Self-modulation of high-frequency electric field and formation of plasma cavities

Self-modulation of the oscillating electric field is observed when its frequency is near the electron plasma frequency. The localized field due to the self-modulation is found to make depressions in the plasma density, and to move in unison with the density dip with approximately the ion-acoustic speed. The maximum amplitudes of the high-frequency field and the density dip are linearly proportional to each other, but the former shows a phase jump at the bottom of the density dip. All these features are qualitatively explained by the results of the recent theory of coupled electron-plasma and ion-acoustic solitary waves. The self-modulation occurs in the overdense region, the parametric decay instability is observed in the underdense region.