A three-dimensional simulation model for investigating multimode operation of a clinotron in the THz frequency range is presented and discussed. The simulation results are validated by comparison with experimental data obtained from a continuous-wave clinotron with a modified interaction cavity operating in the $280-330~\mathrm{GHz}$ frequency range. Both numerical and experimental studies demonstrate that the excitation of higher-order surface modes and their transformation into modes of an oversized cavity have a significant influence on the output characteristics of the 0.33-THz clinotron with the modified cavity. A theoretical analysis of the three-dimensional electromagnetic system of the $0.33-\mathrm{THz}$ clinotron reveals mode interaction and the excitation of hybrid bulk–surface modes. These modes enable an operating regime for THz oscillators that combines the advantages of devices based on either surface-wave or bulk-mode mechanisms.

We propose a novel concept of a Cherenkov vacuum electron device (VED) based on the coupling of leaky-wave spoof surface plasmon polaritons (LW SSPPs) with multiple-order Smith–Purcell radiation (SPR) in a circuit with a biperiodic double grating (DG) for electromagnetic radiation generation in the THz range. We show that both effective feedback and a radiation output are realized when the order of SPR in a dielectric slab is used for a feedback loop while the order of SPR into a free space serves as a radiation output. The employed biperiodic DG provides a strong coupling of the SPR with the LW SSPP in a wide bandwidth as well as an enhanced beam–wave coupling impedance. The carried-out particle-in-cell simulations demonstrate significant increase of output efficiency of up to 3% as well as expansion of frequency tunning range up to 20% (0.53–0.64 THz).

We theoretically investigate the regime of strong coupling between spoof surface plasmon polaritons and multiple-order Smith-Purcell radiation in a THz Cherenkov vacuum electron device in order to optimize its output performance. To achieve this regime, we propose a bi-periodic double grating coated with a dielectric layer. The core idea of this approach is to separate the roles of two distinct diffraction orders–one providing feedback and the other enabling efficient radiation output.

Experimental results of a 330 GHz CW clinotron with a modified cavity are presented. These results are compared with the cold-cavity simulation results that take into account mode reflection and conversion at both the collector and the output ends of the cavity. During the analysis of output characteristics of the clinotron, special attention was paid to the excitation of the high-order surface modes in the beam-wave interaction region, which significantly enhances the electronic frequency-tuning range. The efficiency of mode excitation and conversions was analyzed in the frequency range of 280- 335 GHz, while the width of both the oversized the interaction space and the output waveguide were designed to provide singlemode output radiation.

The results of experimental studies of the clinotron tube with modified cavity operating in the frequency range 285-330 GHz have been presented and discussed. The maximum output power of 71 mW was measured for the accelerating voltage of 3.5 kV that corresponds to the operating frequency of 297.8 GHz; and the output power of 62 mW was detected at 329.4 GHz corresponding to the beam parameters of 4.5 kV and 150 mA. In the frequency range from 298 GHz to 315.7 GHz, there are several operating zones with an output power of 50 mW, while in the frequency ranges 285-295 GHz and 316-327 GHz the output power achieves slightly more than 25 mW.

The design of the 330 GHz continuous-wave clinotron oscillator with the modified circuit is presented together with the simulation results of the cold cavity including the circuit coupled to the power output. The simulations revealed the effective broadband conversion of the surface wave excited by the sheet electron beam moving above the grating to the TE $_{\text{10}}$ mode of the oversized output waveguide. The experimental studies of the clinotron oscillator demonstrated the operating frequency range from 280 to 335 GHz and the output power up to 110 mW. Finally, both simulation and experimental results revealed the excitation of the surface wave with one node of E -component along the grating lamella width that effectively transfers to TE $_{\text{20}}$ mode of the waveguide output.

Brief description of the clinotron operating parameters in millimeter and THz ranges is presented. The various schemes of the clinotron tubes have been considered and discussed, mainly focusing on the features like generation of intensive sheet electron beam and its transportation with a small inclination angle to the surface of the slow-wave circuit, thermal expansion of the slow-wave circuit elements during the clinotron operation and requirements for the water-cooling system, beam-wave interaction efficiency, both electronic and mechanical frequency tuning. The advantages of the operation of the THz clinotron with multi-period grating on the hybrid bulk-surface modes have been shown and discussed.

