Conventional Gyrotron Research Articles

Boosted excitation of the fifth cyclotron harmonic based on frequency multiplication in conventional gyrotrons.

We discovered a specific property of the eigenmodes of a cylindrical waveguide, due to which efficient mode excitation at ultrahigh (s=4n+1, n is integer) multiples of the gyrofrequency can be provided by a polyhelical weakly relativistic electron beam, standardly used for gyrotron operation. In the proof-of-principle experiment with a V-band gyrotron driven by a 25-keV, 2-A beam, about 100 mW radiation power at the fifth cyclotron harmonic (0.22 THz) has been detected in the cw regime. Based on the gyrotron theory, we demonstrate in simulations that, using parameters of existing gyrotrons, this method can be applied for producing cw radiation at a power level of up to several watts in the terahertz gap.

Single-Cavity Gyromultipliers With Asymmetric Electron Beams

A new approach to realization of a high cyclotron harmonic gyro-oscillator based on the frequency multiplication effect is proposed and numerically studied. This approach utilizes axially asymmetric electron beam optics, which nevertheless may be as simple as the one of the conventional gyrotrons. Numerical simulations show that the proposed method can provide a watt-level output power in THz and sub-THz frequency ranges using existing electron guns.

Frequency Multiplication in Planar Gyrotrons as a Method for Production of High-Power Multi-THz Radiation

We propose a novel source of high-power 1–3-THz radiation based on frequency multiplication in a planar gyrotron, with transverse energy extraction. In such a scheme, it is possible to increase significantly the transverse size both of the cavity and of the electron beam, and, correspondingly, the output power at the fundamental harmonic. At the same time, equidistant mode spectrum of a cavity of planar gyrotrons is favorable for generation at high-order cyclotron harmonics in the frequency multiplication regime, providing a much higher efficiency of nonlinear conversion when compared with conventional cylindrical gyrotron schemes. In simulations of a 1-THz planar gyrotron, within the multimode time-domain model, we demonstrate a possibility of kilowatt-level radiation at the second cyclotron harmonic and power of several watts at the third one. The obtained results are partially confirmed based on direct 3-D particle-in-cell (PIC) simulations.

Study on a Depressed Collector for a 75 GHz Low-Voltage Compact Gyrotron for Industrial Application

The low-voltage compact gyrotron is suitable for industrial applications. However, the beam-wave interaction efficiency is low in conventional low-voltage gyrotron. To improve the whole tube efficiency, a compact depressed collector is introduced in developing a 75 GHz low-voltage compact gyrotron. The compact depressed collector is directly connected to the output waveguide. It is grounded and isolated with the cavity by a ceramic ring which is easy to be connected with the application system. The design of the original tube electron beam voltage and electron beam current are 10 kV and 1.2 A. In the particle-in-cell (PIC) simulation, the operating mode is TE0,1 and the generated power is 1.2 kW operated at the frequency of 75.5 GHz, which corresponds to an electron efficiency of 10%. When the depressed collector is performed and the electron reflux is under 5%, the efficiency of the whole tube can reach 30%, and when the reflux rate is controlled at about 15%, the efficiency of the whole tube can reach 50%. The dissipation power would be sufficiently reduced.

High-Harmonic Gyrotrons with Axis-Encircling Electron Beams at IAP RAS

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.

Theoretical Study of a ${W}$ -Band-Covering Frequency-Tunable Gyrotron

The gyrotron has already demonstrated the capability of generating high-power coherent electromagnetic (EM) radiation at high frequencies and finds application in fusion plasma heating and magnetic resonance spectroscopy. In this article, we propose a ${W}$ -band gyrotron, which uses a so-called multimode switching scheme to realize ultrabroadband frequency tuning capability, nearly covering about 70% of the ${W}$ -band (75–110 GHz) range. The tuning strategy used to suppress mode competition and stabilize gun parameters in such an open-cavity multimode switching gyrotron is presented. Theoretical study shows that the multimode switching gyrotron can generate a frequency tuning range much wider than a conventional step-tuning gyrotron or a single-mode tuning gyrotron. In addition, another technology that uses a slot-assisted circuit to select only axis-symmetrical TE operating modes is presented. Using such a circuit, a tuning range of about 25 GHz in the ${W}$ -band is obtainable. The proposed multimode switching gyrotron is a promising ultrabroadband source for millimeter-wave and terahertz-wave applications.

An Experimental Investigation of a 0.8 THz Double-Beam Gyrotron

In this paper, we present and discuss the new experimental results that characterize the output parameters, and the overall performance of the recently developed 0.8 THz double-beam gyrotron operating at the second harmonic of the cyclotron frequency on the TE8,5 mode. Alongside with this design mode, several other resonances excited at both the fundamental and second harmonic operations in a wide frequency range have been studied. These experiments demonstrate that the usage of a double-beam electron-optical system opens more possibilities for realization of such regimes of operation that otherwise are not accessible in a conventional (single-beam) gyrotron due to a severe competition between the neighboring modes and the narrowness of the zones of a stable single mode operation. Most notably, the results prove that second harmonic operation at higher beam currents and respectively at higher levels of the output power are possible as expected from the preceding theoretical considerations and numerical studies. The results of this study are helpful for getting a better insight in the physics of the operation of the double-beam gyrotrons and suggest some possibilities for further optimization of both the design of the tube and the experimental setup.

