Helicon waves are right-hand circularly polarized, whistler-like electromagnetic modes driven by radio-frequency antennas in magnetized plasmas. Their ability to produce high-density plasmas, typically ne ~ 10^18-10^20 m^-3, at moderate input powers makes helicon sources attractive for applications including semiconductor processing, space propulsion, plasma-material interaction studies, auxiliary heating in fusion devices, and scalable plasma sources for next-generation particle accelerators. In all these contexts, reliable prediction of helicon wave propagation, damping, and nonlinear energy redistribution is essential for understanding and controlling plasma production.

This thesis investigates helicon wave propagation and damping in the Resonant Antenna Ion Device (RAID), a linear plasma device at the Swiss Plasma Center, EPFL, operating at ne ~ 10^18-10^19 m^-3, Te ~2-3 eV, and background magnetic fields B0 = 65-540 G. The theoretical framework developed here combines a linear normal-mode solver for the helicon dispersion relation in radially non-uniform density profiles with high-resolution magnetic-probe measurements along the plasma column.

The normal-mode analysis identifies multiple co-propagating radial eigenmodes across the full experimental parameter range, with axial wavelengths in quantitative agreement with the dispersion relation. By imposing theoretically constrained damping relationships between modes, the effective electron collision frequency is extracted from the measured wave field as a single free parameter. Across most of the accessible parameter space, this extracted collision frequency agrees with the classical electron-ion and electron-neutral collision frequency, confirming that collisional dissipation is the dominant spatial damping mechanism for helicon waves under the investigated conditions.

A narrow parameter window is nevertheless identified in which the measured damping rate is reduced by nearly a factor of two relative to the collisional prediction. In this regime, a strong second temporal harmonic of the helicon wave is observed, carrying approximately 10% of the total magnetic-field fluctuation power, with bicoherence signatures confirming resonant three-wave coupling. A weakly nonlinear perturbative extension of the fluid-Maxwell framework shows that this second harmonic is resonantly excited by nonlinear convective and Lorentz-force terms. At resonance, third-order back-coupling redistributes energy from the second harmonic between fundamental helicon modes, reducing the apparent damping rate. The resonant window can be predicted from plasma parameters and wave-vector matching, yielding a device-independent criterion for other helicon sources.

These findings are extended to the Helicon Plasma Source (HPS) at CERN, developed for the AWAKE proton-driven plasma wakefield acceleration experiment. Density characterization confirms operation in the required plasma regime, while second-harmonic excitation is observed under conditions consistent with the resonance criterion derived at RAID. This demonstrates that the nonlinear coupling mechanism identified in this thesis is not specific to RAID, but may occur more generally in helicon plasma sources.