The New Roles of Cavitons in Inhomogeneous Plasmas Under Strong Electromagnetic Irradiation.

The characteristics of electron plasma waves (EPWs) trapped inside density cavities or cavitons which are generated by the ponderomotive forces associated with the localized waves are experimentally investigated in inhomogeneous, unmagnetized plasmas under strong electromagnetic (em) excitations. The trapping of EPWs in cavitons is studied in detail by monitoring the wave evolution after the termination of the excitation field in a capacitor-plate experimental configuration. The existence of caviton eigenmodes for the trapped EPWs is confirmed for the first time by using the novel two-frequency excitation technique. Our results verify quantitatively the predictions of the Morales-Lee model for resonance absorption. In a high-power microwave -plasma system, it is found that deep cavitons of |delta{rm n/n_ c} |>= 0.5 can emit em waves at frequency 3/2)omega_{rm o}, where {rm n_ c} is the critical density and omega _{rm o} is the frequency of the incident microwave. This new phenomenon is interpreted as a result of the degenerate parametric decay instability inside a deep caviton in which the trapped EPW at frequency omega_{rm o} decays into electron oscillations at (1/2)omega _{rm o} via the coupling with the lowest caviton eigenmode. The interaction between the EPW at omega_{rm o } and the electron oscillations at (1/2) omega_{rm o}>=nerates a localized current oscillating at (3/2)omega _{rm o} which emits em waves at (3/2)omega_{rm o}. A bursty nature of (3/2)omega_ {rm o} radiation is accompanied by the production of fast electrons, suggesting that the wave-particle interactions terminate the radiation process. The formation of small-scale cavitons embedded in a large density cavity is observed for the first time during ionospheric heating experiments at the Arecibo Observatory, in which high-power radio waves at frequencies below the maximum electron plasma frequency of the ionosphere are launched from the ground station in order to modify the ionosphere. This demonstrates the universality of the caviton concept in inhomogeneous plasmas under intense em irradiation.