The operational characteristics of a watt-level water-cooled 175-GHz continuous-wave (CW) clinotron are studied experimentally and discussed in this article. The oscillator is designed to work either in the broad frequency range (161–175 GHz) and moderate output power (10–200 mW) or in a high-power regime, which has a maximum power of 0.85 W at 174.6 GHz and up to 1.2 W at 171.4 GHz. The radiation power and the frequency can be tuned by changing an acceleration voltage and a beam current. The studies are focused on the long-term drifts of the output power and frequency during a free-running operation of the clinotron. In particular, the influence of a beam current and a cooling water temperature on the frequency is considered and the contribution of both effects is discussed. The maximal variations of a power level and an oscillation frequency observed within 1 h are less than $3\times 10^{-{2}}$ and $5\times 10^{-{5}}$ , respectively.

The results of the research and development of the interaction cavity of sub-THz clinotrons together with the experimental tests several modifications of the tubes are presented and discussed. The surface wave resonator scheme is used to describe the mechanical frequency tuning in the millimeter-wave clinotron tubes by using the plunger while the resonances in the THz range are described by the transformations and reflections of bulk and surface waves. Explanations of the advantages of the modification of the sub-THz clinotron based on the hybrid bulk-surface modes have been provided.

The study of the emission properties of the 140 GHz corrugated horn antenna in the frequency range 170–174 GHz has been presented and discussed. This extended frequency range is a part of upgrade of the collective Thomson scattering (CTS) diagnostic at Wendelstein 7-X stellarator. The antenna characterization was done using the raster scan method and thermographic measurements with the help of 175 GHz Clinotron oscillator. The experimental results showed the possibility of the antenna application for the new CTS detector in the both frequency ranges at 140 GHz and 174 GHz.

The operation of the THz Cherenkov vacuum electron devices with nonuniform double gratings supporting the hybrid bulk-surface modes has been considered for an increase of a thickness of a sheet electron beam and its coupling with RF field. It is shown that advantages of a double grating configuration may comprise the increase of up to 3 times of a coupling impedance as well as a widening of the operation frequency range in comparison with the single grating configuration.

The results of experimental study of the traveling-wave amplification in the oversized circuit with a nonuniform grating, which has increased depth of every third groove, have been presented and discussed. Measured TWT gain versus both frequency and beam voltage is compared with the calculated dispersion. The peculiarities of the interaction of a sheet electron beam with a traveling wave have been discussed for the region of mode coupling. Obtained results show a good agreement with the preliminary 3D PIC simulations.

The preliminary analysis of operation of the 345 GHz sectioned traveling-wave amplifier with the three-stage nonuniform grating has been presented. The design is based on both the simulation and the experimental results obtained for the W-band amplifier earlier. The THz amplifier is designed for the operation with the nonrelativistic sheet electron beams and the oversized beam-wave system. The result of calculations, carried out with the use of the large- signal theory, demonstrated the power gain up to 23 dB for the frequency region from 335 GHz to 365 GHz in the proposed amplifier. The output power of at least 1 W was predicted for the reasonable level of driving signal, that is less than 50 mW.

Solar-type protostars have been shown to harbor highly deuterated complex organics, as evidenced, for instance, by the high relative abundances of doubly and triply deuterated isotopologs. While this degree of deuteration may provide important clues in studying the formation of these species, spectroscopic information on multiply deuterated isotopologs is often insufficient. In particular, searches for triply deuterated methanol, CD 3 OH, are hampered to a large extent by the lack of intensity information from a spectroscopic model. The aim of the present study is to develop a spectroscopic model of CD 3 OH in low-lying torsional states that is sufficiently accurate to facilitate further searches for CD 3 OH in space. We performed a new measurement campaign for CD 3 OH involving three spectroscopic laboratories that covers the 34 GHz−1.1 THz and the 20−900 cm −1 ranges. The analysis was performed using the torsion-rotation Hamiltonian model based on the rho-axis method. We determined a model that describes the ground and first excited torsional states of CD 3 OH, up to quantum numbers J ≤ 55 and K a ≤ 23, and we derived a line list for radio-astronomical observations. The resulting line list is accurate up to at least 1.1 THz and should be sufficient for all types of radio-astronomical searches for this methanol isotopolog. This line list was used to search for CD 3 OH in data from the Protostellar Interferometric Line Survey of IRAS 16293−2422 using the Atacama Large Millimeter/submillimeter Array. Specifically, CD 3 OH is securely detected in the data, with a large number of clearly separated and well-reproduced lines. We not only detected lines belonging to the ground torsional state, but also several belonging to the first excited torsional state. The derived column density of CD 3 OH and abundance relative to the non-deuterated isotopolog confirm the significant enhancement of this multiply deuterated variant. This finding is in line with other observations of multiply deuterated complex organic molecules and may serve as an important constraint on their formation models.

