Observations of harmonic dispersion and sideband coupling in a plasma

Experiments with large−amplitude electron−wave signals demonstrate the existence of nonlinear Floquet−Bloch harmonics and of a passive four−wave coupling interaction. Propagation of the first spatial−temporal harmonic is verified by direct dispersion measurements. The parametric process acts to couple test−wave excitations at nearby frequencies symmetric about a high−level pump. Several possible models for this effect are examined, and it is shown that a four−wave mode−coupling process provides the only consistent explanation. The observed passive interaction operates at spatial positions and at pump−wave levels preceding the turn−on of the trapped−electron sideband instability, and also in the presence of detrapping interference. These results indicate the indifference of the four−wave mechanism to particle trapping, and lead to the inference that the unstable electron sidebands must also be coupled by this competing nonlinear process. Implications of this conclusion for studies of the sideband instability are discussed.