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).

With rapid advances in THz devices, the human exposure to THz waves increases, necessitating thorough safety assessment before commercialization. Despite the significance of frequencies such as 600 GHz in next-generation technologies, currently, no gyrotron is specifically designed for exposure experiments at such high frequencies. Therefore, we developed a 600-GHz continuous wave (CW) gyrotron with a Gaussian beam output to assess exposure to terahertz radiation. First experiment of this gyrotron was carried out. After axial alignment, a Gaussian-like beam with a frequency of 599.1 GHz was emitted, achieving continuous oscillation for 10 min with a power output of 2.4 W.

The spin pumping effect in antiferromagnets, which ultimately converts THz waves into a spin current, is the key physical mechanism leading to an essential function which harnesses the THz technology and spintronics. Here, we report thorough experimental investigations of the spin current induced by the antiferromagnetic spin pumping effect in epitaxial alpha-Fe2O3 thin films having two distinct dynamic modes and unambiguously show that both the inter- and intrasublattice spin mixing conductance are equally substantial. Our experimental insight is an important advance for understanding the physics of transduction between the spin current and the staggered magnetization dynamics at THz frequency.

The self-consistent problem of electromagnetic (EM) field excitation by a rectilinear electron beam moving along either uniform grating Smith–Purcell radiation (SPR) or bi-periodic grating (leaky wave radiation of spoof surface plasmon polariton) and the influence of the excited EM field on the electron beam has been considered. It is shown that a deceleration of electrons (energy transfer from electrons to EM wave) in both the Smith–Purcell radiation and the leaky wave radiation of the spoof surface plasmon polariton regimes may occur for essentially increased slow wave circuit length in comparison with conventional backward wave oscillator (BWO) and orotrons. This allows either to enhance the output power or to reduce the beam current density of the THz oscillator. The simulations show the output power of 18 W with the corresponding efficiency of 1.5% at 0.64 THz in the case of the circuit length of 28 mm while the current density is 180 A/cm2. Such operation is based on the hybrid bulk-surface mode (HBSM) excitation in the THz oscillator cavity with bi-periodic grating and the top reflector for leaky waves. This is promising for the increase of the output efficiency higher than 1% and providing the Watt level of output power in the THz oscillator based on hybrid bulk-surface modes.

A radiation pattern with two peaks was observed in the experiment of a multi-frequency Gaussian beam gyrotron FU CW GVII, which contains a mode converter designed for a co-rotating mode. To find out the cause of the two peaks, we investigated from both experimental and theoretical perspectives Experimentally, by changing the gun coil current to change the electron incident radius into the cavity, the intensity variation of the two peaks was observed. Theoretically, a code was developed to calculate the radiation pattern of the Gaussian beam converted from the counter-rotating mode. In result, the cause of the two peaks is that the co- and counter-rotating modes simultaneously oscillating in the cavity were independently converted into Gaussian beams by the mode converter, and then output from the window. It is indicated that even a mode converter designed for the co-rotating mode can convert the counter-rotating modes with small azimuthal number into Gaussian beams.

An experiment in fundamental oscillation mode was conducted using Gyrotron FU CW GVII designed to oscillate in the second harmonic. It was observed that 11 fundamental wave modes oscillated in the frequency range of 115 to 205 GHz. Gaussian beams converted by a mode converter, designed for the co-rotating second harmonic TE65 mode, were emitted from the gyrotron window. It was demonstrated that this gyrotron can output Gaussian beams in a wide frequency range. A Vlasov launcher converts to a Gaussian beam even for the oscillation of a counter-rotating mode.

It was shown that a spoof surface plasmon polariton (SSPP) with the uniform RF field distribution compatible with a sheet electron beam is resonantly excited by a bulk wave of an open cavity containing a nonuniform (bi-periodic) grating. This provides an efficient interaction of a wide sheet electron beam with a hybrid bulk surface mode (HBSM) in the THz Cherenkov oscillator that has been demonstrated by 3-D PIC simulations. The presented simulation results showed the excitation of oscillations in the range from 0.628 to 0.648 THz with watt level of output power and output efficiency up to 1% for the 100-mA 17.2–23.2-kV electron beam. The conditions for the suppression of the competing modes having variations across a grating width have been discussed.

A second cyclotron harmonic gyrotron in which the selectivity of generation and the possibility of broadband frequency tuning are ensured by a special deformation of the cavity wall has been designed and numerically studied. According to simulations, it can provide minimum 10 W of the output power in the band of 0.3% near the frequency of 400 GHz. The achievability of the declared characteristics is confirmed by direct three-dimensional PIC simulations. The project is aimed at the 8 T cryomagnet installation at the FIR-FU.

We present two sets of experiments on the multiplication of gyrotron radiation frequency. The first one utilizes the second-order susceptibility of the crystal lattice in InP semiconductor with maximum conversion efficiency estimated at 1%. The second experiment is focused on the effect of simultaneous excitation of modes of the cylindrical resonator, resonant with the first, second, and third cyclotron harmonics with the conversion efficiency of up to 10 −4 .

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 interaction of the modes with different variations along grating lamella occurring in a clinotron with periodically modified grating due to the profile of the clinotron circuit cross-section, has been studied. The simulation results demonstrate the interaction of the sheet electron beam with two modes, which differ in the transverse index, at the same frequency.

