NONLINEAR EFFECTS AND TURBULENT BEHAVIOR IN A BEAM-PLASMA INSTABILITY

Continuing experimental studies of beam-plasma interactions in a magnetic field have shown an assortment of nonlinear effects to be present during a well-developed instability. Observations have been made of: the vanishing of the spatial growth rate; electron cross-field diffusion in synchronism with the unstable electric field; generation of harmomics of the fundamental interaction frequency up to the seventh; similarities in the spatial behavior of all seven harmonics; and the existence of a wavenumber power spectrum whose average behavior over the domain of the harmonic peaks is proportional to k -5±0.5, in agreement with certain theories of plasma turbulence. The data suggest that the large-amplitude fundamental electric field is the driving force for the nonlinearity that generates the harmonics, and that the fundamental, and the harmonics as well, represent waves traveling with a phase velocity equal to that of the slow space-charge wave on the electron beam. An interpretation is offered in terms of an approach to stationary plasma turbulence through a linear beam-plasma interaction, followed by harmonic generation, wave-wave coupling, and ultimate broadening of the emission peaks into a continuous spectrum.