Plasma (physics)
From Wikipedia, the free encyclopedia
Plasma
 
  |
| Top row: both lightning and electric sparks are everyday examples of phenomena made from plasma. Neon lights could more accurately be called "plasma lights", as the light comes from the plasma inside of them. Bottom row: A plasma globe, illustrating some of the more complex phenomena of a plasma, including filamentation.
The colors are a result of relaxation of electrons in excited states to
lower energy states after they have recombined with ions. These
processes emit light in a spectrum characteristic of the gas being excited. The second image is of a plasma trail from Space Shuttle Atlantis during re-entry into the atmosphere, as seen from the International Space Station. |
Plasma (from
Greek πλάσμα, "anything formed"
[1]), according to
natural science, is one of
the four fundamental states of matter (the others being
solid,
liquid, and
gas). When air or gas is
ionized,
plasma forms with conductive properties similar to that of metals.
Plasma is the most abundant form of matter in the Universe, because most
stars are in a plasma state.
[2][3]
Plasma comprises the major state of matter of the
Sun. Heating a gas may
ionize its molecules or atoms (reducing or increasing the number of
electrons in them), thus turning it into a plasma, which contains
charged particles: positive
ions and negative electrons or ions.
[4] Ionization can be induced by other means, such as a strong electromagnetic field applied with a
laser or
microwave generator, and is accompanied by the dissociation of
molecular bonds, if present.
[5] Plasma can also be created by the application of an electric field on a gas, where the underlying process is the
Townsend avalanche.
The presence of a non-negligible number of
charge carriers makes the plasma
electrically conductive so that it responds strongly to
electromagnetic fields. Plasma, therefore, has properties quite unlike those of
solids,
liquids, or
gases and is considered a distinct
state of matter.
Like gas, plasma does not have a definite shape or a definite volume
unless enclosed in a container; unlike gas, under the influence of a
magnetic field, it may form structures such as filaments, beams and
double layers. Some common plasmas are found in
stars and
neon signs. In the
universe, plasma is the most common
state of matter for
ordinary matter, most of which is in the rarefied
intergalactic plasma (particularly
intracluster medium) and in stars. Much of the understanding of plasmas has come from the pursuit of controlled
nuclear fusion and
fusion power, for which plasma physics provides the scientific basis.
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.
[6]
Definition
Plasma is loosely described as an electrically neutral medium of
positive and negative particles (i.e. the overall charge of a plasma is
roughly zero). It is important to note that although they are unbound,
these particles are not ‘free’. When the charges move they generate
electrical currents with magnetic fields, and as a result, they are
affected by each other’s fields. This governs their collective behavior
with many degrees of freedom.
[5][7] A definition can have three criteria:
[8][9]
- The plasma approximation: Charged particles must be close
enough together that each particle influences many nearby charged
particles, rather than just interacting with the closest particle (these
collective effects are a distinguishing feature of a plasma). The
plasma approximation is valid when the number of charge carriers within
the sphere of influence (called the Debye sphere whose radius is the Debye screening length)
of a particular particle is higher than unity to provide collective
behavior of the charged particles. The average number of particles in
the Debye sphere is given by the plasma parameter, "Λ" (the Greek letter Lambda).
- 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.
Ranges of parameters
Plasma parameters can take on values varying by many
orders of magnitude, but the properties of plasmas with apparently disparate parameters may be very similar (see
plasma scaling). The following chart considers only conventional atomic plasmas and not exotic phenomena like
quark gluon plasmas:
Range of plasmas. Density increases upwards, temperature
increases towards the right. The free electrons in a metal may be
considered an electron plasma.
