High-Q Cavity With Distributed Bragg Reflector for Second-Harmonic Gyrotrons Driven by Low-Power Electron Beams

A cavity with distributed Bragg reflector (DBR) resonator is considered as a new design solution intended to improve output efficiencies of terahertz gyrotrons. The DBR is formed by a cylindrical sapphire tube and a hollow layer. It is optimized to provide ultra-low ohmic losses of a selected operating mode due to field shielding of the metal resonator wall. It is shown that the frequency of this mode and its coupling with an electron beam are slightly affected by the DBR. It is found that the DBR resonator initiates minor mode conversion, resulting in high output purity of the operating mode. Using a second-harmonic 0.5-THz gyrotron powered by a 1-kW electron beam as an example, it is shown that the sapphire DBR enables 10-fold increase in the peak output efficiency from 1% to 10%.