Plasma and ionised gases have properties and display behaviours unlike those of the other states, and the transition between them is mostly a matter of nomenclature and subject to interpretation. Based on the surrounding environmental temperature and density, partially ionised or fully ionised forms of plasma may be produced. Neon signs and lightning are examples of partially ionised plasma, while the interior of the Sun is an example of fully ionised plasma, along with the solar corona and stars.
The word plasma comes from Ancient Greekπλάσμα, meaning 'moldable substance' or 'jelly', and describes the behaviour of the ionised atomic nuclei and the electrons within the surrounding region of the plasma. Very simply, each of these nuclei are suspended in a movable sea of electrons. Plasma was first identified in a Crookes tube, and so described by Sir William Crookes in 1879 (he called it "radiant matter"). The nature of this "cathode ray" matter was subsequently identified by British physicist Sir J.J. Thomson in 1897.
The term "plasma" was coined by Irving Langmuir in 1928.Lewi Tonks and Harold Mott-Smith, both of whom worked with Irving Langmuir in the 1920s, recall that Langmuir first used the word "plasma" in analogy with blood. 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.”
Langmuir described the plasma he observed as follows:
"Except near the electrodes, where there are sheaths containing very few electrons, the ionised 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."
Properties and parameters
Artist's 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.
Plasma is an electrically neutral medium of unbound positive and negative particles (i.e. the overall charge of a plasma is roughly zero). Although these particles are unbound, they are not ‘free’ in the sense of not experiencing forces. Moving charged particles generate an electric current within a magnetic field, and any movement of a charged plasma particle affects and is affected by the fields created by the other charges. In turn this governs collective behaviour with many degrees of variation. Three factors define a plasma:
The plasma approximation: The plasma approximation applies when the plasma parameter, Λ, representing the number of charge carriers within a sphere (called the Debye sphere whose radius is the Debye screening length) surrounding a given charged particle, is sufficiently high as to shield the electrostatic influence of the particle outside of the sphere.
Bulk interactions: The Debye screening length (defined above) is short compared to 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.
Plasma frequency: The electron plasma frequency (measuring plasma oscillations of the electrons) is large compared to the electron-neutral collision frequency (measuring frequency of collisions between electrons and neutral particles). When this condition is valid, electrostatic interactions dominate over the processes of ordinary gas kinetics.
Plasma temperature is commonly measured in kelvins or electronvolts and is, informally, a measure of the thermal kinetic energy per particle. High temperatures are usually needed to sustain ionisation, which is a defining feature of a plasma. The degree of plasma ionisation is determined by the electron temperature relative to the ionization energy (and more weakly by the density), in a relationship called the Saha equation. At low temperatures, ions and electrons tend to recombine into bound states—atoms—and the plasma will eventually become a gas.
In most cases the electrons are close enough to thermal equilibrium that their temperature is relatively well-defined, even when there is a significant deviation from a Maxwellian energy distribution function, for example, due to UV radiation, energetic particles, or strong electric fields. Because of the large difference in mass, the electrons come to thermodynamic equilibrium amongst themselves much faster than they come into equilibrium with the ions or neutral atoms. For this reason, the ion temperature may be very different from (usually lower than) the electron temperature. This is especially common in weakly ionised technological plasmas, where the ions are often near the ambient temperature.
Fully vs. partially (weakly) ionized gases
For plasma to exist, ionisation is necessary. The term "plasma density" by itself usually refers to the "electron density", that is, the number of free electrons per unit volume. The degree of ionisation of a plasma is the proportion of atoms that have lost or gained electrons, and is controlled by the electron and ion temperatures and electron-ion vs electron-neutral collision frequencies. The degree of ionisation, , is defined as , where is the number density of ions and is the number density of neutral atoms. The electron density is related to this by the average charge state of the ions through , where is the number density of electrons.
In a plasma, the electron-ion collision frequency is much greater than the electron-neutral collision frequency . Therefore, with a weak degree of ionization , the electron-ion collision frequency can equal the electron-neutral collision frequency: is the limit separating a plasma from being partially or fully ionized.
The term fully ionized gas introduced by Lyman Spitzer does not mean the degree of ionization is unity, but only that the plasma is in a Coulomb-collision dominated regime, i.e. when , which can correspond to a degree of ionization as low as 0.01%.
A partially or weakly ionized gas means the plasma is not dominated by Coulomb collisions, i.e. when .
Most of "technological" (engineered) plasmas are weakly ionized gases.
