THE EXCITATION OF BACKWARD PROPAGATED ION-ACOUSTIC INSTABILITY IN COLLISION-DOMINATED PLASMAS

The process of spontaneous excitation of ion-acoustic waves (IAW) of growing amplitude by electron drift, in collision-dominated weakly ionized plasmas placed in external d.c. electric field is considered from the stand-point of the linear kinetic theory based on the one-particle distribution-function formalism. The longitudinal permeability is evaluated, with both e-n and i-n collisions taken into account, by way of the respective collision integral perturbations. The results are derived for the range of frequencies nu(en) much greater than omega much greater than nu(in) and nu(en)omega much less than k2upsilon(te)2, provided that the electron drift-to-thermal velocity ratio satisfies u/upsilon(te) much less than k(upsilon)te/nu(en) much less than 1. It is found that, for certain forms of the steady-state electron distribution function (EDF), some of which are very probable in experimental situations, the criterion for the onset of the ion-acoustic instability is satisfied not for cos theta > 0, but rather for cos theta < 0 (cos theta = u . k/uk), i.e. one can find instances in which the instability is propagated in the direction opposite to that of the electron drift. In particular, this is found to be the case in some weakly ionized gas-discharge argon plasmas, where the IAW of growing amplitude propagated backwards are expected to set in, if the i-n collision frequency is sufficiently small.