Plasma (physics)

:This article is about plasma in the sense of an ionized gas. For other uses of the term, such as blood plasma, see plasma (disambiguation).

Characteristics

The term plasma is generally reserved for a system of charged particles large enough to behave collectively. 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).

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In technical terms, the typical characteristics of a plasma are:

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• Debye screening lengths that are short compared to the physical size of the plasma.
• Large number of particles within a sphere with a radius of the Debye length.
• Mean time between collisions usually is long when compared to the period of plasma oscillations.

Plasma scaling

Plasmas and their characteristics exist over a wide range of scales (ie. they are scaleable over many orders of magnitude). The following chart deals only with conventional atomic plasmas and not other exotic phenomena, such as, quark gluon plasmas:

Related Topics:
Orders of magnitude - Quark gluon plasma

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Typical plasma scaling ranges: orders of magnitude (OOM)

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CharacteristicTerrestrial plasmasCosmic plasmas

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Sizein metres (m)10-6 m (lab plasmas) to:102 m (lightning) (~8 OOM)10-6 m (spacecraft sheath) to1025 m (intergalactic nebula) (~31 OOM)

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Lifetimein seconds (s)10-12 s (laser-produced plasma) to:107 s (fluorescent lights) (~19 OOM)101 s (solar flares) to:1017 s (intergalactic plasma) (~17 OOM)

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Density in particles percubic metre107 to:1021 (inertial confinement plasma)1030 (stellar core) to:100 (i.e., 1) (intergalactic medium)

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Temperaturein kelvin (K)~0 K (Crystalline non-neutral plasmahttp://sdphca.ucsd.edu/) to:108 K (magnetic fusion plasma)102 K (aurora) to:107 K (Solar core)

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Magnetic fieldsin teslas (T)10-4 T (Lab plasma) to:103 T (pulsed-power plasma)10-12 T (intergalactic medium) to:107 T (Solar core)

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Temperatures

The defining characteristic of a plasma is ionization. Although ionization can be caused by UV radiation, energetic particles, or strong electric fields, processes that tend to result in a non-Maxwellian electron distribution function, it is most commonly caused by heating the electrons in such a way that they are close to thermal equilibrium so the electron temperature is relatively well-defined. Because the large mass of the ions relative to the electrons hinders energy transfer, it is possible for the ion temperature to be very different (usually lower).

Related Topics:
Maxwellian - Distribution function - Thermal equilibrium

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The degree of ionization is determined by the electron temperature relative to the ionization energy (and more weakly by the density) in accordance with the Saha equation. If only a small fraction of the gas molecules are ionized (for example 1%), then the plasma is said to be a cold plasma, even though the electron temperature is typically several thousand degrees. The ion temperature in a cold plasma is often near the ambient temperature. Because the plasmas utilized in plasma technology are typically cold, they are sometimes called technological plasmas. They are often created by using a very high electric field to accelerate electrons, which then ionize the atoms. The electric field is either capacitively or inductively coupled into the gas by means of a plasma source, e.g. microwaves. Common applications of cold plasmas include plasma-enhanced chemical vapor deposition, plasma ion doping, and reactive ion etching.

Related Topics:
Saha equation - Ambient temperature - Plasma source - Chemical vapor deposition - Plasma ion doping - Reactive ion etching

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A hot plasma, on the other hand, is nearly fully ionized. This is what would commonly be known as the "fourth-state of matter". The Sun is an example of a hot plasma. The electrons and ions are more likely to have equal temperatures in a hot plasma, but there can still be significant differences.

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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 angle of the ions through n_e=langle Z angle 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).

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Potentials

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, although where double layers are formed, the potential drop can be large enough to accelerate ions to relativistic velocities and produce synchrotron radiation such as x-rays and gamma rays. This results in the important concept of quasineutrality, which says that, on the one hand, it is a very good approximation to assume that the density of negative charges is equal to the density of positive charges (n_e=langle Z angle n_i), but that, on the other hand, electric fields can be assumed to exist as needed for the physics at hand.

Related Topics:
Debye sheath - Double layer - Synchrotron radiation

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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^{ePhi/k_BT_e}. Differentiating this relation provides a means to calculate the electric field from the density: ec{E} = (k_BT_e/e)( abla n_e/n_e).

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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.

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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 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.

Related Topics:
Astrophysical - Debye screening - Electric fields - Debye length - Magnetic field - Academic discipline - Magnetohydrodynamics

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