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

Helicon discharges are widely used to produce high-density, low-temperature plasmas and are central in a broad range of applications, such as plasma-material interaction studies, electric propulsion, fundamental plasma physics investigations, auxiliary heating in tokamaks, and wake-field particle acceleration. In such plasmas, helicon wave damping is commonly assumed to be dominated by collisional dissipation. In this work, we combine a normal-mode analysis with spatially resolved magnetic field measurements using a movable B-dot probe at the Resonant Antenna Ion Device to investigate helicon wave damping over a broad parameter space. While collisional models accurately describe most operating regimes, we identify a specific parameter window in which the damping of the fundamental helicon mode is reduced by approximately a factor of two relative to collisional predictions. In this regime, a coherent magnetic-field component at the second harmonic frequency 2ω becomes clearly detectable, contributing up to ~10% of the total magnetic signal. Using a weakly nonlinear perturbative framework, we show that this second-harmonic component arises from a resonant wave–wave interaction of the form ω + ω → 2ω. Importantly, the analysis reveals a nonlinear back-coupling from the second harmonic to the fundamental mode, which quantitatively accounts for the observed reduction in damping. Although second-harmonic helicon waves have been reported previously, their role has not been identified as a resonant nonlinear mechanism with a measurable impact on helicon wave dissipation. Based on this analysis, we derive a simple predictive criterion that identifies the parameter range in which this nonlinear interaction is important. Preliminary measurements from the CERN AWAKE Helicon Plasma Source exhibit similar harmonic signatures, suggesting that this mechanism may be robust across different helicon devices.