The research and development of thermionic cathodes for the clinotron tubes operating in millimeter and THz ranges resulted in fabrication of several types of electron emitters based on oxide cathode technology, L-cathode and dispenser cathode. Presented consideration of both advantages and disadvantages of these cathodes allows to optimize the electron optical system of the clinotron tubes of different frequency bands taking into account the requirements for the starting current, output power, radiation spectra etc. The development and application of the dispenser cathode, which emitter part consists of fine spherical powder composed of pure tungsten phase with favorable dispersity, allowed us to obtain the beam current density higher than 50A/cm 2 in the compact diode electron gun in the static magnetic field and to form the intense sheet electron beam with the current up to 200mA at the accelerating voltage up to 6kV.

The results of both numerical simulations and experimental study of the traveling-wave amplification in the ${W}$ -band circuit with the nonuniform grating are presented and discussed. The circuit supports a wide sheet electron beam that is promising for a signal gain increase in terahertz (THz) amplifiers. The small-signal gain up to 30 dB is predicted by the simulations of the two-section amplifier in the frequency range from 94 to 99 GHz. The experimental tests of the 14.3-mm input section, powered by a 3.8 kV, 100-mA sheet electron beam, revealed the linear gain of 12 dB and instantaneous bandwidth of 2 GHz. The full bandwidth of 5 GHz has been experimentally observed by adjusting the beam voltage from 3.6 to 4.1 kV.

The results of the concept development of the universal high-voltage power supply with the output parameters providing the reliable operation of compact THz vacuum electron devices have been presented and discussed. The low-level of high-voltage ripples less than 10 ppm at 6 kV, 250 mA was obtained with the help of the designed high-precision and fast-response stabilization scheme. Real-time stabilization of the output parameters of vacuum electron devices was realized by using the multiloop proportional-integral-differential feedback control and was tested with the continuous-wave clinotron tubes in millimeter range. The developed high-voltage power supply offers the high-voltage modulation mode that allows applying the THz tubes with electronic frequency tuning for frequency-modulated continuous wave radar applications.

The spectral properties of terahertz (THz) continuous-wave (CW) clinotrons have been studied both theoretically and experimentally. The resonant properties of clinotrons having strong influence on the spectral linewidth have been analyzed both for the frequencies below 150 GHz where resonances are determined by reflections of surface wave, and for THz tubes where resonances are caused mainly by the effect of mode transformations in oversized cavities. The influence of beam voltage ripples in THz clinotron on spectral line broadening has been studied and discussed. The clinotron spectral linewidth of 1.18 MHz has been demonstrated at 346.4 GHz with the output power of 100 mW when the output voltage stability of high-voltage power supply (HVPS) was 5 ppm.

The results of the development of THz imaging system based on 140 GHz CW Clinotron tube and quasi-optical antenna have been presented and discussed. The preliminary tests were carried out for a single-frequency mode. The image restoration is performed by using the Wiener deconvolution algorithm showing a good agreement for the test sample.

Subject and purpose. This paper deals with the results recently obtained in Vacuum Electronics Department of O. Ya. Usikov Institute for Radiophysics and Electronics of National Academy of Sciences of Ukraine during the development of compact complexes for generation of electromagnetic radiation in the terahertz frequency range. These complexes with clinotrons as electromagnetic oscillators are intended for carrying out experimental researches in the field of nuclear magnetic resonance spectroscopy using the dynamic nuclear polarization technique. Methods and methodology. To calculate the frequency characteristics, the model of the beam-wave interaction of clinotron was improved by an algorithm that takes into account mutual transformations and reflections of the surface and highest waves in clinotron electromagnetic system. The methods of statistical analysis of the experimental data of developed and tested clinotrons were applied to clarify the obtained results. The values of the space charge, the optimum oscillation phase shift for the SWS period, the length and Pierce gain parameter depending on the operating frequency were determined. Results. Transport of the intense electron beams (EB) in slightly inhomogeneous magnetic focusing fields and interaction of EB with RF fields of SWS were investigated theoretically and experimentally. The propagation of surface and highest waves and their mutual transformations on inhomogeneities of clinotron electromagnetic systems were simulated. The RF ohmic losses caused by the surface roughness of the SWS and thermal effects caused by falling EB were studied. The generation of oscillations in packaged clinotrons in the frequency range 120…410 GHz was experimentally obtained. The developed systems demonstrate the frequency stability of the generated electromagnetic radiation at a level of 1 ... 30 ppm in the frequency range of 120...410 GHz and provide level of output power about 100 mW at a frequency of 300 GHz. Conclusions. The ways of both output power and stability increase in THz clinotrons are proposed. Methods for reduction of the RF ohmic losses effect on the efficiency of beam-wave interaction in THz clinotrons by using multi-stage SWS and electromagnetic systems with a small length parameter have been investigated.