Experimental Studies of Nonlinear Landau Damping and Growth of Plasma Waves in a Magnetic Field

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Abstract
The nonlinear Landau damping and growth of longitudinal waves in a magnetic field is studied experimentally. It is observed that a finite‐amplitude cyclotron harmonic wave may decay nonresonantly into a similar, small‐amplitude wave. This perturbed wave dan be amplified exponentially by increasing the amplitude of the finite‐amplitude wave. Similarly, an externally launched small‐amplitude test wave, having the same frequency as the perturbed wave, can be amplified at a comparable rate. The spatial growth of the test wave is in agreement with the predictions of a third‐order mode‐coupling equation. Using a calibrated interferometer system, the nonlinear Landau damping coupling coefficients (matrix elements) are measured quantitatively, and are found to be in good agreement with theory.