View source for Plasma (physics)

{{Short description|State of matter}}
{{pp|small=yes}}
{{Use dmy dates|date=March 2015}}
{{multiple image|perrow = 2|total_width=300
| image1 = Lightning3.jpg
| image2 = Neon Internet Cafe open 24 hours.jpg
| image3 = Plasma-lamp 2.jpg
| image4 = Space Shuttle Atlantis in the sky on July 21, 2011, to its final landing.jpg
| footer = Top: [[Lightning]] and [[Neon sign|neon lights]] are commonplace generators of plasma. Bottom left: A [[plasma globe]], illustrating some of the more complex plasma phenomena, including [[#Filamentation|filamentation]]. Bottom right: A plasma trail from the [[Space Shuttle]] [[Space Shuttle Atlantis|''Atlantis'']] during re-entry into [[Atmosphere of Earth|Earth's atmosphere]], as seen from the [[International Space Station]].
}}

'''Plasma''' ({{etymology|grc|''{{wikt-lang|grc|πλάσμα}}'' ({{grc-transl|πλάσμα}})|moldable substance}}) is one of four fundamental [[state of matter|states of matter]] (the other three being [[solid]], [[liquid]], and [[gas]]) characterized by the presence of a significant portion of [[charged particle]]s in any combination of [[ion]]s or [[electron]]s.<ref name="πλάσμα">{{Cite book|first1=Henry George |last1=Liddell |first2=Robert |last2=Scott |title=A Greek-English Lexicon |chapter=πλάσμα|chapter-url=http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dpla%2Fsma|access-date=2023-02-10|publisher=Clarendon Press |year=1940}}</ref> It is the most abundant form of [[ordinary matter]] in the [[universe]], mostly in [[star]]s (including the [[Sun]]), but also dominating the rarefied [[intracluster medium]] and [[Outer space#Intergalactic space|intergalactic medium]].<ref name="Itptma2013a">{{cite book
|last1=Chu
|first1=P.K.|last2=Lu
|first2=XinPel
|date=2013
|title=Low Temperature Plasma Technology: Methods and Applications
| page=3
|publisher=CRC Press
|isbn=978-1-4665-0990-0}}</ref><ref name="Piel2010">{{Cite book
|last =Piel
|first=A.
|date=2010
|title= Plasma Physics: An Introduction to Laboratory, Space, and Fusion Plasmas
|pages= 4–5
|publisher=[[Springer Nature|Springer]]
|url=https://books.google.com/books?id=9cA0DwAAQBAJ&pg=PR8
|isbn=978-3-642-10491-6
|url-status=live
|archive-url=https://web.archive.org/web/20160105142523/https://books.google.com/books?hl=en
|archive-date=5 January 2016
 |df = dmy-all
}}</ref><ref name="Phillips1995">
{{Cite book
|last=Phillips
|first=K. J. H.
|date=1995
|title=Guide to the Sun
|page=295
|publisher=[[Cambridge University Press]]
|url=https://books.google.com/books?id=idwBChjVP0gC&q=Guide+to+the+Sun+phillips
|isbn=978-0-521-39788-9
|url-status=live
|archive-url= https://web.archive.org/web/20180115215631/https://books.google.com/books?id=idwBChjVP0gC&printsec=frontcover&dq=Guide+to+the+Sun+phillips&hl=en&sa=X&ved=0ahUKEwiBj4Gbj5bXAhXrrVQKHfnAAKUQ6AEIKDAA
|archive-date=15 January 2018
|df=dmy-all}}</ref><ref name="Aschwanden2004">{{cite book
|last=Aschwanden
|first=M. J.
|year=2004
|title=Physics of the Solar Corona. An Introduction
|publisher=Praxis Publishing
|isbn=978-3-540-22321-4}}</ref>
Plasma can be artificially generated, for example, by heating a neutral gas or subjecting it to a strong [[electromagnetic field]].<ref name="BoPA2015">{{cite book
|last1=Chiuderi
|first1=C.
|last2=Velli
|first2=M.
|date=2015
|title=Basics of Plasma Astrophysics
|page=17
|publisher=[[Springer Nature|Springer]]
|isbn=978-88-470-5280-2}}</ref>

The presence of [[charged particle]]s makes plasma [[electrically conductive]], with the dynamics of individual particles and macroscopic plasma motion governed by collective electromagnetic fields and very sensitive to externally applied fields.<ref name="Itpd2012b">{{cite book |last1=Morozov |first1=A.I.|date=2012 |title=Introduction to Plasma Dynamics |page=30 |publisher=CRC Press| isbn=978-1-4398-8132-3}}</ref> The response of plasma to electromagnetic fields is used in many modern devices and technologies, such as [[plasma display|plasma televisions]] or [[plasma etching]].<ref name="Itptma2013">{{cite book
|last1=Chu
|first1=P.K.
|last2=Lu
|first2=XinPel
|date=2013
|title=Low Temperature Plasma Technology: Methods and Applications
|publisher=CRC Press
|isbn=978-1-4665-0990-0}}</ref>

Depending on temperature and density, a certain number of neutral particles may also be present, in which case plasma is called [[partially ionized]]. [[Neon sign]]s and [[lightning]] are examples of partially ionized plasmas.<ref>{{Cite web | title = How Lightning Works | publisher = HowStuffWorks | url = http://science.howstuffworks.com/nature/natural-disasters/lightning2.htm | url-status=live | archive-url = https://web.archive.org/web/20140407080201/http://science.howstuffworks.com/nature/natural-disasters/lightning2.htm | archive-date = 7 April 2014 | df = dmy-all | date = April 2000 }}</ref>
Unlike the [[phase transition]]s between the other three states of matter, the transition to plasma is not well defined and is a matter of interpretation and context.<ref name="Itpd2012">{{cite book |last1=Morozov |first1=A.I.|date=2012 |title=Introduction to Plasma Dynamics |page=4−5 |publisher=CRC Press|isbn=978-1-4398-8132-3}}</ref> Whether a given degree of ionization suffices to call a substance "plasma" depends on the specific phenomenon being considered.

==Early history==

[[File:Plasma microfields.webm|thumb|Plasma [[electric field|microfields]] calculated by an [[N-body]] [[computer simulation|simulation]]. Note the fast moving electrons and slow ions, resembling a [[body fluid|bodily fluid]].]]

