Free-electron Maser Research Articles

High-Power Spatially Extended Free-Electron Masers with Three-Dimensional Distributed Feedback

High-Power Spatially Extended Free-Electron Masers with Three-Dimensional Distributed Feedback

Nonlinear Dynamics of Radiation in Multiple-Beam Vacuum Electronic Devices

The article is devoted to overview of different types of vacuum electronic devices with two or more charged particle beams. There are travelling wave and backward wave tubes, free electron lasers and masers, volume free electron lasers. Two different cases take place in such situation: multiple-beam instability in such devices and multiple-stream instability. In the first case some charged particle beams moves in the system with different velocities. In the second one there are beams with almost equal velocities (streams). Two systems of equations for volume free electron laser with two electron beams are proposed. Some numerical results of VFEL numerical simulation are given and discussed.

Nonlinear dynamics of radiation of high-current beams of charged particles in spatially periodic structures

Provided a review of physical processes underlying the functioning of vacuum electronic devices, namely, nonlinear processes of radiation of high-current beams of charged particles during their passage through spatially periodic structures (resonators) and their interaction with an electromagnetic field. Principles of functioning of travelling wave tubes, backward wave tubes, multi-wave Cherenkov generators, free electron lasers and free electron masers, volume free electron lasers are considered. These devices operate in a wide spectrum range from microwave to X-ray, have high efficiency, and allow obtaining high radiation powers in narrow spectral ranges. Increase the performance and reliability of electronic vacuum devices is based on improving the complex electromagnetic structures in resonators. The article demonstrates a variety of such devices with an obvious generality of physical principles used in them. Nonlinear chaotic dynamics of their functioning is considered. Particular attention is paid to consideration of principles of operation of volume free electron lasers as well as their difference from other devices.

Oversized Modified Bragg Cavities for High-Power Long-Pulse Subterahertz Free-Electron Lasers

We study a new modification of the Bragg structures based on the coupling of traveling and quasicritical waves, as well as the possibility to use it as an electrodynamic system of free-electron masers (FEMs) and lasers (FELs). It is shown that inclusion of quasicritical waves in the feedback loop allows one to improve significantly the selective characteristics of Bragg structures and ensure the stable regime of single-frequency generation at a rather large (as compared to the wavelength) transverse size of the interaction space. Simulation and “cold” electrodynamic tests performed in the millimeter- and submillimeter-wave ranges confirm high efficiency of application of such structures. Operability of the new feedback mechanism has been confirmed in a series of joint experiments performed by the Joint Institute for Nuclear Research (Dubna, Russia) and the Institute of Applied Physics (IAP) of the Russian Academy of Sciences (Nizhny Novgorod, Russia) with FEL/FEM-based generators in frequency bands from Ka to W for transverse cavity sizes of up to five wavelengths. The project of a high-power long-pulse FEL operated in the subterahertz and terahertz frequency ranges, which is developed jointly by the IAP and the Budker Institute of Nuclear Physics, is discussed. The results of modeling an electrodynamic system based on modified Bragg reflectors having diameters of 20 and 40 wavelengths for implementation of FELs in the 300 and 600 GHz ranges, respectively, are presented.

Powerful oversized W-band free-electron maser with advanced Bragg resonator based on coupling of propagating and cutoff waves

We study, both theoretically and experimentally, a free-electron maser (FEM) with a two-mirror resonator that includes the advanced Bragg structure based on coupling of propagating and cutoff waves as an upstream reflector. Similar to gyrotrons, the presence of a cutoff wave in the feedback loop improves significantly the selectivity of the resonator at large values of the oversize factor and facilitates a stable single-mode oscillation regime in short wavelength bands. Proof-of-principle experiments conducted at the induction linac LIU-3000 (Dubna) demonstrate the operability of the FEM with the proposed modification of the Bragg resonator. At the transverse oversize factor of about 5, stable narrow-band generation was achieved with an output power of up to 7 MW and the oscillation frequency of about 80 GHz corresponding to the frequency of the cutoff mode excited in the advanced Bragg reflector. These investigations encourage the application of the advanced Bragg resonators for the development of high-power long-pulse FELs driven by induction linacs and operating in the subterahertz to terahertz frequency range.

