Excitation and Damping of Oscillations

Abstract

All kinds of plasma oscillations can be supported if energy input is fed into a plasma from outside the system. If there is no such input oscillation in an equilibrium plasma must be damped by various energy-dissipation processes. However, real plasmas are very often not in complete thermodynamic equilibrium. In such nonequilibrium plasmas, oscillations can be excited as well as damped. Assume that a plasma is in hydrodynamic (but not thermodynamic) equilibrium and that oscillation are excited spontaneously. This means that the equilibrium state is unstable. Growing oscillations are merely another form of instability; we may call these oscillatory instabilities.* It follows from the general laws of thermodynamics that a condition of complete thermodynamic equilibrium cannot be unstable. Thus, an equilibrium state can be unstable to oscillation only in the hydrodynamic sense and not in the thermodynamic sense. Growing oscillations always appear as one of the links in the energy-transformation chain.