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Plasma (physics)
For other uses, see Plasma.
Plasma (from Greek , anything formed[1]) isone of the four
fundamental states of matter, the othersbeing solid, liquid, and
gas. A plasma has properties un-like those of the other states.A
plasma can be created by heating a gas or subjectingit to a strong
electromagnetic eld applied with a laseror microwave generator.
This decreases or increases thenumber of electrons, creating
positive or negative chargedparticles called ions,[2] and is
accompanied by the disso-ciation of molecular bonds, if
present.[3]
The presence of a signicant number of charge carriersmakes
plasma electrically conductive so that it respondsstrongly to
electromagnetic elds. Like gas, plasma doesnot have a denite shape
or a denite volume unless en-closed in a container. Unlike gas,
under the inuence ofa magnetic eld, it may form structures such as
laments,beams and double layers.Plasma is the most abundant form of
ordinary matter inthe Universe, most of which is in the rareed
intergalacticregions, particularly the intracluster medium, and in
stars,including the Sun.[4][5] A common form of plasmas onEarth is
seen in neon signs.Much of the understanding of plasmas has come
from thepursuit of controlled nuclear fusion and fusion power,
forwhich plasma physics provides the scientic basis.
1 Properties and parameters
1.1 Denition
Plasma is loosely described as an electrically neutralmedium of
unbound positive and negative particles (i.e.the overall charge of
a plasma is roughly zero). It is im-portant to note that although
they are unbound, these par-ticles are not free in the sense of not
experiencing forces.When the charges move, they generate electrical
currentswith magnetic elds, and as a result, they are aected byeach
others elds. This governs their collective behav-ior with many
degrees of freedom.[3][7] A denition canhave three
criteria:[8][9]
1. The plasma approximation: Charged particlesmust be close
enough together that each particleinuences many nearby charged
particles, rather
Artists rendition of the Earths plasma fountain, showing
oxygen,helium, and hydrogen ions that gush into space from regions
nearthe Earths poles. The faint yellow area shown above the
northpole represents gas lost from Earth into space; the green area
isthe aurora borealis, where plasma energy pours back into
theatmosphere.[6]
than just interacting with the closest particle (thesecollective
eects are a distinguishing feature of aplasma). The plasma
approximation is valid whenthe number of charge carriers within the
sphere ofinuence (called the Debye sphere whose radius isthe Debye
screening length) of a particular particleis higher than unity to
provide collective behavior ofthe charged particles. The average
number of parti-cles in the Debye sphere is given by the plasma
pa-rameter, "" (the Greek uppercase letter Lambda).
2. Bulk interactions: The Debye screening length(dened above) is
short compared to the physicalsize of the plasma. This criterion
means that inter-actions in the bulk of the plasma are more
impor-tant than those at its edges, where boundary eectsmay take
place. When this criterion is satised, theplasma is
quasineutral.
3. Plasma frequency: The electron plasma frequency
1
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2 1 PROPERTIES AND PARAMETERS
(measuring plasma oscillations of the electrons) islarge
compared to the electron-neutral collision fre-quency (measuring
frequency of collisions betweenelectrons and neutral particles).
When this conditionis valid, electrostatic interactions dominate
over theprocesses of ordinary gas kinetics.
1.2 Ranges of parameters
Plasma parameters can take on values varying by manyorders of
magnitude, but the properties of plasmas withapparently disparate
parameters may be very similar (seeplasma scaling). The following
chart considers only con-ventional atomic plasmas and not exotic
phenomena likequark gluon plasmas:
10-2 10-1 100 101 102 103 104 105 eV10-5
100
105
1010
1020
1025
1015
ELEC
TRO
N D
ENSI
TYEl
ectr
ons
per
cubi
c ce
ntim
etre
RANGES OF PLASMAS
TEMPERATURE102 103 104 105 106 107 108 109 K
Photosphere
Flames
Metals
Magnetosphere
Solar wind
Ionosphere
Interstellar
Interplanetary
Galactic
Solar corona
Chromosphere
LasersCentre of Sun
Fusion
e-/cm3
Range of plasmas. Density increases upwards, temperature
in-creases towards the right. The free electrons in a metal may
beconsidered an electron plasma.[10]
1.3 Degree of ionization
For plasma to exist, ionization is necessary. The termplasma
density by itself usually refers to the electrondensity, that is,
the number of free electrons per unitvolume. The degree of
ionization of a plasma is the pro-portion of atoms that have lost
or gained electrons, andis controlled mostly by the temperature.
