Collisional dissipation is expected to be the primary damping mechanism for helicon waves in low-temperature, high-density, moderate magnetic field plasmas. Combining a normal-mode analysis with high-resolution magnetic field measurements, we validate this expectation across a broad parameter space. However, we identify a distinct regime where the measured helicon damping rate is reduced by nearly a factor of 2 compared with collisional predictions. In this regime, a second harmonic at $2\omega$ is observed, contributing about 10% to the total magnetic field amplitude. A weakly nonlinear perturbative analysis shows that this component results from a resonant three-wave coupling ($\omega +\omega \to 2\omega$) that feeds back on the fundamental, accounting for the apparent underdamping and defining a predictive interaction window in helicon devices. As a controlled realization of quadratic three-wave coupling in a magnetized plasma, these results illustrate how resonant harmonic generation can redistribute wave energy and modify effective linear damping.

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• High-frequency & RF discharges in plasmas
• Nonlinear phenomena in plasmas
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