Plasmas

Plasmas are the fourth state of matter, consisting of a partially or fully ionized gas of free electrons and positive ions. They exhibit collective behavior and high electrical conductivity, and respond strongly to electric and magnetic fields. In plasmas, long-range electromagnetic forces dominate over short-range collisions, leading to phenomena such as Debye shielding and plasma oscillations.

Plasmas form when energy input ionizes atoms, which can occur at high temperatures, under intense electrical discharges, or through photoionization. They are ubiquitous in the universe, making up stars and the interstellar medium, and appear in phenomena such as the solar wind, lightning, and auroras. Humans create plasmas in laboratories and industrial settings for study and practical use.

Plasmas vary from partially ionized to fully ionized. The degree of ionization, temperature, and density determine the regime, including low-temperature plasmas used in electronics processing and hot plasmas in fusion devices. Key parameters include the plasma frequency and Debye length, and much of plasma behavior is described by magnetohydrodynamics, which treats the plasma as a conducting fluid interacting with magnetic fields.

Applications span lighting and displays (neon signs, fluorescent lamps), materials processing (etching and deposition), spectroscopy, and sterilization. In research, plasmas enable fusion energy experiments and particle sources; plasma propulsion is explored for spacecraft propulsion. Safety considerations include ultraviolet radiation, high voltages, reactive species, and strong electromagnetic fields.