[10]
Typical ranges of plasma parameters: orders of magnitude (OOM)
| Characteristic |
Terrestrial plasmas |
Cosmic plasmas |
Size
in meters |
10−6 m (lab plasmas) to
102 m (lightning) (~8 OOM) |
10−6 m (spacecraft sheath) to
1025 m (intergalactic nebula) (~31 OOM) |
Lifetime
in seconds |
10−12 s (laser-produced plasma) to
107 s (fluorescent lights) (~19 OOM) |
101 s (solar flares) to
1017 s (intergalactic plasma) (~16 OOM) |
Density
in particles per
cubic meter |
107 m−3 to
1032 m−3 (inertial confinement plasma) |
1 m−3 (intergalactic medium) to
1030 m−3 (stellar core) |
Temperature
in Kelvin |
~0 K (crystalline non-neutral plasma[11]) to
108 K (magnetic fusion plasma) |
102 K (aurora) to
107 K (solar core) |
Magnetic fields
in teslas |
10−4 T (lab plasma) to
103 T (pulsed-power plasma) |
10−12 T (intergalactic medium) to
1011 T (near neutron stars) |
Degree of ionization
For plasma to exist,
ionization
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 ionization
of a plasma is the proportion of atoms that have lost or gained
electrons, and is controlled mostly by the temperature. Even a partially
ionized gas in which as little as 1% of the particles are ionized can
have the characteristics of a plasma (i.e., response to magnetic fields
and high
electrical conductivity). The degree of ionization,
, 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.
Temperatures
Plasma temperature is commonly measured in
Kelvins or
electronvolts
and is, informally, a measure of the thermal kinetic energy per
particle. Very 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 a relationship called the
Saha equation. At low temperatures, ions and electrons tend to recombine into bound states—atoms
[12]—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 ionized technological plasmas, where the ions are often near the
ambient temperature.
Thermal vs. non-thermal plasmas
Based on the relative temperatures of the electrons, ions and
neutrals, plasmas are classified as "thermal" or "non-thermal". Thermal
plasmas have electrons and the heavy particles at the same temperature,
i.e., they are in thermal equilibrium with each other. Non-thermal
plasmas on the other hand have the ions and neutrals at a much lower
temperature (sometimes room temperature), whereas electrons are much
"hotter" (
).
A plasma is sometimes referred to as being "hot" if it is nearly
fully ionized, or "cold" if only a small fraction (for example 1%) of
the gas molecules are ionized, but other definitions of the terms "hot
plasma" and "cold plasma" are common. Even in a "cold" plasma, the
electron temperature is still typically several thousand degrees
Celsius. Plasmas utilized in "plasma technology" ("technological
plasmas") are usually cold plasmas in the sense that only a small
fraction of the gas molecules are ionized.
Plasma Potential
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.
[13] Plasma temperatures in lightning can approach 28,000 Kelvin (27,726.85 °C) (49,940.33 °F) and electron densities may exceed 10
24 m
−3.
Since plasmas are very good
electrical conductors,
electric potentials play an important role. The potential as it exists
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". 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.
In
astrophysical plasmas,
Debye screening prevents
electric fields from directly affecting the plasma over large distances, i.e., greater than the
Debye length. However, the existence of charged particles causes the plasma to generate, and be affected by,
magnetic fields.
This can and does cause extremely complex behavior, such as the
generation of plasma double layers, an object that separates charge over
a few tens of
Debye lengths. The dynamics of plasmas interacting with external and self-generated
magnetic fields are studied in the
academic discipline of
magnetohydrodynamics.
Magnetization
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.
[14]
Comparison of plasma and gas phases
Plasma is often called the
fourth state of matter after solid, liquids and gases.
[15][16] 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:
| Property |
Gas |
Plasma |
| Electrical conductivity |
Very low: Air is an excellent insulator until it breaks down into plasma at electric field strengths above 30 kilovolts per centimeter.[17] |
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. |
| Velocity distribution |
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. |
| Interactions |
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. |
Common plasmas
Plasmas are by far the most common
phase of ordinary matter in the universe, both by mass and by volume.
[18] Our Sun, and all
stars, are made of plasma, much of
interstellar space is filled with a plasma, albeit a very sparse one, and
intergalactic space too. In our solar system,
interplanetary space is filled with the plasma of the
Solar Wind that extends from the Sun out to the
heliopause. Even
black holes, which are not directly visible, are fuelled by accreting ionising matter (i.e. plasma),
[19] and they are associated with
astrophysical jets of luminous ejected plasma,
[20] such as
M87's jet that extends 5,000 light-years.
[21]
Dust and small grains within a plasma will also pick up a net
negative charge, so that they in turn may act like a very heavy negative
ion component of the plasma (see
dusty plasmas).