Thermal vs. nonthermal (cold) plasmas
Based on the relative temperatures of the electrons, ions and neutrals, plasmas are classified as "thermal" or "non-thermal" (also referred to as "cold plasmas").
Thermal plasmas have electrons and the heavy particles at the same temperature, i.e. they are in thermal equilibrium with each other.
Nonthermal plasmas on the other hand are non-equilibrium ionized gases, with two temperatures: ions and neutrals stay at a low temperature (sometimes room temperature), whereas electrons are much hotter. (). A kind of common nonthermal plasma is the mercury-vapor gas within a fluorescent lamp, where the "electrons gas" reaches a temperature of 10,000 kelvins while the rest of the gas stays barely above room temperature, so the bulb can even be touched with hands while operating.
A particular and unusual case of "inverse" nonthermal plasma is the very high temperature plasma produced by the Z machine, where ions are much hotter than electrons.
Lightning is an example of plasma present at Earth's surface. Typically, lightning discharges 30,000 amperes at up to 100 million volts, and emits light, radio waves, X-rays and even gamma rays. Plasma temperatures in lightning can approach 28,000 K (28,000 °C; 50,000 °F) and electron densities may exceed 1024 m−3.
Since plasmas are very good electrical conductors, electric potentials play an important role. The average potential in the space between charged particles, independent of how it can be measured, is called the "plasma potential", or the "space potential". If an electrode is inserted into a plasma, its potential will generally lie considerably below the plasma potential due to what is termed a Debye sheath. The 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 the Debye length there can be charge imbalance. In the special case that double layers are formed, the charge separation can extend some tens of Debye lengths.
The magnitude of the potentials and electric fields must be determined by means other than simply finding the net charge density. A common example is to assume that the electrons satisfy the Boltzmann 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 repulsive electrostatic force.
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 before making a collision, i.e., , where is the "electron gyrofrequency" and 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 high conductivity, the electric field associated with a plasma moving in a magnetic field is given by (where is the electric field, is the velocity, and is the magnetic field), and is not affected by Debye shielding.
Comparison of plasma and gas phases
Plasma is often called the fourth state of matter after solid, liquids and gases, despite plasma typically being an ionised gas. It is distinct from these and other lower-energy states of matter. Although it is closely related to the gas phase in that it also has no definite form or volume, it differs in a number of ways, including the following:
Very low: Air is an excellent insulator until it breaks down into plasma at electric field strengths above 30 kilovolts per centimeter.
Usually 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, influenced by gravity and by collisions with one another.
Two or three: Electrons, ions, protons and neutrons can be distinguished by the sign and value of their charge so that they behave independently in many circumstances, with different bulk velocities and temperatures, allowing phenomena such as new types of waves and instabilities.
Maxwellian: Collisions usually lead to a Maxwellian velocity distribution of all gas particles, with very few relatively fast particles.
Often non-Maxwellian: Collisional interactions are often weak in hot plasmas and external forcing can drive the plasma far from local equilibrium and lead to a significant population of unusually fast particles.
Binary: Two-particle collisions are the rule, three-body collisions extremely rare.
Collective: Waves, or organized motion of plasma, are very important because the particles can interact at long ranges through the electric and magnetic forces.
Although the underlying equations governing plasmas are relatively simple, plasma behaviour is extraordinarily varied and subtle: the emergence of unexpected behaviour from a simple model is a typical feature of a complex system. Such systems lie in some sense on the boundary between ordered and disordered behaviour and cannot typically be described either by simple, smooth, mathematical functions, or by pure randomness. The spontaneous formation of interesting spatial features on a wide range of length scales is one manifestation of plasma complexity. The features are interesting, for example, because they are very sharp, spatially intermittent (the distance between features is much larger than the features themselves), or have a fractal form. Many of these features were first studied in the laboratory, and have subsequently been recognized throughout the universe. Examples of complexity and complex structures in plasmas include:
Filamentation 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 micrometers to kilometers in length. One interesting aspect of the filamentation generated plasma is the relatively low ion density due to defocusing effects of the ionised electrons. (See also Filament propagation)
The strength and range of the electric force and the good conductivity of plasmas usually ensure that the densities of positive and negative charges in any sizeable region are equal ("quasineutrality"). A plasma with a significant excess of charge density, or, in the extreme case, is composed of a single species, is called a non-neutral plasma. In such a plasma, electric fields play a dominant role. Examples are charged particle beams, an electron cloud in a Penning trap and positron plasmas.
Dusty plasma/grain 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.
