Fluctuations and wave scattering in plasmas (hydrodynamic and kinetic approaches)

Abstract

Kinetic and fluid treatments of nonlinear wave interactions (wave mixing and electromagnetic wave scattering) in plasmas are considered and the difference between the two approaches is discussed. Solving the kinetic equation by the method of moments makes it possible to substantiate the validity and to find the applicability range of the fluid approach to the study of nonlinear plasma processes (in particular, electromagnetic wave scattering by plasma fluctuations). Electromagnetic and hydrodynamic fluctuations in plasmas are considered. In the case of wave transformation, when mixed waves satisfy linear equations, both fluid and kinetic approaches yield similar expressions for the inducing current. In the case of electromagnetic wave scattering by thermal fluctuations, the two approaches lead to similar results only in the cold-plasma limiting case, when thermal effects may be completely disregarded (this concerns both thermal corrections to the wave dispersion equation and the Langevin source in the material relation for the fluctuations). The different expressions for the inducing current, obtained from different representations of the fluid equations, reduce to the same formula in the cold-plasma limiting case. An additional effect of electromagnetic wave scattering caused by the thermal dispersion of fluctuations is considered. The current that induces scattered waves is analyzed as a function of the scattered wave frequency.