Direct nonlinear coupling of electromagnetic waves and electrostatic waves in a plasma: experiment

Observations of direct nonlinear coupling between electromagnetic waves and electrostatic waves in the bulk volume of a plasma are reported and compared with the predictions of an accompanying physical theory. The experimental configuration consists of a cylindrical resonant cavity partially filled with a coaxial Q -machine plasma column. Experimental verification of the predicted interaction resonances is presented. The finite interaction length defined by the cavity modifies the theoretical wave number selection rule, imparting a “lineshape” to the resonance which is consistent with observations. Two of the three interacting waves are externally excited and their interaction causes a third wave to grow with distance. The direction of growth depends upon the resonance conditions and choice of excitation, while the rate of growth depends upon the pump level, coupling strength, and linear wave damping. Indirect measurements of the mode power levels are presented which verify in magnitude the theoretically predicted coupling.