MODELING LOOP ANTENNAS FOR HF PLASMA-HEATING IN THE ION-CYCLOTRON FREQUENCY-RANGE

The technique of impedance and admittance matrices allows a simple and elegant solution of the problem of radiation from antennas of arbitrary shape and orientation in plane stratified geometry. The medium towards which the antenna radiates is completely characterized by the values of these matrices at its surface, which for a hot plasma in the IC frequency range we obtain numerically in the finite Larmor radius approximation. In vacuum, the wave admittance and impedance matrices obey simple transformation rules, which are used to solve the matching problem for arbitrary source currents. Combined with the variational approach originally developed for the case of fast wave launching by Teilhaber and Jacquinot, this method allows the determination of the self-consistent current distribution in the antenna, its radiation resistance and its reactive impedance. The flexibility of the approach is illustrated by several examples, both without and with plasma loading. They include a discussion of the characteristics of the folded antenna which will be used for fast-wave ICR heating in the ASDEX Upgrade tokamak, and of T-antennas for launching ion Bernstein waves near the first ion cyclotron harmonic.