Plasma (physics)

Plasma is a state of matter that results from one of the other three states (often, the gaseous one) having undergone an appreciable degree of ionization. It thus consists of a significant portion of charged particles (ions and/or electrons). 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 the physical conditions, 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 three other classical 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.