Characterization of an annular helicon plasma source powered by an outer or inner RF antenna

This paper presents a novel annular helicon plasma source which can be powered by either an outer radio-frequency antenna or an inner antenna. A low pressure argon plasma (5×10^-4 Torr), with an ion mean free path comparable to the dimension of the plasma source, is created by a constant radio-frequency (13.56 MHz) power of 310 W and a convergent-divergent (nozzle shape) magnetic field is used to guide the plasma motion, similar to the parametric setup of previous experiments for helicon thrusters. Three electrostatic probes (a retarding field energy analyser, an emissive probe and a Langmuir probe) are used to spatially characterize the plasma dynamics. The primary result is that there is an ion beam when the outer antenna is powered and no ion beam with the inner antenna powered even though there is a considerable potential drop in both cases. The plasma plume in the diffusion chamber is carefully characterized and compared for the two antenna regimes. Its structure is shown to be greatly influenced by the presence of an ion beam and the variation of antenna location. A plasma wake is also observed in the plume behind the inner tube interface and related cross-field transport of ions and electrons in this region is briefly discussed.