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Plasma (physics)

2007 Schools Wikipedia Selection. Related subjects: General Physics

   A plasma lamp, illustrating some of the more complex phenomena of a
   plasma, including filamentation. The colors are a result of the
   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.
   Enlarge
   A plasma lamp, illustrating some of the more complex phenomena of a
   plasma, including filamentation. The colors are a result of the
   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.

   In physics and chemistry, a plasma is typically an ionized gas, and is
   usually considered to be a distinct phase of matter in contrast to
   solids, liquids, and gases because of its unique properties. " Ionized"
   means that at least one electron has been dissociated from a proportion
   of the atoms or molecules. The free electric charges make the plasma
   electrically conductive so that it responds strongly to electromagnetic
   fields.

   This fourth state of matter was first identified in a discharge tube
   (or Crookes tube), and so described by Sir William Crookes in 1879 (he
   called it "radiant matter"). The nature of the Crookes tube " cathode
   ray" matter was subsequently identified by British physicist Sir J.J.
   Thomson in 1897, and dubbed "plasma" by Irving Langmuir in 1928 ,
   perhaps because it reminded him of a blood plasma . Langmuir wrote:

          "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."

   Plasma typically takes the form of neutral gas-like clouds or charged
   ion beams, but may also include dust and grains (called dusty plasmas).
   They are typically formed by heating and ionizing a gas, stripping
   electrons away from atoms, thereby enabling the positive and negative
   charges to move freely.

Common plasmas

   Plasmas are the most common phase of matter. Some estimates suggest
   that up to 99% of the entire visible universe is plasma. Since the
   space between the stars is filled with a plasma, albeit a very sparse
   one (see interstellar medium and intergalactic space), essentially the
   entire volume of the universe is plasma (see astrophysical plasmas). In
   the solar system, 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. Notable plasma physicist Hannes Alfvén also
   noted that due to their electric charge, very small grains also behave
   as ions and form part of plasma (see dusty plasmas).
                   Common forms of plasma include

                        Artificially produced plasma

     * Those found in plasma displays and TVs
     * Inside fluorescent lamps (low energy lighting), neon signs
     * Rocket exhaust
     * The area in front of a spacecraft's heat shield during reentry into
       the atmosphere
     * Fusion energy research
     * The electric arc in an arc lamp, an arc welder or plasma torch
     * Plasma ball (sometimes called a plasma sphere or plasma globe)
     * Plasma used to etch dielectric layers in the production of
       integrated circuits

   Terrestrial plasmas

     * Lightning
     * Ball lightning
     * St. Elmo's fire
     * Sprites, elves, jets
     * The ionosphere
     * The polar aurorae

   Space and astrophysical plasmas

     * The Sun and other stars
       (which are plasmas heated by nuclear fusion)
     * The solar wind
     * The interplanetary medium
       (the space between the planets)
     * The interstellar medium
       (the space between star systems)
     * The Intergalactic medium
       (the space between galaxies)
     * The Io-Jupiter flux-tube
     * Accretion disks
     * Interstellar nebulae

Plasma properties and parameters

   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-or plasma
   energy pouring back into the atmosphere.
   Enlarge
   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-or plasma
   energy pouring back into the atmosphere.

   Plasma properties are strongly dependent on the bulk (or average)
   parameters. Some of the most important plasma parameters are the degree
   of ionization, the plasma temperature, the density and the magnetic
   field in the plasma region. We explain these parameters, and then
   describe how plasmas interact with electric and magnetic fields and
   outline the qualitative differences between plasmas and gases.

Definition of a plasma

   Although a plasma is loosely described as an electrically neutral
   medium of positive and negative particles, a more rigorous definition
   requires three criteria to be satisfied:
    1. The plasma approximation: Charged particles must be close enough
       together that each particle influences many nearby charged
       particles, rather than just the interacting with the closest
       particle (these collective effects are a distinguishing feature of
       a plasma). The plasma approximation is valid when the number of
       electrons within the sphere of influence (called the Debye sphere
       whose radius is the Debye (screening) length) of a particular
       particle is large. The average number of particles in the Debye
       sphere is given by the plasma parameter, Λ.
    2. 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.
    3. 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, plasmas act to shield charges very rapidly
       (quasineutrality is another defining property of plasmas).

