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The Institute of Applied Physics of the Russian Academy of Sciences has for many years been developing sub-terahertz and terahertz large-orbit gyrotrons that permit selective oscillation at higher cyclotron harmonics than is possible in the conventional gyrotrons. Currently, experimental studies are conducted at two specialized facilities. A prototype universal sub-terahertz source for magnetic resonance spectroscopy is studied using a facility that generates long-pulse and continuous electron beams with particle energies of up to 30 KeV. Continuous selective oscillation at the second and third cyclotron harmonics with frequencies of 0.267 and 0.394 THz was obtained for a radiation power of 900 and 370 W, respectively. New resonators with periodic phase correctors have been developed to increase the efficiency of third-harmonic oscillation and obtain fourth-harmonic oscillation with frequencies of up to 0.65 THz. Using a facility with an electron energy of up to 80 KeV, we study the possibilities of increasing the pulse generation power at the third harmonic at frequencies close to 1 THz to employ in experiments on obtaining a gas discharge in a focused terahertz wave beam and generating high-power extreme ultraviolet radiation.

High-power terahertz radiation finds application in electron paramagnetic resonance spectroscopy and is used to enhance the sensitivity of nuclear magnetic resonance spectroscopy through dynamic nuclear polarization. The compact source of terahertz radiation proposed in this work follows the gyrotron scheme and relies on the proximity of the paramagnetic-resonance and cyclotron frequencies for electrons. However, the design of this terahertz source differs greatly from those of conventional gyrotrons. Simulations of its performance show that terahertz radiation with required parameters can be obtained with applied voltages below 2 kV. Experimental tests demonstrate that stimulated synchrontron radiation can indeed be generated at such nonrelativistic voltages.

Dense short electron bunches produced by state-of-the-art photoinjector-based accelerators are used to create sources of powerful electromagnetic pulses of terahertz frequency range based on spontaneous coherent emission by these bunches. However, there is a problem of stabilizing the phase dimension of a bunch along the space of electron wave interaction. In this work, two means of such stabilization are considered that are based on using the intrinsic electromagnetic fields (quasi-static and radiation) of bunches.

Two experimental gyrotron test benches in the terahertz frequency range with large-orbit electron beams operating at high harmonics of the cyclotron frequency are created at the Russian Academy of Sciences’ Institute of Applied Physics. An overview is presented of work aimed at developing advanced irregular microwave systems for these gyrotrons. The aim of this work is to reduce ohmic losses and improve the selectivity of operating wave excitation at high harmonics.

Despite the high theoretical values of the electron efficiency of the subterahertz gyrotrons working at cyclotron harmonics at low operating voltages, the achievement of acceptable output power levels in such devices is a significant challenge due to the mode competition, the necessity of producing an electron beam with a high current, high ohmic losses in the walls, and the required high accuracy of cavity manufacturing. To solve these problems, we analyze thoroughly variants of low-voltage gyrotrons with conventional cavities and the recently proposed variant of the sectioned cavity, and present a calculation of the electron-optical system for such cavities. It is shown that by ensuring micrometer accuracy of cavity manufacturing at a low operating voltage (5 kV), it is possible to achieve output efficiencies of up to 5% and a power of up to 100 W at frequencies of about 0.4 THz and higher at the second cyclotron harmonic.

We describe new versions of high-harmonic gyrotrons with axis-encircling electron beams developing on the basis of two (80 keV pulsed and 30 keV CW) experimental setups. Selective operation at the second (0.267 THz) and at the third (0.394 THz) cyclotron harmonics were observed in the 30 keV gyrotron. Quasi-regular cavities with periodic phase correctors, where a far-from-cutoff axial mode with a decreased diffraction Q-factor is excited in a gyrotron-like regime, 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. At the pulsed gyrotron setup, a sectioned klystrontype cavity with a decreased diffraction Q-factor was experimentally tested.

The motion and radiation of short dense bunches of ultrarelativistic electrons produced by laser-driven accelerators and moving in an undulator in the form of magnetized helices have been studied. Simulations demonstrate the possibility of generating wideband THz pulses with energies of hundreds of microjoules and relatively high efficiency in regimes close to the group synchronism of electrons with the waveguide mode.

