Instabilities in Homogeneous Plasmas
The source of energy responsible for the onset of a plasma instability may be sought in the excess of the free energy contained in a plasma away from thermodynamic equilibrium. When the plasma is away from thermodynamic equilibrium, a collective mode may in some cases become unstable; the amplitude of a collective mode tends to grow exponentially. Most of the plasmas found in nature are in fact significantly far from thermodynamic equilibrium. Plasmas may also be subjected to the influence of external force fields, which would create flows of particles, momentum, and energy in plasmas. Possibilities of plasma instabilities exist in any of these nonequilibrium circumstances. A typical case of plasma instability, perhaps the simplest from a mathematical viewpoint, occurs in a beam–plasma system. The electron plasma oscillation and the ion-acoustic wave propagate preferentially in the direction of the magnetic field.