Abstract. In addition to the development of various resonators, the concept of a probehead equipped with an additional low noise amplifier (LNA) is becoming increasingly popular to enhance the sensitivity of EPR spectrometers. The low noise detection amplifier makes it possible to measure pulsed EPR signals with high sensitivity. However, a strong reflected pulse signal can cause saturation and deterioration of the LNA characteristics, which requires protection of the LNA (for example, by using a protection switch in front of the LNA), which in turn reduces the signal-to-noise ratio. To overcome these limitations, we propose using an EPR probehead based on a bimodal cavity with strong isolation between the input and output ports, in combination with a low noise amplifier connected to the cavity output. Experiments demonstrate 4-fold increase in the signal-to-noise ratio (SNR) compared to the reflection mode. Performance of the probe was also compared with the Bruker EN 5170 D2 probe available in our laboratory, which showed an improvement that can be achieved by increasing the SNR by 2 times due to additional LNA and isolation of the detection channel from the input signal, and by 3.3 times due to a larger sample volume in the bimodal probe (~20 µl) at Q-band frequencies compared to the Bruker one (~6 µl). The developed probehead can be used together with commercial Bruker ELEXYS EPR spectrometers without modification of the microwave bridge.

In addition to the development of various resonators, the concept of a probehead equipped with an additional low-noise amplifier (LNA) is becoming increasingly popular to enhance the sensitivity of electron paramagnetic resonance (EPR) spectrometers. The low-noise-detection amplifier makes it possible to measure pulsed EPR signals with high sensitivity. However, a strong reflected pulse signal can cause saturation and deterioration of the LNA characteristics, which requires protection of the LNA (for example, by using a protection switch in front of the LNA), which, in turn, reduces the signal-to-noise ratio. To overcome these limitations, we propose using an EPR probehead based on a bimodal cavity with strong isolation between the input and output ports in combination with a low-noise amplifier connected to the cavity output. The experiments demonstrate a 4-fold increase in the signal-to-noise ratio (SNR) of a bimodal probehead operating in transmission mode compared to its operation in reflection mode, which was achieved thanks to the additional use of LNA. The performance of the probe was also compared with the Bruker EN 5107D2 probe available in our laboratory, which showed an improvement that can be achieved by increasing the SNR by 2 times due to additional LNA and isolation of the detection channel from the input signal and by 3.3 times due to a larger sample volume in the bimodal probe (∼ 20 µL) at Q-band frequencies compared to the Bruker one (∼ 6 µL). The developed probehead can be used together with commercial Bruker ELEXYS EPR spectrometers without modification of the microwave bridge.

We suggest using emission from the photoinjector-formed electron bunches moving through micro-undulators for generation of powerful super-radiant pulses in the terahertz/far infrared frequency range. Within the time-domain quasi-optical approach, we demonstrate the spatial coherence of emission with narrow angular spectrum from the electron bunches with transverse size limited by the Fresnel parameter NF ∼ 1, when the diffraction effects together with slippage provide synchronization of radiation from the entire volume of the extended bunch. For picosecond-duration, 5 MeV, 250 pC electron bunches, and a micro-undulator with a period of 3 mm, the peak power of 15 THz SR pulses can be about of 20 MW.

The mode selective properties of open cavity can be improved by a rectangular groove with specially selected sizes made in resonator wall parallel to electric field. This method is attractive for use in gyrotrons to obtain single-mode operation at high cyclotron harmonics. The influence of a resonant scattering element in the form of a rectangular groove on the mode Q-factor of open resonator is theoretically and experimentally studied. The analytical expressions for groove optimal dimensions are obtained for both Fabry-Perot two-mirror cavity and cylindrical open cavity of the gyrotron. The groove can provide mode selection when its width is between 1/2 and 3/2 of operating mode wavelength. The analytical formulas are confirmed by numerical simulations based on FDTD method. Both the theoretical formula and the simplified calculation method allow the fast design of resonators with sparse mode spectrum. The calculations are verified experimentally. The mirrors with grooves with various widths with design frequency of 135GHz were irradiated by Gaussian beams formed by corrgugated horn and 3D-printed plastic lens, and the reflection coefficients were measured. The obtained experimental data is in good agreement with theoretical and numerical models. The method promises the capability of selective excitation of high cyclotron harmonics in sub-terahertz gyrotrons with oversized cavities.

We present the results of studies aimed at creation of accelerating structures for a photoinjector accelerator that is currently under construction at the IAP RAS. Based on the general theory of coupled oscillators, a simple method for diagnosing the resonant properties of two-cell accelerating structures is given. Electrodynamic measurements of the manufactured photogun at a low power level confirm the ability to achieve the required acceleration gradient, although they show quite a notable difference between its resonant properties and the design. These measurements are in good agreement with analytical estimates and numerical simulations. The project of an additional accelerating structure designed to further accelerate the photoinjector electron bunches up to the energy providing their efficient injection into a wakefield plasma accelerator is also presented.

