Comparison between the dielectric‐filled rectangular and vacuum folded waveguide as fast‐wave launchers in the ICRF

In reactor situations, a waveguide may have certain advantages (such as power handling capability and structural rigidity) over loop antennas as fast wave launchers in tokamaks. We carry out a numerical analysis of a dielectric‐filled rectangular waveguide, using a formalim that gives the wave reflection coefficient from the plasma (or the equivalent plasma impedance), once the plasma surface admittance tensor and the eigenmodes of the unloaded launcher are known. In the context of a slab model, we obtain the plasma surface admittance tensor by solving the wave differential equations in a cold plasma, assuming a radiation condition from inside the plasma. The eigenfunctions of the rectangular waveguide are readily written down. For the vacuum folded waveguide, semi‐analytical expressions for the TE mode eigenfunctions hasve been obtained by a Ritz variational method. Some problems concerning the calculation of the reflection coefficient for a folded waveguide are discussed.