Introduction to the Plasma Issue

Abstract

The study of the physics of ionized gases has had a long and complicated history. The word "plasma" was first coined by Langmuir and Tonks in 1929 to denote a gas in which an important fraction of the molecules are dissociated into ions and electrons, the gas as a whole remaining electrically neutral. The laboratory study of plasmas, of course, had been pursued long before that, many important discoveries in the realm of gas discharge phenomena having been made in the 1800's. These studies, continuing into the Twentieth Century as exemplified by the work of Langmuir, served as the foundation for many practical electronic devices used for the generation, rectification, and control of electrical energy. The plasmas used in these devices usually have a low-charge density, and the fractional ionization is ordinarily less than one per cent. This small percentage of ionization is sufficient to provide good electrical conductivity which can be controlled externally, but it is difficult to study theoretically because of the numerous competing processes involving neutral atoms, metastable atoms, ions, electrons, and collective oscillations of ions and electrons.