smallness of the ratio of the plasma electrostatic and thermal energy densities. Such plasmas are called weakly coupled. *Bulk interactions: The
Debye length In plasmas and electrolyte An electrolyte is a substance that produces an electrically conducting solution when dissolved in a polar solvent, such as water. The dissolved electrolyte separates into cations and anions, which disperse uniformly th ...
is much smaller than the physical size of the plasma. This criterion means that interactions in the bulk of the plasma are more important than those at its edges, where boundary effects may take place. When this criterion is satisfied, the plasma is quasineutral. *Collisionlessness: The electron plasma frequency (measuring
plasma oscillationPlasma oscillations, also known as Langmuir waves (after Irving Langmuir), are rapid oscillations of the electron density in conducting media such as plasmas or metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, ...
s of the electrons) is much larger than the electron–neutral collision frequency. When this condition is valid, electrostatic interactions dominate over the processes of ordinary gas kinetics. Such plasmas are called collisionless.

## Non-neutral plasma

The strength and range of the electric force and the good conductivity of plasmas usually ensure that the densities of positive and negative charges in any sizeable region are equal ("quasineutrality"). A plasma with a significant excess of charge density, or, in the extreme case, is composed of a single species, is called a non-neutral plasma. In such a plasma, electric fields play a dominant role. Examples are charged
particle beam A particle beam is a stream of charged or neutral particles, in many cases moving at near the speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant important in many areas of physics. Its exact va ...
s, an electron cloud in a
Penning trapA Penning trap is a device for the storage of charged particles using a homogeneous axial magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materi ...
and positron plasmas.

## Dusty plasma

A
dusty plasmaA dusty plasma is a plasma containing micrometer (10−6) to nanometer (10−9) sized particles suspended in it. Dust particles are charged and the plasma and particles behave as a plasma. Dust particles may form larger particles resulting in "grain ...

contains tiny charged particles of dust (typically found in space). The dust particles acquire high charges and interact with each other. A plasma that contains larger particles is called grain plasma. Under laboratory conditions, dusty plasmas are also called ''complex plasmas''.

# Properties and parameters

## Density and ionization degree

For plasma to exist,
ionization Ionization or ionisation is the process by which an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday ...
is necessary. The term "plasma density" by itself usually refers to the electron density $n_e$, that is, the number of charge-contributing electrons per unit volume. The degree of ionization $\alpha$ is defined as fraction of neutral particles that are ionized: $\alpha = \frac,$ where $n_i$ is the ion density and $n_n$ the neutral density (in number of particles per unit volume). In the case of fully ionized matter, $\alpha = 1$. Because of the quasineutrality of plasma, the electron and ion densities are related by $n_e = \langle Z_i\rangle n_i$, where $\langle Z_i\rangle$ is the average ion charge (in units of the
elementary charge The elementary charge, usually denoted by or sometimes e is the electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positiv ...
).

## Temperature

Plasma temperature, commonly measured in
kelvin The kelvin is the base unit of temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal en ...

or
electronvolt In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular suc ...
s, is a measure of the thermal kinetic energy per particle. High temperatures are usually needed to sustain ionization, which is a defining feature of a plasma. The degree of plasma ionization is determined by the
electron temperature that may develop in a plasma (', 1976) Plasma parameters define various characteristics of a Plasma (physics), plasma, an electrically conductive collection of charged particles that responds ''collectively'' to electromagnetic forces. Plasma ...
relative to the
ionization energy In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and forc ...
(and more weakly by the density). In
thermal equilibrium Two physical system A physical system is a collection of physical objects. In physics, it is a portion of the physical universe chosen for analysis. Everything outside the system is known as the environment (systems), environment. The enviro ...

, the relationship is given by the Saha equation. At low temperatures, ions and electrons tend to recombine into bound states—atoms—and the plasma will eventually become a gas. In most cases, the electrons and heavy plasma particles (ions and neutral atoms) separately have a relatively well-defined temperature; that is, their energy distribution function is close to a Maxwellian even in the presence of strong
electric Electricity is the set of physics, physical Phenomenon, phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnet ...

or
magnetic Magnetism is a class of physical attributes that are mediated by magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For in ...

fields. However, because of the large difference in mass between electrons and ions, their temperatures may be different, sometimes significantly so. This is especially common in weakly ionized technological plasmas, where the ions are often near the
ambient temperature Ambient or Ambiance or Ambience may refer to: Music and sound * Ambience (sound recording)In filmmaking, ambience (also known as atmosphere, atmos, or background) consists of the sounds of a given location or space. It is the opposite of "silence" ...
while electrons reach thousands of kelvin. The opposite case is the
z-pinch 300px, Desk top sized inductively coupled current driven toroidal Z-pinch in a krypton plasma showing intense glow from plasma filament. In fusion power Fusion power is a proposed form of power generation that would generate electricity by usin ...
plasma where the ion temperature may exceed that of electrons.

