Wave-wave interaction in plasmas

The paper elucidates the general topic of wave-wave interaction in plasmas by discussing some basic questions entering the analysis of nonlinear interactions of waves. It considers coherent nonlinear wave-wave interaction for nonlinearly stable as well as (explosively) unstable situations. Effects of saturation (due to nonlinear phase shifts) and dissipation (with mutually different linear damping of waves) are included. Particular attention is paid to the dynamics of the phases of the waves. For the problem of external excitation of the nonlinear effects, e.g. the stimulated Brillouin and Raman scattering processes occurring in laser-plasma interaction, the influence of deviation from perfect coherency of the pump wave (the incident laser beam) is considered, including the effects of pump depletion during the nonlinear coupling process. The results of a numerical analysis of the problem of transition between a coherent wave state of interacting waves and a random phase situation are also described. Finally, the nonlinear excitation of a nonresonant beat-wave, which interacts strongly with the plasma particles is discussed. Such a process is of considerable interest in connection with the heating of a plasma. It can also be viewed as a mechanism responsible for the stimulated scattering of waves by electrons, i.e., for the stimulated Compton scattering, and for beat-wave heating of plasmas. In fact, the problem of the Landau-damped nonresonant beat-wave may be viewed as a link between the descriptions of wave-wave and wave-particle interactions. (Auth.)