Search

A microwave system in the form of a quasi-optical (QO) transmission line is proposed as an interaction circuit of an amplifier or an oscillator based on the cyclotron resonance maser (CRM) instability. The circuit consists of periodically spaced along the horizontal ${z}$ -axis focusing mirrors, which transport the Gaussian wave beam along a zigzag path with vertical and inclined segments. A beam of electrons having sufficiently high perpendicular velocities is guided by a static B-field along the ${z}$ -axis so that the e-beam periodically crosses the wave beam. The CRM instability occurs in regions where an electromagnetic wave having a Bz-component, as in a gyrotron, propagates strictly across the e-beam resulting in the interaction with minimal sensitivity to particle velocity spread. Three-dimensional Particle-In-Cell (3-D PIC) simulations demonstrate the attractiveness of this “zigzag” CRM for the implementation of relatively broadband amplifiers or oscillators with unique (more than an octave bandwidth) frequency tunability at wavelengths of 1–2 mm.

To cover the so-called terahertz gap in powerful sources of coherent electromagnetic waves, a second-harmonic gyrotron operating in a 10 T liquid helium-free superconducting magnet has been designed, manufactured, and tested. With an operating voltage of 15 kV and a beam current of 0.6 A, the gyrotron generated coherent continuous-wave radiation at a frequency of 0.526 Terahertz with an output power of 250 Watts and an efficiency of 2.7%.

We propose a robust way of advancement of the gyrotrons into the region of terahertz frequencies and MW-level power of radiation while retaining high efficiency and high stability of the frequency. These gyrotrons may be used for electron cyclotron resonance heating and current drive systems (ECRH&CD) in future fusion setups, in large complexes of coherently radiating gyrotrons for THz data and energy transmission, for THz particles acceleration. We at first demonstrate the advantages of this way at the example of the gyrotron with very-high-order operating mode TE56.24, driven by a relatively weak external monochromatic signal, and show that in the phase- and frequency-locking mode a significant enhancement of radiation parameters – frequency, power and efficiency – can be achieved.

Experimental details are discussed and millimeter wave measurements presented of a W-band gyrotron traveling-wave tube (gyro-TWT) that uses a helically corrugated interaction waveguide operating at the second electron cyclotron harmonic. A method for increasing the gain by using an interaction circuit with two sections separated by a long sub-cutoff drift section has been experimentally verified. A 10-mW driving source was sufficient to enable an amplified signal output power of 3 kWs. The continuous-wave (CW) operation with a single-stage depressed collector and deposited beam power of about 20 kW is believed to be the first demonstration of its kind for a W-band gyro-TWT.

Three different designs of an external microwave system for feeding and extracting power to and from a gyrotron traveling-wave tube (gyro-TWT) through one oversized port are presented and discussed. As it shown, such a microwave system should operate as a splitter which splits two wavebeams with mutually orthogonal polarizations incoming through one port into two spatially separated ports or transmission lines transporting each wavebeam individually. The orthomode splitters presented in this paper were designed to be used with a gyro-TWT producing 350- and 50-kW maximum pulsed and average powers, respectively, with a 3-dB bandwidth of 10 GHz at the center frequency of 95 GHz. Such a high-power level and wide bandwidth at short millimeter waves make quasi-optical components to be the most appropriate solutions in contrast to low-mode standard waveguide circuits.

