Nonlinear Wave Interaction and Fluctuations in Plasma

Collective fluctuations in nonequilibrium plasma with nonlinear wave interaction are considered. The four-wave interaction is the main nonlinear effect when three-wave resonances are absent. This interaction causes an eigen frequency shift, proportional to the oscillation intensity. The proportionality coefficient is determined by nonlinear susceptibilities of the plasma. The spectral distribution of field fluctuations is derived with the Langmuir oscillation nonlinear frequency shift taken into account. It is shown that a stationary turbulent state of plasma with the fluctuational Langmuir oscillation level much greater than the thermal one is possible.