ABSTRACT
A plasma, for the physicist, is not a jelly-like substance. It is a gas containing a very high density of electrons and ions. The name ‘plasma’ for such a gas was coined by the late Irving Langmuir in the course of his theoretical and experimental investigations of gas discharges at General Electric Research Laboratories during the 1920s. In a gaseous discharge, such as one finds in a fluorescent light, only a minute fraction of the atoms present are ionized, that is disassociated into positive ions and electrons; none the less a study of the motion of the ions and electrons shows that many new and interesting phenomena can take place. In most highly-ionized gases, such as one finds in the ionosphere (the layer of free ions and electrons present toward the top of our atmosphere), the motion of the electrons and ions is, in fact, organized to a remarkable extent. The organization takes two forms, neither of which is characteristic of ordinary dilute gases made up of neutral atoms. First, a given particle, ion or electron, does not move independently of its neighbors. Rather, such a particle is always accompanied by a cloud of other particles, which move along with it in such a way as to screen out the electric field produced by its charge. Second, the electrons carry out long-wavelength, high-frequency oscillations, which involve the coherent motion of many thousands of particles. Langmuir’s studies of the possibilities for organized behavior in such a system led him to believe that here was a new state of matter – neither solid, liquid or gas. He called it plasma.
