Plasmas Generated by Axisymmetric Helicon Antenna

Abstract

A theoretical model of the plasmas generated by a sheath-helix antenna is developed for axisymmetric perturbations. The system configuration consists of a cylindrical plasma column inside a dielectric tube of radius Rc. The eigenvalue equation is obtained inside the plasma column and the eigenfunction is identified as the Bessel function of the first kind of order zero. The radial wave numbers for the eigenfunction are described in terms of the rf frequency and plasma density. An algebraic dispersion relation is analytically obtained. Several eigenmodes are identified from the dispersion relation. They are the low-frequency whistler-like mode, the Trivelpiece-Gould mode (TG), and the lower-hybrid mode. Properties of the plasmas and perturbed fields excited by a single-loop antenna are experimentally investigated. A cross-sectional view of the light emission indicates that the helicon-plasma density generated at the lower-hybrid frequency has a hollow profile. The azimuthal component of the perturbed electric field is obtained experimentally and its radial profile is found to be similar to the theoretical model. The emission peak coincides with the radial location of the strongest electric-field intensity. The experimental observations of the field profiles agree remarkably well with theoretical predictions.