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 fusion plasma from the reactor walls. However, later it was found that the external magnetic fields in this configuration could induce kink instabilities in the plasma and subsequently lead to an unexpectedly high heat loss to the walls.
In 2013, a group of materials scientists reported that they have successfully generated stable impermeable plasma with no magnetic confinement using only an ultrahigh-pressure blanket of cold gas. While spectroscopic data on the characteristics of plasma were claimed to be difficult to obtain due to the high pressure, the passive effect of plasma on 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.
The complex self-constricting magnetic field lines and current paths in a field-aligned Birkeland current that can develop in a plasma.
To completely describe the state of a plasma, all of the
particle locations and velocities that describe the electromagnetic field in the plasma region would need to be written down.
However, it is generally not practical or necessary to keep track of all the particles in a plasma.
Therefore, plasma physicists commonly use less detailed descriptions, of which
there are two main types:
Fluid 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 picture, 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 layers, nor resolve wave-particle effects.
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 gyrokinetic approach can substantially reduce the computational expense of a fully kinetic simulation.
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, RF and microwave
The pressure they operate at—vacuum pressure (< 10 mTorr or 1 Pa), moderate pressure (≈1 Torr or 100 Pa), atmospheric pressure (760 Torr or 100 kPa)
The degree of ionisation within the plasma—fully, partially, or weakly ionised
The temperature relationships within the plasma—thermal plasma (), non-thermal or "cold" plasma ()
The electrode configuration used to generate the plasma
The magnetization of the particles within the plasma—magnetized (both ion and electrons are trapped in Larmor orbits by the magnetic field), partially magnetized (the electrons but not the ions are trapped by the magnetic field), non-magnetized (the magnetic field is too weak to trap the particles in orbits but may generate Lorentz forces)
Just like the many uses of plasma, there are several means for its generation, however, one principle is common to all of them: there must be energy input to produce and sustain it. For this case, plasma is generated when an electric current is applied across a dielectric gas or fluid (an electrically non-conducting material) as can be seen in the adjacent image, which shows a discharge tube as a simple example (DC used for simplicity).
The potential difference and subsequent electric field pull the bound electrons (negative) toward the anode (positive electrode) while the cathode (negative electrode) pulls the nucleus. As the voltage increases, the current stresses the material (by electric polarization) beyond its dielectric limit (termed strength) into a stage of electrical breakdown, marked by an electric spark, where the material transforms from being an insulator into a conductor (as it becomes increasingly ionised). The underlying process is the Townsend avalanche, where collisions between electrons and neutral gas atoms create more ions and electrons (as can be seen in the figure on the right). The first impact of an electron on an atom results in one ion and two electrons. Therefore, the number of charged particles increases rapidly (in the millions) only "after about 20 successive sets of collisions", mainly due to a small mean free path (average distance travelled between collisions).
Cascade process of ionisation. Electrons are ‘e−’, neutral atoms ‘o’, and cations ‘+’.
Avalanche effect between two electrodes. The original ionisation event liberates one electron, and each subsequent collision liberates a further electron, so two electrons emerge from each collision: the ionising electron and the liberated electron.
With ample current density and ionisation, this forms a luminous electric arc (a continuous electric discharge similar to lightning) between the electrodes.[Note 1]Electrical resistance along the continuous electric arc creates heat, which dissociates more gas molecules and ionises the resulting atoms (where degree of ionisation is determined by temperature), and as per the sequence: solid-liquid-gas-plasma, the gas is gradually turned into a thermal plasma.[Note 2] A thermal plasma is in thermal equilibrium, which is to say that the temperature is relatively homogeneous throughout the heavy particles (i.e. atoms, molecules and ions) and electrons. This is so because when thermal plasmas are generated, electrical energy is given to electrons, which, due to their great mobility and large numbers, are able to disperse it rapidly and by elastic collision (without energy loss) to the heavy particles.[Note 3]
Glow discharge plasmas: non-thermal plasmas generated by the application of DC or low frequency RF (<100 kHz) electric field to the gap between two metal electrodes. Probably the most common plasma; this is the type of plasma generated within fluorescent light tubes.
Capacitively coupled plasma (CCP): similar to glow discharge plasmas, but generated with high frequency RF electric fields, typically 13.56 MHz. These differ from glow discharges in that the sheaths are much less intense. These are widely used in the microfabrication and integrated circuit manufacturing industries for plasma etching and plasma enhanced chemical vapor deposition.
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 metallurgical processes. For example, it is used to smelt minerals containing Al2O3 to produce aluminium.
