On Temporal Evolution of Plasma Density Fluctuations in the Presence of a Strong Electromagnetic Wave

Abstract

Plasma heating experiments at the lower hybrid frequency have lately been carried out in many laboratories. The advantages of this frequency region are well known. as is the danger of anomalous peripheral absorption of energy due to parametric instabilities. For comparatively small amplitudes of the pump wave the non-linear mechanism of such absorption can be described in terms of the theory of weak plasma turbulence /1,2/. In this case a stationary spectrum of high-frequency noises is supposed to exist in a plasma, transferring its energy to particles mainly through collisional damping. At higher amplitude levels qualitative changes in the character of plasma turbulence occur, in particular, strong density fluctuations accompanied by distortions in the spatial distribution of high-frequency oscillations .. In this paper we shall consider an instability of this type driven directly by the pump wave .. We shall ignore completely the possible presence of a turbulent background, implying that the instability in question either has the fastest growth rate or may occur independently of the short-wavelength noise. Instabilities caused by pondermotive forces of highly intense waves (self-modulation, filamentation, soliton formation and collapse) have been widely discussed; however, the case of a monochromatic quasi-electrostatic pump reveals some new features which we shall first consider qualitatively.