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.

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.

Collective Thomson Scattering (CTS) diagnostics measure the scattering spectrum of monochromatic incident radiation off collective fluctuations in the plasma. In this contribution, we present the first results from the upgraded CTS diagnostic at Wendelstein 7-X (W7-X) operating in the frequency range between 172 and 176 GHz. This frequency range allows for minimization of noise originating from the electron cyclotron emission in the plasma. Consequently, the good signal-to-noise ratio allows measurements of fast ions or bulk plasma parameters with higher temporal resolution compared with the previously used 140 GHz system.

In this paper, we present the design and commissioning results of the upgraded collective Thomson scattering diagnostic at the Wendelstein 7-X stellarator. The diagnostic has a new radiometer designed to operate between the second and third harmonics of the electron cyclotron emission from the plasma at 171–177 GHz, where the emission background has a minimum and is of order 10–100 eV. It allows us to receive the scattered electromagnetic field with a significantly improved signal-to-noise ratio and extends the set of possible scattering geometries compared to the case of the original instrument operated at 140 GHz. The elements of the diagnostic are a narrowband notch filter and a frequency stabilized probing gyrotron that will allow measuring scattered radiation spectra very close to the probing frequency. Here, we characterize the microwave components applied to the radiometer and demonstrate the performance of the complete system that was achieved during the latest experimental campaign, OP2.1.

Considered in this chapter, classical vacuum electron devices based on "electron beam-slow-wave" synchronism, such as backward wave oscillator (BWO) [1], Carcinotron [2], clinotron [3], orotron [4], and diffraction radiation oscillator (DRO) [5, 6], have been widely used in microwaves due to outstanding performances combining high levels of output power and wide frequency tuning range. Therefore, at present many research and development activities are focused on filling the THz gap with the help of mentioned above devices. However, there are several reasons for a drastic output power drop in those devices with the wavelength shortening that results in small efficiency of those tubes in the THz frequency range and, therefore, it limits their practical applications. Among these reasons, the most special ones are as follows: (1) technological constraints in the manufacturing of small-scale slow-wave circuits and other components of the tubes; (2) requirements in generation and transportation of the intense electron beams including the problems of fabrication of magnetic system; (3) increase in electromagnetic wave attenuation caused by ohmic losses due to metal surface roughness and skin layer effects; (4) problems of electromagnetic energy extraction from the small-scaled slow-wave circuits, as well as mode competitions, etc.

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 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.

Two promising methods for generation of THz radiation are presented and discussed. The hybrid bulk-surface modes excited in a cavity with bi-periodic grating have been considered. Such modes appear due to Electrodynamic interaction of the bulk modes with surface-wave resonator modes (i.e., leaky spoof surface plasmon polariton of an open grating). The potential for the effective generation of the THz radiation in a Clinotron by the excitation of the hybrid modes has been demonstrated. Theory of a new instability in reversed biased pn-junctions with impact ionization was developed and applied for investigation of THz oscillators design.

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 properties of hybrid bulk-surface modes have been studied and their potential for the efficient interaction with wide sheet electron beams in the THz range has been shown. Main emphasis is put on the interaction of the transverse fundamental and higher modes with respect to the grating width.

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.

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.

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.

We report the design and initial experimental results of an 800-GHz second harmonic gyrotron prototype, in which a 15-T pulsed magnet is applied. This gyrotron prototype is developed at the Wuhan National High Magnetic Field Center (WHMFC). An in-house software for analyzing and designing terahertz gyrotrons and an experimental research platform are developed. In addition, alignment adjustments, eddy current effects, and radiation tests are discussed. The initial experimental results show that the prototype gyrotron can irradiate ~40 W at fundamental harmonic 383.7-GHz (TE 1,4 ) waves at 14.14 T and a few milliwatts at second harmonic 0.8-THz (TE 8,5 ) waves at 14.86 T.