The possibility of a smooth electronic frequency tuning for medium-power gyrotrons operated at the fundamental cyclotron resonance has been demonstrated in the numerical simulations. The use of a low operating voltage allows a drastic increase in the gyrotron operation efficiency at the higher axial modes and, hence, in the bandwidth of continuous frequency tuning. A gyrotron cavity with power output in the direction of the cathode end can provide a smoother power versus frequency dependence in the gyro-backward wave oscillator regime than a cavity with the conventional output in the direction of the collector. Simulations for a 2-kV gyrotron with a frequency of about 264-GHz demonstrate 3-GHz full-width at half-maximum frequency tuning with a power level of 15 to 30 W, which are attractive parameters for spectroscopic applications.

The formation and utilization of a helical electron beam are studied theoretically for a gyrotron with a very low operating voltage in a range 1.5–1.8 kV. Such a gyrotron (“gyrotrino”) was earlier proposed for operation inside a magnetic system of an NMR spectrometer with a dynamic nuclear polarization upgrade. Despite the very low voltage, the optimization of the electrode shape can provide velocity and positional electron spreads not exceeding these values for conventional high-voltage gyrotrons. A very small cathode-anode separation makes the gyrotrino very sensitive to thermal expansion of the gun elements that should be compensated by movement of the cathode. Estimations for long-pulse and CW regimes of the gyrotrino operation show that the ion background significantly decreases the reduction of the beam potential and leads to an acceptable drift of the electron cyclotron frequency at the voltage front. A satisfactory thermal load on the waste-beam collector located in a strong uniform magnetic field can be achieved due to the omnidirectional heat flow regime occurring in the case of thin beam footprint.

Feasibility of a gyrotron for the dynamic nuclear polarization (DNP) purpose, integrated with nuclear magnetic resonance (NMR) spectrometer inside a single cryomagnet, is analyzed on the basis of numerical simulations. The necessary condition for DNP is matching of the gyrotrino and DNP frequencies. This imposes a strong restriction on the gyrotron operating voltage, which should be less than 2 kV. The most part of the uniform magnetic field region in the cryomagnet is occupied by a sample with NMR probe, so there is a very limited space for the gyrotron cavity. This dictates a number of peculiarities for the gyrotrino design, in particular, the diffraction power output from the cathode end of the cavity and collecting of a thin electron beam in a strong magnetic field. According to simulations, the gyrotrino operating at the fundamental cyclotron resonance with a voltage of 1.5 kV can provide an output power of 10–20 W at a frequency of 264 GHz, which is suitable for many NMR-DNP experiments.

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.

Large ohmic losses in the cavities of terahertz gyrotrons may lead to the overlapping of the axial mode spectra. In a number of gyrotron experiments, this effect has been used to provide a fairly broadband frequency tuning by changing appropriately the operating magnetic field and/or accelerating voltage of the gyrotron. Similar to the systems with nonfixed axial structure of the RF electromagnetic field and low diffraction quality, which are due to weak reflections of the operating wave from the collector end of the electrodynamic system, this changing leads to a monotonic change in the axial index of the operating wave and transition from the gyrotron regime to the gyro-BWO regime. According to a theoretical comparison of these two methods performed on the basis of generalization of self-consistent gyrotron equations with allowance for variations in the axial electron momenta, low-reflection systems can provide a higher efficiency and monotonicity of the frequency tuning.

To implement an efficient source of coherent radiation with negative mass longitudinal stabilization, the methods of the formation and injection of a dense electron bunch onto a stationary helical trajectory in a combined helical undulator and a strong uniform magnetic field are studied. Using a magnetic or electric lens permits sending particles almost along the converging lines of the magnetic field (magnetic following) and obtaining a nearly rectilinearly moving compressed bunch inside a solenoid. After that, the bunch can be injected into an adiabatically increasing field of the helical undulator. In this way, it is possible to excite operating undulator oscillations of particles, significantly mitigating the effects of destructive bunch expansion, the excitation of parasitic cyclotron oscillations, and velocity spread, thereby providing stabilization and terahertz radiation of a dense bunch. Due to a significant mode selection for an axis-encircling bunch, as well as due to the long-term interaction of the particles with a dominant mode, which is closest to the group synchronism conditions, radiation with a relatively narrow spectrum and high efficiency can be obtained even in a strongly oversized waveguide. An additional efficiency enhancement can be obtained due to the reduction of the velocity spread caused by the mutual Coulomb repulsion of electrons during injection due to the initial energy chirp of the bunch.

