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This article is about the ionized gas. See also the Disambiguation page " Plasma " for other uses and meanings , illustrating some of the more complex phenomena of a plasma, including ''filamentation'']] blasts plasma throughout the solar system.]] 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. " Ionized " means that at least one Electron has been dissociated from a proportion of the atoms or molecules. The free Electric Charge s make the plasma Electrically Conductive so that it responds strongly to Electromagnetic Field s. This fourth state of matter was first identified in a discharge tube (or G. L. Rogoff, Ed., ''IEEE Transactions on Plasma Science'', vol. 19, p. 989, Dec. 1991. See extract at http://www.plasmacoalition.org/what.htm. More specifically, a plasma is an electrically conductive collection of Charged Particle s that responds ''collectively'' to Electromagnetic Force s. Plasma typically takes the form of neutral gas-like clouds or charged Ion Beam s, but may also include dust and grains (called Dusty Plasma s) Peratt, Anthony, ''Physics of the Plasma Universe'' (1992); They are typically formed by heating and ionizing a gas, stripping Electron s away from Atom s, thereby enabling the positive and negative charges to move freely. COMMON PLASMAS Plasmas are the most common and Intergalactic Medium ), essentially the entire volume of the universe is plasma (see Astrophysical Plasma s). 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 . Alfvén also noted that due to their electric charge, very small grains also behave as ions and form part of plasma (see Dusty Plasma s).
CHARACTERISTICS Plasmas possess several bulk (or average) characteristics. Some of the most important plasma characteristics are the degree of ionization, the plasma temperature, the density and the magnetic field in the plasma region. These characteristics can be used to identify plasma and to classify plasma behaviour. Plasma characteristics can take on values varying by many Orders Of Magnitude . The following chart deals only with conventional atomic plasmas and not other exotic phenomena, such as, Quark Gluon Plasma s:
Rigorous definition of a plasma Although the term plasma is often used loosely to describe any collection of charged particles, a system can only be rigorously called a plasma if the following technical criteria are satisfied: # Debye Screening lengths are short compared to the physical size of the plasma. # Large number of particles within any sphere with radius of the Debye Length . # Mean time between collisions usually long when compared to the period of Plasma Oscillation s. Degree of ionization For plasma to exist, ionisation 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). Temperatures , showing a glowing blue plasma streaming upwards. 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.]] 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 . Temperature controls the 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. 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 of the ions through . (See quasineutrality below.) The third important quantity is the density of neutrals . In a hot plasma this is small, but may still determine important physics. The degree of ionization is . Potentials is an example of plasma present at Earth's surface. Typically, lightning discharges 30 thousand amps, 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 kelvins and electron densities may exceed 1024/m3. ] 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 (), but on the scale of the Debye length there can be charge imbalance. In the special case that '' Double Layer s'' 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, 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 Field s 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 Field s. 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 Length s. The dynamics of plasmas interacting with external and self-generated Magnetic Field s 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: . 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 not affected by Debye Shielding .Richard Fitzpatrick, ''Introduction to Plasma Physics'', Magnetized plasmas 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:
COMPLEX PLASMA PHENOMENA of Tycho's Supernova , a huge ball of expanding plasma. Langmuir coined the name ''plasma'' because of its similarity to blood plasma, and Hannes Alfvén noted its cellular nature. Note also the filamentary blue outer shell of X-ray emitting 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:
Ultracold plasma 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 Beam s and an electron cloud in a Penning Trap . 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 neccesary 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 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 seperately. 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 cannot capture velocity space structures like interpenetrating beams, or resolve wave-particle effects. Kinetic 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. FIELDS OF ACTIVE RESEARCH . The electric field in a plasma Double Layer is so effective at accelerating ions, that electric fields are used in Ion Drive s]] This is just a partial list of topics. A more complete and organised list can be found on the Web site for Plasma science and technology Web site for Plasma science and technology .
FOOTNOTES SEE ALSO can be considered to be a very low temperature partial plasma.]]
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