The role of the lower hybrid resonance in helicon plasmas

A numerical study is carried out to investigate the eigenmode characteristics of helicon plasmas near the lower hybrid resonance using the analytic solutions of the wave equation for uniform plasmas. It is shown that there are innumerable or a few isolated eigenmodes depending on the value of the wave frequency whether it is higher or not than that of the lower hybrid frequency. The plasma resistance is usually large with a sharp peak near the lower hybrid frequency, but it depends on the plasma density. Accordingly, the wave equation is solved self-consistently with the particle and power balance equations, and it is shown that there exists a threshold frequency for efficient ionization near the lower hybrid resonance. This may explain the existence of the optimum frequencies of helicon discharge which have been experimentally found to be near the lower hybrid frequencies.