Ranges of plasma 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:
       Typical ranges of plasma parameters: orders of magnitude (OOM)
              Characteristic Terrestrial plasmas Cosmic plasmas
   Size
   in metres 10^−6 m (lab plasmas) to
   10^2 m (lightning) (~8 OOM) 10^−6 m (spacecraft sheath) to
   10^25 m (intergalactic nebula) (~31 OOM)
   Lifetime
   in seconds 10^−12 s (laser-produced plasma) to
   10^7 s (fluorescent lights) (~19 OOM) 10^1 s (solar flares) to
   10^17 s (intergalactic plasma) (~17 OOM)
   Density
   in particles per
   cubic metre 10^7 m^-3 to
   10^32 m^-3 (inertial confinement plasma) 10^0 (i.e., 1) m^-3
   (intergalactic medium) to
   10^30 m^-3 (stellar core)
   Temperature
   in kelvins ~0 K (crystalline non-neutral plasma) to
   10^8 K (magnetic fusion plasma) 10^2 K (aurora) to
   10^7 K (solar core)
   Magnetic fields
   in teslas 10^−4 T (lab plasma) to
   10^3 T (pulsed-power plasma) 10^−12 T (intergalactic medium) to
   10^11 T (near neutron stars)

Degree of ionization

   For plasma to exist, ionization is necessary. The degree of ionization
   of a plasma is the proportion of atoms which 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. respond to
   magnetic fields and be highly electrically conductive). The degree of
   ionization, α is defined as α = n[i]/(n[i] + n[a]) where n[i] is the
   number density of ions and n[a] is the number density of neutral atoms.

Temperatures

   A candle flame. Fire can be considered to be a low temperature partial
   plasma.
   Enlarge
   A candle flame. Fire can be considered to be a low temperature partial
   plasma.

   Plasma temperature is commonly measured in Kelvin or electron volts,
   and is (roughly speaking) a measure of the thermal kinetic energy per
   particle. 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 among
   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.

   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
   (normally room temperature) whereas electrons are much "hotter".

   Temperature controls the degree of plasma ionization. In particular,
   plasma ionization is determined by the electron temperature relative to
   the ionization energy (and more weakly by the density) in accordance
   with the Saha equation. 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 in this sense.

Densities

   Next to the temperature, which is of fundamental importance for the
   very existence of a plasma, the most important property is the density.
   The word "plasma density" by itself usually refers to the electron
   density, that is, the number of free electrons per unit volume. The ion
   density is related to this by the average charge state \langle Z\rangle
   of the ions through n_e=\langle Z\rangle n_i . (See quasineutrality
   below.) The third important quantity is the density of neutrals n[0].
   In a hot plasma this is small, but may still determine important
   physics. The degree of ionization is n[i] / (n[0] + n[i]).

Potentials

   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 Kelvin and
   electron densities may exceed 1024/m3.
   Enlarge
   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 Kelvin and
   electron densities may exceed 10^24/m^3.

   Since plasmas are very good 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 the development of a
   Debye sheath. Due to the good electrical conductivity, the electric
   fields in plasmas tend to be very small. This results in the important
   concept of quasineutrality, which says that it is a very good
   approximation to assume that the density of negative charges is equal
   to the density of positive charges over large volumes of the plasma (
   n_e=\langle Z\rangle n_i ), 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:

          n_e \propto e^{e\Phi/k_BT_e} .

   Differentiating this relation provides a means to calculate the
   electric field from the density:

          \vec{E} = (k_BT_e/e)(\nabla n_e/n_e) .

