Parametric Mode-Mode Coupling between Drift Waves in Plasmas

The nonlinear phenomenon of parametric excitation of stable resonant drift modes through mode‐mode coupling is examined in the potassium plasma of a Q device. The scheme, which was extensively investigated, is the degenerate case of three‐mode parametric coupling in which the stable resonant mode (m = 1) at ω1 is excited by the externally driven pump mode (m = 2) at ω0 = 2ω1. The usual frequency and wavenumber matching conditions are satisfied. The additional new aspects examined are the enhancement of thermal fluctuations, the threshold for excitation, and the power balance between the coupled modes and the external source. The main results obtained are the measurement of the coupling coefficient and the determination of the functional behavior of the modes. General agreement between the experimental results and theory derived from the expansion of the governing fluid equations to second order is attained. To describe the steady‐state amplitudes observed experimentally, nonlinear saturation phenomena are...