Interaction between a Langmuir wave and a ballistic perturbation

The theoretical solutions of the Landau–Vlasov initial value problem giving mode‐mode coupling usually neglect the free‐streaming contribution. The problem is solved theoretically including the ballistic terms. A mode, resulting from the interaction between the ballistic perturbation of the frequency ω′ associated with a launched wave and the Landau component of frequency ω′′ of a second launched wave, appears only if ω′′≳ω′. Its frequency is ω=ω′′−ω′ and its phase velocity is identical to that of the Landau mode ω/k=ω′′/k′′. The amplitude of this mode is calculated as a function of distance from the launching probe and this is compared with the amplitude measured experimentally in a plasma column.