Plasma was first identified in laboratory by [[Sir William Crookes]]. Crookes presented a [[lecture]] on what he called "radiant matter" to the [[British Association for the Advancement of Science]], in Sheffield, on Friday, 22 August 1879.<ref>{{cite web |url=http://www.worldcatlibraries.org/wcpa/top3mset/5dcb9349d366f8ec.html |title=Find in a Library: On radiant matter a lecture delivered to the British Association for the Advancement of Science, at Sheffield, Friday, August 22, 1879 |access-date=2006-05-24 |url-status=live |archive-url=https://web.archive.org/web/20060709162459/http://www.worldcatlibraries.org/wcpa/top3mset/5dcb9349d366f8ec.html |archive-date=9 July 2006 |df=dmy-all }} {{cite web |url=http://www.tfcbooks.com/mall/more/315rm.htm |title=Radiant Matter |access-date=24 May 2006 |url-status=live |archive-url=https://web.archive.org/web/20060613212651/http://tfcbooks.com/mall/more/315rm.htm |archive-date=13 June 2006 |df=dmy-all }}</ref>
Systematic studies of plasma began with the research of [[Irving Langmuir]] and his colleagues in the 1920s. Langmuir also introduced the term "plasma" as a description of ionized gas in 1928:<ref name="langmuir1928">{{Cite journal | last1 = Langmuir | first1 = I. | title = Oscillations in Ionized Gases | doi = 10.1073/pnas.14.8.627 | journal = Proceedings of the National Academy of Sciences | volume = 14 | issue = 8 | pages = 627–637 | year = 1928 | pmid =  16587379| pmc =  1085653| bibcode = 1928PNAS...14..627L | df = dmy-all | doi-access = free }}</ref>

{{Blockquote|Except near the electrodes, where there are ''sheaths'' containing very few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is very small. We shall use the name ''plasma'' to describe this region containing balanced charges of ions and electrons.}}

[[Lewi Tonks]] 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]].<ref>{{cite journal |first=Lewi |last=Tonks |title=The birth of "plasma" |year=1967 |journal=American Journal of Physics |volume=35 |issue=9 |pages=857–858 |doi=10.1119/1.1974266|bibcode=1967AmJPh..35..857T }}</ref><ref>{{cite book|author=Brown, Sanborn C.|chapter=Chapter 1: A Short History of Gaseous Electronics|editor1=Hirsh, Merle N. |editor2=Oskam, H. J.|title=Gaseous Electronics|volume=1|publisher=Academic Press|date=1978|isbn=978-0-12-349701-7|chapter-url=https://books.google.com/books?id=C1UmeQ_E0_AC&pg=PA1|url-status=live|archive-url=https://web.archive.org/web/20171023230956/https://books.google.co.uk/books?hl=en&lr=&id=C1UmeQ_E0_AC&oi=fnd&pg=PA1&ots=vwabB53YqL&sig=SI8DiBRSQI_yGy_DrspkxNLR0rs#v=onepage&q=blood&f=false|archive-date=23 October 2017|df=dmy-all}}</ref> 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."<ref>{{cite journal |first=Harold M.|last=Mott-Smith |journal=Nature |volume=233 |issue=5316 |page=219 |year=1971 |title=History of "plasmas" |bibcode=1971Natur.233..219M |doi=10.1038/233219a0 |pmid=16063290 |doi-access=free }}</ref>

{{Continuum mechanics|fluid}}

==Definitions==
===The fourth state of matter===
Plasma is called the ''fourth [[state of matter]]'' after [[solid]], [[liquid]], and [[gas]].<ref>{{cite book
 |last        = Frank-Kamenetskii
 |first       = David A.
 |author-link  = David A. Frank-Kamenetskii
 |orig-year   = 1961–1963
 |year        = 1972
 |edition     = 3rd
 |title       = Plasma-The Fourth State of Matter
 |language    = en
 |publisher   = Plenum Press
 |url         = https://books.google.com/books?id=Q_vpBwAAQBAJ&q=%22Plasma-The+Fourth+State+of+Matter%22+Frank-Kamenetskii
 |place       = New York
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20180115215631/https://books.google.com/books?id=Q_vpBwAAQBAJ&printsec=frontcover&dq=%22Plasma-The+Fourth+State+of+Matter%22+Frank-Kamenetskii&hl=en&sa=X&ved=0ahUKEwi8gerahpbXAhXT31QKHdlfB5oQ6AEIKDAA
 |archive-date = 15 January 2018
 |df          = dmy-all
|isbn       = 9781468418965
 }}</ref><ref>Yaffa Eliezer, Shalom Eliezer, ''The Fourth State of Matter: An Introduction to the Physics of Plasma'', Publisher: Adam Hilger, 1989, {{ISBN|978-0-85274-164-1}}, 226 pages, page 5</ref><ref>{{cite book|author=Bittencourt, J.A.|title=Fundamentals of Plasma Physics|publisher=Springer|date=2004|isbn=9780387209753|page=1|url=https://books.google.com/books?id=qCA64ys-5bUC&pg=PA1|url-status=live|archive-url=https://web.archive.org/web/20170202072845/https://books.google.com/books?id=qCA64ys-5bUC&pg=PA1|archive-date=2 February 2017|df=dmy-all}}</ref> It is a state of matter in which an [[ionized]] substance becomes highly [[Electrical resistivity and conductivity|electrically conductive]] to the point that long-range [[electromagnetism|electric and magnetic fields]] dominate its behaviour.<ref name="Chen, Francis F. 2016 2–3">{{cite book | title=Introduction to Plasma Physics and controlled fusion | author=Chen, Francis F. | year=1984 | publisher=Springer International Publishing | url=https://books.google.com/books?id=WGbaBwAAQBAJ&q=editions:9PGss7GnX-MC | pages=2–3 | url-status=live | archive-url=https://web.archive.org/web/20180115215631/https://books.google.com/books?id=WGbaBwAAQBAJ&printsec=frontcover&dq=editions:9PGss7GnX-MC&hl=en&sa=X&ved=0ahUKEwimuOfm_pXXAhVrzFQKHTrOCaUQ6AEIKDAA | archive-date=15 January 2018 | df=dmy-all | isbn=9781475755954 }}</ref><ref name="Freidberg, Jeffrey P. 2008 121">{{cite book | title=Plasma Physics and Fusion Energy | author=Freidberg, Jeffrey P. | year=2008 | publisher=Cambridge University Press | url=https://books.google.com/books?id=Vyoe88GEVz4C | page=121 | url-status=live | archive-url=https://web.archive.org/web/20161224204205/https://books.google.com/books?id=Vyoe88GEVz4C | archive-date=24 December 2016 | df=dmy-all | isbn=9781139462150 }}</ref>

Plasma is typically an electrically quasineutral medium of unbound positive and negative [[particle]]s (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 current]]s, and any movement of a [[Charged particle|charged]] plasma particle affects and is affected by the [[Electric field|fields]] created by the other charges. In turn, this governs collective behaviour with many degrees of variation.<ref name="Sturrock">{{cite book |title=Plasma Physics: An Introduction to the Theory of Astrophysical, Geophysical & Laboratory Plasmas |last=Sturrock |first=Peter A. |date=1994 |publisher=Cambridge University Press |isbn=978-0-521-44810-9}}</ref><ref>{{cite book |title=The Framework of Plasma Physics |author=Hazeltine, R.D. |author2=Waelbroeck, F.L. |date=2004 |publisher=Westview Press |isbn=978-0-7382-0047-7}}
</ref>

