I. Introduction
At present, the sources of terahertz radiation are in great demand due to their numerous scientific and technological applications, including spectroscopy, plasma diagnostics, communications, and medicine. Over the last few years, there has been significant progress in the development of short-wave gyrotrons operating at frequencies of 0.4–1.3 THz [1]–[8]. The maximum radiated power was achieved in fundamental harmonic gyrotrons with the pulsed magnetic fields [3], [4]. At the same time, for continuous-wave (CW) generation in the terahertz band, operation at the harmonics of cyclotron frequency is inevitable. However, in this regime, a number of well-known problems arise. Among them, the lower efficiency of interaction and the mode competition are the major problems. For suppression of the competing modes, two groups of methods, namely, electron selection (e.g., using an axis-encircling electron beam that couples only to the corotating modes with azimuthal index equal to the harmonic number in the large-orbit gyrotrons [5], [6]) and, alternatively, electrodynamic selection (e.g., through an appropriate profiling of the cavity resonator and the field profile inside it [9], [10]). Unfortunately, the first method requires a complicated electron optical system (EOS) that forms an axis-encircling beam (using a cusp gun or a kicker) instead of the helical electron beam of a conventional EOS from a magnetron injection gun (MIG). The second method also has a significant drawback because it demands an extremely high accuracy of manufacturing of the circuit components.