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Description
Radio-frequency (RF) helicon sources usually display three modes of operation: a capacitive mode at low RF power $P_{RF}$, an inductive mode at intermediate power and a wave-sustained (W) mode, above a critical power $P_c$. The W-mode is reached when the plasma density allows for solutions to the dispersion relation of the natural helicon/Trivelpiece Gould (W) modes, which depend on the antenna design and the cavity geometry. A large density jump occurs at the transition to the W mode. A strong hysteresis is usually observed, and high-density natural W-modes are triggered in the ramp-down, when $P_{RF}$ is further decreased.
We demonstrate that W-modes are triggered well below the critical power $P_c$, when transiently and locally increasing the plasma density, either by current injection on-axis, or using a small secondary axial inductive source. When the perturbation stops, the primary RF source maintained at $P_{RF} < P_c$ sustains the high-density natural W-modes observed during the ramp-down. A thorough experimental characterization of spatially-resolved plasma density and wave magnetic fields structure is detailed for a half-helical antenna in a $1 \mathrm{\,m}$ long, $20 \mathrm{\,cm}$ in diameter plasma vessel, operating in the range of $0.1 \mathrm{\,Pa}$ with a static magnetic field of $180 \mathrm{\, G}$. We stress that a few tens of Watts of auxiliary power allows to trigger a W-mode at $P_{RF} = 500 \mathrm{\,W} = 0.5P_c$.
The underlying mechanism lies in the modification of the density profile, which is more centrally peaked in the presence of the axial perturbations, and modifies the wave coupling. This mechanism has been validated using the HELIC code, which solves Maxwell's equations with boundary conditions for given plasma density profiles and antenna design. The computation of the absorbed power using experimentally informed density profiles demonstrates that high density W-modes are triggered at lower $P_{RF}$ in the presence of peaked profiles. In this simplified model, the predicted steady-state working points reproduce the experimental features. When injection is stopped, the high density in the cavity allows the sub-critical natural modes to be maintained. These results potentially open new avenues for the development of efficient helicon sources at low RF power.