Terahertz generation of high- and low-frequency pulses from an ultrarelativistic electron bunch, which move in a waveguide placed in an undulator, has been studied. If the length of the bunch is less than the low-frequency wavelength, its radiation under certain conditions stabilizes the length of the bunch and largely compensates for the velocity spread introduced by the particle repulsion. This leads to a significant improvement in the micro-bunching of electrons and increases the energy of the radiated high-frequency pulses.

A helix magnetized along the axis can be used as an undulator with a fairly strong transverse field. Two such helices with the opposite direction of magnetization and shifted relative to each other create a field that is twice as strong. It has been shown, both theoretically and experimentally, that helical undulator fields can also be created due to the redistribution of the uniform magnetic field of the solenoid by ferromagnetic or well-conducting insertions as well as by various combinations of these elements. Use of such undulators in electron sources of terahertz radiation is promising for a significant enhancement of both emitted power and frequency.

The use of both spatial premodulation and negative mass instability is promising for the energy enhancement and spectrum narrowing of the undulator radiation from dense electron bunches formed in modern laser-driven photoinjectors. Combining these two methods will permit the formation of long-lived trains of short and dense bunches with very large charges, which are able to efficiently emit a much more powerful and narrow-band terahertz radiation than single bunches with the same total charge, or premodulated bunches in the positive mass oscillation regime. The development of radiation instability in the negative mass regime can enable significant quasiperiodic self-modulation of relatively long nonmodulated bunches resulting in their fairly efficient and selective terahertz superradiance.

We describe high-harmonic gyrotrons with axis-encircling electron beams developing on the basis of two (80 keV pulsed and 30 keV Cw) experimental setups. Selective operation at the second (0.267 THz) and at the third (0.394 THz) cyclotron harmonics were observed in the 30 keV gyrotron. 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. At the pulsed gyrotron setup, a sectioned cavity with a decreased diffractive Q-factor was experimentally tested.

Backward Wave Oscillators (BWO's) utilizing moderately relativistic (~550kV), high-current (~10 kA) electron beams are capable of producing hundreds of MWs of pulsed radiation in the centimeter wavelength range. Such relativistic BWOs (RBWOs) allow for broadband, smooth, frequency-tuning via adjustment of the accelerating potential; making them an attractive source for use in frequency-swept pulse compression. This paper presents results of a 2.86m long 5-fold helically corrugated, dispersive pulse compressor where a maximum power compression ratio of 25 was achieved by compressing an input microwave pulse of 80 ns duration swept from 9.65 GHz to 9.05 GHz into a 1.6ns Gaussian-envelope pulse. For an average input power of 5.8 kW generated by a conventional traveling wave tube amplifier, a peak pulse output power of 145 kW was measured corresponding to an energy efficiency of 66%. An X-band relativistic BWO, designed to drive a 0.97m long 5-fold compressor, was built and tested using the accelerating potential generated by a SINUS-6 highcurrent accelerator. The experimental RBWO operated close to the predicted power of 700MW with its oscillation frequency varied from 10 to 9.6GHz via the falling edge of the voltage pulse. It was demonstrated that the ~15ns duration frequency-swept part of the RBWO pulse was effectively compressed resulting in a 4.5-fold peak power increase with a maximum power of 3.2 GW generated. The potential for a 5-fold helical waveguide to compress longer duration pulses generated by a RBWO is discussed.

A possibility of gyrotron operation at a very low voltage, 1.5–2 kV, is considered in the context of dynamic nuclear polarization (DNP) application. Simulations predict that a low-voltage device has some specific but useful features like efficiency operation at high axial modes and capability of wide-band frequency tuning.

The use of the “negative mass” regime can provide stabilization of longitudinal size of dense short photo-injector electron bunches at long distances as well as compression of the longer bunches. This allows one to increase significantly the power capabilities of a terahertz source based on coherent spontaneous undulator emission from a short bunch. The required configuration of the magnetic field can be achieved in a specially designed undulator.

