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In the last few years, a considerable progress has been achieved in the development of terahertz-range gyrotrons [1-4]. At the frequency of 1 THz, the radiation power amounts several kilowatts in conventional gyrotrons with tubular helical beams formed by magnetron injection guns [1-3] and up to hundreds watts in large-orbit gyrotrons (LOG) with axis-encircling electron beams [4]. All these experiments were performed in the pulse regime. Recently, terahertz-range radiation at the second cyclotron harmonic was achieved. In the latter experiment, a superconductive magnet was used. The typical feature of such magnets is fairly large diameter of the warm bore of 5-10 cm. At the same time, the transverse cross-sections of conventional terahertz gyrotrons with cylindrical resonator are quite limited by several millimeters (Fig.1a). It is caused by problems with mode selection that restricted admissible waveguide radius and correspondingly the driving current. In order to provide starting conditions, one should increase interaction length above an optimal value that together with rather small cross-section results in substantial Ohmic losses. According to simulations for experimental conditions corresponding to [5], the Ohmic losses amount 80% of radiation power. Obviously, the improvement of mode selection is the key issue in the further development of short-wavelength gyrotrons.

Development of Terahertz-Range Planar Gyrotrons with Transverse Energy Extraction Operating at Cyclotron Harmonics