Behavior of the sideband instability

The mechanism of the trapped−electron instability of a large−amplitude wave is revealed by the following experimental observations: Sideband waves exhibit an initial enhanced damping (examined by the use of linear test excitations) for a distance ∼λb′ the large−wave oscillation length. Beyond this point, the unstable waves grow with a rate linearly proportional to the main−wave electric potential. Measurements show that the instability growth depends on the initial amplitude of the large wave and can persist after this wave field has decayed by tens of decibels. This implication that the driving force of the instability resides in the particles is substantiated by direct measurements of the electron distribution function which, in the presence of a large wave, is found to develop a ’’bump−on−the−tail’’ of sufficient magnitude to account for the observed sideband growth.