The current consensus is that about 96% of the total energy density
in the universe is not plasma or any other form of ordinary matter, but a
combination of
cold dark matter and
dark energy.
In our Solar System, however, the density of ordinary matter is much
higher than average and much higher than that of either dark matter or
dark energy. The planet
Jupiter accounts for most of the
non-plasma, only about 0.1% of the mass and 10
−15% of the volume within the orbit of
Pluto.
Complex plasma phenomena
Although the underlying equations governing plasmas are relatively
simple, plasma behavior is extraordinarily varied and subtle: the
emergence of unexpected behavior 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 behavior and cannot typically be described either by simple,
smooth, mathematical functions, or by pure randomness. The spontaneous
formation of interesting spatial features on a wide range of length
scales is one manifestation of plasma complexity. The features are
interesting, for example, because they are very sharp, spatially
intermittent (the distance between features is much larger than the
features themselves), or have a
fractal
form. Many of these features were first studied in the laboratory, and
have subsequently been recognized throughout the universe. Examples of
complexity and complex structures in plasmas include:
Filamentation
Striations or string-like structures,
[25] also known as
birkeland currents, are seen in many plasmas, like the
plasma ball, the
aurora,
[26] lightning,
[27] electric arcs,
solar flares,
[28] and
supernova remnants.
[29] They are sometimes associated with larger current densities, and the interaction with the magnetic field can form a
magnetic rope structure.
[30] High power microwave breakdown at atmospheric pressure also leads to the formation of filamentary structures.
[31] (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
micrometers to kilometers in length.
[32]
One interesting aspect of the filamentation generated plasma is the
relatively low ion density due to defocusing effects of the ionized
electrons.
[33] (See also
Filament propagation)
Shocks or double layers
Plasma properties change rapidly (within a few
Debye lengths) across a two-dimensional sheet in the presence of a (moving) shock or (stationary)
double layer.
Double layers involve localized charge separation, which causes a large
potential difference across the layer, but does not generate an
electric field outside the layer. Double layers separate adjacent plasma
regions with different physical characteristics, and are often found in
current carrying plasmas. They accelerate both ions and electrons.
Electric fields and circuits
Quasineutrality of a plasma requires that plasma currents close on themselves in electric circuits. Such circuits follow
Kirchhoff's circuit laws and possess a
resistance and
inductance.
These circuits must generally be treated as a strongly coupled system,
with the behavior in each plasma region dependent on the entire circuit.
It is this strong coupling between system elements, together with
nonlinearity, which may lead to complex behavior. Electrical circuits in
plasmas store inductive (magnetic) energy, and should the circuit be
disrupted, for example, by a plasma instability, the inductive energy
will be released as plasma heating and acceleration. This is a common
explanation for the heating that takes place in the
solar corona.
Electric currents, and in particular, magnetic-field-aligned electric
currents (which are sometimes generically referred to as "
Birkeland currents"), are also observed in the Earth's aurora, and in plasma filaments.
Cellular structure
Narrow sheets with sharp gradients may separate regions with
different properties such as magnetization, density and temperature,
resulting in cell-like regions. Examples include the
magnetosphere,
heliosphere, and
heliospheric current sheet.
Hannes Alfvén
wrote: "From the cosmological point of view, the most important new
space research discovery is probably the cellular structure of space. As
has been seen in every region of space accessible to in situ
measurements, there are a number of 'cell walls', sheets of electric
currents, which divide space into compartments with different
magnetization, temperature, density, etc."
[34]
Critical ionization velocity
The
critical ionization velocity
is the relative velocity between an ionized plasma and a neutral gas,
above which a runaway ionization process takes place. The critical
ionization process is a quite general mechanism for the conversion of
the kinetic energy of a rapidly streaming gas into ionization and plasma
thermal energy. Critical phenomena in general are typical of complex
systems, and may lead to sharp spatial or temporal features.
Ultracold plasma
Ultracold plasmas are created in a
magneto-optical trap (MOT) by trapping and cooling neutral
atoms, to temperatures of 1
mK or lower, and then using another
laser to
ionize the atoms by giving each of the outermost electrons just enough energy to escape the electrical attraction of its parent ion.
