plasma

Plasma is the fourth state of matter. It is a gas in which a significant portion of atoms are ionized, creating free electrons and positively charged ions, with a population of neutral particles as well. This ionization endows the medium with electrical conductivity and a strong response to electric and magnetic fields. Plasmas emit light when excited species return to lower energy levels, which is the basis of neon lights and many astrophysical phenomena.

Plasmas are produced when energy is sufficient to remove electrons from atoms, through high temperature, electrical discharges, or intense radiation. They can be very hot, like the interiors of stars, or relatively cool, like the low-temperature plasmas used in lighting, surface treatments, and sterilization. In many plasmas, electrons are much hotter than ions, creating non-thermal conditions that still sustain reactive chemistry.

In nature, plasmas are ubiquitous: stars and interstellar gas, the solar wind, the Earth's ionosphere and magnetosphere, and lightning. Humans use plasmas in industry and research: lighting and displays, plasma etching and deposition in semiconductor fabrication, plasma cutting and welding, fluorescent lamps, and fusion experiments that aim to confine hot plasma with magnetic fields.

Plasmas are typically quasi-neutral on macroscopic scales, but local charge separations and collective electromagnetic effects dominate their behavior. They can be categorized into thermal (high-temperature) plasmas and non-thermal or cold plasmas, as well as magnetized and partially ionized varieties. The term plasma was coined by Irving Langmuir in 1928.