Spontaneous Excitation of the Long-Wave Ion-Cyclotron and Ion-Acoustic Oscillations in Fully Ionized Plasmas

For model wavelengths longer than the electron mean free path, the damping coefficients of both longitudinal ion-cyclotron and ion-acoustic waves are close to zero. As in the collisionless plasma these coefficients may become negative provided that the electron drift velocity exceeds some threshold value, due to the normal Doppler effect. The threshold drift for both modes mentioned is evaluated, assuming that the plasma is fully ionized, i.e., using one-particle distribution function formalism and kinetic equations with Landau collision integrals. It is shown that the spontaneous excitation of the ion-cyclotron oscillations first sets in at the long wave limit, as the electron drift is gradually augmented. On the contrary, spontaneous excitation of ion-acoustic waves is first noted at short wavelengths. Conditions of existence of the long wave limits for both modes are discussed.