Low Ohmic Losses and Mode Selectivity Provided by a Distributed Bragg Reflector for Cavities of Terahertz Gyrotrons

A new high- dielectric-loaded cavity is considered an alternative to conventional cavities for terahertz gyrotrons. The cavity consists of a two-layered distributed Bragg reflector (DBR) resonator jointed to standard input and output cavity sections. The DBR is formed by a cylindrical dielectric tube and a hollow layer. The main function of the DBR is to shield the metal wall of the resonator from the field of the selected operating mode. It is shown that this mode can be efficiently excited by the same electron beam in both the conventional uniform resonator and the DBR resonator. It is found that the mode conversion at the junctions between DBR resonator and standard cavity sections is low, resulting in more than 99% output purity of the operating mode. It is established that the characteristics of both the conventional and dielectric-loaded cavities exhibit similar robustness against dimensional errors. It is shown that, unlike the competing modes, the operating mode benefits from exceptionally low ohmic losses, which favors improved mode selection in the dielectric-loaded cavity. Using the cavity of an existing second-harmonic 0.5-THz gyrotron as an example, it is shown that sapphire DBR makes it possible to increase the ohmic quality factor of the operating mode and gyrotron efficiency by a factor of 8 and 6, respectively.