A radio frequency plasma source with a non-uniform magnetic field was operated at 27.12 MHz, a peak magnetic field of 530 G, RF powers from 100 to 500 W, and increasing argon pressures, to investigate hybrid coupling modes and remote ionization under various experimental conditions. Plasma diagnostics revealed a smooth transition from a capacitive to inductive and helicon coupled discharge as the power increased, suggesting a superposition of coupling modes. Plasma densities on the order of ∼1018 m−3 could be achieved at relatively low RF powers. Measurements of power transfer efficiency indicated efficient energy transfer near the antenna, while the plasma developed an on-axis density peak at the magnetic nozzle throat, demonstrating non-local ionization even when the discharge operated in capacitive mode. Despite the excitation of an m=0 helicon wave for PRF200 W, classic collisional and collision-less wave damping mechanisms were insufficient to explain the observed wave decay, hinting to more complex energy deposition mechanisms at play. Analysis of non-local electrons kinetics suggest that remote ionization might be sustained by non-local electron transport rather than by helicon heating, and that increasing neutral pressure does not significantly alter the discharge dynamics, implying a limited role of ion-neutral collisions. Overall, the data hint that high-density plasmas could be achieved in non-uniform magnetic-nozzle devices even with absent or limited helicon wave contributions. These findings also highlight the complexity of the nonlinear processes involved (non-local particle transport/electrostatic instabilities/hybrid coupling modes) and motivate further experimental and numerical work to assess their role in RF power deposition in magnetic nozzles.
Caldarelli et al. (Fri,) studied this question.