Even a par-tially ionized gas in which as little as 1% of the
parti-cles are ionized can have the characteristics of a
plasma(i.e., response to magnetic elds and high electrical
con-ductivity). The degree of ionization, , is dened as = nini+nn ,
where ni is the number density of ions andnn is the number density
of neutral atoms. The electrondensity is related to this by the
average charge state hZiof the ions through ne = hZini , where ne
is the numberdensity of electrons.
1.4 TemperaturesSee also: Nonthermal plasma
Plasma temperature is commonly measured in kelvins
orelectronvolts and is, informally, a measure of the ther-mal
kinetic energy per particle. Very high temperaturesare usually
needed to sustain ionization, which is a den-ing feature of a
plasma. The degree of plasma ioniza-tion is determined by the
electron temperature relativeto the ionization energy (and more
weakly by the den-sity), in a relationship called the Saha
equation. At lowtemperatures, ions and electrons tend to recombine
intobound statesatoms[12]and the plasma will eventuallybecome a
gas.In most cases the electrons are close enough to
thermalequilibrium that their temperature is relatively well-dened,
even when there is a signicant deviation froma Maxwellian energy
distribution function, for example,due to UV radiation, energetic
particles, or strong electricelds. Because of the large dierence in
mass, theelectrons come to thermodynamic equilibrium
amongstthemselves much faster than they come into equilibriumwith
the ions or neutral atoms. For this reason, the iontemperature may
be very dierent from (usually lowerthan) the electron temperature.
This is especially com-mon in weakly ionized technological plasmas,
where theions are often near the ambient temperature.
1.4.1 Thermal vs. non-thermal plasmas
Based on the relative temperatures of the electrons, ionsand
neutrals, plasmas are classied as thermal or non-thermal. Thermal
plasmas have electrons and the heavyparticles at the same
temperature, i.e. they are in ther-mal equilibrium with each other.
Non-thermal plas-mas on the other hand have the ions and neutrals
at amuch lower temperature (sometimes room temperature),whereas
electrons are much hotter ( Te Tn ).A plasma is sometimes referred
to as being hot if it isnearly fully ionized, or cold if only a
small fraction (forexample 1%) of the gas molecules are ionized,
but otherdenitions of the terms hot plasma and cold plasmaare
common. Even in a cold plasma, the electron tem-perature is still
typically several thousand degrees Celsius.Plasmas utilized in
plasma technology (technologicalplasmas) are usually cold plasmas
in the sense that onlya small fraction of the gas molecules are
ionized.
1.5 Plasma potentialSince plasmas are very good electrical
conductors, elec-tric potentials play an important role. The
potential as itexists on average in the space between charged
particles,independent of the question of how it can be measured,is
called the plasma potential, or the space potential.
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1.6 Magnetization 3
Lightning is an example of plasma present at Earths
surface.Typically, lightning discharges 30,000 amperes at up to 100
mil-lion volts, and emits light, radio waves, X-rays and even
gammarays.[13] Plasma temperatures in lightning can approach
28,000K (28,000 C; 50,000 F) and electron densities may exceed
1024m3.
If an electrode is inserted into a plasma, its potential
willgenerally lie considerably below the plasma potential dueto
what is termed a Debye sheath. The good electricalconductivity of
plasmas makes their electric elds verysmall. This results in the
important concept of quasineu-trality, which says the density of
negative charges is ap-proximately equal to the density of positive
charges overlarge volumes of the plasma ( ne = hZini ), but onthe
scale of the Debye length there can be charge imbal-ance. In the
special case that double layers are formed, thecharge separation
can extend some tens of Debye lengths.The magnitude of the
potentials and electric elds mustbe determined by means other than
simply nding the netcharge density. A common example is to assume
that theelectrons satisfy the Boltzmann relation:
ne / ee/kBTe :
Dierentiating this relation provides a means to calculatethe
electric eld from the density:
~E = (kBTe/e)(rne/ne):
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 bevery low, or it must be
very small, otherwise it will bedissipated by the repulsive
electrostatic force.In astrophysical plasmas, Debye screening
preventselectric elds from directly aecting the plasma overlarge
distances, i.e., greater than the Debye length. How-ever, the
existence of charged particles causes the plasmato generate, and be
aected by, magnetic elds. Thiscan and does cause extremely complex
behavior, suchas the generation of plasma double layers, an object
thatseparates charge over a few tens of Debye lengths. Thedynamics
of plasmas interacting with external and self-generated magnetic
elds are studied in the academic dis-cipline of
magnetohydrodynamics.