A 45-GHz gyrotron-based microwave oscillator setup has been purposefully developed for powering of a superconducting electron cyclotron resonance (ECR) ion source. The setup provides a smoothly regulated output power in the 0.1–20-kW range (up to 26 kW in manual mode) with an efficiency of up to 50% and collector depression in both continuous wave (CW) and pulse modes. Pulse mode specifications: duration 5–200 ms, leading edge less than ${2}~\mu \text{s}$ , trailing edge less than ${1}~\mu \text{s}$ , repetition rate 1–10 Hz, and triggered operation using an external reference oscillator. The set of high-voltage power supplies (HVPSs) is equipped with a fast electronic protective system to ensure trouble-free operation of the gyrotron. The functional structure of the setup corresponds to the general design method for feedback-controlled microwave sources. The feedback loop consists of a power meter (or a detector unit, to the user’s choice), a control system, and an anode HVPS, the regulated voltage of which controls the output power of the gyrotron. The feedback characteristic time is less than 1 s. Feedback control is provided for both CW and pulse modes. The setup includes a unique quasi-optical line that transmits the microwave beam through a 2-m air gap, as well as a synthesized mode converter from the Gaussian beam to the TE01 or TE11 mode (to the user’s choice) installed on a 300-kV platform. During the tests, the developed setup proved to be a reliable and user-friendly tool applicable for long-term powering of the fourth-generation ECR ion sources.

We are considering the possibility of ultrawideband millimeter-wave generation in microwave oscillators consisting of two coupled helical-waveguide gyro-TWTs, one of which operating as an amplifier whereas the other acting as a saturable absorber or as a nonlinear converter. In the first case, the regime of passive mode-locking is realized when the radiation represents a periodical train of ultrashort electromagnetic pulses. The spectral domain corresponds to the formation of a microwave “frequency comb.” On the other hand, if the second gyro-TWT acts as a nonlinear converter with a fast variation of the phase characteristic, it leads to the generation of chaotic signals with quasi-continuous spectrum and a bandwidth of up to ~10% which is defined by the bandwidth of the gyro-TWTs.

In order to realize a high-power (100 s of kilowatts at the W-band) wideband microwave amplifier, we suggest to use a cascade of two gyrotron traveling-wave tubes (gyro-TWTs), one of which possesses a relatively high gain (40–50 dB) while the other provides a high output power at a moderate (15–20 dB) gain. Both gyro-TWTs are assumed to use helically corrugated waveguides and operate at the second cyclotron harmonic. A principal scheme of such an amplifier chain is discussed, including issues on the microwave input, output, and transmission between the tubes. Computer modeling of the beam-wave interaction using CST Particle Studio PIC Solver shows the possibility of achieving of 200–370-kW output power with 8–10-GHz bandwidth when driving by a 50-mW input source at the W-band.

Previously, relativistic gyrotrons with thermionic magnetron-injection electron guns have been demonstrated successfully to produce radiation in the cm and long-mm wavebands with the pulse power exceeding 10 MW. This brief presents a new step toward generating higher-frequency microwaves with a relativistic gyrotron operated in the 3-mm wavelength band. The gyrotron employed with a 250 kV, 100 A, $1~\mu \text{s}$ helical electron beam. The operating wave was the rotating TE12,5,1 mode of a circular waveguide cavity. The mode was transformed into the Gaussian wave beam by a built-in mode converter. In the optimized single-mode regime, the gyrotron delivered 5.6 MW of the peak power in pulses with durations of about 0.5 $\mu \text{s}$ .

In this paper, we present a design of a broadband powerful $W$ -band gyroamplifier with an interaction circuit in the form of a helically corrugated waveguide operating at the third cyclotron harmonic. An analysis of stability of the gyrotron traveling wave tube (gyro-TWT) with respect to parasitic wave-beam interactions at the operating and nonoperating cyclotron harmonics has been performed, and the range of parameters at which the gyro-TWT is zero-drive stable is found. Moreover, a comparison with a traditional smooth-waveguide gyro-TWT revealed that the helical-waveguide gyro-TWT possess superior stability characteristics at higher cyclotron harmonics. For a set of parameters attractive for possible applications, the 3-D particle-in-cell simulations were performed, according to which the proposed gyro-TWT yielded a 6% amplification bandwidth with the maximum output power of 80 kW and gain of 25 dB for a 10-A, 70-kV electron beam.

