  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.
