ABSTRACT
In any dynamical system, when a state of equilibrium or steady motion has been discovered, an important question is whether the state will be stable or unstable. In the case of plasmas the investigation of possible instabilities is of great practical interest, but unfortunately very involved. Besides the familiar electrostatic two-stream instability, a plasma without magnetic field may also be susceptible to unstable waves or oscillations which are partly or wholly transverse. There are microinstabilities which depend on a non-uniform equilibrium state. These require the solution of Boltzmann's equation with spatial gradients in the unperturbed state. The term "universal" has been applied to these instabilities. Their growth rates are often smaller than those typical of hydromagnetic or pure velocity-space instabilities, yet they are important in the sense that they are likely to be present as a residual effect when steps have been taken to exclude the others.
