Modeling of Selective Photonic-structure Cavities for Sub-terahertz High-cyclotron-harmonic Gyrotrons

Typically, cavities of sub-terahertz electron cyclotron masers (gyrotrons) have oversized dimensions. That leads to a problem of the mode selectivity in the process of realization of gyrotrons operating at high cyclotron harmonics. In this work, we study a cavity based on a photonic structure formed by two parallel corrugated mirrors acting as two diffraction gratings. Such a system supports only high-Q transverse modes with cutoff frequencies placed in a narrow band close to the corresponding Bragg-type resonance. In this work, a 2D model of a 90GHz Bragg-type gyrotron cavity has been developed and simulated. On the basis of HFSS modeling, the data of eigenfrequencies and Q-factors for such a system are systematized. In addition, estimations for starting currents of gyrotrons based on the photonic-structure cavity are obtained.