The use of hollow electron beams permits a significant increase in the diameter of the beam tunnel in comparison with the conventional pencil-beam slow-wave electron devices. As a result, the current density and the heating of the microwave structure are decreased, which allows increasing the average radiation power at high frequencies. To demonstrate this capability, two versions of W-band oscillators, namely, an axisymmetric orotron and a backward-wave oscillator (BWO), have been designed. The electron beam with an outer diameter of 1.6 mm and a 0.1-mm thick wall could be produced in a 30 kV/1 A Pierce-like electron gun with the hundredfold magnetic compression. Oscillators use microwave structures with an azimuthally symmetric corrugation, sinusoidal for the orotron and rectangular for the BWO. Simulations based on the averaged equations and 3-D PIC-code predict an output power up to 1 kW for the orotron and 0.7 kW for the BWO, whereas the thermal regime in the BWO is easier for continuous-wave operation. Wideband frequency tuning of the BWO is simulated. To reduce ohmic losses, a smooth downtaper of the corrugation in the orotron and an abrupt cutoff with a specially optimized last tooth in the BWO were designed.

Dynamic nuclear polarization in strong-field nuclear magnetic resonance (NMR) spectroscopy requires terahertz radiation with moderate power levels. Nowadays, conventional gyrotrons are used almost exclusively to generate such radiation. In this review paper, we consider alternative variants of electronic microwave oscillators which require much weaker magnetic fields for their operation, namely, large-orbit gyrotrons operated at high cyclotron-frequency harmonics and Cerenkov-type devices, such as a backward-wave oscillator and a klystron frequency multiplier with tubular electron beams. Additionally, we consider the possibility to use the magnetic field created directly by the solenoid of an NMR spectrometer for operation of both the gyrotron and the backward-wave oscillator. Location of the oscillator in the spectrometer magnet makes it superfluous to use an additional superconducting magnet creating a strong field, significantly reduces the length of the radiation transmission line, and, in the case of Cerenkov-type devices, allows one to increase considerably the output-signal power. According to our calculations, all the electronic devices considered are capable of ensuring the power required for dynamic nuclear polarization (10 W or more) at a frequency of 260 GHz, whereas the gyrotrons, including their versions proposed in this paper, remain a single option at higher frequencies.

Dynamic nuclear polarization (DNP) in high-field nuclear magnetic resonance (NMR) spectroscopy requires medium-power terahertz radiation, which nowadays can be provided basically by gyrotrons with superconducting magnets. As the electron cyclotron frequency is very close to the frequency of electron paramagnetic resonance for the same magnetic field, under certain conditions the gyrotron can be installed inside the same solenoid used for NMR spectrometer. This eliminates the need for an additional superconducting magnet, results in a shorter terahertz transmission line, and can make DNP systems practical. In addition to an extremely low-voltage gyrotron (“gyrotrino”), we analyze also advantages of strong magnetic field for a slow-wave electron device as an alternative terahertz source.

A feasibility of combining the gyrotron for dynamic nuclear polarization (DNP) application and the NMR spectrometer in the single superconducting magnet is considered. Simulations predict that a compact extremely low-voltage gyrotron (“gyrotrino”) can provide THz radiation at the frequency of 263 GHz required for DNP with power level of the order of 10 W.

