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The Conversion of Langmuir Waves in the Inhomogeneous Plasma

The conversion of Langmuir waves on sinusoidal density perturbations in a plasma with immobile ions was studied in numerical experiments (particle simulation). It was shown that the conversion has no threshold. It leads to the excitation of short wavelength modes of the wave (direct and backward waves) with wave numbers kN = ke ± βki, β = 1, 2, 3.... The time evolution of the mode amplitude can be described by the modulus of β - order Bessel function - EβN|Eo = Jβ[½(ñ/no)ωoet]|. The conversion leads to the localization of the electric field in the density cavity and to the formation of an envelope-soliton-like structure. In cases with high density perturbations this structure divides to more small-dimensional structures up to the beginning of the damping. It was shown that the damping is due to the trapping and acceleration of nonresonant electrons. The new phenomenon in a plasma with mobile ions is the formation of smalldimensional density perturbations in the region of electric field localization due to the action of ponderomotive force. The conversion on these perturbations leads to rapid damping of electric field.