1.6 Magnetization
Plasma with a magnetic eld strong enough to inuencethe motion of
the charged particles is said to be magne-tized. A common
quantitative criterion is that a parti-cle on average completes at
least one gyration around themagnetic eld before making a
collision, i.e., !ce/vcoll >1 , where !ce is the electron
gyrofrequency and vcoll isthe electron collision rate. It is often
the case that theelectrons are magnetized while the ions are not.
Mag-netized plasmas are anisotropic, meaning that their prop-erties
in the direction parallel to the magnetic eld aredierent from those
perpendicular to it. While electricelds in plasmas are usually
small due to the high conduc-tivity, the electric eld associated
with a plasma movingin a magnetic eld is given by E = v B (where E
isthe electric eld, v is the velocity, and B is the magneticeld),
and is not aected by Debye shielding.[14]
1.7 Comparison of plasma and gas phases
Plasma is often called the fourth state of matter aftersolid,
liquids and gases.[15][16] It is distinct from these andother
lower-energy states of matter. Although it is closelyrelated to the
gas phase in that it also has no denite formor volume, it diers in
a number of ways, including thefollowing:
2 Common plasmasFurther information: Astrophysical plasma,
Interstellarmedium and Intergalactic space
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4 3 COMPLEX PLASMA PHENOMENA
Plasmas are by far the most common phase of ordinarymatter in
the universe, both by mass and by volume.[18]Essentially, all of
the visible light from space comes fromstars, which are plasmas
with a temperature such that theyradiate strongly at visible
wavelengths. Most of the or-dinary (or baryonic) matter in the
universe, however, isfound in the intergalactic medium, which is
also a plasma,but much hotter, so that it radiates primarily as
X-rays.In 1937, Hannes Alfvn argued that if plasma pervadedthe
universe, it could then carry electric currents capableof
generating a galactic magnetic eld.[19] After winningthe Nobel
Prize, he emphasized that:
In order to understand the phenomena ina certain plasma region,
it is necessary to mapnot only the magnetic but also the electric
eldand the electric currents. Space is lled witha network of
currents which transfer energyand momentum over large or very large
dis-tances. The currents often pinch to lamen-tary or surface
currents. The latter are likely togive space, as also interstellar
and intergalacticspace, a cellular structure.[20]
By contrast the current scientic consensus is that about96% of
the total energy density in the universe is notplasma or any other
form of ordinary matter, but a com-bination of cold dark matter and
dark energy. Our Sun,and all stars, are made of plasma, much of
interstellarspace is lled with a plasma, albeit a very sparse one,
andintergalactic space too. Even black holes, which are notdirectly
visible, are thought to be fuelled by accreting ion-ising matter
(i.e. plasma),[21] and they are associated withastrophysical jets
of luminous ejected plasma,[22] such asM87s jet that extends 5,000
light-years.[23]
In our solar system, interplanetary space is lled with theplasma
of the SolarWind that extends from the Sun out tothe heliopause.
However, the density of ordinary matteris much higher than average
and much higher than thatof either dark matter or dark energy. The
planet Jupiteraccounts for most of the non-plasma, only about 0.1%
ofthe mass and 1015% of the volume within the orbit ofPluto.Dust
and small grains within a plasma will also pick upa net negative
charge, so that they in turn may act likea very heavy negative ion
component of the plasma (seedusty plasmas).
3 Complex plasma phenomenaAlthough the underlying equations
governing plasmas arerelatively simple, plasma behavior is
extraordinarily var-ied and subtle: the emergence of unexpected
behaviorfrom a simple model is a typical feature of a
complexsystem. Such systems lie in some sense on the boundary
between ordered and disordered behavior and cannot typ-ically be
described either by simple, smooth, mathemat-ical functions, or by
pure randomness. The spontaneousformation of interesting spatial
features on a wide rangeof length scales is one manifestation of
plasma complex-ity. The features are interesting, for example,
becausethey are very sharp, spatially intermittent (the
distancebetween features is much larger than the features
them-selves), or have a fractal form. Many of these featureswere
rst studied in the laboratory, and have subsequentlybeen recognized
throughout the universe. Examples ofcomplexity and complex
structures in plasmas include:
3.1 Filamentation
Striations or string-like structures,[27] also known asbirkeland
currents, are seen in many plasmas, likethe plasma ball, the
aurora,[28] lightning,[29] electricarcs, solar ares,[30] and
supernova remnants.[31] Theyare sometimes associated with larger
current densities,and the interaction with the magnetic eld can
forma magnetic rope structure.[32] High power microwavebreakdown at
atmospheric pressure also leads to the for-mation of lamentary
structures.[33] (See also Plasmapinch)Filamentation also refers to
the self-focusing of a highpower laser pulse. At high powers, the
nonlinear part ofthe index of refraction becomes important and
causes ahigher index of refraction in the center of the laser
beam,where the laser is brighter than at the edges, causing
afeedback that focuses the laser even more. The tighterfocused
laser has a higher peak brightness (irradiance)that forms a plasma.