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:

{| class="wikitable"
|-
! {{diagonal split header|Property|State}} !! Gas !! Plasma
|-
! Interactions
| '''Short-range''': Two-particle (binary) [[collision]]s are the rule.
| '''Long-range''': Collective motion of particles is ubiquitous in plasma, resulting in various [[waves in plasma|waves]] and other types of collective phenomena.
|-
! Electrical conductivity
| '''Very low''': Gases are excellent [[Insulator (electricity)|insulators]] up to electric field strengths of tens of kilovolts per centimetre.<ref>{{cite web
| url=https://hypertextbook.com/facts/2000/AliceHong.shtml
| title=Dielectric Strength of Air
| first=Alice
| last=Hong
| year=2000
| website=The Physics Factbook
| editor-last=Elert
| editor-first=Glenn
| access-date=2018-07-06
}}</ref>
| '''Very high''': For many purposes, the conductivity of a plasma may be treated as infinite.
|-
! Independently acting species
| '''One''': All gas particles behave in a similar way, largely influenced by collisions with one another and by [[gravity]].
| '''Two or more''': [[Electron]]s and [[ion]]s possess different [[electric charge|charge]]s and vastly different masses, so that they behave differently in many circumstances, with various types of plasma-specific waves and [[Instability|instabilities]] emerging as a result.
|}

===Ideal plasma===
Three factors define an ideal plasma:<ref name="Hazeltine">{{cite book|author=Dendy, R. O.|title=Plasma Dynamics|url=https://books.google.com/books?id=puuQM4Dx0zYC&q=plasma+dynamics+dendy&pg=PR19|publisher=Oxford University Press|date=1990|isbn=978-0-19-852041-2|url-status=live|archive-url=https://web.archive.org/web/20180115215631/https://books.google.com/books?id=puuQM4Dx0zYC&pg=PR19&dq=plasma+dynamics+dendy&hl=en&sa=X&ved=0ahUKEwjlvfbU_JXXAhVJxVQKHVSwC5kQ6AEILTAB|archive-date=15 January 2018|df=dmy-all}}</ref><ref>{{cite book|author=Hastings, Daniel|author2=Garrett, Henry|name-list-style=amp |title=Spacecraft-Environment Interactions|isbn=978-0-521-47128-2|publisher=Cambridge University Press|date=2000}}</ref>

*'''The plasma approximation''': The plasma approximation applies when the [[plasma parameter]] Λ,<ref>{{Cite book|url=https://books.google.com/books?id=WGbaBwAAQBAJ&q=editions:9PGss7GnX-MC|title=Introduction to plasma physics and controlled fusion|author=Chen, Francis F.|date=1984|publisher=Plenum Press|others=Chen, Francis F., 1929-|isbn=978-0306413322|edition=2nd|location=New York|oclc=9852700|url-status=live|archive-url=https://web.archive.org/web/20180115215631/https://books.google.com/books?id=WGbaBwAAQBAJ&printsec=frontcover&dq=editions:9PGss7GnX-MC&hl=en&sa=X&ved=0ahUKEwimuOfm_pXXAhVrzFQKHTrOCaUQ6AEIKDAA|archive-date=15 January 2018|df=dmy-all}}</ref> representing the number of charge carriers within the [[Debye sphere]] is much higher than unity.<ref name="Chen, Francis F. 2016 2–3"/><ref name="Freidberg, Jeffrey P. 2008 121"/> 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.<ref>{{Cite book| publisher = WORLD SCIENTIFIC| isbn = 978-981-02-3305-1|id={{isbnt| 978-981-281-554-5}}| last1 = Fortov| first1 = Vladimir E| last2 = Iakubov| first2 = Igor T| title = The Physics of Non-Ideal Plasma| access-date = 2021-03-19| date = November 1999| doi = 10.1142/3634| url = http://www.worldscientific.com/worldscibooks/10.1142/3634}}</ref>
*'''Bulk interactions''': The [[Debye length]] 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.<ref>{{Cite web|url=http://www.plasma-universe.com/Quasi-neutrality|title=Quasi-neutrality - The Plasma Universe theory (Wikipedia-like Encyclopedia)|website=www.plasma-universe.com|language=en|access-date=2017-10-25|url-status=live|archive-url=https://web.archive.org/web/20171026001653/http://www.plasma-universe.com/Quasi-neutrality|archive-date=26 October 2017|df=dmy-all}}</ref>
*'''Collisionlessness''': The electron plasma frequency (measuring [[plasma oscillation]]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.<ref>{{Cite journal| doi = 10.1070/PU1997v040n01ABEH000200| issn = 1063-7869| volume = 40| issue = 1| pages = 21–51| last = Klimontovich| first = Yu L.| title = Physics of collisionless plasma| journal = Physics-Uspekhi| access-date = 2021-03-19| date = 1997-01-31| s2cid = 250739485| url = http://iopscience.iop.org/article/10.1070/PU1997v040n01ABEH000200/meta}}</ref>

===Non-neutral plasma===
{{main|Non-neutral plasmas}}
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 plasmas|non-neutral plasma]]. In such a plasma, electric fields play a dominant role. Examples are charged [[particle beam]]s, an electron cloud in a [[Penning trap]] and positron plasmas.<ref>{{Cite journal | last1 = Greaves | first1 = R. G. | last2 = Tinkle | first2 = M. D. | last3 = Surko | first3 = C. M. | doi = 10.1063/1.870693 | title = Creation and uses of positron plasmas | journal = Physics of Plasmas | volume = 1 | issue = 5 | pages = 1439 | year = 1994 |bibcode = 1994PhPl....1.1439G }}</ref>

===Dusty plasma===
{{main|Dusty plasma}}
A [[dusty plasma]] 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''.<ref>{{cite journal |last1=Morfill |first1=G. E. |title= Complex plasmas: An interdisciplinary research field |journal=Reviews of Modern Physics |volume=81 |date=2009 |pages=1353–1404 |doi=10.1103/RevModPhys.81.1353|bibcode = 2009RvMP...81.1353M |first2=Alexei V. |last2=Ivlev |issue=4 }}</ref>

==Properties and parameters==
[[File:plasma fountain.gif|thumb|upright=1.2|right|Artist's [[artistic rendering|rendition]] of the Earth's [[plasma fountain]], showing oxygen, helium, and hydrogen ions that gush into space from regions near the Earth's poles. The faint yellow area shown above the north pole represents gas lost from Earth into space; the green area is the [[aurora borealis]], where plasma energy pours back into the atmosphere.<ref>{{Cite web|title=Plasma Fountain|url=https://pwg.gsfc.nasa.gov/istp/news/9812/solar1.html|access-date=2023-02-10|website=pwg.gsfc.nasa.gov}}</ref>]]

===Density and ionization degree===
For plasma to exist, [[ionization]] is necessary. The term "plasma density" by itself usually refers to the electron density <math>n_e</math>, that is, the number of charge-contributing electrons per unit volume. The degree of ionization <math>\alpha</math> is defined as fraction of neutral particles that are ionized:

<math display="block">\alpha = \frac{n_i}{n_i + n_n},</math>

where <math>n_i</math> is the ion density and <math>n_n</math> the neutral density (in number of particles per unit volume). In the case of fully ionized matter, <math>\alpha = 1</math>. Because of the quasineutrality of plasma, the electron and ion densities are related by <math>n_e = \langle Z_i\rangle n_i</math>, where <math>\langle Z_i\rangle</math> is the average ion charge (in units of the [[elementary charge]]).

