On the density limit in the helicon plasma sources

Abstract

Existence of the density limit in the helicon plasma sources is revisited. The low- and high-frequency regimes of a helicon plasma source operation are distinguished. In the low-frequency regime with ω<ωciωce, the density limit is deduced from the Golant-Stix criterion of the accessibility of the lower hybrid resonance. In the high-frequency case, ω>ωciωce, an appropriate limit is given by the Shamrai-Taranov criterion. Both these criteria are closely related to the phenomenon of the coalescence of the helicon wave with the Trivelpiece-Gould mode. We draw a conclusion that the derived density limits are not currently achieved in existing devices, perhaps, because of high energy cost of gas ionization.