Corona discharge: this is a non-thermal discharge generated by the application of high voltage to sharp electrode tips. It is commonly used in ozone generators and particle precipitators.
Dielectric barrier discharge (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. It is also widely used in the web treatment of fabrics. The application of the discharge to synthetic fabrics and plastics functionalizes the surface and allows for paints, glues and similar materials to adhere. The dielectric barrier discharge was used in the mid-1990s to show that low temperature atmospheric pressure plasma is effective in inactivating bacterial cells. This work and later experiments using mammalian cells led to the establishment of a new field of research known as plasma medicine. The dielectric barrier discharge configuration was also used in the design of low temperature plasma jets. These plasma jets are produced by fast propagating guided ionisation waves known as plasma bullets.
Capacitive discharge: this is a nonthermal 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.
"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.
Such ionized gases used in "plasma technology" ("technological" or "engineered" plasmas) are usually weakly ionized gases in the sense that only a tiny fraction of the gas molecules are ionized. These kinds of weakly ionized gases are also nonthermal "cold" plasmas. In the presence of magnetics fields, the study of such magnetized nonthermal weakly ionized gases involves resistive magnetohydrodynamics with low magnetic Reynolds number, a challenging field of plasma physics where calculations require dyadic tensors in a 7-dimensionalphase space. When used in combination with a high Hall parameter, a critical value triggers the problematic electrothermal instability which limited these technological developments.
Plasmas are the object of study of the academic field of plasma science or plasma physics, including sub-disciplines such as space plasma physics. It currently involves the following fields of active research and features across many journals, whose interest includes:
^The material undergoes various ‘regimes’ or stages (e.g. saturation, breakdown, glow, transition and thermal arc) as the voltage is increased under the voltage-current relationship. The voltage rises to its maximum value in the saturation stage, and thereafter it undergoes fluctuations of the various stages; while the current progressively increases throughout.
^Across literature, there appears to be no strict definition on where the boundary is between a gas and plasma. Nevertheless, it is enough to say that at 2,000°C the gas molecules become atomized, and ionised at 3,000 °C and "in this state, [the] gas has a liquid like viscosity at atmospheric pressure and the free electric charges confer relatively high electrical conductivities that can approach those of metals."
^Note that non-thermal, or non-equilibrium plasmas are not as ionised and have lower energy densities, and thus the temperature is not dispersed evenly among the particles, where some heavy ones remain ‘cold’.
^Brown, Sanborn C. (1978). "Chapter 1: A Short History of Gaseous Electronics". In HIRSH, Merle N. e OSKAM, H. J. Gaseous Electronics. 1. Academic Press. ISBN 978-0-12-349701-7. Archived from the original on 23 October 2017.
^von Engel, A. and Cozens, J.R. (1976) "Flame Plasma" in Advances in electronics and electron physics, L. L. Marton (ed.), Academic Press, ISBN 978-0-12-014520-1, p. 99Archived 2 December 2016 at the Wayback Machine.
^Mészáros, Péter (2010) The High Energy Universe: Ultra-High Energy Events in Astrophysics and Cosmology, Publisher: Cambridge University Press, ISBN 978-0-521-51700-3, p. 99Archived 2 February 2017 at the Wayback Machine..
^Raine, Derek J. and Thomas, Edwin George (2010) Black Holes: An Introduction, Publisher: Imperial College Press, ISBN 978-1-84816-382-9, p. 160Archived 2 December 2016 at the Wayback Machine.
^Moss, G. D.; Pasko, V. P.; Liu, N.; Veronis, G. (2006). "Monte Carlo model for analysis of thermal runaway electrons in streamer tips in transient luminous events and streamer zones of lightning leaders". Journal of Geophysical Research. 111: A02307. Bibcode:2006JGRA..111.2307M. doi:10.1029/2005JA011350.
^ abcGomez, E.; Rani, D. A.; Cheeseman, C. R.; Deegan, D.; Wise, M.; Boccaccini, A. R. (2009). "Thermal plasma technology for the treatment of wastes: A critical review". Journal of Hazardous Materials. 161 (2–3): 614–626. doi:10.1016/j.jhazmat.2008.04.017. PMID18499345.
^Park, J.; Henins, I.; Herrmann, H. W.; Selwyn, G. S.; Hicks, R. F. (2001). "Discharge phenomena of an atmospheric pressure radio-frequency capacitive plasma source". Journal of Applied Physics. 89: 20. Bibcode:2001JAP....89...20P. doi:10.1063/1.1323753.
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