## Plasma potential

Since plasmas are very good
electrical conductor In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force ...
s, electric potentials play an important role. The average potential in the space between charged particles, independent of how it can be measured, is called the "plasma potential", or the "space potential". If an electrode is inserted into a plasma, its potential will generally lie considerably below the plasma potential due to what is termed a
Debye sheath The Debye sheath (also electrostatic sheath) is a layer in a plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark–gluon plasma, ...
. The good electrical conductivity of plasmas makes their electric fields very small. This results in the important concept of "quasineutrality", which says the density of negative charges is approximately equal to the density of positive charges over large volumes of the plasma ($n_e = \langle Z\rangle n_i$), but on the scale of the
Debye length In plasmas and electrolyte An electrolyte is a substance that produces an electrically conducting solution when dissolved in a polar solvent, such as water. The dissolved electrolyte separates into cations and anions, which disperse uniformly th ...
, there can be charge imbalance. In the special case that '' double layers'' are formed, the charge separation can extend some tens of Debye lengths. The magnitude of the potentials and electric fields must be determined by means other than simply finding the net
charge density In electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is c ...

. A common example is to assume that the electrons satisfy the
Boltzmann relation In a plasma, the Boltzmann relation describes the number densityThe number density (symbol: ''n'' or ''ρ''N) is an intensive quantity used to describe the degree of concentration In chemistry Chemistry is the scientific discipline invol ...
: $n_e \propto e^.$ Differentiating this relation provides a means to calculate the electric field from the density: $\vec = (k_BT_e/e)(\nabla n_e/n_e).$ It is possible to produce a plasma that is not quasineutral. An electron beam, for example, has only negative charges. The density of a non-neutral plasma must generally be very low, or it must be very small, otherwise, it will be dissipated by the repulsive
electrostatic force ''F'' between two point charges ''q''1 and ''q''2 is directly proportional to the product of the magnitudes of charges and inversely proportional to the square of the distance between them. Like charges repel each other, and opposite charges mut ...

.

## Magnetization

The existence of charged particles causes the plasma to generate, and be affected by,
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

s. Plasma with a magnetic field strong enough to influence the motion of the charged particles is said to be magnetized. A common quantitative criterion is that a particle on average completes at least one gyration around the magnetic-field line before making a collision, i.e., $\nu_ / \nu_ > 1$, where $\nu_$ is the electron
gyrofrequency Cyclotron resonance describes the interaction of external forces with charged particles experiencing a magnetic field, thus already moving on a circular path. It is named after the cyclotron, a cyclic particle accelerator that utilizes an oscillatin ...
and $\nu_$ is the electron collision rate. It is often the case that the electrons are magnetized while the ions are not. Magnetized plasmas are ''
anisotropic Anisotropy () is the property of a material which allows it to change or assume different properties in different directions as opposed to isotropy Isotropy is uniformity in all orientations; it is derived from the Greek ''isos'' (ἴσος, ...
'', meaning that their properties in the direction parallel to the magnetic field are different from those perpendicular to it. While electric fields in plasmas are usually small due to the plasma high conductivity, the electric field associated with a plasma moving with velocity $\mathbf$ in the magnetic field $\mathbf$ is given by the usual $\mathbf = -\mathbf\times\mathbf$, and is not affected by Debye shielding.

