The presence ofcharged particles makes plasmaelectrically conductive, with the dynamics of individual particles and macroscopic plasma motion governed by collective electromagnetic fields and very sensitive to externally applied fields.[7] The response of plasma to electromagnetic fields is used in many modern devices and technologies, such asplasma televisions orplasma etching.[8]
Depending on temperature and density, a certain number of neutral particles may also be present, in which case plasma is calledpartially ionized.Neon signs andlightning are examples of partially ionized plasmas.[9]Unlike thephase transitions between the other three states of matter, the transition to plasma is not well defined and is a matter of interpretation and context.[10] Whether a given degree of ionization suffices to call a substance "plasma" depends on the specific phenomenon being considered.
Plasma was first identified in laboratory bySir William Crookes. Crookes presented alecture on what he called "radiant matter" to theBritish Association for the Advancement of Science, in Sheffield, on Friday, 22 August 1879.[11]Systematic studies of plasma began with the research ofIrving Langmuir and his colleagues in the 1920s. Langmuir also introduced the term "plasma" as a description of ionized gas in 1928:[12]
Except near the electrodes, where there aresheaths 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 nameplasma 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 theblood plasma.[13][14] 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."[15]
Plasma is typically an electrically quasineutral medium of unbound positive and negativeparticles (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 generateelectric currents, and any movement of acharged plasma particle affects and is affected by thefields created by the other charges. In turn, this governs collective behaviour with many degrees of variation.[21][22]
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:
State
Property
Gas
Plasma
Interactions
Short-range: Two-particle (binary)collisions are the rule.
Long-range: Collective motion of particles is ubiquitous in plasma, resulting in variouswaves and other types of collective phenomena.
Electrical conductivity
Very low: Gases are excellentinsulators up to electric field strengths of tens of kilovolts per centimetre.[23]
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 bygravity.
Two or more:Electrons andions possess differentcharges and vastly different masses, so that they behave differently in many circumstances, with various types of plasma-specific waves andinstabilities emerging as a result.
The plasma approximation: The plasma approximation applies when theplasma parameter Λ,[26] representing the number of charge carriers within theDebye sphere is much higher than unity.[19][20] 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.[27]
Bulk interactions: TheDebye 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.[28]
Collisionlessness: The electron plasma frequency (measuringplasma oscillations 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.[29]
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 anon-neutral plasma. In such a plasma, electric fields play a dominant role. Examples are chargedparticle beams, an electron cloud in aPenning trap and positron plasmas.[30]
Adusty 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 calledcomplex plasmas.[31]
Properties and parameters
Artist'srendition of the Earth'splasma 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 theaurora borealis, where plasma energy pours back into the atmosphere.[32]
Density and ionization degree
For plasma to exist,ionization is necessary. The term "plasma density" by itself usually refers to the electron density, that is, the number of charge-contributing electrons per unit volume. The degree of ionization is defined as fraction of neutral particles that are ionized:
where is the ion density and the neutral density (in number of particles per unit volume). In the case of fully ionized matter,. Because of the quasineutrality of plasma, the electron and ion densities are related by, where is the average ion charge (in units of theelementary charge).
Temperature
Plasma temperature, commonly measured inkelvin orelectronvolts, 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 theelectron temperature relative to theionization energy (and more weakly by the density). Inthermal equilibrium, the relationship is given by theSaha equation. At low temperatures, ions and electrons tend to recombine into bound states—atoms[33]—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 energydistribution function is close to aMaxwellian even in the presence of strongelectric ormagnetic 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 theambient temperature while electrons reach thousands of kelvin.[34] The opposite case is thez-pinch plasma where the ion temperature may exceed that of electrons.[35]
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.[36] Plasma temperatures can approach 30000 K and electron densities may exceed 1024 m−3.
Since plasmas are very goodelectrical conductors, electric potentials play an important role.[clarification needed] 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 aDebye 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 (), but on the scale of theDebye length, there can be charge imbalance. In the special case thatdouble layers are formed, the charge separation can extend some tens of Debye lengths.[37]
The magnitude of the potentials and electric fields must be determined by means other than simply finding the netcharge density. A common example is to assume that the electrons satisfy theBoltzmann relation:
Differentiating this relation provides a means to calculate the electric field from the density:
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 repulsiveelectrostatic force.[38]
Magnetization
The existence of charged particles causes the plasma to generate, and be affected by,magnetic fields. 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.,, where is the electrongyrofrequency and is the electron collision rate. It is often the case that the electrons are magnetized while the ions are not. Magnetized plasmas areanisotropic, 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 in the magnetic field is given by the usualLorentz formula, and is not affected byDebye shielding.[39]
Mathematical descriptions
The complex self-constricting magnetic field lines and current paths in a field-alignedBirkeland current that can develop in a plasma.[40]
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] 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 (seePlasma parameters). One simple fluid model,magnetohydrodynamics, treats the plasma as a single fluid governed by a combination ofMaxwell's equations and theNavier–Stokes equations. A more general description is the two-fluid plasma,[41] 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 aMaxwell–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 ordouble layers, nor resolve wave-particle effects.[citation needed]
Kinetic model
Kinetic models describe the particle velocity distribution function at each point in the plasma and therefore do not need to assume aMaxwell–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 theparticle-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. TheVlasov equation may be used to describe the dynamics of a system of charged particles interacting with an electromagnetic field.In magnetized plasmas, agyrokinetic approach can substantially reduce the computational expense of a fully kinetic simulation.[citation needed]
Plasma science and technology
Plasmas are studied by the vastacademic field ofplasma science orplasma physics, including several sub-disciplines such asspace plasma physics.