===Temperature===
Plasma temperature, commonly measured in [[kelvin]] or [[electronvolt]]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]] relative to the [[ionization energy]] (and more weakly by the density). In [[thermal equilibrium]], the relationship is given by the [[Saha equation]]. At low temperatures, ions and electrons tend to recombine into bound states—atoms<ref name="Nicholson">{{cite book |title=Introduction to Plasma Theory |last=Nicholson |first= Dwight R. |date=1983 |publisher=John Wiley & Sons |isbn=978-0-471-09045-8}}</ref>—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 (physics)|distribution function]] is close to a [[Maxwell–Boltzmann distribution|Maxwellian]] even in the presence of strong [[electric field|electric]] or [[magnetic field|magnetic]] 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]] while electrons reach thousands of kelvin.<ref>{{Cite book |last=Hamrang |first=Abbas |title=Advanced Non-Classical Materials with Complex Behavior: Modeling and Applications, Volume 1 |publisher=CRC Press |year=2014 |pages=10}}</ref> The opposite case is the [[z-pinch]] plasma where the ion temperature may exceed that of electrons.<ref>{{Cite journal| doi = 10.1063/5.0009432| issn = 1070-664X| volume = 27| issue = 6| pages = 060901| last = Maron| first = Yitzhak| title = Experimental determination of the thermal, turbulent, and rotational ion motion and magnetic field profiles in imploding plasmas| journal = Physics of Plasmas| date = 2020-06-01| bibcode = 2020PhPl...27f0901M| doi-access = free}}</ref>

{{see also|Nonthermal plasma|Anisothermal plasma}}

===Plasma potential===
[[File:Bliksem in Assen.jpg|thumb|upright=1.6|[[Lightning]] as an example of plasma present at Earth's surface:
Typically, lightning discharges 30 kiloamperes at up to 100 megavolts, and emits radio waves, light, X- and even gamma rays.<ref>{{Cite web | author= NASA Administrator |date=2013-06-07 |title=Flashes in the Sky: Earth's Gamma-Ray Bursts Triggered by Lightning |url=http://www.nasa.gov/vision/universe/solarsystem/rhessi_tgf.html |access-date=2023-02-10 |website=NASA |language=en}}</ref> Plasma temperatures can approach 30000 K and electron densities may exceed 10<sup>24</sup> m<sup>−3</sup>.]]
Since plasmas are very good [[electrical conductor]]s, electric potentials play an important role.{{clarify|what role?|date=October 2017}} 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 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 (<math>n_e = \langle Z\rangle n_i</math>), but on the scale of the [[Debye length]], there can be charge imbalance. In the special case that ''[[Double layer (plasma)|double layers]]'' are formed, the charge separation can extend some tens of Debye lengths.<ref>{{Cite journal| doi = 10.1007/BF00642580 | issn = 1572-946X| volume = 55| issue = 1| pages = 59–83| last = Block| first = Lars P.| title = A double layer review| journal = Astrophysics and Space Science| accessdate = 2021-07-15| date = 1978| s2cid = 122977170| url = https://doi.org/10.1007/BF00642580}}</ref>

The magnitude of the potentials and electric fields must be determined by means other than simply finding the net [[charge density]]. A common example is to assume that the electrons satisfy the [[Boltzmann relation]]:
<math display="block">n_e \propto \exp(e\Phi/k_\text{B} T_e).</math>

Differentiating this relation provides a means to calculate the electric field from the density:
<math display="block">\vec{E} = \frac{k_\text{B} T_e}{e} \frac{\nabla n_e}{n_e}.</math>

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]].<ref>{{Cite book|title=Plasma science: from fundamental research to technological applications |date=1995 | publisher=National Academy Press | others=National Research Council (U.S.). Panel on Opportunities in Plasma Science and Technology |isbn=9780309052313 |location=Washington, D.C. |pages=51 |oclc=42854229}}</ref>

===Magnetization===
The existence of charged particles causes the plasma to generate, and be affected by, [[magnetic field]]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., <math>\nu_{\mathrm{ce}} / \nu_{\mathrm{coll}} > 1</math>, where <math>\nu_{\mathrm{ce}}</math> is the electron [[gyrofrequency]] and <math>\nu_{\mathrm{coll}}</math> is the electron collision rate. It is often the case that the electrons are magnetized while the ions are not. Magnetized plasmas are ''[[anisotropic]]'', 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 <math>\mathbf{v}</math> in the magnetic field <math>\mathbf{B}</math> is given by the usual [[Lorentz force|Lorentz formula]] <math>\mathbf{E} = -\mathbf{v}\times\mathbf{B}</math>, and is not affected by [[Debye shielding]].<ref>{{Cite web|first=Richard |last=Fitzpatrick |website=Introduction to Plasma Physics |title=Magnetized Plasmas|url=https://farside.ph.utexas.edu/teaching/plasma/lectures/node10.html|access-date=2023-02-10}}</ref>

==Mathematical descriptions==
[[File:Magnetic rope.svg|thumb|The complex self-constricting magnetic field lines and current paths in a field-aligned [[Birkeland current]] that can develop in a plasma.<ref>{{Cite web|title=chapter 15|url=https://history.nasa.gov/SP-345/ch15.htm#250|access-date=2023-02-10|website=history.nasa.gov|date=January 1976 |last1=Alfven |first1=H. |last2=Arrhenius |first2=G. }}</ref>]]
{{main|Plasma modeling}}
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.{{Citation needed|date=January 2021}} 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]]). One simple fluid model, [[magnetohydrodynamics]], treats the plasma as a single fluid governed by a combination of [[Maxwell's equations]] and the [[Navier–Stokes equations]]. A more general description is the two-fluid plasma,<ref>{{cite journal |last1=Roy |first1=Subrata |last2=Pandey |first2=B. P. |title=Numerical investigation of a Hall thruster plasma |journal=Physics of Plasmas |date=September 2002 |volume=9 |issue=9 |pages=4052–4060 |doi=10.1063/1.1498261|bibcode=2002PhPl....9.4052R |hdl=2027.42/70486 |hdl-access=free }}</ref> 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]]. 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 layer (plasma)|double layer]]s, nor resolve wave-particle effects.{{Citation needed|date=January 2021}}

===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]]. 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-cell]] (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 equation]] may be used to describe the dynamics of a system of charged particles interacting with an electromagnetic field.
In magnetized plasmas, a [[gyrokinetics|gyrokinetic]] approach can substantially reduce the computational expense of a fully kinetic simulation.{{Citation needed|date=January 2021}}

==Plasma science and technology==

Plasmas are studied by the vast [[academic field]] of ''plasma science'' or ''plasma physics'', including several sub-disciplines such as [[space plasma physics]].

