Impact of resonant second-harmonic generation on helicon-wave damping.

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 2ω 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 (ω+ω→2ω) 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.