Decay of longitudinal plasma oscillations to ion-sound oscillations

Abstract

Recently, the question of the nonlinear relation between various plasma oscillations has been the subject of much attention as a result of a series of circumstances. The most important of these is the fact that in the majority of experiments on beam instabilities [1, 2] the intensity of the oscillations excited is very large, so that nonlinear effects in the interaction of oscillations must be significant. It should be noted that beam instability is not the only method of exciting highfrequency plasma oscillations. As was shown in [3], very intense oscillations may also be excited by beams of transverse waves of various frequency ranges, among which are powerful light beams [4]. Finally, excitation is possible by means of shock waves [5] and large-amplitude waves propagating through a plasma. Nonlinear coupling of plasma and low-frequency ion-sound oscillations leads, in particular, to the generation of the latter [6]. On the one hand, this is of interest as regards the problem of turbulent heating of a plasma, since the absorption of ion-sound oscillations in a plasma is usually stronger than the absorption of plasma oscillations. On the other hand, ion-sound oscillations may bring about the acceleration of low-energy ions due to the effects of induced Cerenkov absorption and radiation of waves by ions, as considered in the work of one of the authors [7], Although plasma oscillations accelerate particles more effectively [8], the injection conditions in the configuration for acceleration by plasma oscillations are very stringent v > ve. The number of ions with such velocity for small ion temperatures Ti is small. Thus, the acceleration of ions will arise in this case as a result of the interaction of ion-sound oscillations until such time as their velocity reaches values of the order ve. This question is of interest not only for the acceleration of ions (heating) in the presence of high-frequency turbulence created by beams of charged particles or as a result of the action of powerful radiation on a plasma, but also for the problem of neutron radiation from powerful impulse discharges in a plasma and for a series of astrophysical problems. In what follows we consider a number of one-dimensional self-consistent problems regarding the interaction (decay and fusion) of plasma and ion-sound oscillations resulting from the induced Raman scattering of the former by the latter. It is shown that the development of instability in a turbulent plasma with a high level of excited plasma oscillations leads both to the excitation of ion-sound oscillations, and also to the appearance in the plasma oscillation spectrum of satellites differing from the basic frequency ω0e by a frequency of the order ω0i and with greater intensities for the lower frequencies. The qualitative change of the plasma oscillation spectrum may serve as an immediate indication of the excitation of ion-sound oscillations in the system. The results obtained allow one to trace the process of development of instabilities. It is shown that in a plasma with a high level of ion-sound oscillations “violet” satellites are excited in the plasma oscillation spectrum, while the intensities of the violet satellites have a tendency to level out and form a satellite plateau if the level of ion-sound waves is high enough.