Plasmas can appear in nature in various forms and locations, with a few examples given in the following table:

{{clear}}

{| class="wikitable"
|+ Common forms of plasma
|-
! style="width: 40%;"|Artificially produced
! style="width: 30%;"|Terrestrial plasmas
! style="width: 30%;"|Space and astrophysical plasmas
|- style="vertical-align: top;"
|
*In [[plasma displays]], including TV screens.
*Inside [[fluorescent lamp]]s (low energy lighting), [[neon sign]]s
*Rocket exhaust and [[ion thruster]]s
*The area in front of a [[spacecraft]]'s [[heat shield]] during [[Atmospheric entry|re-entry]] into the [[earth's atmosphere|atmosphere]]
*Plasmas in [[fusion energy]] research
*[[Plasma globe]] (sometimes called plasma sphere or plasma ball)
*[[Laser]]-produced plasmas (LPP), found when high power lasers interact with materials
|
*[[Lightning]]
*The [[magnetosphere]] contains plasma in the Earth's surrounding space environment
*The [[ionosphere]]
*The [[Aurora (astronomy)|polar aurorae]]
*[[Upper-atmospheric lightning]], including [[Sprite (lightning)|sprites]], blue jets, blue starters, gigantic jets, ELVESs
*[[St. Elmo's fire]]
*[[Fire]] (if sufficiently hot)
|
*[[Star]]s (plasmas heated by [[nuclear fusion]])
*The [[solar wind]]
*The [[interplanetary medium]] (space between planets)
*The [[interstellar medium]] (space between star systems)
*The [[Outer space#Intergalactic space|Intergalactic medium]]<br />(space between galaxies)
*[[Accretion disk]]s
*Interstellar [[nebula]]e
|}

===Space and astrophysics===

{{further|Astrophysical plasma}}
Plasmas are by far the most common [[Phase (matter)|phase of ordinary matter]] in the universe, both by mass and by volume.<ref>{{Cite book|last1=Gurnett|first1=D. A.|url=https://books.google.com/books?id=VcueZlunrbcC&pg=PA2%257CPAGE=2%257CISBN=978-0-521-36483-6%257CPUBLISHER|title=Introduction to Plasma Physics: With Space and Laboratory Applications|last2=Bhattacharjee|first2=A.|date=2005-01-06|publisher=Cambridge University Press|isbn=978-0-521-36483-6|language=en}}{{cite book|last1=Scherer |first1=K. |last2=Fichtner |first2=H. |last3=Heber |first3=B. |title=Space Weather: The Physics Behind a Slogan|date=2005|url=https://books.google.com/books?id=irHgIUtLi0gC&pg=PA138|page=138|isbn=978-3-540-22907-0|publisher=Springer|location=Berlin}}.</ref>

Above the Earth's surface, the ionosphere is a plasma,<ref>{{cite book |last=Kelley |first=M. C. |title=The Earth's Ionosphere: Plasma Physics and Electrodynamics |date=2009 |publisher=Academic Press |isbn=9780120884254 |edition=2nd}}</ref> and the magnetosphere contains plasma.<ref>{{cite journal|last=Russell|first=C.T.|title=The Magnetopause|journal=Physics of Magnetic Flux Ropes|series=Geophysical Monograph Series|date=1990|volume=58|pages=439–453|doi=10.1029/GM058p0439|bibcode=1990GMS....58..439R|isbn=0-87590-026-7|url=http://www-ssc.igpp.ucla.edu/ssc/tutorial/magnetopause.html|access-date=25 August 2018|archive-url=https://web.archive.org/web/20120503220342/http://www-ssc.igpp.ucla.edu/ssc/tutorial/magnetopause.html|archive-date=3 May 2012|url-status=dead}}</ref> Within our Solar System, [[interplanetary space]] is filled with the plasma expelled via the [[solar wind]], extending from the Sun's surface out to the [[Heliopause (astronomy)|heliopause]]. Furthermore, all the distant [[star]]s, and much of [[Interstellar medium|interstellar space]] or [[Outer space#Intergalactic space|intergalactic space]] is also filled with plasma, albeit at very low densities. [[Astrophysical plasma]]s are also observed in [[accretion disk]]s around stars or compact objects like [[white dwarf]]s, [[neutron star]]s, or [[black holes]] in close [[binary star]] systems.<ref>{{Cite book|last=Mészáros|first=Péter|url=https://books.google.com/books?id=NXvE_zQX5kAC&dq=%2522Black%2520hole%2522%2520plasma%2520acreting&pg=PA99|title=The High Energy Universe: Ultra-High Energy Events in Astrophysics and Cosmology|date=2010-09-16|publisher=Cambridge University Press|isbn=978-1-139-49072-6|language=en}}</ref> Plasma is associated with ejection of material in [[astrophysical jets]], which have been observed with accreting black holes<ref>{{Cite book|last1=Raine|first1=Derek J.|url=https://books.google.com/books?id=O3puAMw5U3UC&pg=PA160|title=Black Holes: An Introduction|last2=Thomas|first2=Edwin George|date=2010|publisher=Imperial College Press|isbn=978-1-84816-382-9|language=en}}</ref> or in active [[galaxy|galaxies]] like [[Messier 87#Jet|M87's jet]] that possibly extends out to 5,000 light-years.<ref>{{Cite web|title=APOD: 2004 December 11 - M87's Energetic Jet|url=https://apod.nasa.gov/apod/ap041211.html|access-date=2023-02-10|website=apod.nasa.gov}}</ref>

===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{{Citation needed|date=January 2021}}
*The pressure they operate at—vacuum pressure (< 10&nbsp;mTorr or 1 Pa), moderate pressure (≈1&nbsp;Torr or 100 Pa), atmospheric pressure (760&nbsp;Torr or 100 kPa){{Citation needed|date=January 2021}}
*The degree of ionization within the plasma—fully, partially, or weakly ionized{{Citation needed|date=January 2021}}
*The temperature relationships within the plasma—thermal plasma (<math>T_e = T_i = T_\text{gas}</math>), non-thermal or "cold" plasma (<math>T_e \gg T_i = T_\text{gas}</math>){{Citation needed|date=January 2021}}
*The electrode configuration used to generate the plasma{{Citation needed|date=January 2021}}
*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 force]]s){{citation needed|date=February 2017}}

====Generation of artificial plasma====
[[File:Simple representation of a discharge tube - plasma.png|thumb]]
[[File:Plasma jacobs ladder.jpg|thumb|alt=Artificial plasma produced in air by a Jacob's Ladder|Artificial plasma produced in air by a [[Electric arc#Visual entertainment|Jacob's Ladder]]]]
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.<ref name="Hippler" /> 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).{{Citation needed|date=January 2021}}