The plasma has an index of refrac-tion lower than one, and causes a
defocusing of the laserbeam. The interplay of the focusing index of
refraction,and the defocusing plasma makes the formation of a
longlament of plasma that can be micrometers to kilometersin
length.[34] One interesting aspect of the lamentationgenerated
plasma is the relatively low ion density due todefocusing eects of
the ionized electrons.[35] (See alsoFilament propagation)
3.2 Shocks or double layers
Plasma properties change rapidly (within a few Debyelengths)
across a two-dimensional sheet in the presenceof a (moving) shock
or (stationary) double layer. Doublelayers involve localized charge
separation, which causesa large potential dierence across the
layer, but does notgenerate an electric eld outside the layer.
Double layersseparate adjacent plasma regions with dierent
physicalcharacteristics, and are often found in current
carryingplasmas. They accelerate both ions and electrons.
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3.7 Non-neutral plasma 5
3.3 Electric elds and circuits
Quasineutrality of a plasma requires that plasma currentsclose
on themselves in electric circuits. Such circuits fol-low Kirchhos
circuit laws and possess a resistance andinductance. These circuits
must generally be treated as astrongly coupled system, with the
behavior in each plasmaregion dependent on the entire circuit. It
is this strongcoupling between system elements, together with
non-linearity, which may lead to complex behavior. Elec-trical
circuits in plasmas store inductive (magnetic) en-ergy, and should
the circuit be disrupted, for example,by a plasma instability, the
inductive energy will be re-leased as plasma heating and
acceleration. This is acommon explanation for the heating that
takes place inthe solar corona. Electric currents, and in
particular,magnetic-eld-aligned electric currents (which are
some-times generically referred to as "Birkeland currents"),
arealso observed in the Earths aurora, and in plasma la-ments.
3.4 Cellular structure
Narrow sheets with sharp gradients may separate regionswith
dierent properties such as magnetization, densityand temperature,
resulting in cell-like regions. Examplesinclude the magnetosphere,
heliosphere, and heliosphericcurrent sheet. Hannes Alfvn wrote:
From the cos-mological point of view, the most important new
spaceresearch discovery is probably the cellular structure ofspace.
As has been seen in every region of space ac-cessible to in situ
measurements, there are a number of'cell walls, sheets of electric
currents, which divide spaceinto compartments with dierent
magnetization, temper-ature, density, etc.[36]
3.5 Critical ionization velocity
The critical ionization velocity is the relative velocity
be-tween an ionized plasma and a neutral gas, above which arunaway
ionization process takes place. The critical ion-ization process is
a quite general mechanism for the con-version of the kinetic energy
of a rapidly streaming gasinto ionization and plasma thermal
energy. Critical phe-nomena in general are typical of complex
systems, andmay lead to sharp spatial or temporal features.
3.6 Ultracold plasma
Ultracold plasmas are created in a magneto-optical trap(MOT) by
trapping and cooling neutral atoms, to temper-atures of 1 mK or
lower, and then using another laser toionize the atoms by giving
each of the outermost electronsjust enough energy to escape the
electrical attraction ofits parent ion.
One advantage of ultracold plasmas are their well charac-terized
and tunable initial conditions, including their sizeand electron
temperature. By adjusting the wavelengthof the ionizing laser, the
kinetic energy of the liberatedelectrons can be tuned as low as 0.1
K, a limit set by thefrequency bandwidth of the laser pulse. The
ions inheritthe millikelvin temperatures of the neutral atoms, but
arequickly heated through a process known as disorder in-duced
heating (DIH). This type of non-equilibrium ul-tracold plasma
evolves rapidly, and displays many otherinteresting
phenomena.[37]
One of the metastable states of a strongly nonideal plasmais
Rydberg matter, which forms upon condensation of ex-cited
atoms.