State-of-the-Art of High-Power Gyro-Devices and Free Electron Masers

This paper presents a review of the experimental achievements related to the development of high-power gyrotron oscillators for long-pulse or CW operation and pulsed gyrotrons for many applications. In addition, this work gives a short overview on the present development status of frequency step-tunable and multi-frequency gyrotrons, coaxial-cavity multi-megawatt gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, large orbit gyrotrons (LOGs), quasi-optical gyrotrons, fast- and slow-wave cyclotron autoresonance masers (CARMs), gyroklystrons, gyro-TWT amplifiers, gyrotwystron amplifiers, gyro-BWOs, gyro-harmonic converters, gyro-peniotrons, magnicons, free electron masers (FEMs), and dielectric vacuum windows for such high-power mm-wave sources. Gyrotron oscillators (gyromonotrons) are mainly used as high-power millimeter wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control, and diagnostics of magnetically confined plasmas for clean generation of energy by controlled thermonuclear fusion. The maximum pulse length of commercially available 140 GHz, megawatt-class gyrotrons employing synthetic diamond output windows is 30 min (CPI and European KIT-SPC-THALES collaboration). The world record parameters of the European tube are as follows: 0.92 MW output power at 30-min pulse duration, 97.5% Gaussian mode purity, and 44% efficiency, employing a single-stage depressed collector (SDC) for energy recovery. A maximum output power of 1.5 MW in 4.0-s pulses at 45% efficiency was generated with the QST-TOSHIBA (now CANON) 110-GHz gyrotron. The Japan 170-GHz ITER gyrotron achieved 1 MW, 800 s at 55% efficiency and holds the energy world record of 2.88 GJ (0.8 MW, 60 min) and the efficiency record of 57% for tubes with an output power of more than 0.5 MW. The Russian 170-GHz ITER gyrotron obtained 0.99 (1.2) MW with a pulse duration of 1000 (100) s and 53% efficiency. The prototype tube of the European 2-MW, 170-GHz coaxial-cavity gyrotron achieved in short pulses the record power of 2.2 MW at 48% efficiency and 96% Gaussian mode purity. Gyrotrons with pulsed magnet for various short-pulse applications deliver Pout = 210 kW with τ = 20 μs at frequencies up to 670 GHz (η ≅ 20%), Pout = 5.3 kW at 1 THz (η = 6.1%), and Pout = 0.5 kW at 1.3 THz (η = 0.6%). Gyrotron oscillators have also been successfully used in materials processing. Such technological applications require tubes with the following parameters: f > 24 GHz, Pout = 4–50 kW, CW, η > 30%. The CW powers produced by gyroklystrons and FEMs are 10 kW (94 GHz) and 36 W (15 GHz), respectively. The IR FEL at the Thomas Jefferson National Accelerator Facility in the USA obtained a record average power of 14.2 kW at a wavelength of 1.6 μm. The THz FEL (NOVEL) at the Budker Institute of Nuclear Physics in Russia achieved a maximum average power of 0.5 kW at wavelengths 50–240 μm (6.00–1.25 THz).

Experimental demonstration of free electron maser operation in the regime of non-resonant trapping

The regime of nonresonant trapping aimed at improving the operation of the amplifying scheme of free electron devices has been developed and experimentally demonstrated in the Ka-band. In this regime, the wiggler is profiled in such a way that the electron beam is out of resonance with the operating wave at the beginning of the wiggler, and different electron fractions gradually get into synchronism as the wiggler's period decreases. According to the simulations, such “nonresonant” trapping provides higher efficiency, a broader amplification band, and lower sensitivity to the beam quality as compared to conventional regimes of interaction. The conclusions of the theoretical analysis are confirmed by the results of joint experiments between the Joint Institute for Nuclear Research and the Institute of Applied Physics of the Russian Academy of Sciences, in which a free-electron maser utilizing this regime was implemented: Using a set of kilowatt magnetrons with frequencies in the range from 30 to 36 GHz, an output power of up to 25–28 MW with a gain of up to 36–37 dB was demonstrated. This makes the investigated regime attractive for shorter wavelength designs of free-electron devices whose efficiency is strongly affected by the electron beam quality.

