Modelling of folded waveguide antennas

Abstract

The power handling and coupling of a folded waveguide (FWG) antenna array in the ion cyclotron range of frequencies (ICRF) are analysed numerically for reactor plasma conditions. The fields inside the FWG unit are approximated in the model of the usual rectangular waveguide that will result from unfolding the FWG, keeping the corresponding dimensions. Including the surface impedance for a hot reactor sized plasma and the external current simulating the short probe source inside the waveguide allows one to investigate the coupling between units, the input impedance and the matching of a system consisting of several FWG units. Reconstructing the field structure both inside the waveguides and in the vacuum layer (between the coupler front and the plasma), the poloidal and toroidal spectra of the waves launched into the plasma are computed in the case where the effect of the poloidal magnetic field is neglected. Assuming 60 MHz frequency and specific ITER relevant waveguide and plasma parameters with 5 cm scrape-off decay length and 10 cm vacuum gap between the plasma and the waveguide, the present model with idealized non-perturbed feeder excitation predicts a voltage of less than 40 kV along the current probe source, a maximum electric field below 30 kV'cm-l and a maximum voltage of the order of 250 kV inside the FWG with ten folds for 10 MW radiated power per unit. For a waveguide complex composed of eight units, the mutual coupling between the units is found to be non-negligible, indicating a need for tuning in the system circuit.