Micro-undulators allow reducing the particles energy of electron beams used for operation of short-wavelength (including terahertz) free electron lasers (FELs). Planar micro-undulators with a period of 1–5 mm were recently implemented at IAP RAS. Such planar micro-undulators are compatible with planar waveguides open in transverse (lateral) direction, which provides diffractive mode selectivity. In terahertz frequency band, the feedback and mode selection with respect to both longitudinal and second transverse (directed normally to the plates) co-ordinates can be provided by the advanced Bragg structures based on coupling of the propagating and the quasi-cutoff modes. These structures are realized by applying to the waveguide wall of a corrugation with a period of the wavelength. To feed the FEL with the planar micro-undulator, we suggest to use a ribbon high-current relativistic electron beam. Based on simulations, we demonstrate that such a beam with particles energy less than 1 MeV and current of 1–2 kA is sufficient for providing of 10–15 MW of radiation power at the frequency of 0.5 THz.

In this paper, we present the results of two experiments with sub-terahertz gyrotrons with wide frequency tuning range provided by the use of short cavities. The first experiment is carried out on a 163 GHz gyrotron operating in a TE 1,3 mode. In this tube, the excitation of five axial modes provided the frequency tuning band of 2.1 GHz and average output power at high-order modes of 100 W. The second experiment was carried out with the gyrotron designed for second-harmonic 527 GHz operation but switched to the regime of operation at the fundamental cyclotron resonance with TE 5,2 mode and 250 GHz frequency. In this gyrotron, the obtained frequency tuning range was 3.8 GHz with output power about of 50 W. Both experiments prove the feasibility of the developed approach and open up new possibilities of gyrotron applications in spectroscopy.

This brief presents a millimeter-wave gyrotron that operates at a low voltage (below 5 kV) and achieves medium-power, high-efficiency, continuous-wave output. The gyrotron reaches a peak output power of 140 W at a frequency of 30.192 GHz, with an operating voltage and beam current of 4.2 kV and 280 mA, respectively, resulting in an impressive interaction efficiency of 11.9%. Remarkably, even at a reduced voltage of 1.9 kV, the gyrotron can still produce an output power of 5.0 W. The numerical calculation and particle-in-cell (PIC) simulation are conducted to verify the experiment. These results indicate that the gyrotron has the potential to generate medium power with high efficiency at low voltages. The size and cost of the power supply unit for gyrotrons can be significantly reduced. This investigation could advance the development of compact gyrotrons for various applications.

We propose and test experimentally the design of a planar micro-undulator, which allows ensuring a simple profile of the undulator parameter with a value of about unity at a period of 1 mm, as well as the possibility to operate in a regime with a repetition frequency of tens of hertz. Along with small-size sources of high-density beams of accelerated electrons, such as the photo-injection accelerator or the plasma wakefield accelerator, this micro-undulator can be used to make small-size sources of the terahertz and X-ray radiation. It is shown that the Joule heating inhibits the further decrease in the period value. Analytical estimates of the undulator parameter agree well with both the results of numerical simulations and the results of experimental tests.

A new scheme of the decelerating system of the surface wave oscillator (SWO) of planar geometry is proposed, which allows to realize the output of the generated radiation along the normal to the ribbon electron beam powering the generator. This effect can be achieved by adding an supplementary harmonic corrugation, which connects waves propagates in directions collinear to the motion of the electron beam and perpendicular to the electrodynamic system. The study of the electrodynamic characteristics of one-dimensional (ID) Bragg structures was carried out, their parameters were optimized, and the possibility of deducing the main part of the radiation generated in the SWO from the normal to the direction of electron motion was shown. The results obtained within the framework of the analytical theory of the coupled wave approach are confirmed by direct numerical modeling based on the commercial CST Microwave Studio program. A modification of the decelerating system based on two-dimensional periodic (2D) Bragg structures is proposed, which implements the transverse output of energy into the SWO. To verify the operability of these structures, numerical modeling was carried out within the framework of the CST package.

