Design and Characterization of a 170-GHz Resonant Diplexer for High-Power ECRH Systems

The development of resonant diplexers for millimeter waves can improve the flexibility, simplify the system complexity, and extend the performance of electron cyclotron resonance heating (ECRH) systems. A 170-GHz quasi-optical resonant diplexer consisting of two phase inversion mirrors and two planar phase gratings, which is compatible with ECHR for the international experimental tokamak reactor (ITER), has been realized and tested. In order to make the structure of the diplexer simple, the LP01 beam is used directly without being converted into a Gaussian-beam. The beam propagation inside of the diplexer is presented and the working principles of each component are discussed in details. A resonator technique is applied to study the transmission characteristics by creating a high-Q-factor resonator with high sensitivity to any small variations of the diplexer. Numerical simulations and experimental measurements are in good agreement, showing a transmission efficiency of the resonant channel and the nonresonant channel of 0.95 and 0.97, respectively. The mode output and mode filtering of the resonant channel is investigated by changing the input modes, and the average wrong mode suppression for low-order modes is 8 dB and for very high-order modes reach 17.5 dB. The isolation of the diplexer is tested by measuring the power of the LP01 mode in the isolated port for different loads at the outputs. The obtained results demonstrate the possibility of reducing the complexity of large ECRH systems, filtering the wrong modes in the transmission line system, as well as receiving electron cyclotron emission radiation from the plasma by sharing the launcher with the transmitted ECRH.