High-power microwave diplexers for advanced ECRH systems

Combination of the power of two (or more) gyrotrons is an attractive feature for large ECRH systems, e.g. for ITER. Fast switching of the power from continuously operating gyrotrons between two antennas synchronous to the rotation of the magnetic islands in tokamaks maximizes the efficiency for stabilization of neoclassical tearing modes. Both tasks can be fulfilled with a high-power four-port diplexer, utilizing small frequency differences or small frequency-shift keying (some tens of MHz, by modulation of a supply voltage) of the gyrotrons. In the paper, high-power diplexer designs for quasi-optical and corrugated waveguide transmission systems including numerical and experimental results are discussed. In more detail, the principle and the design of a quasi-optical resonator diplexer is presented. Low-power measurements at 140 GHz of switching contrast, mode purity and efficiency are shown to agree well with theory. Results from high-power experiments on fast switching and power combination of two gyrotrons using the ECRH system for the stellarator W7-X are presented. Near-term plans for high-power experiments at ASDEX Upgrade and FTU are described (Note that there are also plans for in-line ECE on ASDEX Upgrade). Finally, applications of diplexers for the ECRH system on ITER assuming conventional gyrotrons are discussed, and future possibilities offered by phase-controlled gyrotrons like multibeam combining and switching are highlighted.