Design of mode-selective directional couplers for Electron Cyclotron Resonance Heating on the Mirror Fusion Test Facility

The Electron Cyclotron Resonance Heating (ECRH) System will provide microwave energy at 28, 35, and 56 GHz to heat electrons in the thermal barrier and maximum potential regions in the end-plugs of the Mirror Fusion Test Facility (MFTF-B) tandem mirror machine. To design an effective quasi-optical antenna system for ECRH which will convert the TE/sub On/ output of overmoded circular waveguide to a linearly polarized collimated microwave beam requires an accurate determination of the modal content of microwave energy present in the overmoded circular waveguide. To determine the modal content in the waveguide system, mode-selective directional couplers have been designed and fabricated for the three ECRH operating frequencies. The directional couplers are designed to minimize mode conversions in the circular primary waveguide and to excite only the TE/sub 10/ mode in the overmoded rectangular secondary waveguide. Because of the configuration of the directional couplers, fabrication techniques for the attainment of extremely close tolerances and excellent surface finishes were developed. Design and fabrication techniques, as well as experimental results, are presented