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.<ref name="Chen">{{cite book |title=Plasma Physics and Controlled Fusion |last=Chen |first=Francis F. |date=1984 |publisher=Plenum Press |isbn=978-0-306-41332-2 |url=https://books.google.com/books?id=WGbaBwAAQBAJ&q=editions:9PGss7GnX-MC |url-status=live |archive-url=https://web.archive.org/web/20180115215631/https://books.google.com/books?id=WGbaBwAAQBAJ&printsec=frontcover&dq=editions:9PGss7GnX-MC&hl=en&sa=X&ved=0ahUKEwimuOfm_pXXAhVrzFQKHTrOCaUQ6AEIKDAA |archive-date=15 January 2018 |df=dmy-all }}</ref> 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",<ref name="Leal-Quiros" /> mainly due to a small mean free path (average distance travelled between collisions).{{Citation needed|date=January 2021}}

=====Electric arc=====
[[File:Cascade-process-of-ionization.svg|thumb|Cascade process of ionization. Electrons are "e−", neutral atoms "o", and cations "+".|left]]
[[File:Townsend Discharge.svg|thumbnail|300px|Avalanche effect between two electrodes. The original ionization event liberates one electron, and each subsequent collision liberates a further electron, so two electrons emerge from each collision: the ionizing electron and the liberated electron.]]
[[Electric arc]] is a continuous electric discharge between two electrodes, similar to [[lightning]].
With ample current density, the discharge forms a luminous arc, where the inter-electrode material (usually, a gas) undergoes various stages — saturation, breakdown, glow, transition, and thermal arc. The voltage rises to its maximum in the saturation stage, and thereafter it undergoes fluctuations of the various stages, while the current progressively increases throughout.<ref name="Leal-Quiros">{{cite journal |author=Leal-Quirós, Edbertho |date=2004 |title=Plasma Processing of Municipal Solid Waste |journal= Brazilian Journal of Physics |volume=34 |issue=4B |pages=1587–1593 |bibcode = 2004BrJPh..34.1587L |doi=10.1590/S0103-97332004000800015|doi-access=free }}</ref> [[Electrical resistance]] along the arc creates [[heat]], which dissociates more gas molecules and ionizes the resulting atoms. Therefore, the [[electrical energy]] is given to electrons, which, due to their great mobility and large numbers, are able to disperse it rapidly by [[elastic collision]]s to the heavy particles.<ref name="Gomez" />

====Examples of industrial plasma====
Plasmas find applications in many fields of research, technology and industry, for example, in industrial and extractive [[metallurgy]],<ref name="Gomez">{{Cite journal | last1 = Gomez | first1 = E. | last2 = Rani | first2 = D. A. | last3 = Cheeseman | first3 = C. R. | last4 = Deegan | first4 = D. | last5 = Wise | first5 = M. | last6 = Boccaccini | first6 = A. R. | doi = 10.1016/j.jhazmat.2008.04.017 | title = Thermal plasma technology for the treatment of wastes: A critical review | journal = Journal of Hazardous Materials | volume = 161 | issue = 2–3 | pages = 614–626 | year = 2009 | pmid = 18499345| s2cid = 206069219 }}</ref><ref name="Szałatkiewicz">{{Cite journal | last1 = Szałatkiewicz | first1 = J. | doi = 10.3390/ma9080683 | title = Metals Recovery from Artificial Ore in Case of Printed Circuit Boards, Using Plasmatron Plasma Reactor | journal = Materials | volume = 9 | issue = 8| pages = 683–696 | year = 2016 | pmid = 28773804| pmc = 5512349| bibcode = 2016Mate....9..683S| doi-access = free }}</ref> surface treatments such as [[plasma spraying]] (coating), [[etching]] in microelectronics,<ref name="NRC">{{cite book |author= National Research Council |date=1991 |title=Plasma Processing of Materials : Scientific Opportunities and Technological Challenges |publisher=National Academies Press |isbn= 978-0-309-04597-1}}</ref> metal cutting<ref name="Nemchinsky">{{Cite journal | last1 = Nemchinsky | first1 = V. A. | last2 = Severance | first2 = W. S. | doi = 10.1088/0022-3727/39/22/R01 | title = What we know and what we do not know about plasma arc cutting | journal = Journal of Physics D: Applied Physics | volume = 39 | issue = 22 | pages = R423 | year = 2006 |bibcode = 2006JPhD...39R.423N | s2cid = 116995929 }}</ref> and [[welding]]; as well as in everyday [[Vehicle emissions control|vehicle exhaust cleanup]] and [[Fluorescent lamp|fluorescent]]/[[Electroluminescence|luminescent]] lamps,<ref name="Hippler">{{cite book |editor=Hippler, R. |editor2=Kersten, H. |editor3=Schmidt, M. |editor4=Schoenbach, K.M. |date=2008 |title=Low Temperature Plasmas: Fundamentals, Technologies, and Techniques |chapter=Plasma Sources |publisher=Wiley-VCH |edition=2nd |isbn=978-3-527-40673-9}}</ref> fuel ignition, and even in [[Scramjet|supersonic combustion engines]] for [[aerospace engineering]].<ref name="Peretich">{{cite journal |author=Peretich, M.A. |author2=O'Brien, W.F. |author3=Schetz, J.A. |date=2007 |title=Plasma torch power control for scramjet application |publisher=Virginia Space Grant Consortium |url=http://www.vsgc.odu.edu/src/SRC07/SRC07papers/Mark%20Peretich%20_%20PaperFinal%20Report.pdf |access-date=12 April 2010 |url-status=dead |archive-url=https://web.archive.org/web/20100629193719/http://www.vsgc.odu.edu/src/SRC07/SRC07papers/Mark%20Peretich%20_%20PaperFinal%20Report.pdf |archive-date=29 June 2010 |df=dmy-all }}</ref>

