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

Improved Mode Selection in Coaxial Cavities for Subterahertz Second-Harmonic Gyrotrons

A coaxial metal rod with partial dielectric coating is considered as a means for efficient suppression of all volume competing modes in cavities for second-harmonic gyrotrons operated in whispering gallery modes. The rod radius is selected small enough to have only a slight effect on operating mode, which therefore remains insensitive to fabrication tolerances and a misalignment of the coaxial insert. By contrast, for the competing modes such a rod is shown to reduce the effective cavity length, thereby greatly increasing the starting currents. Such a method of mode selection is demonstrated to be more versatile, when compared to that provided by a tapered coaxial conductor. The advantage of a dielectric-coated coaxial insert is illustrated by the example of a cavity for a 100-kW 300-GHz pulsed gyrotron operated in the second-harmonic mode

Starting currents of modes in cylindrical cavities with mode-converting corrugations for second-harmonic gyrotrons

A self-consistent system of equations (known as single-mode gyrotron equations) is extended to describe the beam-wave interaction in a cylindrical gyrotron cavity with mode-converting longitudinal corrugations, which produce coupling of azimuthal basis modes. The system of equations is applied to investigate the effect of corrugations on starting currents of the cavity modes. For these modes, eigenvalues, ohmic losses, field structure, and beam-wave coupling coefficients are investigated with respect to the corrugation parameters. It is shown that properly sized mode-converting corrugations are capable of improving the selectivity properties of cylindrical cavities for second-harmonic gyrotrons