ABSTRACT
This chapter investigates the conditions under which onsets of various plasma instabilities may be expected. It considers instabilities characteristic of inhomogeneous plasmas. The nature of the instabilities should therefore depend critically on the interplay between the magnetic field and the spatial inhomogeneities. The ion-acoustic wave instability is a typical plasma-wave instability arising in these circumstances. A drift motion between electrons and ions produces an electric current in a plasma. When the drift velocity is smaller than a thermal velocity of the particles, the velocity distributions significantly overlap between the two species. The existence of such an ion-acoustic wave instability under the action of an applied electric field has been considered by many investigators both for gaseous plasmas and for solid-state plasmas. When the drift velocity becomes comparable to or greater than either of the thermal spreads, an instability with a different classification, namely a two-stream instability, takes place; the thermal effects are less important in this instability.
