Abstract
The influence of the presence of a large amplitude plasma wave eψl exp (ωet-er) on the damping of another wave (ωk, k) in a plasma is considered. It is seen that the usual Landau's damping coefficient is increased by a factor [1+eψl/hωk] 2exp [- (kD/k) 2 (eψl/hωk) 2] if e and k are related by Pe/m_??_p k/m-eψl/h, the result being due to the fact that the resonant momentum p of electrons for emission (upper sign) and absorption (lower sign) of-plasmon plasmon (ωk. k) is determined by a conservation equation W (p+h, e) +hωk-W (p.e) =0 where W=W (p, e.;ψl) is the dispersion relation of an electron of momentum p in the periodic electric field associated with the plasma wave (ωe, e). The diffusion in the velocity space of electrons in a collisionless plasma is next considered. Various features of relaxation processes of electron velocity distribution may be understood by imagining an electron undergoing a sequence of displacements (recoils) in the velocity space, each recoil being either in the backward, or in “forward” direction depending on whether the electron emits or absorbs a plasmon. The probability that the electron suffers a forward (backward) displacement by absorbing (emiting) a plasmon is governed by a distribution in (phase) velocity space of plasmons, since, for instance, electrons cannot absorb plasmons from plasmon vacuum and whenever they arrive at a region of plasmon vacuum in the velocity space after a sequence of forward displacements by successive absorption of plasmons they suddenly become incapable of sufferring further forward displacements.