High-power broadband 30-GHz FEM amplifier operated in the grazing incident regime

A high-power broadband free electron maser (FEM) amplifier has been realized in collaboration between JINR (Dubna) and IAP RAS (Nizhny Novgorod) based on the linac LIU-3000 (0.8 MeV/200 A/250 ns). To achieve a wide frequency amplification band, the regime characterized by grazing of the dispersion curves of the beam electrons to the operating wave (the so-called grazing regime) was studied in the FEM. According to the simulations, such an operating regime allows an instant amplification band of up to 5%–7% with output power at the level ∼20 MW and a gain of about 30–35 dB to be obtained in the Ka band. The proof-of-principle experiments examined this scheme in the 30-GHz frequency range and confirmed the simulations. As a result, the predicted power level with design parameters has been achieved, and fast frequency tuning within the feeding magnetron band ±0.5 GHz has been demonstrated.

Development of a High-Power Wideband Amplifier on the Basis of a Free-Electron Maser Having an Operating Frequency Near 30 GHz: Modeling and Results of the Initial Experiments

We develop a high-power wideband amplifier based on a free-electron maser for particle acceleration, which will be operated in the 30 GHz frequency band, on the basis of the LIU-3000 linear induction accelerator forming an electron beam with an electron energy of 0.8 MeV, a current of 250 A, and a pulse duration of 200 ns. As the operating regime, we chose the regime of grazing of dispersion curves, since, according to the modeling performed, it allows one to ensure an instantaneous amplification band of about 5–7% in an undulator with regular winding for an output radiation power at a level of 20 MW and a gain of 30–35 dB. The results of the first experiments studying this FEM-based scheme are presented, in which the specified power level is achieved in the range around 30 GHz, and fast tuning of ±0.5 GHz in the band of variations in the frequency of the master magnetron is demonstrated. Modeling shows that the use of the non-resonance trapping/braking regime, which is realized in an undulator with profiled parameters, allows one to expect an increase in the radiation power of up to 35–40 MW with simultaneous widening of the amplification band up to 30% under the conditions of the LIU-3000 experiments.

Using Two-Dimensional Distributed Feedback for Synchronization of Radiation from Two Parallel-Sheet Electron Beams in a Free-Electron Maser.

A spatially extended planar 75GHz free-electron maser with a hybrid two-mirror resonator consisting of two-dimensional upstream and traditional one-dimensional downstream Bragg reflectors and driven by two parallel-sheet electron beams 0.8 MeV/1 kA has been elaborated. For the highly oversized interaction space (cross section 45×2.5 vacuum wavelengths), the two-dimensional distributed feedback allowed realization of stable narrow-band generation that includes synchronization of emission from both electron beams. As a result, spatially coherent radiation with the output power of 30-50MW and a pulse duration of ∼100 ns was obtained in each channel.

Mode splitting effect in FEMs with oversized Bragg resonators

Splitting of the fundamental mode in an oversized Bragg resonator with a step of the corrugation phase, which operates over the feedback loop involving the waveguide waves of different transverse structures, was found to be the result of mutual influence of the neighboring zones of the Bragg scattering. Theoretical description of this effect was developed within the framework of the advanced (four-wave) coupled-wave approach. It is shown that mode splitting reduces the selective properties, restricts the output power, and decreases the stability of the narrow-band operating regime in the free-electron maser (FEM) oscillators based on such resonators. The results of the theoretical analysis were confirmed by 3D simulations and “cold” microwave tests. Experimental data on Bragg resonators with different parameters in a 30-GHz FEM are presented. The possibility of reducing the mode splitting by profiling the corrugation parameters is shown. The use of the mode splitting effect for the output power enhancement by passive compression of the double-frequency pulse generated in the FEM with such a resonator is discussed.