# Mathematical descriptions

To completely describe the state of a plasma, all of the particle locations and velocities that describe the electromagnetic field in the plasma region would need to be written down. However, it is generally not practical or necessary to keep track of all the particles in a plasma. Therefore, plasma physicists commonly use less detailed descriptions, of which there are two main types:

## Fluid model

Fluid models describe plasmas in terms of smoothed quantities, like density and averaged velocity around each position (see
Plasma parameters that may develop in a plasma (', 1976) Plasma parameters define various characteristics of a Plasma (physics), plasma, an electrically conductive collection of charged particles that responds ''collectively'' to electromagnetic force Elect ...
). One simple fluid model,
magnetohydrodynamics Magnetohydrodynamics (MHD; also magneto-fluid dynamics or hydro­magnetics) is the study of the magnetic properties and behaviour of electrically conducting Electrical resistivity (also called specific electrical resistance or volume res ...
, treats the plasma as a single fluid governed by a combination of
Maxwell's equations Maxwell's equations are a set of coupled partial differential equation In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), ...
and the
Navier–Stokes equations In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...
. A more general description is the two-fluid plasma, where the ions and electrons are described separately. Fluid models are often accurate when collisionality is sufficiently high to keep the plasma velocity distribution close to a
Maxwell–Boltzmann distribution In physics (in particular in statistical mechanics), the Maxwell–Boltzmann distribution is a particular probability distribution named after James Clerk Maxwell and Ludwig Boltzmann. It was first defined and used for describing particle speeds i ...
. Because fluid models usually describe the plasma in terms of a single flow at a certain temperature at each spatial location, they can neither capture velocity space structures like beams or double layers, nor resolve wave-particle effects.

## Kinetic model

Kinetic models describe the particle velocity distribution function at each point in the plasma and therefore do not need to assume a
Maxwell–Boltzmann distribution In physics (in particular in statistical mechanics), the Maxwell–Boltzmann distribution is a particular probability distribution named after James Clerk Maxwell and Ludwig Boltzmann. It was first defined and used for describing particle speeds i ...
. A kinetic description is often necessary for collisionless plasmas. There are two common approaches to kinetic description of a plasma. One is based on representing the smoothed distribution function on a grid in velocity and position. The other, known as the
particle-in-cellThe particle-in-cell (PIC) method refers to a technique used to solve a certain class of partial differential equations In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), m ...
(PIC) technique, includes kinetic information by following the trajectories of a large number of individual particles. Kinetic models are generally more computationally intensive than fluid models. The
Vlasov equationThe Vlasov equation is a differential equation In mathematics, a differential equation is an functional equation, equation that relates one or more function (mathematics), functions and their derivatives. In applications, the functions generally r ...
may be used to describe the dynamics of a system of charged particles interacting with an electromagnetic field. In magnetized plasmas, a gyrokinetic approach can substantially reduce the computational expense of a fully kinetic simulation.

# Plasma science and technology

Plasmas are the object of study of the
academic field An academic discipline or academic field is a subdivision of knowledge that is Education, taught and researched at the college or university level. Disciplines are defined (in part) and recognized by the academic journals in which research is p ...
of ''plasma science'' or ''plasma physics'', including sub-disciplines such as space plasma physics. It currently involves the following fields of active research and features across many journals, whose interest includes: *Plasma theory **
Plasma equilibria and stability The stability of a plasma is an important consideration in the study of plasma physics. When a system containing a Plasma (physics), plasma is at Mechanical equilibrium, equilibrium, it is possible for certain parts of the plasma to be disturbed by ...
**Plasma interactions with waves and beams ** Guiding center **
Adiabatic invariantA property of a physical system In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics) ...
**
Debye sheath The Debye sheath (also electrostatic sheath) is a layer in a plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark–gluon plasma, ...
**
Coulomb collisionA Coulomb collision is a binary elastic collision between two charged particles interacting through their own electric field An electric field (sometimes E-field) is the physical field that surrounds each electric charge Electric charge is t ...
*Plasmas in nature **
Astrophysical plasma Astrophysical plasma is plasma outside of the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the name ...
** Northern and southern (polar) lights **The Earth's
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere The thermosphere is the layer in the directly above the and below the . Within this layer of the atmosphere, ...
**
Interplanetary medium The interplanetary medium (IPM) or interplanetary space consists of the mass and energy which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planets, asteroids, and comets, move. The IPM sto ...
**Planetary
magnetosphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses m ...

s ** Space plasma *Industrial plasmas ** Plasma chemistry **
Plasma processing Plasma processing is a Plasma (physics), plasma-based material processing technology that aims at modifying the chemical and physical properties of a surface. Plasma processing techniques include: *Plasma activation *Plasma ashing *Plasma cleaning ...
**
Plasma spray Thermal spraying techniques are coating processes in which melted (or heated) materials are sprayed onto a surface. The "feedstock" (coating precursor) is heated by electrical (plasma or arc) or chemical means (combustion flame). Thermal sprayin ...
**
Plasma displayA plasma display panel (PDP) is a type of flat panel display that uses small cells containing Plasma (physics), plasma: ionized gas that responds to electric fields. Plasma TVs were the first large (over 32 inches diagonal) flat panel displays to b ...

