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Weakness of the electron-wave coupling in compact low-power terahertz-frequency-range gyrotrons due to relatively low electron beam currents and operation at high cyclotron harmonics results in the use of long-length cavities with high diffraction ${Q}$ -factors, and, therefore, with a high share of the ohmic loss of the radiated wave power. In this work, we demonstrate a possibility for stable operation at higher axial modes possessing relatively low diffraction ${Q}$ -factors. Enhanced efficiency of the electron-wave interaction is provided due to the excitation of higher axial modes in the traveling-wave-tube regime.

A method of improving the selectivity of open cavities of electron cyclotron masers (gyrotrons) operating at high cyclotron harmonics in the terahertz frequency range is proposed and described. This method is purely electrodynamical and based on a simple deformation of the cavity wall. The proposed approach paves the way for the increase in the gyrotron frequencies and for enhancing the selectivity of open cavities in other applications.

Recently, 30-keV/0.7-A continuous wave (CW) subterahertz large-orbit gyrotron was experimentally realized. In this experiment, two different modes at different (second and third) cyclotron harmonics were excited in the same cavity separately at slightly different operating magnetic fields. This work is dedicated to description and theoretical explanation of various regimes of competition of these two modes observed in the experiment.

A free-electron maser based on a new generation of induction linacs capable of forming 5 - 20 MeV / 2 kA / 200 ns electron beams is currently under development. In this work, we study possibilities for realization of powerful THz two-wave oscillators based on the use of the regime of frequency multiplication.

Amplification of a long THz-frequency-range wave pulse coming from a continuous-wave gyrotron by a short electron bunch is studied in the situation, when the axial electron velocity is close to the wave group velocity. The possibility of realization of two different regimes of the electron-wave interaction at different slippages between the electron bunch and the wave is shown. In the case of a short bunch and the zero slippage, a quasi-regular amplification is possible; in this regime, the amplitude increases in a region around the moving bunch and, therefore, a wave pulse with a relatively long length (as compared to the electron bunch length) is formed. The second regime is realized in the case when the bunch is relatively long, and it slightly slips with respect to the wave. In this case, a short (as compared to the bunch length) wave pulse is formed on one of fronts of the bunch and amplifies as it "runs" along the bunch. The peak power of this wave pulse can be several orders of magnitude higher than the saturated power achieved in the quasi-regular amplification regime.

An electrodynamic method based on azimuthally asymmetric cavities is proposed to provide selective excitation of symmetric TE waves in fundamental-harmonic gyrotrons. The presence of small azimuthal inhomogeneities in a cavity with a "close-to-circular" cross-section does not affect on excitation of the operating TE 0,p mode, but increases the starting current of excitation of the parasitic TE 2,p mode. This approach can ensure also mechanical adjustment of the frequency of the operating TE 0,p mode.

We describe a series of works aimed at creating high-harmonic gyrotrons with cavities of complicated shapes as a way to increase efficiencies of their operation and to improve their selective properties. These works are mainly aimed at the implementation of the large-orbit gyrotrons developed at our Institute on the basis of two experimental installations. At the same time, some approaches have proved attractive also for traditional gyrotron configurations.

Using non-axisymmetric electron beams in single-cavity self-exciting gyromultipliers with simple axisymmetric cavities allows the realization of an arbitrary multiplication factor. The most efficient is using a pencil-like electron beam displaced from the cavity axis as it allows, according to simulations, achieving a power of up to several watts in the THz frequency range when operating at the fourth cyclotron harmonic.

A two-stage accelerator complex based on the photoinjection gun is under development in the Institute of Applied Physics at present. It is designed to accelerate the electron bunches with a charge of up to 100 pC to the particle energy of ~3–5 MeV after the first stage and ~20 MeV after the second stage. The report presents the design and design parameters of the accelerating microwave sections, as well as the results of "cold" testing of the first stage key components.