   It is, of course, 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 (ie. greater than
   the Debye length). But the existence of charged particles causes the
   plasma to generate and be affected by magnetic fields. This can and
   does cause extremely complex behaviour, 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

   A plasma in which the magnetic field is 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 (ie.
   ω[ce] / ν[coll] > 1 where ω[ce] is the "electron gyrofrequency" and
   ν[coll] 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 E = -V x B (where E is the electric
   field, V is the velocity, and B 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. It is distinct from
   the three lower-energy phases of matter; solid, liquid, and gas,
   although it is closely related to the gas phase in that it also has no
   definite form or volume. There is still some disagreement as to whether
   a plasma is a distinct state of matter or simply a type of gas. Most
   physicists consider a plasma to be more than a gas because of a number
   of distinct properties including the following:
                             Property Gas Plasma
   Electrical Conductivity Very low

          The air is quite a good insulator, as demonstrated by high
          voltage electric power transmission where wires typically carry
          110,000 volts. High voltages may lead to electrical breakdown,
          as can lower pressures in fluorescent lights and neon signs

   Very high
    a. For many purposes the electric field in a plasma may be treated as
       zero, although when current flows the voltage drop, though small,
       is finite, and density gradients are usually associated with an
       electric field according to the Boltzmann relation.
    b. Any electric currents in the plasma "couple" (ie., connect and
       influence) strongly to magnetic fields, resulting in a large
       variety of structures such as filaments, sheets, and jets.
    c. Collective phenomena are common because the electric and magnetic
       forces are both long-range and potentially many orders of magnitude
       stronger than gravitational forces.

   Independently acting species One

          All gas particles behave in a similar way, influenced by
          gravity, and collisions with one another

   Two or three
   Electrons, ions, and neutrals can be distinguished by the sign of their
   charge so that they behave independently in many circumstances, having
   different velocities or even different temperatures, leading to
   phenomenon such as new types of waves and instabilities
   Velocity distribution
   Maxwellian

          The velocity distribution of all gas particles has a
          characteristic shape:

   May be non-Maxwellian
   Whereas collisional interactions always lead to a Maxwellian velocity
   distribution, electric fields influence the particle velocities
   differently. The velocity dependence of the Coulomb collision cross
   section can amplify these differences, resulting in phenomena like
   two-temperature distributions and run-away electrons.
   Interactions Binary
   Two-particle collisions are the rule, three-body collisions extremely
   rare. Collective
   Each particle interacts simultaneously with many others. These
   collective interactions are about ten times more important than binary
   collisions.

Complex plasma phenomena

   The remnant of Tycho's Supernova, a huge ball of expanding plasma. The
   blue outer shell arises from X-ray emission by high-speed electrons.
   Enlarge
   The remnant of Tycho's Supernova, a huge ball of expanding plasma. The
   blue outer shell arises from X-ray emission by high-speed electrons.

   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 recognised throughout the
   universe. Examples of complexity and complex structures in plasmas
   include:

Filamentation

   The striations or "stringy" things, seen in many plasmas, like the
   plasma ball (image above), the aurora, lightning, electric arcs, solar
   flares, and supernova remnants They are sometimes associated with
   larger current densities, and are also called magnetic ropes,. (See
   also Plasma pinch)

Shocks or double layers

   Narrow sheets with sharp gradients, such as shocks or double layers
   which support rapid changes in plasma properties. Double layers involve
   localised 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.
   A schematic representation of the Heliospheric current sheet, the
   largest structure in the Solar System, resulting from the influence of
   the Sun's rotating magnetic field on the plasma in the interplanetary
   medium (Solar Wind). It is sometimes informally refered to as the
   'Ballerina Skirt' model. . Enlarge
   A schematic representation of the Heliospheric current sheet, the
   largest structure in the Solar System, resulting from the influence of
   the Sun's rotating magnetic field on the plasma in the interplanetary
   medium ( Solar Wind). It is sometimes informally refered to as the
   'Ballerina Skirt' model. .