Summary form only given. There has been a drive in recent years to produce ultra-high power short pulses for a range of applications. These high power pulses can be produced by microwave pulse compression. Sweep-frequency based microwave pulse compression using smooth bore hollow waveguides is one technique of pulse compression, however at very high powers this method has some limitation due to its operation close to cut-off. In optimum cases, the frequency at the beginning of an input pulse should be only 0.5-1% above the cutoff frequency. If a Cherenkov type TWT is used to drive the compressor then it is inevitable that the low-frequency part of the amplification band is below the cutoff which then reflected from the compressor back to the amplifier resulting in its possible parasitic self-oscillation. If a relativistic BWO is used as a source of frequency-modulated pulses for a smooth-waveguide compressor, then the necessary frequency sweep that would be required can only be produced by using a difficult-to-realize voltage modulation on the BWO. The two combined problems of wave reflection from a compressor and optimum frequency modulation can be solved using a waveguide with a special helical corrugation of its inner surface. A special helical corrugation of a circular waveguide ensures an eigenwave having a strongly frequency-dependent group velocity far from cut-off, which makes the helically corrugated waveguide attractive for using as a compressor of pulses from very high power amplifiers and oscillators. The results of proof-of-principle experiments and calculations of the wave dispersion using a PIC code are presented. In the experiments a 70 ns 1 kW pulse from a conventional TWT was compressed in a 2 metre long helical waveguide. The compressed pulse had a peak power of 10.9 kW and duration of 3 ns. In order to find the optimum pulse compression ratio the waveguide's dispersion characteristics must be well known. The dispersion of the helix was calculated using the PIC code MAGIC and verified using an experimental technique. For a helical compressor the optimum negative frequency sweep can quite naturally be realized at the falling edge of a relativistic BWO pulse. For example, if a BWO generates hundreds of MW during 50 ns over a voltage drop from 700 kV to 400 kV, this frequency modulated radiation can be compressed up to 10-20 times in power with efficiency higher than 50%. Future work detailing plans to produce short, ultra-high power (multi-GW) pulses discussed.

This paper describes the use of a helically corrugated waveguide as a dispersive medium for microwave pulse compression. The helically corrugated waveguide has a large variation of group velocity with frequency, but in a region where the group velocity remains large. Therefore this compressor does not suffer from reflections associated with cut-off scenarios at frequencies close to its operating regime and may be used in conjunction with high power wideband tunable microwave sources and amplifiers. © 2005 American Institute of Physics.

A novel interaction cavity has been designed for a gyro-monotron, allowing co-harmonic generation at the 2nd and 4th harmonic resonances of the cyclotron frequency. The output aperture of the cavity has been designed to trap the lower harmonic, whilst allowing output of the upper harmonic. Results from recent numerical simulations, performed using MAGIC 3-D, are presented. The intended co-harmonic behaviour is observed, with simultaneous excitation of the 2nd and 4th harmonics. Refinement of the output structure is now being undertaken to ensure only the 4th harmonic signal is emitted. (2 pages)

Simulations of coherent spontaneous undulator radiation in a waveguide demonstrate that the use of negative mass instability (NMI) for retaining longitudinal sizes of dense electron bunches, which are formed in laser-driven photoinjectors, allows one to increase power capabilities of a terahertz radiation source by many times. The NMI is realized in an undulator with combined helical and over-resonance uniform longitudinal magnetic fields due to nonisochronous longitudinal oscillations of electrons, whose frequencies increase/decrease with increasing/decreasing particle energy. In such conditions, an effective longitudinal size of the bunches can be preserved at long distance even at an extremely high electron density. Correspondingly, an energy extraction efficiency of more than 20% is revealed at a narrow frequency radiation spectrum, suggesting realization of a compact and powerful THz source.

A magnetic cusp gun (MCG) is being developed to generate an axis-encircling electron beam, which is called the large orbit beam, which is going to drive a 0.396-THz fourth-harmonic gyrotron. Developing an MCG imposes crucial challenges on a simultaneously minimizing guiding center deviation and velocity spread of the electron beam, particularly because an ultrahigh magnetic compression ratio is unavoidable, as is the case for a terahertz (THz) gyrotron. The study of the electron dynamics in the MCG reveals that, close to the emitter, a pair of focusing electrodes are employed to construct a special focusing and accelerating electric field as a way to balance the space-charge influence and guiding center deviation. Investigation indicates that both the electron-beam generalized-angular-momentum spread and the guiding center distribution are the critical factors contributing to beam parameter spread. Intensive optimization generates a high-power MCG with a pitch factor of 1.5, the highest magnetic field of 4 T, minimum transverse velocity spread of 1.1%, and a beam current of 2 A. The key parameters exhibit excellent stability tuning over a wide range of beam current and magnetic field. These merits enable the harmonic gyrotrons or even the frequency-tunable THz gyrotrons to be developed.

The review summarizes recent experimental results in the field of THz gyrotrons developed for various applications. A CW gyrotron with the operation frequency of 0.26 THz has been successfully used for DNP spectroscopy A pulsed high-harmonic Large Orbit Gyrotron (LOG) with the frequency of 0.55 THz and kW level of output power has been used for THz breakdown and obtaining dense plasma in gases. A powerful pulsed 0.67 THz/200 kW gyrotron is under development for remote detection of ionization sources.

