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Coherent subterahertz radiation is achieved in an electron cyclotron maser (large-orbit gyrotron configuration) operating at a higher cyclotron harmonic in the continuous-wave generation regime. This auto-oscillator is based on the use of a gun with a magnetic field cusp and a section for adiabatic magnetic compression with a factor of 1000, forming a 30 keV/0.7 A axis-encircling electron beam in a magnetic field of 5 T. A stable single-mode generation is achieved in the case of operation at the third and second cyclotron harmonics at frequencies of 0.394 THz and 0.267 THz with radiation powers of 0.37 kW and 0.9 kW, respectively.

We consider specific features of the development of a continuous-wave 258 GHz gyrotron operated at the second gyrofrequency harmonic, which are related to ensuring of high stability of its output parameters. The requirements to accuracy of manufacture of the cavity and adjustment of the electron beam are evaluated, as well as the necessary parameters of the power sources of the gyrotron complex, which should maintain the required level and stability of the generated power and frequency. The influence of the effect of spurious-mode reflection from the output window of the gyrotron is studied.

We consider some effects connected with the axial misalignment of the electron beam and the cavity in gyrotrons. These effects are studied by the example of three gyrotrons with different parameters of the electron beam and output characteristics. The experimental data and the results of computer calculations are compared on the basis of a multi-mode nonstationary model of the electron-wave interaction in the gyrotron cavity.

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