**Plasma sources **
Dusty plasmaA dusty plasma is a plasma containing micrometer (10−6) to nanometer (10−9) sized particles suspended in it. Dust particles are charged and the plasma and particles behave as a plasma. Dust particles may form larger particles resulting in "grain ...

s *
Plasma diagnosticsPlasma diagnostics are a pool of methods, instruments, and experimental techniques used to measure properties of a Plasma (physics), plasma, such as plasma components' density, distribution function over energy (temperature), their spatial profiles a ...
**
Thomson scattering Thomson scattering is the elastic scattering of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural sc ...
**
Langmuir probe 100px, One of two Langmuir probes from the Swedish Institute of Space Physics in Uppsala on board ESA's space vehicle Rosetta (spacecraft), Rosetta, due for a 67P/Churyumov-Gerasimenko, comet. The probe is the spherical part, 50 mm in diameter an ...
**
Ball-pen probe A ball-pen probe is a modified Langmuir probe used to measure the plasma potential in magnetized plasmas. The ball-pen probe balances the electron and ion saturation currents, so that its floating potential is equal to the plasma potential. Becau ...
**
Faraday cup A Faraday cup is a metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts Electrical resistiv ...

**
Spectroscopy Spectroscopy is the study of the interaction Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way ...

**
Interferometry Interferometry is a technique which uses the ''interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or dis ...
** Ionospheric heating ** Incoherent scatter radar *Plasma applications **
Dielectric barrier discharge Dielectric-barrier discharge (DBD) is the electrical discharge An electric discharge is the release and transmission of electricity in an applied electric field An electric field (sometimes E-field) is the physical field that surrounds each e ...
**
Enhanced oil recovery Enhanced oil recovery (abbreviated EOR), also called tertiary recovery, is the extraction of crude oil Petroleum (), also known as crude oil and oil, is a naturally occurring, yellowish-black liquid found in geological formations beneath t ...
**
Fusion power Fusion power is a proposed form of that would generate by using heat from . In a fusion process, two lighter combine to form a heavier nucleus, while releasing energy. Devices designed to harness this energy are known as fusion reactors. Fusi ...
**Plasma Actuator (e.g.
Serpentine geometry plasma actuator The serpentine plasma actuator represents a broad class of plasma actuator. The actuators vary from the standard type in that their electrode geometry has been modified in to be periodic across its span. History This class of plasma actuators was ...
) ***
Magnetic fusion energy Magnetic confinement fusion is an approach to generate thermonuclear Thermonuclear fusion is a way to achieve nuclear fusion by using extremely high temperatures. There are two forms of thermonuclear fusion: ''uncontrolled'', in which the resulti ...
(MFE) — ****
Tokamak A tokamak (; russian: токамáк) is a device which uses a powerful magnetic field to confine plasma (physics), plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement fusion, magnetic confinement devices ...
****
Stellarator A stellarator is a Plasma (physics), plasma device that relies primarily on external magnets to confine a plasma. Scientists researching magnetic confinement fusion aim to use stellarator devices as a vessel for nuclear fusion reactions. The na ...
****
Reversed field pinch File:Revese Field Pinch P-Field.png, thumbnail, The poloidal field in a reversed field pinch A reversed-field pinch (RFP) is a device used to produce and contain near-thermonuclear Plasma (physics), plasmas. It is a Pinch (magnetic fusion), toroi ...
****
Magnetic mirror A magnetic mirror, known as a magnetic trap (магнитный захват) in Russia and briefly as a pyrotron in the US, is a type of magnetic confinement, magnetic confinement device used in fusion power to trap high temperature plasma (phys ...
****
Dense plasma focusA dense plasma focus (DPF) is a type of plasma generating system originally developed as a fusion power Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion, nuclear fusion reactions ...
***
Inertial confinement fusion Inertial confinement fusion (ICF) is a type of fusion energy research that attempts to initiate nuclear fusion reactions by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium an ...