A gun with the magnetic-field cusp in the anode-cathode gap is developed. A near-axis beam with an electron energy of 50–80 keV, a current of up to 0.7 A, a pitch factor of 1.5, and a relatively small spread of rotational velocities is generated using the gun in magnetic field of 10.5–14 T. The gyrotron with a working frequency of up to 1.0 THz that operates at the third cyclotron harmonic is demonstrated for the first time.

The effect of passive compression of frequency-modulated pulses in dispersive media is used to raise the microwave radiation peak power to a multigigawatt level. A waveguide with a helically corrugated surface is applied as a dispersive medium, and a relativistic 3-cm backward-wave oscillator with an accelerating voltage decaying within the pulse duration serves as a source of frequency-modulated pulses. The compression of pulses to an FWHM of 2.2 ns attended by a rise in the peak power by a factor of 4.5 (to 3.2 GW) is demonstrated with a SINUS-6 accelerator.

Powerful gyrotrons with radiation frequencies in the range 0.33–0.65 THz were demonstrated at the IAP as early as in the 1970–1980s. This trend has recently been renewed in connection with a significant increase in interest in terahertz frequency range. In the course of new experiments, the radiation frequency of pulsed gyrotrons was increased up to 1.3 THz and 1 THz at the fundamental and third cyclotron harmonics, respectively. In addition, gyrotrons operated in CW regime with a frequency of 0.3 THz for technological applications (in collaboration with the University of Fukui, Japan) and 0.26 THz for the dynamic nuclear polarization at a high-field NMR were implemented. Designs of a pulsed fundamental-harmonic gyrotron with MW-level power at 0.3 THz and a CW kW-level third-harmonic gyrotron with a frequency of 0.4 THz are currently developed. Estimates show that modern techniques for the creation of strong magnetic fields now make it possible to realize gyrotrons with an operating frequency at least up to 1–1.5 THz. Such generators utilize a relatively low particle energy and can provide higher average power than the existing FELs.

An analytical theory describing the dynamics of relativistic moving 1D electron pulses (layers) with the density modulation affected by a space charge has been revised and generalized for its application to the formation of dense picosecond bunches from linear accelerators with laser-driven photo injectors, and its good agreement with General Particle Tracer simulations has been demonstrated. Evolution of quasi-one-dimensional bunches (disks), for which the derived formulas predict longitudinal expansion, is compared with that for thin and long electron cylinders (threads), for which the excitation of non-linear waves with density spikes was found earlier by Musumeci et al. [Phys. Rev. Lett. 106(18), 184801 (2011)] and Musumeci et al. [Phys. Rev. Spec. Top. --Accel. Beams 16(10), 100701 (2013)]. Both types of bunches can be used for efficiency enhancement of THz sources based on the Doppler frequency up-shifted coherent spontaneous radiation of electrons. Despite the strong Coulomb repulsion, the periodic...

A relativistic backward wave oscillator for the 10-GHz range with the oscillation frequency tuned within about 5% by changing the accelerating voltage from 600 to 350 kV has been developed. Discrete variations in the voltage and the corresponding frequency tuning from pulse to pulse is rapidly performed by changing the anode-cathode distance in the vacuum diode without breaking vacuum in the working volume. During this, the electron beam power remains almost constant, while the output microwave power varies within 0.4–0.8 GW. The introduction of a dielectric cylinder into the accelerating gap provides a smooth voltage drop from 600 to 350 kV with the corresponding frequency tuning during a 20-ns pulse.

We review briefly the main ideas and achievements in the field of physics related to shortwavelength large-orbit gyrotrons, in which the coupling of electrons with the working mode and the discrimination of parasitic modes in the case of resonance at the high cyclotron harmonic are more efficient compared with conventional gyrotrons. The results of studying a new large-orbit gyrotron with moderate electron energies of 50–80 keV and comparatively low magnetic fields of 10.5–14 T are presented. In this gyrotron, high-power single-mode generation was obtained at the second and third cyclotron harmonics in the frequency range 0.55–1.00 THz. The prospects of development and application of short-wavelength large-orbit gyrotrons are discussed.

