On the density limit in the helicon plasma sources

Abstract

Existence of the density limit in the helicon plasma sources is critically revisited. The low- and high-frequency regimes of a helicon plasma source operation are distinguished. In the low-frequency regime with $\omega <\sqrt{{\omega }_{ci}{\omega }_{ce}}$ the density limit is deduced from the Golant-Stix criterion of the accessibility of the lower hybrid resonance. In the high-frequency case, $\omega >\sqrt{{\omega }_{ci}{\omega }_{ce}}$, 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 argue that theoretical density limits are not achieved in existing devices but might be met in the future with the increase of applied rf power.

Submitted to Physics of Plasmas