(ICF) ***
Plasma weapon A directed-energy weapon (DEW) is a ranged weapon A ranged weapon is any weapon A weapon, arm or armament is any implement or device that can be used with the intent to inflict physical damage or harm. Weapons are used to increase th ...
s **
Ion implantation technological facility in Toulouse, France. Ion implantation is a low-temperature process by which ion (physics), ions of one element are accelerated into a solid target, thereby changing the physical, chemical, or electrical properties of the t ...
**
Ion thruster An ion thruster, ion drive, or ion engine is a form of electric propulsion An electrically powered spacecraft propulsion system uses electrical, and possibly also magnetic fields, to change the velocity of a spacecraft 275px, The US ...
**
MAGPIE Magpies are birds of the Corvidae family. Like other members of their family, they are widely considered to be intelligent creatures. The Eurasian magpie, for instance, is thought to rank among the world's Animal cognition, most intelligent crea ...

(Implosion experiments) **
Plasma ashing In semiconductor A semiconductor material has an Electrical resistivity and conductivity, electrical conductivity value falling between that of a Electrical conductor, conductor, such as metallic copper, and an insulator (electricity), insulator ...
**Food processing *** Nonthermal plasma or "cold plasma" **
Plasma arc waste disposal Plasma or plasm may refer to: Science * Plasma (physics) Plasma ( 1, where \nu_ is the electron gyrofrequency and \nu_ is the electron collision rate. It is often the case that the electrons are magnetized while the ions are not. Magnetized pl ...
, recycling. **
Plasma acceleration Plasma acceleration is a technique for accelerating charged particles, such as electrons, positrons, and ions, using the electric field associated with plasma oscillation, electron plasma wave or other high-gradient plasma structures (like shock and ...
** Plasma medicine (e. g. Dentistry) ** Plasma window Plasmas can appear in nature in various forms and locations, which can be usefully broadly summarised in the following Table:

## Space and astrophysics

Plasmas are by far the most common phase of ordinary matter in the universe, both by mass and by volume. Above the Earth's surface, the ionosphere is a plasma, and the magnetosphere contains plasma. Within our Solar System,
interplanetary space Interplanetary may refer to: *Interplanetary space, the space between the planets of the Solar System *Interplanetary spaceflight, travel between planets *The interplanetary medium, the material that exists in interplanetary space *The InterPlaneta ...
is filled with the plasma expelled via the
solar wind The solar wind is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, i ...

, extending from the Sun's surface out to the heliopause. Furthermore, all the distant
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

s, and much of
interstellar space Outer space, commonly shortened to space, is the expanse that exists beyond Earth and Earth atmosphere, its atmosphere and between astronomical object, celestial bodies. Outer space is not completely empty—it is a hard vacuum containing a ...
or
intergalactic space Outer space is the expanse that exists beyond Earth and between celestial bodies. Outer space is not completely empty—it is a hard vacuum containing a low density of particles, predominantly a plasma of hydrogen Hydrogen is the c ...
is also likely filled with plasma, albeit at very low densities.
Astrophysical plasma Astrophysical plasma is plasma outside of the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the name ...
s are also observed in
Accretion disk An accretion disk is a structure (often a ) formed by diffuse material in al motion around a massive . The central body is typically a . causes orbiting material in the disk to spiral inward towards the central body. Gravitational and frictional ...

s around stars or compact objects like
white dwarf A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter Degenerate matter is a highly dense state of fermionic matter in which the Pauli exclusion principle exerts significant ...

s,
neutron star A neutron star is the collapsed core Core or cores may refer to: Science and technology * Core (anatomy) In common parlance, the core of the body is broadly considered to be the torso. Functional movements are highly dependent on this par ...
s, or in close
binary star A binary star is a star system consisting of two stars orbiting around their common barycenter. Systems of two or more stars are called ''Star system#Hierarchical systems, multiple star systems''. These systems, especially when more distant, oft ...

systems. Plasma is associated with ejection of material in astrophysical jets, which have been observed with accreting black holes or in active galaxy, galaxies like Messier 87#Jet, M87's jet that possibly extends out to 5,000 light-years.

