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 and the eigenfunction is identified as the Bessel function J0(x) of the first kind of order zero. The radial wave numbers ξ and η for the eigenfunction are described in terms of the rf frequency and plasma density. A full dispersion relation is analytically obtained, including influence of the finite plasma size, antenna effects, the electron inertia, and ion mass contributions. It is shown from the dispersion relation that the radial mode number ξ approaches infinity at the lower-hybrid frequency, exhibiting a resonance condition. Meanwhile, the radial wave number η approaches 3.83/Rc at the lower-hybrid frequency. A cross-sectional view of the light emission in experiment indicates that the helicon-plasma density at the lower-hybrid frequency has a hollow profile. The azimuthal component Eθ(r) of the perturbed electric field observed experimentally is very similar to the theoretical model of J1(3.83r/Rc) at the lower-hybrid frequency. The emission peak coincides with the radial location of the strongest electric-field intensity.
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