Selectivity Properties of Cylindrical Waveguides with Longitudinal Wall Corrugations for Second-Harmonic Gyrotrons Tetiana I. Tkachova 1 & Vitalii I. Shcherbinin 1 & Viktor I. Tkachenko 1,2 Received: 6 June 2019 /Accepted: 29 August 2019/ # Springer Science+Business Media, LLC, part of Springer Nature 2019 Abstract Spatial harmonic method is applied to investigate selectivity properties of longitudinally corrugated waveguides for potential application in second-harmonic gyrotrons. The effect of corrugations on frequencies, ohmic losses, and mode conversion of guiding TE modes is studied in details. Numerical results are presented for operating second-harmonic and com- peting first-harmonic modes of a 0.4-THz gyrotron with corrugated RF structure. It is shown that longitudinal wall corrugations of proper dimensions can ensure increase in ohmic losses and decrease in beam-wave coupling strength for the first-harmonic modes, while their effect on the operating mode is only slight. This demonstrates improved selectivity properties of corrugated waveguides for second-harmonic gyrotrons. Keywords Gyrotron . Cyclotron harmonic . Corrugated waveguide . Eigenvalue . Ohmic losses . Mode conversion 1 Introduction Gyrotrons are the most powerful radiation sources in microwave to terahertz frequency band [1, 2]. For the most part, this explains their wide use in various applications, ranging from fusion plasma heating and material processing in microwave band to remote detection of concealed radioactive materials for terahertz frequencies. As far as size and cost are concerned, the most attractive oscillators are gyrotrons operated at the second (or higher) harmonic of cyclotron frequency. This is because second-harmonic gyrotrons require half as much mag- netic field as the first-harmonic counterparts. For this reason, such gyrotrons are now used Journal of Infrared, Millimeter, and Terahertz Waves https://doi.org/10.1007/s10762-019-00623-y * Vitalii I. Shcherbinin vshch@ukr.net 1 National Science Center “Kharkiv Institute of Physics and Technology” of National Academy of Science of Ukraine, 1 Akademicheskaya St., Kharkiv 61008, Ukraine 2 V.N. Karazin Kharkiv National University, 4 Svobody Sq., Kharkiv 61022, Ukraine