## Artificial plasmas

Most artificial plasmas are generated by the application of electric and/or magnetic fields through a gas. Plasma generated in a laboratory setting and for industrial use can be generally categorized by: *The type of power source used to generate the plasma—DC, AC (typically with radio frequency (RF)) and microwave *The pressure they operate at—vacuum pressure (< 10 mTorr or 1 Pa), moderate pressure (≈1 Torr or 100 Pa), atmospheric pressure (760 Torr or 100 kPa) *The degree of ionization within the plasma—fully, partially, or weakly ionized *The temperature relationships within the plasma—thermal plasma ($T_e = T_i = T_$), non-thermal or "cold" plasma ($T_e \gg T_i = T_$) *The electrode configuration used to generate the plasma *The magnetization of the particles within the plasma—magnetized (both ion and electrons are trapped in Gyroradius, Larmor orbits by the magnetic field), partially magnetized (the electrons but not the ions are trapped by the magnetic field), non-magnetized (the magnetic field is too weak to trap the particles in orbits but may generate Lorentz forces)

### Generation of artificial plasma

Just like the many uses of plasma, there are several means for its generation. However, one principle is common to all of them: there must be energy input to produce and sustain it. For this case, plasma is generated when an electric current is applied across a dielectric gas or fluid (an electrically Electrical conductor, non-conducting material) as can be seen in the adjacent image, which shows a discharge tube as a simple example (direct current, DC used for simplicity). The potential difference and subsequent electric field pull the bound electrons (negative) toward the anode (positive electrode) while the cathode (negative electrode) pulls the nucleus. As the voltage increases, the current stresses the material (by electric polarization) beyond its dielectric strength, dielectric limit (termed strength) into a stage of electrical breakdown, marked by an electric spark, where the material transforms from being an insulator (electrical), insulator into a Electrical conductor, conductor (as it becomes increasingly ionized). The underlying process is the Townsend avalanche, where collisions between electrons and neutral gas atoms create more ions and electrons (as can be seen in the figure on the right). The first impact of an electron on an atom results in one ion and two electrons. Therefore, the number of charged particles increases rapidly (in the millions) only "after about 20 successive sets of collisions", mainly due to a small mean free path (average distance travelled between collisions).

### =Electric arc

= With ample current density and ionization, this forms a luminous electric arc (a continuous electric discharge similar to
lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden flow of electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan ...

) between the electrodes. Electrical resistance along the continuous electric arc creates heat, which dissociates more gas molecules and ionizes the resulting atoms (where degree of ionization is determined by temperature), and as per the sequence:
solid Solid is one of the four fundamental states of matter 4 (four) is a number A number is a mathematical object A mathematical object is an abstract concept arising in mathematics. In the usual language of mathematics, an ''object'' is an ...

-
liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical objects, ...

-
gas Gas is one of the four fundamental states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

-plasma, the gas is gradually turned into a thermal plasma. A thermal plasma is in
thermal equilibrium Two physical system A physical system is a collection of physical objects. In physics, it is a portion of the physical universe chosen for analysis. Everything outside the system is known as the environment (systems), environment. The enviro ...

, which is to say that the temperature is relatively homogeneous throughout the heavy particles (i.e. atoms, molecules and ions) and electrons. This is so because when thermal plasmas are generated, electrical energy is given to electrons, which, due to their great mobility and large numbers, are able to disperse it rapidly and by elastic collision (without energy loss) to the heavy particles.Note that non-thermal, or non-equilibrium plasmas are not as ionized and have lower energy densities, and thus the temperature is not dispersed evenly among the particles, where some heavy ones remain "cold".

### Examples of industrial/commercial plasma

Because of their sizable temperature and density ranges, plasmas find applications in many fields of research, technology and industry. For example, in: industrial and extractive metallurgy, surface treatments such as plasma spraying (coating), etching in microelectronics, metal cutting and welding; as well as in everyday Vehicle emissions control, vehicle exhaust cleanup and Fluorescent lamp, fluorescent/Electroluminescence, luminescent lamps, fuel ignition, while even playing a part in Scramjet, supersonic combustion engines for aerospace engineering.