We report high-power microwave oscillation of a 94 GHz gyrotron, contributing to the study on air breakdown plasma in Microwave Rocket. The gyrotron is demountable, adopting a diode magnetron injection gun and a built-in quasi-optical mode converter, installed in a compact superconducting magnet with a bore diameter of 100 mm. In this study, microwave signal was detected using a pyroelectric detector and a liquid thermograph sheet at the beam voltage of 40 kV and beam current of about 20 A. Besides, air breakdown plasma, useful for the thrust generation of Microwave Rocket, was successfully ignited at atmospheric pressure. The observed plasma's filamentary structure at the propagation velocity of 720 m/s was different from 170 GHz, which indicates a frequency dependence of the plasma structure.

Mode selection is a key issue for any powerful subterahertz oscillator utilizing oversized cavity. In particular, severe mode competition restrains gyrotron operation at high cyclotron harmonics. To improve the mode selectivity for the open cavities, we use a small resonant groove in the cavity wall in order to scatter all the modes except the chosen one. The proposed method is demonstrated in the experiment with pulsed second-harmonic gyrotron at 1.2 THz, and is used for the design of CW 400 GHz frequency-tunable gyrotron.

To realize the GW-class power source for Microwave Rocket, the cost reduction of gyrotrons is one of the solutions to lower the threshold to realize the beam station. In this study, a sub-MW-class gyrotron which adapted a smaller bore diameter of the superconducting magnet was designed and developed. The output power, pulse duration, and frequency are respectively 600 kW, 100 μs, and 94 GHz. The electron current is driven by a charged capacitor bank and in-house IGBT switches. The gyrotron tube was aligned with the superconducting magnet to achieve high electric efficiency.

Collective Thomson scattering (CTS) is one of attractive diagnostics for measuring locally and directly the fuel temperature and the velocity distribution of fast ions in fusion plasmas. A mega-watt class source of millimeter or sub-millimeter waves is required to detect a weak scattered radiation superimposed on background radiation owing to electron cyclotron emissions (ECEs) from plasmas. Based on electron cyclotron resonance heating (ECRH) system with the frequencies of 77 GHz and 154 GHz in the Large Helical Device (LHD), the CTS diagnostic system has been developed to measure bulk ion temperatures from a few keV to similar to 10 keV and fast ions originated from 180 keV-neutral beam injection in the LHD. The measured CTS spectra and their time evolutions are analyzed with the electrostatic scattering theory. The bulk ion temperatures obtained from CTS spectra increase with the neutral beam injections and decrease with the heating terminated. The velocity map of simulated fast ions explains that the bumps on tail of measured CTS spectra are caused by the co- and counter-fast ions. A new prescription for anisotropic velocity distribution function is proposed. As for 154 GHz bands, the CTS spectrum broadenings for D and H plasmas are distinguished reasonably at the same temperature, and its ion temperatures are comparable to those of the charge exchange recombination spectroscopy. As reactor-relevant diagnostics, a 300 GHz gyrotron and a corresponding receiver system have been implemented in LHD to access high density plasmas with low background ECEs. The recent progress for CTS diagnostics and their spectrum analysis with the probe frequencies of 77 GHz, 154 GHz, and 300 GHz in the LHD experiments is described.

We observed yellow light emission from ZnO single crystals with high conductivity by the irradiation of sub-terahertz waves from gyrotrons. The emitted spectrum has a broad peak around 2 eV. As the spatial distribution of yellow light reflects the gyrotron beam profile and is visible with bare eyes, the high-conductive ZnO single crystals may be applied as an imager of gyrotron beam patterns usable for quick beam diagnosis.

In this paper, we present and discuss some operation modes arising in the oscillator with a sheet electron beam and a periodically modified grating. Special attention has been paid to the modes when surface plasmon polaritons are coupled to the radiating wave. The presented simulation results indicate high potential of the proposed oscillator for an efficient generation of THz radiation.

Second-harmonic 0.4-THz gyrotron with complex-cavity resonator with TE8,3-TE8,5 mode conversion is studied. Cold-cavity electromagnetic parameters are simulated and start-oscillation current is calculated. The numerical results are compared with the experimental measurements.

In this paper, we present and discuss the latest experimental results from the investigation of the operational performance of a 0.8 THz double-beam gyrotron, which has been developed and manufactured recently in the framework of an international research project led by IAP-RAS and FIR UF. After the initial tests, the current experiments have been focused not only on the study of the operation at the design mode TE8,5 (at the second harmonic of the cyclotron frequency) but also of several other modes the generated radiation of which could be appropriate for various other applications besides the envisaged 1.2 GHz DNP-NMR spectroscopy. The obtained results give a deeper insight into the specifics of the operation of the double-beam gyrotron and especially on its capability to provide an improved mode selection and therefore a single mode second harmonic operation.

Till 2005, Research Center for Development of Far-Infrared Region, University of Fukui (herein referred to as FIR-UF) had developed gyrotrons to aim at increasing their frequency. For this purpose, the Gyrotron FU series were developed[1-3]. Fundamental, second and third harmonic oscillations were observed, and the breakthrough of 1 THz oscillation in gyrotron was achieved for the first time in the world[4]. Since 2006, gyrotrons have been developed in FIR-UF for the purpose of applications such as DNPNMR spectroscopy[5,6], sintering and direct measurement of energy level of positronium hyperfine structure[7]. Gyrotrons developed for this purpose are Gyrotron FU CW series[8-13], Gyrotron FU CW G series[14-19] and pulsed gyrotrons[20-28].