High-power free-electron maser operated in a two-mode frequency-multiplying regime

The frequency multiplication effects in high-power free-electron masers (FEM) with Bragg cavities were studied to provide the advance of the oscillators into short-wavelength bands. Theoretical analysis of frequency-multiplying FEMs was carried out within the framework of the averaged coupled-wave approach. Proof-of-principle experiments were performed based on a moderately relativistic induction linac LIU-3000 (JINR). As a result, an FEM multiplier operated with a megawatt power level in the 6-mm and 4-mm wavelength bands at the second and third harmonics, respectively, was realized. The possibility of using two-mode bichromatic FEMs for powering a double-frequency accelerating structure was discussed.

Peculiarities of the Mode Spectrum in Free-Electron Masers Based on Oversized Bragg Resonators with a Corrugation Phase Step

We study the operating mode splitting caused by interaction of the neighboring Bragg scattering zones in an oversized Bragg resonator with a corrugation phase step, which is operated at the coupled forward and backward waveguide modes with different transverse structures. This effect is described within the framework of the coupled-wave approach using an advanced four-wave model. It is shown that this effect deteriorates the selective properties of the resonator and, finally, restricts the output power and reduces stability of the narrow-band operating regime in the free-electron masers (FEMs) based on such resonators. The results of the theoretical analysis were corroborated by 3D simulations and “cold” electrodynamic tests. Experimental studies of 30-GHz FEMs with the Bragg resonators having different corrugation depths demonstrated the onset of both narrow-band single-mode and multifrequency multimode oscillation regimes in such resonators. The possibility of power enhancement by using passive compression of the FEM output pulse in a double-frequency oscillation regime is discussed.

Modeling of a High-Power Wideband Free-Electron Maser Amplifier with an Operating Frequency of 30 GHz to be Used in Particle Acceleration Experiments

We study the possibility to develop a high-power free-electron maser amplifier with an operating frequency of 30 GHz to be used in particle acceleration experiments. Various operation regimes are considered, which are capable of ensuring a wide instantaneous amplification band with a high electron efficiency. It is shown that it is possible to achieve a gain ratio of more than 25 dB and an output power of 20 MW in a frequency band with a width of up to 20% of the operating frequency in the so-called dispersion curve contact regime by using a moderately relativistic electron beam formed by the LIU-3000 induction accelerator in an undulator with a regular winding. Application of the profiled undulator allows one to achieve a radiation power of up to 50 MW in a band having a width of more than 50% of the operating frequency in the regime of non-resonant trapping and deceleration of particles.

Two Ways for High-Power Generation of Subterahertz Radiation by Usage of Strong Relativistic Electron Beams

This paper describes results of studies on application of high current (1–10 kA) relativistic electron beams to generate millimeter and subterahertz (sub-THz) radiation in two ways. The first one is realized at the ELMI-device where the scheme of a free electron maser (FEM) with a sheet electron beam is applied for the case of a planar two-beam-channel geometry. The second way is based on emission of sub-THz radiation from plasmas with turbulence induced by a strong relativistic electron beam. These experiments are carried out at the GOL-3 facility. We present experimental results obtained at these two devices and briefly describe their theoretical analysis.

Observation of the stimulated coherent diffraction radiation in an open resonator at LUCX facility

We present an initial test of a new type of a pre-bunched beam pumped free electron maser based on Stimulated Coherent Diffraction Radiation (SCDR) generated in an open resonator. A fast Schottky Barrier Diode (time response <1ns) has enabled us to investigate the properties of the radiation stored in the cavity as well as the intrinsic properties of the cavity itself. We observed a turn-by-turn SCDR generated by a multibunch beam. When the cavity length was exactly a half of the bunch spacing a clear resonance was observed. Moreover, turn-by-turn measurements revealed the cavity quality factor of 72.88, which was rather high for an open resonator in the wavelength range of 3–5mm. An exponential growth of the photon intensity as a function of the number of bunches was also demonstrated.