=====Low-pressure discharges=====
*''[[Glow discharge]] plasmas'': non-thermal plasmas generated by the application of DC or low frequency RF (<100&nbsp;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.<ref>{{cite web |url=http://www-spof.gsfc.nasa.gov/Education/wfluor.html |title=The Fluorescent Lamp: A plasma you can use |author=Stern, David P. |access-date=2010-05-19 |url-status=live |archive-url=https://web.archive.org/web/20100530165958/http://www-spof.gsfc.nasa.gov/Education/wfluor.html |archive-date=30 May 2010 |df=dmy-all }}</ref>
*''[[Capacitively coupled plasma]] (CCP)'': similar to glow discharge plasmas, but generated with high frequency RF electric fields, typically [[ISM band|13.56&nbsp;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.<ref>{{cite journal |last1=Sobolewski |first1=M.A. |last2=Langan & Felker |first2=J.G. & B.S. |date=1997 |title=Electrical optimization of plasma-enhanced chemical vapor deposition chamber cleaning plasmas |journal=Journal of Vacuum Science and Technology B |volume=16 |issue=1 |pages=173–182 |url=http://physics.nist.gov/MajResProj/rfcell/Publications/MAS_JVSTB16_1.pdf | archive-url=https://web.archive.org/web/20090118212957/http://www.physics.nist.gov/MajResProj/rfcell/Publications/MAS_JVSTB16_1.pdf | archive-date=18 January 2009 |doi=10.1116/1.589774|bibcode = 1998JVSTB..16..173S }}</ref>
*''[[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.<ref>{{Cite journal | last1 = Okumura | first1 = T. | doi = 10.1155/2010/164249 | title = Inductively Coupled Plasma Sources and Applications | journal = Physics Research International | volume = 2010 | pages = 1–14 | year = 2010 | doi-access = free }}</ref>
*''[[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).<ref>{{cite book|title=Plasma Chemistry|date=2008|publisher=Cambridge University Press|page=229|url=https://books.google.com/books?id=ZzmtGEHCC9MC&pg=PA229|isbn=9781139471732|url-status=live|archive-url=https://web.archive.org/web/20170202060021/https://books.google.com/books?id=ZzmtGEHCC9MC&pg=PA229|archive-date=2 February 2017|df=dmy-all}}</ref>

=====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 Al<sub>2</sub>O<sub>3</sub> to produce [[aluminium]].{{Citation needed|date=January 2021}}
*''[[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.{{Citation needed|date=January 2021}}
*''[[Dielectric barrier discharge]] (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.<ref>{{Cite journal | last1 = Roy | first1 = S. | last2 = Zhao | first2 = P. | last3 = Dasgupta | first3 = A. | last4 = Soni | first4 = J. | title = Dielectric barrier discharge actuator for vehicle drag reduction at highway speeds| doi = 10.1063/1.4942979 | journal = AIP Advances | volume = 6 | issue = 2 | pages = 025322 | year = 2016 | bibcode = 2016AIPA....6b5322R | doi-access = free }}</ref> It is also widely used in the web treatment of fabrics.<ref>{{Cite journal | last1 = Leroux | first1 = F. | last2 = Perwuelz | first2 = A. | last3 = Campagne | first3 = C. | last4 = Behary | first4 = N. | s2cid = 137392051 | title = Atmospheric air-plasma treatments of polyester textile structures | doi = 10.1163/156856106777657788 | journal = Journal of Adhesion Science and Technology | volume = 20 | issue = 9 | pages = 939–957 | year = 2006 }}</ref> The application of the discharge to synthetic fabrics and plastics functionalizes the surface and allows for paints, glues and similar materials to adhere.<ref>{{Cite journal
| last1 = Leroux | first1 = F. D. R.
| last2 = Campagne | first2 = C.
| last3 = Perwuelz | first3 = A.
| last4 = Gengembre | first4 = L. O.
| title = Polypropylene film chemical and physical modifications by dielectric barrier discharge plasma treatment at atmospheric pressure
| doi = 10.1016/j.jcis.2008.09.062
| journal = Journal of Colloid and Interface Science
| volume = 328
| issue = 2
| pages = 412–420
| year = 2008
| pmid = 18930244
| bibcode = 2008JCIS..328..412L
}}</ref> The dielectric barrier discharge was used in the mid-1990s to show that low temperature atmospheric pressure plasma is effective in inactivating bacterial cells.<ref>{{cite journal|doi=10.1109/27.533129|title=Sterilization of contaminated matter with an atmospheric pressure plasma|journal=IEEE Transactions on Plasma Science|volume=24|issue=3|pages=1188–1191|year=1996|last1=Laroussi|first1=M.|bibcode=1996ITPS...24.1188L}}</ref> 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.<ref>{{cite journal|doi=10.1016/j.physrep.2014.02.006|title=Guided ionization waves: Theory and experiments|journal=Physics Reports|volume=540|issue=3|pages=123|year=2014|last1=Lu|first1=X.|last2=Naidis|first2=G.V.|last3=Laroussi|first3=M.|last4=Ostrikov|first4=K.|bibcode=2014PhR...540..123L}}</ref>
*''[[Capacitive discharge]]:'' this is a [[nonthermal plasma]] generated by the application of RF power (e.g., [[ISM band|13.56&nbsp;MHz]]) to one powered electrode, with a grounded electrode held at a small separation distance on the order of 1&nbsp;cm. Such discharges are commonly stabilized using a noble gas such as helium or argon.<ref>{{Cite journal | last1 = Park | first1 = J. | last2 = Henins | first2 = I. | last3 = Herrmann | first3 = H. W. | last4 = Selwyn | first4 = G. S. | last5 = Hicks | first5 = R. F. | title = Discharge phenomena of an atmospheric pressure radio-frequency capacitive plasma source | doi = 10.1063/1.1323753 | journal = Journal of Applied Physics | volume = 89 | issue = 1 | pages = 20 | year = 2001 |bibcode = 2001JAP....89...20P | url = https://zenodo.org/record/1231852 }}</ref>
*"[[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.{{Citation needed|date=January 2021}}

====MHD converters====
{{main|magnetohydrodynamic converter|magnetohydrodynamic generator|magnetohydrodynamic drive}}
{{see also|Electrothermal instability}}
A world effort was triggered in the 1960s to study [[magnetohydrodynamic converter]]s 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 wave]]s, lower thermal transfer and reduce [[Drag (physics)|drag]].{{Citation needed|date=January 2021}}

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.<ref>{{Cite book|title=Plasma Scattering of Electromagnetic Radiation: Theory and Measurement Techniques |date=2011|publisher=Academic Press/Elsevier |url=https://books.google.com/books?id=1NS5Fxam1lkC&pg=PA273 |first1=J. |last1=Sheffield |first2=D. |last2=Froula |first3=S. H. |last3=Glenzer |first4=N. C. Jr. |last4=Luhmann |isbn=978-0080952031|edition=1st ed., 2nd|location=Burlington, MA|pages=273|oclc=690642377}}</ref> 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 tensor]]s 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.{{Citation needed|date=January 2021}}

==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.{{Citation needed|date=January 2021}} Examples of complexity and complex structures in plasmas include:

