Backward-scattering instability of cyclotron harmonic waves in nonlinear decay processes

Experimental results are presented which demonstrate that a finite-amplitude traveling cyclotron harmonic wave may decay into two similar waves by the process of resonant mode-mode coupling. The results presented here are restricted to cases where the group velocity of one of the perturbed waves is in the opposite direction from that of the other perturbed wave. The important feature of this process is that considerable growth, both in time and space, can occur for the perturbed waves. The spatial growth of these waves is observed, and the wave amplitudes are in qualitative agreement with theory. Furthermore, the experiments provide a quantitative measure of the resonant mode-mode coupling coefficient for waves that propagate perpendicularly to the magnetic field in a warm plasma.