Plasma Instabilities

Abstract

The subject of plasma instabilities has undergone a development in the last decade which at first sight seems chaotic. This apparent chaos is due to the rapid growth in the number of known instabilities. In a recent review by Lehnert¹ thirty-one plasma instabilities are listed. Of these, all but eleven have been discovered since 1958. (When we say “discovered” we mean discovered theoretically. Many of them have not been unambiguously identified experimentally.) However, along with this growth in the number of known instabilities, there has been a growth in understanding of the relationships existing within families of instabilities. For instance, there are a number of instabilities with frequencies near the ion cyclotron frequency and its harmonics. These were discovered by different people at different times and have a variety of names, such as “anisotropy”, “loss cone”, “drift cyclotron” etc. All of these may be described in terms of emission and absorption of plasma waves by energetic particles. When the emission exceeds the absorption, the wave grows (is unstable). Conditions under which emission may exceed absorption may be achieved in various ways, such as making the distribution functions anisotropic, putting a beam through the plasma, having spatial gradients of density or temperature, etc. In a sense, all of these instabilities depend upon an “inverted population” such as one has in masers and lasers.