Plasma (physics)
Plasma is a state of matter that results from a gaseous state having undergone some degree of ionization. It thus consists of a significant portion of charged particles. While rarely encountered on Earth, it is estimated that 99.9% of all ordinary matter in the universe is plasma. Stars are almost pure balls of plasma, and plasma dominates the rarefied intracluster medium and intergalactic medium. Plasma can be artificially generated, for example, by heating a neutral gas or subjecting it to a strong electromagnetic field.
The presence of charged particles makes plasma electrically conductive, with the dynamics of individual particles and macroscopic plasma motion governed by collective electromagnetic fields and very sensitive to externally applied fields. The response of plasma to electromagnetic fields is used in many modern devices and technologies, such as plasma televisions or plasma etching.
Depending on temperature and density, a certain number of neutral particles may also be present, in which case plasma is called partially ionized. Neon signs and lightning are examples of partially ionized plasmas.
Unlike the phase transitions between the other three states of matter, the transition to plasma is not well defined and is a matter of interpretation and context. Whether a given degree of ionization suffices to call a substance "plasma" depends on the specific phenomenon being considered.
Early history
While a student at the age of 24, in 1816, Michael Faraday delievered a series of lectures with one of them titled "On Radiant Matter" where he hypothesised about the existence of such a matter, distinguished from the gaseous form "as far beyond vaporisation as that is above fluidity". He further expanded on this notion in 1819, listing it—while admitting being purely hypothetical—among "the four states of matter". One finds in the notes to this lecture "Reasons for belief in existence. Experimental evidence. Kinds of radiant matter admitted.", likely related to the light he observed in electrical discharges in air and other gases at different pressures.Plasma was first identified in laboratory by Sir William Crookes, who presented a lecture to the British Association for the Advancement of Science, in Sheffield, on Friday, 22 August 1879. Crookes used the "radiant matter" term, paying tribute to Faraday and his far-reaching speculations.
File:Plasma microfields.webm|thumb|Plasma microfields calculated by an N-body simulation. Note the fast moving electrons and slow ions, resembling a bodily fluid.
Systematic studies of plasma began with the research of Irving Langmuir and his colleagues in the 1920s. Langmuir also introduced the term "plasma" as a description of ionized gas in 1928:
Lewi Tonks and Harold Mott-Smith, both of whom worked with Langmuir in the 1920s, recall that Langmuir first used the term by analogy with the blood plasma. 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."
Definitions
The fourth state of matter
Plasma is called the fourth state of matter after solid, liquid, and gas. It is a state of matter in which an ionized substance becomes highly electrically conductive to the point that long-range electric and magnetic fields dominate its behaviour.Plasma is typically an electrically quasineutral medium of unbound positive and negative particles. Although these particles are unbound, they are not "free" in the sense of not experiencing forces. Moving charged particles generate electric currents, 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.
Plasma is distinct from the other states of matter. In particular, describing a low-density plasma as merely an "ionized gas" is wrong and misleading, even though it is similar to the gas phase in that both assume no definite shape or volume. The following table summarizes some principal differences:
| Gas | Plasma | |
| Interactions | Short-range: Two-particle collisions are the rule. | Long-range: Collective motion of particles is ubiquitous in plasma, resulting in various waves and other types of collective phenomena. |
| Electrical conductivity | Very low: Gases are excellent insulators up to electric field strengths of tens of kilovolts per centimetre. | 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, largely influenced by collisions with one another and by gravity. | Two or more: Electrons and ions possess different charges and vastly different masses, so that they behave differently in many circumstances, with various types of plasma-specific waves and instabilities emerging as a result. |
Ideal plasma
Three factors define an ideal plasma:- The plasma approximation: The plasma approximation applies when the plasma parameter Λ, representing the number of charge carriers within the Debye sphere is much higher than unity. It can be readily shown that this criterion is equivalent to smallness of the ratio of the plasma electrostatic and thermal energy densities. Such plasmas are called weakly coupled.
- Bulk interactions: The Debye length is much smaller than 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.
- Collisionlessness: The electron plasma frequency is much larger than the electron–neutral collision frequency. When this condition is valid, electrostatic interactions dominate over the processes of ordinary gas kinetics. Such plasmas are called collisionless.
Non-neutral plasma
Dusty plasma
A dusty plasma contains tiny charged particles of dust. 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.Properties and parameters
Density and ionization degree
For plasma to exist, ionization is necessary. The term "plasma density" by itself usually refers to the electron density, that is, the number of charge-contributing electrons per unit volume. The degree of ionization is defined as the fraction of neutral particles that are ionized:where is the ion density and the neutral density. In the case of fully ionized matter,. Because of the quasineutrality of plasma, the electron and ion densities are related by, where is the average ion charge.
Temperature
Plasma temperature, commonly measured in kelvin or electronvolts, is a measure of the thermal kinetic energy per particle. 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. In thermal equilibrium, the relationship is given by 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 and heavy plasma particles separately have a relatively well-defined temperature; that is, their energy distribution function is close to a Maxwellian even in the presence of strong electric or magnetic fields. However, because of the large difference in mass between electrons and ions, their temperatures may be different, sometimes significantly so. This is especially common in weakly ionized technological plasmas, where the ions are often near the ambient temperature while electrons reach thousands of kelvin. The opposite case is the z-pinch plasma where the ion temperature may exceed that of electrons.
Plasma potential
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.