Detailed Investigations on a Multisection Cavity for a Continuously Frequency-Tunable Gyrotron

The effects of the slightly up-tapered sections in a multisection gyrotron cavity on the resonant frequency and the quality factor are investigated in detail. The main role of these sections is to change the effective cavity length or the total reflection. The slightly up-tapered section near the input section plays a role of increasing the effective cavity length and the diffractive quality factor, and the increase for a higher order axial mode is more obvious. The slightly up-tapered main section plays a role of reducing the diffractive quality factor, and the decrease for a lower order axial mode is more notable. The role of the slightly up-tapered section near the output end is complicated; when the angle is small, the main role of this section is to increase the effective cavity length; however, with the increase of the angle, the main role is to reduce the reflection from the open end. In a conventional three-section gyrotron cavity, the quality factor of the low-order axial mode is dozens of times of that of the high-order axial mode, and in an optimized multisection gyrotron cavity, the quality factor for all axial modes can maintain an order of magnitude in the cold cavity approximation; it means that the operating current for all axial modes can keep an order of magnitude.

Terahertz Broadband Whispering-Gallery Mode Gyrotron Backward-Wave Oscillator

Conventional gyrotron backward-wave oscillators (gyro-BWOs) operate in a low-order mode, e.g., TE $_{0,1}$ mode. As the operating frequency extends to the terahertz (THz) band, the transverse size of low-order mode cavity shrinks, and the power capability is reduced, consequently. A solution to adopt an overmoded interaction cavity with a significantly enlarged index of the operating mode is valid on the condition that the challenging problem of mode competition can be controlled during the broadband frequency tuning. In this paper, a high-order whispering-gallery mode (WGM) THz gyro-BWO with a cathode-end output circuit is investigated. A segment-tapered circuit is applied to suppress the ${Q}$ factors of competing modes and to obtain a the competition-free stable start-oscillation scenario. The theoretical result predicts that the effective frequency tuning range continuously covers between 252.3 and 260 GHz when the ${B}$ -field is changed from 9.41 to 9.96 T. Our studies are beneficial to the development of high-performance sources for THz biomedical and material science applications.

A Novel Approach for Computation of High-Order Axial Modes in a Gyrotron Resonator

This paper presents a novel approach for computation of high-order axial modes (HOAMs) in a conventional and uptapered gyrotron resonator. A code has been developed which computes the eigenfrequency, Q-factor and axial field profile of HOAMs in the absence of electron beam in the resonator. Cyclic coordinate search method has been used for the optimization of complex eigenfrequencies, which obviates the computation of gradients of radiation boundary conditions and, hence, reduces computational complexity. HOAMs have been computed for resonators operating at 10, 132, 203, and 424 GHz, and the results obtained have been found to be in good quantitative agreement with the experimental and numerical data available in the published literature. Broadband continuous frequency tuning bandwidth found for subterahertz resonators also agrees well with the published results. HOAMs computed for 1-THz gyrotron resonator designed at CSIR-CEERI have also been presented.

A Compact THz Source for Enhancing the Sensitivity of Nuclear Magnetic Resonance Spectroscopy with Dynamic Nuclear Polarization

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.

Prospective THz Gyrotrons for High-Field Magneto-Resonance Spectroscopy

A high-harmonic Large Orbit Gyrotron and a low-voltage gyrotrino placed inside a spectrometer cryomagnet enable greatly simplify terahertz systems for magneto-resonance spectrometers. Large Orbit Gyrotrons provide a powerful third-harmonic generation at frequencies of 1 THz and 0.394 THz in pulsed and CW regimes, respectively, at significantly lower magnetic fields than conventional gyrotrons. According to simulations the gyrotrino with the voltage of 1.5 kV and frequency of 0.264 THz can generate a power of tens of watts; a possibility to operate at such a low voltage is demonstrated in the existing gyrotron with three-electrode magnetron-injection gun.

Competition-free second harmonic mode THz orbital angular momentum gyrotron with dual-mode operation by a perturbed cavity

We propose a dual-mode orbital angular momentum (OAM) scheme at 400 GHz that uses a gyrotron. A conventional gyrotron with two modes of operation has a second harmonic resonance that competes with the fundamental mode. We used a perturbed cavity to reduce the fundamental mode competition in a second harmonic gyrotron by allowing higher-order axial mode operation. Based on a simulation using an in-house self-consistent gyrotron code, we demonstrated that the operating dual second harmonic modes TE6,4(+)/TE12,2(+) of the OAM source in the perturbed cavity at around 400 GHz can be excited without competition from the fundamental mode TE4,2(−).

