- in geology, a green type of chalcedony (rare - first used in 1772),
- in medicine, the fluid part of blood: see blood plasma,
- lymph or milk (first used in 1845),
- an ionized gas. The remainder of this article is concerned with this last meaning.
Plasma is the fourth state of matter. It is distinct from the three lower-energy phases of matter solid, liquid, and gas. Plasmas are the most common form of matter, comprising more than 99% of the known visible universe. Commonly encountered forms of plasma include the Sun and other stars (which are plasmas heated by nuclear fusion), lit fluorescent lamps, lightning, the Aurora borealis, the solar wind, and interstellar nebulae. A plasma is also generated in front of a spacecraft's heat shield on reentering the atmosphere.
In astrophysical plasmas, Debye screening prevents electric fields from affecting the plasma very much, 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. The dynamics of plasmas interacting with external and self-generated magnetic fields are studied in the academic discipline of magnetohydrodynamics.
There are two broad categories of plasma, hot plasmas and cold plasmas. The Sun is an example of a hot plasma. Full ionization takes place, and the ions and the electrons are in thermal equilibrium. This is what would commonly be known as the "fourth-state of matter". A cold plasma is one where only a small fraction of the atoms in a gas are ionized, and the electrons reach a very high temperature, whereas the ions remain at the ambient temperature. These plasmas can be created by using a very high electric field to accelerate electrons which ionize the atoms. The electric field is either capacitively or inductively coupled into the gas by means of a plasma source. Common applications of cold plasmas include Plasma-Enhanced Chemical Vapor Deposition, Plasma Ion Doping, and Reactive Ion Etching.
The term plasma is generally reserved for a system of charged particles large enough to behave collectively, excluding microscopically small collections of charged particles. The typical characteristics of a plasma are:
- 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 are long when compared to the period of plasma oscillations.