A natural problem arising in the case of realization of a electron maser operating in the terahertz frequency range is the use of oversized microwave system (with characteristic sized significantly exceeding the operating wavelength). In this situation, it is difficult to provide selective excitation of a definite operating transverse mode. We propose to give up working on a fixed transverse mode, and to use excitation of a supermode formed by a fixed set of several transverse modes of an oversized waveguide. The use of the Talbot effect is proposed to create oversized microwave systems that provide high Q-factors for operating supermode. As an example, in this work we describe our simulations for a 2 THz Free-Electron Maser fed by a high-current relativistic electron beam. In addition, we discuss possibilities for using this approach in THz gyrotrons.

The possibility of implementing an intense source of extreme ultraviolet radiation (EUV) based on a gas-discharge plasma is investigated. The radiation source is a large-orbit pulsed gyrotron (LOG) implemented in 2008, operating on the third cyclotron harmonic with a radiation power of 0.4 kW at an operating frequency of 1 THz. Modification of the LOG allows increasing the generation power up to 4 kW by increasing the power of the electron beam and reducing the share of ohmic losses in the resonator. The proposed methods of selection of oscillation in the resonator of the gyrotron. The LOG is designed to work with an electron beam of 100 kV/1.2 A in a pulsed magnetic field of 14 T. The possibility of high-quality focusing of LOG radiation is shown. It is estimated that with a good coupling to the 1 THz radiation plasma, the plasma density will significantly exceed 1016 cm-3 at an electron temperature of 50 eV. This should provide a discharge of extreme ultraviolet light with a power of about 100 W in the range of 13–17 nm for xenon.

We describe two high-harmonic large-orbit gyrotron (LOG) installations developed in the Institute of Applied Physics for various physical applications. The pulsed 80–100 keV/0.7–1.0 A LOG is aimed to provide high-power radiation at frequencies up to 1 THz. Now we plan to increase the output power up to the level of several kW in order to use this gyrotron in plasma applications. The 30 keV/0.7 A CW sub-THz LOG is being created as a prototype of an universal multi-frequency source for DNP/NMR applications. At the moment, selective continuous-wave operation at the second and third cyclotron harmonics (0.267 THz and 0.394 THz) was achieved. Special cavities are designed to achieve the fourth-harmonic operation at frequencies of up to 0.65 THz.

The 30 keV / 0.7 A CW gyrotron is developed for the spectroscopy applications. Recently, selective operation at the second (0.267 THz) and at the third (0.394 THz) cyclotron harmonics was achieved. Special quasi-regular cavities are designed to achieve the fourth-harmonic operation at frequencies of up to 0.65 THz. The pulsed 80-100 keV / 0.7-1.0 A gyrotron is aimed to provide high-power pulses of radiation at the third cyclotron harmonic at frequencies close to 1 THz. Now we study possibilities to increase the peak level of the output power up to the level of several kW in order to use this gyrotron in plasma applications.

We study spontaneous coherent super-radiative undulator emission in the terahertz frequency range from a short (as compared to the wavelength of the radiated wave), dense electron bunch. Since the group velocity of the wave is close to the bunch velocity, this is a process of spontaneous radiation followed by amplification of a single wave cycle. Despite the Coulomb repulsion of electrons inside the bunch, its compactness, which is necessary to ensure the spontaneous coherent character of the radiation process, is provided by the compression of the bunch under the action of its own radiation fields. As a result, formation of an ultra-short (several cycles long) powerful wave packet occurs when the bunch moves through several undulator periods with high (∼20% in optimized profiled systems) efficiency of extraction of the electron energy and high intensity (∼100 MV/m) of the peak wave field.

We propose a high-efficiency regime of a “multi-stage” trapping in FELs. This FEL scheme use strongly tapered flying RF undulator sections to be fed by short (nanosecond) high-power RF pulses produced by already existing BWOs. In this regime, phase locking of the RF sources is not necessary. Moreover, this regime provides an effective amplification of a multi-frequency wave signal in the SASE regime used in typical short-wavelength FELs. In this work, we describe the proposed multi-stage regime, as well as profiled microwave system designed for the realization of tapered rf undulators.

