Rectangular waveguide launchers for ion-cyclotron-resonance heating of tokamaks

Two types of waveguides that have been proposed as launchers for fast-wave ion-cyclotron-resonance heating of large tokamaks are examined. The coupling of power from a transmission line to the waveguide via coupling probes, and from the waveguide to a plasma with parameters representative of the proposed ITER device, are calculated. These calculations are applied to model a waveguide filled with a dielectric, and a vacuum waveguide containing a polarization-rotating grid. It is found that the power reflected to the transmission line by a dielectric-filled waveguide can be reduced to negligible levels by using an open-ended or shorted coupling probe of suitable dimensions. However, if the dielectric chosen is water, losses in the waveguide reach an unacceptable level. These losses could be reduced by a large factor by using a suitable ceramic material as the dielectric. The waveguide incorporating a polarization-rotating grid was found not to be a suitable launcher for ion-cyclotron-resonance heating, since only a tiny fraction of the incident power is transmitted to the plasma by the grid-Faraday screen system. (orig.)