Plasmas can appear in nature in various forms and locations, with a few examples given in the following table:
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 withradio frequency (RF)) and microwave[citation needed]
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)[citation needed]
The degree of ionization within the plasma—fully, partially, or weakly ionized[citation needed]
The temperature relationships within the plasma—thermal plasma (), non-thermal or "cold" plasma ()[citation needed]
The electrode configuration used to generate the plasma[citation needed]
The magnetization of the particles within the plasma—magnetized (both ion and electrons are trapped inLarmor 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 generateLorentz forces)[citation needed]
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.[48] For this case, plasma is generated when anelectric current is applied across adielectric gas or fluid (an electricallynon-conducting material) as can be seen in the adjacent image, which shows adischarge tube as a simple example (DC used for simplicity).[citation needed]
Thepotential difference and subsequentelectric field pull the bound electrons (negative) toward theanode (positive electrode) while thecathode (negative electrode) pulls the nucleus.[49] As thevoltage increases, the current stresses the material (byelectric polarization) beyond itsdielectric limit (termed strength) into a stage ofelectrical breakdown, marked by anelectric spark, where the material transforms from being aninsulator into aconductor (as it becomes increasinglyionized). The underlying process is theTownsend 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",[50] mainly due to a small mean free path (average distance travelled between collisions).[citation needed]
Electric arc
Cascade process of ionization. Electrons are "e−", neutral atoms "o", and cations "+".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 tolightning.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.[50]Electrical resistance along the arc createsheat, which dissociates more gas molecules and ionizes the resulting atoms. Therefore, theelectrical energy is given to electrons, which, due to their great mobility and large numbers, are able to disperse it rapidly byelastic collisions to the heavy particles.[51]
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 withinfluorescent light tubes.[56]
Capacitively coupled plasma (CCP): similar to glow discharge plasmas, but generated with high frequency RF electric fields, typically13.56 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.[57]
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.[58]
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 inmetallurgical processes. For example, it is used to smelt minerals containing Al2O3 to producealuminium.[citation needed]
Corona discharge: this is a non-thermal discharge generated by the application of high voltage to sharp electrode tips. It is commonly used inozone generators and particle precipitators.[citation needed]
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 aplasma actuator for vehicle drag reduction.[60] It is also widely used in the web treatment of fabrics.[61] The application of the discharge to synthetic fabrics and plastics functionalizes the surface and allows for paints, glues and similar materials to adhere.[62] The dielectric barrier discharge was used in the mid-1990s to show that low temperature atmospheric pressure plasma is effective in inactivating bacterial cells.[63] This work and later experiments using mammalian cells led to the establishment of a new field of research known asplasma 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.[64]
Capacitive discharge: this is anonthermal plasma generated by the application of RF power (e.g.,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.[65]
"Piezoelectric direct discharge plasma:" is anonthermal 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]
A world effort was triggered in the 1960s to studymagnetohydrodynamic converters in order to bringMHD power conversion to market with commercial power plants of a new kind, converting thekinetic energy of a high velocity plasma intoelectricity with nomoving parts at a highefficiency. 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 activeflow control around vehicles or projectiles, in order to soften and mitigateshock waves, lower thermal transfer and reducedrag.[citation needed]
Such ionized gases used in "plasma technology" ("technological" or "engineered" plasmas) are usuallyweakly ionized gases in the sense that only a tiny fraction of the gas molecules are ionized.[66] 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 involvesresistive magnetohydrodynamics with lowmagnetic Reynolds number, a challenging field of plasma physics where calculations requiredyadic tensors in a7-dimensionalphase space. When used in combination with a highHall parameter, a critical value triggers the problematicelectrothermal instability which limited these technological developments.[citation needed]
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 acomplex 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 afractal form. Many of these features were first studied in the laboratory, and have subsequently been recognized throughout the universe.[citation needed] Examples of complexity and complex structures in plasmas include:
Filamentation also refers to the self-focusing of a high power laser pulse. At high powers, the nonlinear part of theindex 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 bemicrometers to kilometers in length.[73] One interesting aspect of the filamentation generated plasma is the relatively low ion density due to defocusing effects of the ionized electrons.[74] (See alsoFilament 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 byHannes Alfvén in 1960s and 1970s for its possible applications in insulation offusion plasma from the reactor walls.[75] However, later it was found that the externalmagnetic fields in this configuration could inducekink instabilities in the plasma and subsequently lead to an unexpectedly high heat loss to the walls.[76]
In 2013, a group of materials scientists reported that they have successfully generated stable impermeable plasma with nomagnetic 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 onsynthesis of differentnanostructures clearly suggested the effective confinement. They also showed that upon maintaining the impermeability for a few tens of seconds, screening ofions 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 complexnanomaterials.[77]
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