Selective Microwave Two-Mirror System for Subterahertz Gyrotrons with High Cyclotron Harmonics

It is a well-known fact that in cavities of sub-terahertz electron cyclotron masers (for example, gyrotrons operating at high cyclotron harmonics) a selectivity problem arises due to their oversized dimensions. In this work, a 2D schematic of a $90 \mathbf{G H z}$ Bragg-type gyrotron cavity has been developed and simulated. The open resonator consists of two opposite corrugated metal surfaces, inclined at an angle of 45 degrees and acting as two diffraction gratings. Such a system supports only high-Q transverse modes with cutoff frequencies placed in a relatively narrow band close to the corresponding Bragg-type resonance. A simple theory of eigenfrequencies and Q-factor has been developed for such a system and a comparison of these results with data obtained based on HFSS modeling is presented. Moreover, estimations for starting currents of gyrotrons based on the photonic-structure cavity for two main types (an axis-encircling beam and a tubular beam) of electron beam are obtained.