### =Low-pressure discharges

= *''Glow discharge plasmas'': non-thermal plasmas generated by the application of DC or low frequency RF (<100 kHz) electric field to the gap between two metal electrodes. Probably the most common plasma; this is the type of plasma generated within fluorescent light tubes. *''Capacitively coupled plasma (CCP)'': similar to glow discharge plasmas, but generated with high frequency RF electric fields, typically ISM band, 13.56 MHz. These differ from glow discharges in that the sheaths are much less intense. These are widely used in the microfabrication and integrated circuit manufacturing industries for plasma etching and plasma enhanced chemical vapor deposition. *''Cascaded Arc Plasma Source'': a device to produce low temperature (≈1eV) high density plasmas (HDP). *''Inductively coupled plasma (ICP)'': similar to a CCP and with similar applications but the electrode consists of a coil wrapped around the chamber where plasma is formed. *''Wave heated plasma'': similar to CCP and ICP in that it is typically RF (or microwave). Examples include helicon discharge and electron cyclotron resonance (ECR).

### =Atmospheric pressure

= *''Arc discharge:'' this is a high power thermal discharge of very high temperature (≈10,000 K). It can be generated using various power supplies. It is commonly used in Metallurgy, metallurgical processes. For example, it is used to smelt minerals containing Al2O3 to produce aluminium. *''Corona discharge:'' this is a non-thermal discharge generated by the application of high voltage to sharp electrode tips. It is commonly used in ozone generators and particle precipitators. *''
Dielectric barrier discharge Dielectric-barrier discharge (DBD) is the electrical discharge An electric discharge is the release and transmission of electricity in an applied electric field An electric field (sometimes E-field) is the physical field that surrounds each e ...
(DBD):'' this is a non-thermal discharge generated by the application of high voltages across small gaps wherein a non-conducting coating prevents the transition of the plasma discharge into an arc. It is often mislabeled 'Corona' discharge in industry and has similar application to corona discharges. A common usage of this discharge is in a plasma actuator for vehicle drag reduction. It is also widely used in the web treatment of fabrics. The application of the discharge to synthetic fabrics and plastics functionalizes the surface and allows for paints, glues and similar materials to adhere. The dielectric barrier discharge was used in the mid-1990s to show that low temperature atmospheric pressure plasma is effective in inactivating bacterial cells. This work and later experiments using mammalian cells led to the establishment of a new field of research known as plasma medicine. The dielectric barrier discharge configuration was also used in the design of low temperature plasma jets. These plasma jets are produced by fast propagating guided ionization waves known as plasma bullets. *''Capacitive discharge:'' this is a nonthermal plasma generated by the application of RF power (e.g., ISM band, 13.56 MHz) to one powered electrode, with a grounded electrode held at a small separation distance on the order of 1 cm. Such discharges are commonly stabilized using a noble gas such as helium or argon. *"Piezoelectric direct discharge plasma:" is a nonthermal plasma generated at the high-side of a piezoelectric transformer (PT). This generation variant is particularly suited for high efficient and compact devices where a separate high voltage power supply is not desired.

### MHD converters

A world effort was triggered in the 1960s to study magnetohydrodynamic converters in order to bring magnetohydrodynamic generator, MHD power conversion to market with commercial power plants of a new kind, converting the kinetic energy of a high velocity plasma into electricity with no moving parts at a high efficiency. Research was also conducted in the field of supersonic and hypersonic aerodynamics to study plasma interaction with magnetic fields to eventually achieve passive and even active Flow control (fluid), flow control around vehicles or projectiles, in order to soften and mitigate shock waves, lower thermal transfer and reduce Drag (physics), drag. Such ionized gases used in "plasma technology" ("technological" or "engineered" plasmas) are usually ''weakly ionized gases'' in the sense that only a tiny fraction of the gas molecules are ionized. These kinds of weakly ionized gases are also nonthermal "cold" plasmas. In the presence of magnetics fields, the study of such magnetized nonthermal weakly ionized gases involves Magnetohydrodynamics#Ideal and resistive MHD, resistive magnetohydrodynamics with low magnetic Reynolds number, a challenging field of plasma physics where calculations require dyadic tensors in a n-dimensional space, 7-dimensional phase space. When used in combination with a high Hall effect, Hall parameter, a critical value triggers the problematic electrothermal instability which limited these technological developments.