This paper presents the results of theoretical and experimental studies of a surface-wave oscillator (SWO) of planar geometry excited by a ribbon high-current relativistic electron beam. Within the framework of the quasi-optical approach and direct three-dimensional particle-in-cell modeling, we demonstrate the advantages of open transverse edged configuration against the closed one for effective mode selection at a fairly large oversize factor. In the experiments carried out on the basis of the SINUKI accelerator (Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, 1 kA/650 keV/17 ns), we form a magnetically guided ribbon electron beam with a cross-section of 0.3 × 20 mm2, which moves parallel to a slow-wave structure with a period of 1.75 mm. A planar W-band SWO both of open and closed edged configuration has been experimentally tested. In full agreement with modeling, the open configuration exhibits much more stable operation, where at a frequency of 75 GHz, we observe pulse generation with duration of about 7 ns. The output power measured by the calorimetric method reaches 25 MW.

An increase in the accelerating gradient in hollow metal structures powered by an RF field is associated with the development of sources of high-power short-pulse high-frequency radiation. To date, the most powerful (multi gigawatts) nanosecond-scale microwave pulses are produced based on the effect of Cherenkov super-radiance (SR). We consider the possibility of experimental observation of high-gradient acceleration of electrons by Ka-band SR pulses in a combined generator–accelerator scheme with two coaxial electron beams formed by a single cathode. The outer tubular beam is used to generate the SR pulse in periodical slow-wave structure, while the inner one is accelerated in a “pill-box” resonator. The main parameters of the proposed scheme are determined based on full-scale particle-in-cell simulations, according to which accelerating gradient can reach 400 MV/m as some fraction of electrons passing the resonator increases energy from 250 keV to 1.85 MeV. Using the obtained data, injector of the coaxial beams and the sensor of accelerated electrons are developed and tested.

The development of the microwave devices requires testing of many variants of microwave components. Therefore, the additive technologies allowing fast and non-expensive component fabrication is of great interest. To evaluate the performance of the 3D printed and metalized millimeter-wave elements, manufactured by chemical metallization of photopolymer-based structures technology (CMPS), single-mode rectangular waveguides were fabricated and measured at the frequencies from 26 to 110 GHz. The measurement results show that the parameters of 3D-printed waveguides are close to the parameters of fully-metallic components produced by conventional techniques. Since the ohmic losses are low in the Ka, W-band, a closed cavity was used for accurate losses evaluation.

On the basis of numerical simulations, we develop an experimental scheme for high-gradient acceleration of electrons by Ka-band GW-level sub-nanosecond pulses of microwave superradiance (SR). The scheme represents a “generator-accelerator” unit, combined in one vacuum volume and powered by two coaxial electron beams. The SR pulse, generated by the external beam, pumps the “pill-box” resonator where the internal test beam is accelerated. The electron energy was determined based on measurements of the internal beam current after it passed through aluminum filters (foils) of various thicknesses. We have been experimentally demonstrated the increase in electron energy above 1 MeV at an acceleration gradient of 250 MV/m. The obtained gradient value significantly exceeds the record parameters achieved on the basis of long-wavelength klystrons.

We show numerically that the gyrotron cavity with specially designed rectangular grooves provides a selective gyrotron excitation at the second cyclotron harmonic and allows frequency tuning. Simulations predict that the output power higher than 10 W can be obtained in the 0.3% band near the frequency of 400 GHz for a 15-kV, 0.5-A electron beam. Results obtained by analytical model calculations and particle-in-cell (PIC)-code are in good agreement.

We propose a high-power extended interaction klystron (EIK) amplifier exploiting open gratings as surface-wave cavities driven by a relativistic high-current sheet electron beam. For in- and out-coupling of the radiation, an additional subharmonic corrugation with a period twice larger than the period of the grating is proposed. Simulations based on both averaged quasi-optical model and particle-in-cell (PIC) code demonstrate the feasibility of the 150-GHz amplifier with 20–40-MW output power and 20–30-dB linear gain in 1% bandwidth.

Low operating voltage is highly attractive for medium-power millimeter-wave gyrotrons since it can reduce their size and cost, increase their safety, and, thus, improve usability for applications. However, at low voltages, the voltage depression caused by DC space-charge fields significantly limits the electron current and transverse power in the beam. Moreover, this current limitation is more pronounced for a beam with a higher pitch factor. As a result, for a given anode voltage, there is a pitch factor at which the transverse beam power in the gyrotron cavity is the maximum. This ultimate transverse power is found analytically in the non-relativistic approximation. Such a power is reached when the pitch factor calculated without taking into account voltage depression is only 0.82; voltage depression decreases the axial electron velocities, thus, increasing the actual pitch factor value in the cavity up to 1.4. As a result of this effect, high power and high efficiency cannot be obtained simultaneously in a low-voltage gyrotron. Using particle-in-cell simulations, two variants of low-voltage (5 kV) gyrotrons have been designed, namely, a device with higher power and an optimal pitch factor of 0.82 in the cavity and a device with a high pitch factor and high efficiency, but lower power.