One advantage of ultracold plasmas are their well characterized and
tunable initial conditions, including their size and electron
temperature. By adjusting the wavelength of the ionizing laser, the
kinetic energy of the liberated electrons can be tuned as low as 0.1 K, a
limit set by the frequency bandwidth of the laser pulse. The ions
inherit the millikelvin temperatures of the neutral atoms, but are
quickly heated through a process known as disorder induced heating
(DIH). This type of non-equilibrium ultracold plasma evolves rapidly,
and displays many other interesting phenomena.
[35]
One of the metastable states of a strongly nonideal plasma is
Rydberg matter, which forms upon condensation of excited atoms.
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.
[36]
Dusty plasma and 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.
[37]
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
fusion plasma from the reactor walls.
[38] 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.
[39] 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.
[40]
Mathematical descriptions
The complex self-constricting magnetic field lines and current paths in a field-aligned
Birkeland current that can develop in a plasma.
[41]
To completely describe the state of a plasma, we would need to write
down all the particle locations and velocities and describe the
electromagnetic field in the plasma region. However, it is generally not
practical or necessary to keep track of all the particles in a plasma.
Therefore, plasma physicists commonly use less detailed descriptions, of
which there are two main types:
Fluid model
Fluid models describe plasmas in terms of smoothed quantities, like density and averaged velocity around each position (see
Plasma parameters). 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 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
gyrokinetic approach can substantially reduce the computational expense of a fully kinetic simulation.
Artificial plasmas
Most artificial plasmas are generated by the application of electric
and/or magnetic fields. 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 ionization within the plasma—fully, partially, or weakly ionized
- 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)
- The application.
Generation of artificial plasma
Just like the many uses of plasma, there are several means for its
generation, however, one principle is common to all of them: there must
be energy input to produce and sustain it.
[42] For this case, plasma is generated when an
electrical current is applied across a
dielectric gas or fluid (an electrically
non-conducting material) as can be seen in the image below, 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.
[43] 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
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”,
[44] mainly due to a small mean free path (average distance travelled between collisions).
Electric arc
Cascade process of ionization. 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 ionization, 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 ionizes the resulting atoms
(where degree of ionization 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.
[45][Note 3]
Examples of industrial/commercial plasma
Because of their sizable temperature and density ranges, plasmas find
applications in many fields of research, technology and industry. For
example, in: industrial and extractive
metallurgy,
[45] surface treatments such as
plasma spraying (coating),
etching in microelectronics,
[46] metal cutting
[47] and
welding; as well as in everyday
vehicle exhaust cleanup and
fluorescent/
luminescent lamps,
[42] while even playing a part in
supersonic combustion engines for
aerospace engineering.
[48]
Low-pressure discharges
- Glow discharge plasmas:
non-thermal plasmas generated by the application of DC or low frequency
RF (<100 a="" between="" class="mw-redirect" common="" electric="" electrodes.="" field="" gap="" generated="" href="http://en.wikipedia.org/wiki/Fluorescent_light" is="" khz="" metal="" most="" nbsp="" of="" plasma="" probably="" the="" this="" title="Fluorescent light" to="" two="" type="" within="">fluorescent light
tubes.
[49]
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.[50]
Cascaded Arc Plasma Source: a device to produce low temperature (~1eV) high density plasmas.
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.[51]
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).[52]
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 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.[53]
The application of the discharge to synthetic fabrics and plastics
functionalizes the surface and allows for paints, glues and similar
materials to adhere.[54]
- 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.[55]
History
Plasma was first identified in a
Crookes tube, and so described by
Sir William Crookes in 1879 (he called it "radiant matter").
[56] The nature of the Crookes tube "
cathode ray" matter was subsequently identified by British physicist
Sir J.J. Thomson in 1897.
[57] The term "plasma" was coined by
Irving Langmuir in 1928,
[58] perhaps because the glowing discharge molds itself to the shape of the Crooks tube (
Gr. πλάσμα – a thing moulded or formed).
[59] Langmuir described his observations as:
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.[58]
Fields of active research