Terahertz broadband tunable pulse gyrotron

Based on the principle of a relativistic electron cyclotron maser, gyrotrons can generate high-power coherent radiation in the millimeter-terahertz (THz) waveband. A pulse magnet can generate an ultra-high field strength, and simultaneously reduces the volume by several times compared with a conventional superconducting magnet, which promotes a THz gyrotron to break the 1 THz barrier. However, only an extremely short duration around the peak field of the pulse magnet can be used for a conventional open-cavity gyrotron fixed-frequency operation. In this letter, a novel gyrotron interaction scheme is proposed to excite the broadband THz radiation by integrating a broadband pre-bunched interaction circuit with a pulse magnet, which is a promising way to expand the frequency tuning bandwidth, enlarge the magnetic field by utilizing the range of the pulse magnet, extend the operating pulse duration of a gyrotron, and realize the quasi-continuous operation of a pulse magnet gyrotron. After an investigation into the frequency and time domains, a broadband pulse gyrotron driven by a 20 kV low-voltage electron beam is predicted to generate radiation with a frequency of between 0.328 and 0.338 THz, with a peak power of 2.1 kW in a 6 ms pulse duration.

Electron-Optical System of the Gyrotron Designed for Operation in the DNP-NMR Spectrometer Cryomagnet (“Gyrotrino”)

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.

Spontaneous emission of radiation by relativistic electrons in a gyro-klystron

Abstract In this paper, we study spontaneous emission of radiation by relativistic electrons in a gyro-klystron. The scheme consists of two solenoid sections separated by a dispersive section. In the dispersive section the electrons are made non resonant with the radiation. The dispersive section transforms a small change of the velocity into changes of the phases of the electrons.This leads to enhanced radiation as compared to a conventional gyrotron type device driven by cyclotron maser interaction. It is shown that the klystron modulated spectrum depends on the dispersive field strength, finite perpendicular velocity component and length of the solenoids but do not depend on the axial magnetic field strength. The analysis is further extended to include the combined effects of the undulator aided gyrotron klystron radiation.

Design of a Second Harmonic Double-Beam Continuous Wave Gyrotron with Operating Frequency of 0.79 THz

This paper presents the most essential steps of a design study of a novel second harmonic gyrotron operating in CW (continuous wave) regime at a frequency of 0.79 THz and an output power of 1–100 W. It is based on a novel idea for suppression of the parasitic modes using a double-beam electron-optical system (EOS). It includes a triode magnetron injection gun (MIG), which forms two high-quality helical electron beams (HEB). Different schemes, namely one with two generating beams and another with one generating and one absorbing beam, have been investigated and compared. It has been shown that the scheme with two generating beams is more advantageous since it allows an effective suppression of the parasitic modes and a stable single-mode operation at the second harmonic resonance. A MIG which is appropriate for the realization of the latter scheme has been optimized using numerical codes for computer-aided design (CAD). It forms beams with practically equal pitch factors and moderate velocity spread. The construction of the gun is not sensitive to small misalignments and shifts of the electrodes and the magnetic field. Among the most promising characteristics of the presented design are an improved mode selection and a stable single-mode generation at currents that are two to three times higher than the currents in the single-beam (i.e., conventional) gyrotrons.

Experiment and Simulation of a W-Band CW 30 kW Low-Voltage Conventional Gyrotron

In this paper, a continuous wave (CW) low-voltage fundamental harmonic W-band gyrotron has been designed and simulated with the help of a particle-in-cell (PIC) software CHIPIC. The output power of 33.4 kW with an electron efficiency 37.1% in the PIC simulation is obtained by the designed conventional gyrotron employing a 30-kV, 3-A electron beam which is produced by a triode magnetron injection gun. In addition, a high-power capacity collector being one of the key components of CW gyrotrons has been evaluated and tested. The peak power density on the collector wall using the spreading electron system is about 390 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in the hot test. The CW output power of 15.8 kW is achieved with the condition of a 29-kV, 1.8-A electron beam in the hot-test experiment of the designed prototype.

New Versions of Terahertz Radiation Sources for Dynamic Nuclear Polarization in Nuclear Magnetic Resonance Spectroscopy

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.

Post-Amplification of a Gyrotron RF Beam by a Sheet Electron Beam

The availability of high-power microwave sources like gyrotrons makes it now possible to use the Gaussian output RF beam of a conventional gyrotron propagating in free space as an input for the interaction with a sheet electron beam (e-beam) drifting along an external magnetic field, with the aim of generating even higher RF power. Since this interaction is from the beginning in the nonlinear high-efficiency regime, the corresponding field amplitude is adequate to extract a significant amount of power from the high-current e-beam with no need for feedback from the walls of any cavity or resonator. The radiation field excited by the nonlinear perturbed electron motion has been calculated using a self-consistent iterative scheme and is seen to maintain the basic features of the initial RF beam (such as the Gaussian profile), but at a much higher power level as is desired for a fusion reactor.