We describe high-harmonic gyrotrons with axis-encircling electron beams developing on the basis of two experimental setups. The 30 keV / 0.7 A CW gyrotron is developed for the spectroscopy applications. Recently, selective operation at the second (0.267 THz) and at the third (0.394 THz) cyclotron harmonics were observed in a series of experiments. Quasi-regular cavities with periodic phase correctors are designed to improve the operation at the third harmonic, as well as to achieve the fourth-harmonic operation at frequencies of up to 0.65 THz. The pulsed 80 keV / 0.7 A gyrotron is aimed to provide high-power (hundreds Watts) microwave pulses at the third cyclotron harmonic at frequencies close to 1 THz. Recently, a sectioned cavity with a decreased diffractive Q-factor was experimentally tested at this setup. Now we study possibilities to increase the peak level of the output power up to the level of several kW in order to use this gyrotron in plasma applications.

Powerful high-efficiency FEM -amplifiers operating over Ka frequency range have been developed in collaboration between IAP RAS and JINR. Potential to reach a broad amplification band in different regimes of e-beam/RF -wave interaction was investigated theoretically and examined in the series of experiments. In the FEM with a regular wiggler operating in the so-called grazing incident regime, output power of 20 MW was simulated in the frequency range of 30 ± 2 GHz. In the experiments, the designed power level and the instantaneous band of about 1 GHz (restricted by the tuning band of the driving magnetron) were achieved. According to the simulations, the use of original regime of non-resonant trapping in tapered wiggler allows enhance in the output power up to 35 – 40 MW with simultaneous widening of the amplification band. In the experimental realization of this novel FEM scheme, an output power on the level of 25 – 27 MW was measured when various magnetrons with the frequencies in the range from 29 GHz to 35 GHz were utilized to drive the amplifier.

Powerful high-efficiency FEM-amplifier operating over Ka frequency range and aimed on accelerating applications is developed in collaboration between IAP RAS and JINR. To achieve a broad amplification band in a regular wiggler, we utilized the so-called grazing regime, in which about of 20 MW of output power was simulated in the frequency range of 30 ± 3 GHz. In the first stage of experiments, the designed power level and the instantaneous band of about 1 GHz (restricted by the tuning band of the driving magnetron) have been demonstrated. According to the simulations, the use of the regime of non-resonant trapping in tapered wiggler allows enhance in the output power up to 35–40 MW with simultaneous widening of the amplification band. Experimental realization of this novel FEM-amplifier scheme is currently in progress.

The negative-mass instability developing in a weakly-relativistic electron beam during its drift along an inhomogeneous magnetostatic field under the effect of space-charge fields is considered. Using the assumption of the convective character of the instability, simple spatial equations of the electron motion in magnetostatic and space-charge fields are obtained. The effect of the magnetic field inhomogeneity on the increase of the spread in electron energy, caused by the instability, as well as on the evolution of the spatial structure of electron density perturbations, is studied for both linear and saturation stages of the instability. It is shown that the spread in electron energy, caused by the negative-mass instability in magnetron-injection guns, can be up to several percents.

Numerical simulations using Magic 3-D have been conducted on a promising three cavity gyro-multiplier arrangement. Results have predicted output of the 4th harmonic, at a frequency of 1.37 THz, with output powers on the order of 100 W.

Numerical simulations using the PiC code Magic 3-D have investigated two distinct gyro-multiplier schemes. Subsequent results have predicted the operation of a single cavity device operating at the 2nd and 4th harmonics, at frequencies of 37.5 GHz and 75 GHz, respectively. As a result, this device is currently under experimental investigation. Additionally, a sectioned cavity device has shown promising output, delivering 4th harmonic output at ∼1.36 THz. Results of both schemes will be presented.