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
   behaviour 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 behaviour. 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 which 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 which is 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 ."

Critical ionization velocity

   The Critical ionization velocity is the relative velocity between an
   ionized plasma and a neutral gas. It is sufficient to substantially
   energise any neutrals which lose an electron. This energisation feeds
   back to cause yet more ionization, and the process can run away, to
   almost completely ionize the gas. Critical phemonema in general are
   typical of complex systems, and may lead to sharp spatial or temporal
   features.

Ultracold plasma

   Saturn's rings in which certain effects have been suggested are due to
   dusty plasmas (false colour image)
   Enlarge
   Saturn's rings in which certain effects have been suggested are due to
   dusty plasmas (false colour image)

   It is possible to create ultracold plasmas, by using lasers to trap and
   cool neutral atoms to temperatures of 1 mK lower. Another laser then
   ionizes the atoms by giving each of the outermost electrons just enough
   energy to escape the electrical attraction of its parent ion.

   The key point about ultracold plasmas is that by manipulating the atoms
   with lasers, the kinetic energy of the liberated electrons can be
   controlled. Using standard pulsed lasers, the electron energy can be
   made to correspond to a temperature of as low as 0.1 K, a limit set by
   the frequency bandwidth of the laser pulse. The ions, however, retain
   the millikelvin temperatures of the neutral atoms. This type of
   non-equilibrium ultracold plasma evolves rapidly, and many fundamental
   questions about its behaviour remain unanswered. Experiments conducted
   so far have revealed surprising dynamics and recombination behaviour
   that are pushing the limits of our knowledge of plasma physics.

Non-neutral plasma

   The strength and range of the electric force and the good conductivity
   of plasmas usually ensure that the density of positive and negative
   charges in any sizeable region are equal ("quasineutrality"). A plasma
   that has a significant excess of charge density or that is, in the
   extreme case, composed of only a single species, a 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 and grain plasma

   A dusty plasma is one containing tiny charged particles of dust
   (typically found in space) that also behaves like a plasma. A plasma
   containing larger particles is called a grain plasma.

Mathematical descriptions

   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 known as models, 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 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 how a system of particles evolves in an electromagnetic
   environment.

Fields of active research

   Hall effect thruster. The electric field in a plasma double layer is so
   effective at accelerating ions, that electric fields are used in ion
   drives
   Enlarge
   Hall effect thruster. The electric field in a plasma double layer is so
   effective at accelerating ions, that electric fields are used in ion
   drives

   This is just a partial list of topics. A more complete and organized
   list can be found on the Web site for Plasma science and technology .
     * Plasma theory
          + Plasma equilibria and stability
          + Plasma interactions with waves and beams
          + Guiding centre
          + Adiabatic invariant
          + Debye sheath
          + Coulomb collision
     * Plasmas in nature
          + The Earth's ionosphere
          + Space plasmas, e.g. Earth's plasmasphere (an inner portion of
            the magnetosphere dense with plasma)
          + Plasma cosmology
          + Plasma Astronomy
          + Industrial plasmas
               o Plasma chemistry
               o Plasma processing
               o Vacuum plasmaspraying
               o Plasma display

     * Plasma sources
     * Dusty Plasmas
     * Plasma diagnostics
          + Thomson scattering
          + Langmuir probe
          + Spectroscopy
          + Interferometry
          + Ionospheric heating
          + Incoherent scatter radar
     * Plasma applications
          + Fusion power
               o Magnetic fusion energy (MFE) — tokamak, stellarator,
                 reversed field pinch, magnetic mirror, dense plasma focus
               o Inertial fusion energy (IFE) (also Inertial confinement
                 fusion — ICF)
               o Plasma-based weaponry
          + Food processing ( Nonthermal plasma)

Pop. culture

     * Plasma is often the discharge of rayguns
     * The Metroid series often has a plasma-based weapon, except in
       Echoes, where it was replaced by a laser based weapon, which
       behaved like plasma.

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