Admittance of a plasma-covered cylindrical antenna

Based on a variational formulation, an expression is derived for the admittance of a circumferential gap in an infinite cylinder covered by a set of coaxial dielectric or plasma layers. The gap is fed by the dominant TEM mode in a narrowly spaced, parallel-plate, radial waveguide. Numerical computations are presented for a multiple-layer model of a lossy plasma and for a dielectric layer which supports surface waves. These computations are compared with admittance measurements of a similarly excited metal cylinder surrounded by an annular plasma. The plasma is produced in a long double-wall cylindrical vessel by a pulsed d-c discharge in argon at 0.4 Torr. In the experiment, the radial guide is excited by a small axial probe fed by a conventional coaxial air line. The admittance viewed in the coaxial line is related to the gap admittance by an experimentally determined two-port network representing the junction region. The measurements are in fair agreement with calculated values based on a three-layer model of the inhomogeneous plasma and the discharge tube.