The maximal frequency of radiation higher than 1 THz has been recently obtained in pulse gyrotrons both at IAP (Nizhny Novgorod, Russia) and FIR (Fukui, Japan). CW generation at a 2.2-kW power level is radiated from a 300-GHz gyrotron and used for technological applications. New gyrotrons demonstrate single-mode operation at the second cyclotron harmonic with a frequency of 395 GHz in the CW regime with a power of 100 W and at the third harmonic with frequencies of 371-414 GHz in 10-mus pulses with power of 10-20 kW. Methods of selective excitation of higher cyclotron harmonics, frequency multiplication, and smooth frequency tuning in terahertz gyrotrons are also discussed in the review.

We study theoretically an electron frequency self-multiplier in which a surface mode of a periodic system is self-excited at a low frequency for a comparatively low current. The electron bunches, which appear as a result of this, excite the volume mode of an open resonator at the doubled frequency (coherent Smith-Purcell radiation). The open-resonator scheme allows one to obtain the higher power and coherence degree of radiation compared with the presently popular frequency multiplication scheme with an open periodic system (diffraction grating). The weakly relativistic and relativistic variants of the multipliers with a two-mirror open resonator designed for obtaining a high-power coherent radiation in the short-wavelength part of the millimeter and submillimeter ranges are studied numerically. The developed approach can also be used for designing high-power frequency multipliers on the basis of an array of nonlinear solid-state elements.

We review the studies of gyrotron-type microwave devices whose electrodynamic system has the form of an oversized metal waveguide with a helically corrugated internal surface. For certain parameters, such a corrugation changes radically the waveguide dispersion ensuring an almost constant group velocity of the eigenmode for a small (close to zero) longitudinal wave number in a wide frequency band. The use of “helical” waveguides along with electron optical systems which form near-axis electron beams makes it possible to create high-efficiency amplifiers based on gyro-traveling-wave tubes (gyro-TWTs) with a wide instantaneous frequency band of amplification and gyro-backward-wave oscillators (gyro-BWOs) with continuous wideband tuning of the oscillation frequency. The studied devices are superior to the well-studied microwave sources of this type (gyroklystrons and gyrotrons) in frequency band, by more than an order of magnitude, and are not inferior to them in efficiency even for a wide spread of electron velocities.

We show theoretically that the use of a circular oversized waveguide with a helically corrugated internal surface as a dispersive medium ensures efficient compression of frequency-modulated microwave pulses up to peak powers of about 10 GW. According to the calculations, a pulse with the required frequency modulation can be obtained in a relativistic backward-wave oscillator operated in the 3-cm wavelength range and producing an output power of hundreds of megawatts. In a demonstration experiment, a 80-ns pulse of kilowatt power with frequency modulation in a 5% band was compressed to a 1.5-ns pulse with a 25-fold power amplification.

Metal waveguide can be used as a dispersive medium to convert long duration, lower power pulses into short, higher peak power pulses. This provides an advanced method to generate radiation with gigawatts power in the millimeter and sub-millimeter wavelength range by compressing a megawatt level long duration pulse. In this paper, a five-fold helically corrugated waveguide operating in X-band was designed and constructed. The experiments conducted show that a 5.75 kW average power microwave pulse with a 6% bandwidth and duration of 80 ns can be compressed into a 144.8 kW, 1.6 ns pulse with a power compression factor of 25.2.

The results of experiments with THz-range Large-orbit gyrotrons (LOGs), which have been carried out over years in the Institute of Applied Physics, Russia, and the new projects of the LOGs operating at up to 4th cyclotron harmonic are presented.

The characteristics of spontaneous coherent undu-lator radiation of a short electron bunch can be essentially improved in the presence of guiding magnetic field. In proper regime, the axial Coulomb repulsion of the electrons leads to their mutual attraction, which slows down bunch degradation and increases the radiated energy.

We have developed a series of low-voltage orotrons operated in the short-wave part of the millimeter and long-wave section of the submillimeter wavelength range. The use of an open resonator as the electrodynamic system of the orotron ensures high stability of the radiation frequency and a wide band of frequency tuning. The output orotron power achieved experimentally amounts to hundreds of milliwatts, which is sufficient for many promising spectroscopy methods.