# Complex plasma phenomena

Although the underlying equations governing plasmas are relatively simple, plasma behaviour is extraordinarily varied and subtle: the emergence of unexpected behaviour from a simple model is a typical feature of a complex system. Such systems lie in some sense on the boundary between ordered and disordered behaviour and cannot typically be described either by simple, smooth, mathematical functions, or by pure randomness. The spontaneous formation of interesting spatial features on a wide range of length scales is one manifestation of plasma complexity. The features are interesting, for example, because they are very sharp, spatially intermittent (the distance between features is much larger than the features themselves), or have a fractal form. Many of these features were first studied in the laboratory, and have subsequently been recognized throughout the universe. Examples of complexity and complex structures in plasmas include:

## Filamentation

Striations or string-like structures, also known as Birkeland currents, are seen in many plasmas, like the plasma globe, plasma ball, the Aurora (astronomy), aurora,
lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden flow of electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan ...

, electric arcs, solar flares, and supernova remnants. They are sometimes associated with larger current densities, and the interaction with the magnetic field can form a magnetic rope structure. High power microwave breakdown at atmospheric pressure also leads to the formation of filamentary structures. (See also Plasma pinch) Filamentation also refers to the self-focusing of a high power laser pulse. At high powers, the nonlinear part of the index of refraction becomes important and causes a higher index of refraction in the center of the laser beam, where the laser is brighter than at the edges, causing a feedback that focuses the laser even more. The tighter focused laser has a higher peak brightness (irradiance) that forms a plasma. The plasma has an index of refraction lower than one, and causes a defocusing of the laser beam. The interplay of the focusing index of refraction, and the defocusing plasma makes the formation of a long filament of plasma that can be micrometer (unit), micrometers to kilometers in length. One interesting aspect of the filamentation generated plasma is the relatively low ion density due to defocusing effects of the ionized electrons. (See also Filament propagation)

## Impermeable plasma

Impermeable plasma is a type of thermal plasma which acts like an impermeable solid with respect to gas or cold plasma and can be physically pushed. Interaction of cold gas and thermal plasma was briefly studied by a group led by Hannes Alfvén in 1960s and 1970s for its possible applications in insulation of Nuclear fusion, fusion plasma from the reactor walls. However, later it was found that the external magnetic fields in this configuration could induce Kink instability, kink instabilities in the plasma and subsequently lead to an unexpectedly high heat loss to the walls. In 2013, a group of materials scientists reported that they have successfully generated stable impermeable plasma with no magnetic confinement using only an ultrahigh-pressure blanket of cold gas. While spectroscopic data on the characteristics of plasma were claimed to be difficult to obtain due to the high pressure, the passive effect of plasma on Chemical synthesis, synthesis of different nanostructures clearly suggested the effective confinement. They also showed that upon maintaining the impermeability for a few tens of seconds, screening of ions at the plasma-gas interface could give rise to a strong secondary mode of heating (known as viscous heating) leading to different kinetics of reactions and formation of complex nanomaterials.

# Gallery

File:HallThruster 2.jpg, Hall effect thruster. The electric field in a plasma double layer is so effective at accelerating ions that electric fields are used in ion drives. File:Wispy 'Plasma Dancer' on the limb of the Sun.ogv, Solar plasma File:Plasma Spraying Process.jpg, Plasma spraying File:MAST plasma image.jpg, Tokamak plasma in nuclear fusion research File:Argon Plasma.jpg, Argon Plasma in the Hawkeye Linearly Magnetized Experiment (HLMX) at the University of Iowa

*Plasma torch *Ambipolar diffusion *Hannes Alfvén Prize *Plasma channel *
Plasma parameters that may develop in a plasma (', 1976) Plasma parameters define various characteristics of a Plasma (physics), plasma, an electrically conductive collection of charged particles that responds ''collectively'' to electromagnetic force Elect ...
*Plasma nitriding *Magnetohydrodynamics, Magnetohydrodynamics (MHD) *Magnetohydrodynamic converter *Marklund convection *Electrically powered spacecraft propulsion *Plasma propulsion engine *Electric field screening *List of plasma physicists *List of plasma physics articles *List of publications in physics#Plasma physics, Important publications in plasma physics *IEEE Nuclear and Plasma Sciences Society *LULI *Quark–gluon plasma, Quark-gluon plasma *Nikola Tesla *Space physics *Total electron content *
Plasma displayA plasma display panel (PDP) is a type of flat panel display that uses small cells containing Plasma (physics), plasma: ionized gas that responds to electric fields. Plasma TVs were the first large (over 32 inches diagonal) flat panel displays to b ...

*Aurora