Power coupling to helicon and Trivelpiece–Gould modes in helicon sources

Abstract

The effects of finite electron mass on the helicon mode and the role of the Trivelpiece-Gould (TG) mode in helicon sources are considered. In an unbounded plasma these waves are commonly referred to as whistler waves. A simple cold plasma antenna-wave coupling code gives results which compare favorably with experimental observations of the whistler wave resonance cone. The connection between these cones and the Trivelpiece-Gould eigenmodes of bounded plasmas is discussed. The theory is applied to the study of antenna-wave coupling in helicon sources. It reveals that finite electron mass effects do not significantly improve antenna-wave coupling for ω/ωce<0.5. It is also suggested that, because of the resonance cone dispersion, eigenmode cavity resonances of the Trivelpiece-Gould mode could be difficult to excite in practice. This precludes geometric resonances of the antenna impedance due to the existence of the TG mode as an explanation for the high plasma production efficiency of helicon sources.