3.7 Non-neutral plasma
The strength and range of the electric force and the
goodconductivity of plasmas usually ensure that the densitiesof
positive and negative charges in any sizeable regionare equal
(quasineutrality). A plasma with a signicantexcess of charge
density, or, in the extreme case, is com-posed of a single species,
is called a non-neutral plasma.In such a plasma, electric elds play
a dominant role. Ex-amples are charged particle beams, an electron
cloud in aPenning trap and positron plasmas.[38]
3.8 Dusty plasma/grain plasma
A dusty plasma contains tiny charged particles of dust(typically
found in space). The dust particles acquire highcharges and
interact with each other. A plasma that con-tains larger particles
is called grain plasma. Under labo-ratory conditions, dusty plasmas
are also called complexplasmas.[39]
3.9 Impermeable plasma
Impermeable plasma is a type of thermal plasma whichacts like an
impermeable solid with respect to gas or coldplasma and can be
physically pushed. Interaction of coldgas and thermal plasma was
briey studied by a groupled by Hannes Alfvn in 1960s and 1970s for
its possibleapplications in insulation of fusion plasma from the
reac-tor walls.[40] However, later it was found that the
externalmagnetic elds in this conguration could induce kink
in-stabilities in the plasma and subsequently lead to an
unex-pectedly high heat loss to the walls.[41] In 2013, a group
ofmaterials scientists reported that they have
successfullygenerated stable impermeable plasma with no
magneticconnement using only an ultrahigh-pressure blanket ofcold
gas. While spectroscopic data on the characteristicsof plasma were
claimed to be dicult to obtain due to thehigh-pressure, the passive
eect of plasma on synthesisof dierent nanostructures clearly
suggested the eective
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6 5 ARTIFICIAL PLASMAS
connement. They also showed that uponmaintaining
theimpermeability for a few tens of seconds, screening ofions at
the plasma-gas interface could give rise to a strongsecondary mode
of heating (known as viscous heating)leading to dierent kinetics of
reactions and formationof complex nanomaterials.[42]
4 Mathematical descriptions
The complex self-constricting magnetic eld lines and
currentpaths in a eld-aligned Birkeland current that can develop in
aplasma.[43]
Main article: Plasma modeling
To completely describe the state of a plasma, we wouldneed to
write down all the particle locations and veloci-ties and describe
the electromagnetic eld in the plasmaregion. However, it is
generally not practical or neces-sary to keep track of all the
particles in a plasma. There-fore, plasma physicists commonly use
less detailed de-scriptions, of which there are two main types:
4.1 Fluid modelFluid models describe plasmas in terms of
smoothedquantities, like density and averaged velocity around
eachposition (see Plasma parameters). One simple uidmodel,
magnetohydrodynamics, treats the plasma as asingle uid governed by
a combination of Maxwellsequations and the NavierStokes equations.
A more gen-eral description is the two-uid plasma picture, where
theions and electrons are described separately. Fluid mod-els are
often accurate when collisionality is sucientlyhigh to keep the
plasma velocity distribution close to aMaxwellBoltzmann
distribution. Because uid modelsusually describe the plasma in
terms of a single ow ata certain temperature at each spatial
location, they canneither capture velocity space structures like
beams ordouble layers, nor resolve wave-particle eects.
4.2 Kinetic modelKinetic models describe the particle velocity
distributionfunction at each point in the plasma and therefore do
notneed to assume aMaxwellBoltzmann distribution. A ki-netic
description is often necessary for collisionless plas-mas. There
are two common approaches to kinetic de-scription of a plasma. One
is based on representing thesmoothed distribution function on a
grid in velocity andposition. The other, known as the
particle-in-cell (PIC)technique, includes kinetic information by
following thetrajectories of a large number of individual
particles. Ki-netic models are generally more computationally
inten-sive than uid models. The Vlasov equation may be usedto
describe the dynamics of a system of charged particlesinteracting
with an electromagnetic eld. In magnetizedplasmas, a gyrokinetic
approach can substantially reducethe computational expense of a
fully kinetic simulation.
5 Articial plasmasMost articial plasmas are generated by the
applicationof electric and/or magnetic elds. Plasma generated in
alaboratory setting and for industrial use can be
generallycategorized by:
The type of power source used to generate theplasmaDC, RF and
microwave
The pressure they operate atvacuum pressure (