Generation of a spatially coherent field structure in free-electron masers with 2D distributed feedback

Nonlinear dynamics of free-electron masers (FEMs) is studied in the planar geometry with 2D distributed feedback (DFB). As is distinct from previous works, the field structure is not fixed with respect to the three spatial coordinates including the coordinate that is orthogonal to surfaces of the plates of the 2D Bragg resonator. Conditions on the allowed oversize parameter (ratio of the gap between the resonator plates to wavelength) under which the steady-state generation remains stable upon variation in electron-beam parameters are derived. It is demonstrated that, at a relatively large gap, variations in the mismatch lead to the jumps of oscillation frequency that correspond to the excitation of bunches of modes with different transverse indices of partial waves. The results of simulation using a particle-in-cell method are presented for a FEM prototype with 2D DFB that is created using an ELMI accelerator at the Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences. The simulated results show that narrow-band spatially coherent radiation can be generated at experimental parameters of the electron beam and electrodynamic system. The advantages of 2D Bragg structures in comparison with conventional 1D structures are demonstrated for FEMs.

Generation of powerful narrow-band 75-GHz radiation in a free-electron maser with two-dimensional distributed feedback

A planar generator based on a free-electron maser (FEM generator) is developed employing a relativistic sheet electron beam (0.8 MeV/1 kA/4 μs) formed by an ELMI accelerator. A hybrid two-mirror resonator consisting of a two-dimensional upstream and a one-dimensional downstream Bragg reflectors is used as the electrodynamic system. The use of two-dimensional distributed feedback implemented in the upstream Bragg structure has made it possible to achieve stable well reproducible single mode generation regime with transverse dimensions of the system of ∼25 × 2.5 wavelengths. Experiments carried out at a frequency of 75 GHz have yielded narrow-band radiation with spectrum width of ∼20 MHz in pulses with a duration of ∼100–200 ns and power of 30–50 MW.

Open Bragg Cavity with Gaussian Wavebeam for Free Electron Maser of Centimeter Wavelength Range

This paper describes a highly selective electromagnetic cavity for a 30 GHz free electron maser (FEM). The cavity consists of two Bragg reflectors with overlapping reflection bands. Reflectors have been synthesized with the help of coupled waves method. For the sake of mode selectivity reflectors are separated by a gap which is wide in the wavelength scale. The operating mode in the interspace between the reflectors represents a Gaussian wavebeam of small diameter, so that its field is detached from the cavity walls, and the loss of radiation through the gap is negligible. At the same time, spurious modes undergo strong diffraction losses through the gap due to different transverse structure of the field. In the calculations, the Q-factor of the operating mode was shown to be at least three times as big as the Q-factors of all spurious modes within the Bragg band. The tested stainless still cavity contained a cylindrical middle-section covered with the absorbing material alsifer to emulate the gap. In low-power tests, the only high-Q mode was observed in the Bragg band, with the frequency of that mode corresponding to the calculated frequency of the operating mode. High-power testing of the cavity is planned in the nearest future

Frequency Selectivity of a Normal and a Hybrid Bragg Resonators

A hybrid Bragg resonator with an advanced and a traditional Bragg reflectors was designed to develop the frequency selectivity of the free electron maser with frequency of 40 GHz using an intense relativistic electron beam. A normal Bragg resonator with two traditional Bragg reflectors was also designed for the same frequency. The interaction between a propagating wave and a quasi-cutoff mode in an advanced Bragg reflector was expected to provide higher frequency selectivity than a traditional Bragg reflector. In the cold test, the frequency resolution of the hybrid Bragg resonator was appeared to be ± 0.05 GHz, while that of the normal one was ± 0.5 GHz.