Gyrotrons are among the most powerful sources of coherent radiation that operate in CW and long pulse regimes in the sub-THz and the THz frequency ranges of the electromagnetic spectrum, i.e. between 0.3 THz and 3.0 THz (corresponding to wavelengths from 1.0 to 0.1 mm). This region, which spans between the frequency bands occupied by various electronic and photonic devices, respectively, is habitually called a THz power gap. The underlying mechanism of the operation of the gyrotron involves a formation of bunches of electrons gyrating in a helical electron beam and their synchronous interaction with a fast (i.e. having a superluminal phase velocity) electromagnetic wave, producing a bremsstrahlung radiation. In contrast to the slow-wave tubes, which utilize tiny structures with dimensions comparable to the wavelength of the radiation, the gyrotrons have a simpler resonant system (cavity resonator) with dimensions that are much greater than the wavelength. This allows much more powerful electron beams to be used and thus higher output powers to be achieved. Although in comparison with the classical microwave tubes the gyrotrons are characterized by greater volume and weight due to the presence of bulky parts (such as superconducting magnets and massive collectors where the energy of the spent electron beam is dissipated) they are much more compact and can easily be embedded in a sophisticated laboratory equipment (e.g. spectrometers, technological systems,

etc.) than other devices such as free-electron lasers (FEL) and radiation sources based on electron accelerators. Nowadays, the gyrotrons are used as powerful sources of coherent radiation in the wide fields of high-power sub-THz and THz science and technologies ^[1][2][3].

) than other devices such as free-electron lasers (FEL) and radiation sources based on electron accelerators. Nowadays, the gyrotrons are used as powerful sources of coherent radiation in the wide fields of high-power sub-THz and THz science and technologies [1-3].

[1] Glyavin M.Y., Idehara T., Sabchevski S.P., "Development of THz Gyrotrons at IAP RAS and FIR UF and Their Applications in Physical Research and High-Power THz Technologies," IEEE Trans. on Terahertz Science and Technology, vol.　5　, no. 5 (2015) 788-797. DOI: 10.1109/TTHZ.2015.2442836I.

[2] Idehara T., Sabchevski S.P. “Gyrotrons for High-Power Terahertz Science and Technology at FIR UF,” Journal of Infrared, Millimeter, and Terahertz Waves, vol. 38, n. 1 (2017) 62–86. DOI: 10.1007/s10762-016-0314-5.

[3] Idehara T., Sabchevski S.P., "Development and Application of Gyrotrons at FIR UF," IEEE Transactions on Plasma Science, vol. 46, no. 7 (2018) 2452-2459. DOI: 10.1109/TPS.2017.2775678.