On the density limit in the helicon plasma sources

TL;DR

This paper critically revisits the density limit in helicon plasma sources, analyzing low- and high-frequency regimes and their associated criteria, and suggests future possibilities with increased RF power.

Contribution

It distinguishes the density limits in different frequency regimes and relates them to wave coalescence phenomena, providing a theoretical framework for future device improvements.

Findings

Density limits are derived from Golant-Stix and Shamrai-Taranov criteria.

Current devices do not reach these theoretical density limits.

Future increases in RF power could meet these density limits.

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.