Plasma (physics)

Plasma (physics), in physics, usually gaseous state of matter in which a part or all of the atoms or molecules are dissociated to form ions (see Ionization). Plasmas consist of a mixture of neutral particles, positive ions (atoms or molecules that have lost one or more electrons), and negative electrons. A plasma is a conductor of electricity, but a volume with dimensions greater than the so-called Debye length exhibits electrically neutral behavior. At a microscopic level, corresponding to distances shorter than the Debye length, the particles of a plasma do not exhibit collective behavior but instead react individually to a disturbance, for example, an electric field.

On the earth, plasmas usually do not occur naturally except in the form of lightning bolts, which consist of narrow paths of air molecules of which approximately 20 percent are ionized, and in parts of flames. The free electrons in a metal can also be considered as a plasma. Most of the universe, however, consists of matter in the plasma state. The ionization is caused either by high temperatures, such as inside the sun and stars, or by radiation, such as the ionization of interstellar gases or, closer to the earth, the upper layers of the atmosphere (see Ionosphere), producing the aurora.

Plasmas can be created by applying an electric field to a low-pressure gas, as in neon or fluorescent tubes (see Neon Lamp). A plasma can also be created by heating a neutral gas to very high temperatures. Usually the required temperatures are too high to be applied externally, and the gas is heated internally by the injection of high-speed ions or electrons that collide with the gas particles, increasing their thermal energy. The electrons in the gas can also be accelerated by external electric fields. Ions from such plasmas are used in the semiconductor industry for etching surfaces and otherwise altering the properties of materials.

In very hot plasmas the particles acquire enough energy to engage in nuclear reactions with each other during collisions. Such fusion reactions are the heat source in the sun's core, and scientists are trying to create artificial plasmas in the laboratory in which fusion reactions would produce energy for the production of electricity.

See also Fusion; Nuclear Energy; Physics: Developments in Physics Since 1930: Plasma Physics.