===Filamentation===
Striations or string-like structures,<ref>{{cite journal|author=Dickel, J. R.|bibcode=1990BAAS...22..832D |title=The Filaments in Supernova Remnants: Sheets, Strings, Ribbons, or?|date=1990|journal=Bulletin of the American Astronomical Society|volume= 22|page=832}}</ref> also known as [[Birkeland current]]s, are seen in many plasmas, like the [[plasma globe|plasma ball]], the [[Aurora (astronomy)|aurora]],<ref>{{Cite journal | last1 = Grydeland | first1 = T. | doi = 10.1029/2002GL016362 | title = Interferometric observations of filamentary structures associated with plasma instability in the auroral ionosphere | journal = Geophysical Research Letters | volume = 30 | issue = 6 | pages = 1338 | year = 2003 | bibcode=2003GeoRL..30.1338G| doi-access = free }}</ref> [[lightning]],<ref>{{Cite journal | last1 = Moss | first1 = G. D. | last2 = Pasko | first2 = V. P. | last3 = Liu | first3 = N. | last4 = Veronis | first4 = G. | title = Monte Carlo model for analysis of thermal runaway electrons in streamer tips in transient luminous events and streamer zones of lightning leaders | doi = 10.1029/2005JA011350 | journal = Journal of Geophysical Research | volume = 111 | issue = A2 | pages = A02307 | year = 2006 | bibcode=2006JGRA..111.2307M| doi-access = free }}</ref> [[electric arc]]s, [[solar flares]],<ref>{{cite journal|author=Doherty, Lowell R.|doi=10.1086/148107|title=Filamentary Structure in Solar Prominences|date=1965|journal=The Astrophysical Journal|volume=141|page=251|last2=Menzel|first2=Donald H.|bibcode=1965ApJ...141..251D}}</ref> and [[supernova remnant]]s.<ref>{{cite web |url=http://seds.lpl.arizona.edu/messier/more/m001_hst.html |title=Hubble views the Crab Nebula M1: The Crab Nebula Filaments |access-date=2017-01-26 |url-status=bot: unknown |archive-url=https://web.archive.org/web/20091005084515/http://seds.lpl.arizona.edu/messier/more/m001_hst.html |archive-date=5 October 2009 |df=dmy-all }}. The University of Arizona</ref> They are sometimes associated with larger current densities, and the interaction with the magnetic field can form a [[magnetic rope]] structure.<ref>{{Cite journal | last1 = Zhang | first1 = Y. A. | last2 = Song | first2 = M. T. | last3 = Ji | first3 = H. S. | doi = 10.1016/S0275-1062(02)00095-4 | title = A rope-shaped solar filament and a IIIb flare | journal = Chinese Astronomy and Astrophysics | volume = 26 | issue = 4 | pages = 442–450 | year = 2002 |bibcode = 2002ChA&A..26..442Z }}</ref> (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.<ref>{{Cite book|author=Chin, S. L. |title=Progress in Ultrafast Intense Laser Science III|url=http://icpr.snu.ac.kr/resource/wop.pdf/J01/2006/049/S01/J012006049S010281.pdf|journal=Journal of the Korean Physical Society|volume=49|date=2006|page=281|chapter=Some Fundamental Concepts of Femtosecond Laser Filamentation|bibcode=2008pui3.book..243C|doi=10.1007/978-3-540-73794-0_12|series=Springer Series in Chemical Physics|isbn=978-3-540-73793-3}}</ref> One interesting aspect of the filamentation generated plasma is the relatively low ion density due to defocusing effects of the ionized electrons.<ref>{{Cite journal | last1 = Talebpour | first1 = A. | last2 = Abdel-Fattah | first2 = M. | last3 = Chin | first3 = S. L. | doi = 10.1016/S0030-4018(00)00903-2 | title = Focusing limits of intense ultrafast laser pulses in a high pressure gas: Road to new spectroscopic source | journal = Optics Communications | volume = 183 | issue = 5–6 | pages = 479–484 | year = 2000 | bibcode=2000OptCo.183..479T}}</ref> (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.<ref>{{cite journal |last1=Alfvén |first1=H. |last2=Smårs |first2=E.|s2cid=26797662 |title= Gas-Insulation of a Hot Plasma |journal=Nature |volume=188 |date=1960 |pages=801–802 |doi=10.1038/188801a0|bibcode = 1960Natur.188..801A |issue=4753 }}</ref> 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.<ref>{{cite journal |last1=Braams |first1=C.M. |title= Stability of Plasma Confined by a Cold-Gas Blanket |journal=Physical Review Letters |volume=17 |issue=9 |date=1966 |pages=470–471 |doi=10.1103/PhysRevLett.17.470|bibcode = 1966PhRvL..17..470B }}</ref> 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]].<ref>{{cite journal |last1=Yaghoubi |first1=A. |last2=Mélinon |first2=P.|title= Tunable synthesis and in situ growth of silicon-carbon mesostructures using impermeable plasma |journal=Scientific Reports |volume=3 |pages=1083 |date=2013 |doi=10.1038/srep01083|bibcode = 2013NatSR...3E1083Y |pmid=23330064 |pmc=3547321}}</ref>

==Gallery==
<gallery>
File:HallThruster 2.jpg|[[Hall-effect thruster]]
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|Plasma in a [[tokamak]]
File:Argon Plasma.jpg|Laboratory plasma
</gallery>

==See also==
{{portal|Physics}}
{{colbegin|colwidth=20em}}
*[[Quark–gluon plasma|Quark-gluon plasma]]
{{colend}}
{{Table_of_phase_transitions}}

==References==
{{Reflist|30em}}

==External links==
{{Sister project links | wikt=plasma | commons=Category:Plasma physics | b=Wikijunior:The Elements/Plasma | q=no | s=Special:Search/Plasma physics | v=Plasma | d=y| n=Special:Search/Plasma physics}}
*[http://fusedweb.llnl.gov/CPEP/Chart_Pages/5.Plasma4StateMatter.html Plasmas: the Fourth State of Matter] {{Webarchive|url=https://web.archive.org/web/20190930230004/http://fusedweb.llnl.gov/CPEP/Chart_Pages/5.Plasma4StateMatter.html |date=30 September 2019 }}
*Introduction to Plasma Physics: [https://web.archive.org/web/20100104142353/http://farside.ph.utexas.edu/teaching/plasma/lectures/lectures.html Graduate course given by Richard Fitzpatrick]|[http://silas.psfc.mit.edu/introplasma/index.html M.I.T. Introduction by I.H.Hutchinson]
*[https://web.archive.org/web/20050308192828/http://starfire.ne.uiuc.edu/ Plasma Material Interaction]
*[http://c3po.barnesos.net/homepage/lpl/grapeplasma/ How to make a glowing ball of plasma in your microwave with a grape] {{Webarchive|url=https://web.archive.org/web/20050906205014/http://c3po.barnesos.net/homepage/lpl/grapeplasma/ |date=6 September 2005 }}|[http://stewdio.org/plasma/ More (Video)]
*[http://comphys.narod.ru OpenPIC3D&nbsp;– 3D Hybrid Particle-In-Cell simulation of plasma dynamics]
*[http://plasma-gate.weizmann.ac.il/pf/ Plasma Formulary Interactive]

{{States of matter}}
{{Branches of physics}}

{{Authority control}}

[[Category:Electromagnetism]]
[[Category:Articles containing video clips]]
[[Category:Astrophysics]]
[[Category:Electrical conductors]]
[[Category:Gases]]
[[Category:Phases of matter|*Plasma]]
[[Category:Plasma physics| ]]