Generation of density cavities and localized electric fields in a nonuniform plasma

A simple model is presented which is capable of describing the detailed space-time evolution of the interaction of long wavelength electromagnetic radiation with an unmagnetized plasma having a nonuniform density profile. The model consists of describing the electric field via the nonlinear Schrödinger equation and the density changes through the ion-acoustic wave equation with the ponderomotive force effects included self-consistently. In the linear regime, this formulation explains the time evolution of the mode-conversion process that leads to the excitation of a short wavelength Langmuir wave in the neighborhood of the resonance layer where the frequency of the external radiation matches the local value of the electron plasma frequency. In the nonlinear regime, the model predicts the generation of density cavities and the associated spatial localization of the electric field. These features are in good agreement with the experimental results of Kim, Wong, and Stenzel. In addition, the model predicts a variety of new interesting phenomena such as the excitation of ion-acoustic oscillations and nonlinear relaxation oscillations which are amenable to experimental observation.