The results of experimental investigation of two Ka-band gyrotron traveling-wave tube (gyro-TWT) amplifiers with helically corrugated waveguides are presented. The first tube produces pulsed output power of 130-160 kW within the frequency range of 33.1-35.5 GHz and is capable of operating with a 10% duty factor. Reliability of its major components in the high average power operation regime (about 10 kW) was proven in a continuous-wave (CW) experiment. The second gyro-TWT amplifier delivered CW power of up to 7.7 kW with -3-dB bandwidth of 2.6 GHz and -1-dB bandwidth of 2.1 GHz. Effective implementation of single-stage depressed collectors (to the best of our knowledge, for the first time for gyro-TWTs) enabled the electron efficiencies as high as 36% for the pulsed tube and 33% for the CW tube to be achieved at operation at the second cyclotron harmonic.

A millimeter-wave tunable notch filter using several extensions of the fundamental-mode rectangular waveguide has been developed to protect plasma diagnostics techniques from stray gyrotron radiation. The design utilizes the resonant behavior of the TE \(_{301}\) mode excited by cutoff coupling holes in a symmetrical smooth H -plane extension of waveguide. Proper choice of the shape and sizes of the coupling elements allows one to tune the operating frequency, notch depth, and notch band. Low-power experiments with the six-cavity filters show a maximal notch depth of below −120 dB at 70 and 170 GHz, and a notch depth of below −90 dB at frequency bands of 70 ± 0.05 and 170 ± 0.2 GHz. The filter design is rather simple and can be advanced into a higher frequency band up to the 400 GHz.

A method, which enables feeding and extraction of microwave radiation to and from gyrotron traveling wave tubes (gyro-TWT) using a single highly oversized barrier window is proposed. The essential conditions for effective implementation of this method are as follows: 1) the operating mode of the amplifier should be a circularly polarized one and 2) the “cold” losses of its interaction circuit should be sufficiently low. A detailed explanation of the method and discussion of the major microwave components are presented.

We propose principles which represent a systematic approach to the synthesis of multimode waveguides converting a specific input field into a different output field. Using these principles, fast and efficient methods of synthesis with specified properties can be constructed for any field analysis method. Several new methods of synthesis for most important applications are offered, and synthesized waveguide components are presented to demonstrate their efficiency.

A multifrequency electron cyclotron resonance heating (ECRH) system is currently under construction at the ASDEX Upgrade tokamak experiment. The system employs depressed collector gyrotrons, step tunable in the range of 105-140 GHz, with a maximum output power of 1 MW and a pulse length of 10 s. One two-frequency GYCOM gyrotron has been in routine operation at ASDEX Upgrade since 2006. A further extension of the system with three more gyrotrons is underway. An in situ calibration scheme for the broadband torus window has been developed. The system is equipped with fast steerable mirrors for real-time MHD control. The gyrotron and the mirrors are fully integrated into the discharge control system. The ECRH system turned out to be essential for the operation of H-modes after covering the plasma facing components of ASDEX Upgrade with tungsten. Deposition of ECRH inside rhotor < 0.2 is necessary to prevent accumulation of W in plasmas with high pedestal temperatures. With respect to the limited loop voltage available in ITER, the use of ECRH for neutral-gas preionization to facilitate plasma breakdown and its application during the current ramp-up to increase the conductivity in order to save transformer flux have been demonstrated successfully for 105 GHz, 3.2 T (O1-mode) and 140 GHz, 2.2 T (X2-mode), corresponding to 170 GHz at ITER with the full and half values of its foreseen toroidal field of 5.3 T.

A new millimeter-wave barrier window in an oversized metallic circular waveguide has been developed for high-power transmission. The novel design utilizes the traveling-wave regime in an oversized profiled guide and synthesized TE0n mode mixing to minimize field intensity at the alumina ceramics surface. Proper choice of ceramics thickness provides necessary broadening of the operation frequency band. Low-power measurements show a reflection level below -25 dB at the operating frequency band of 34.272 plusmn 0.06 GHz

This paper presents the results of an experiment in which the effect of generation of picosecond superradiation pulses was first observed during the induced backward scattering of a high-power pump microwave by a short high-density relativistic electron beam. These investigations became feasible due to an experimental setup built around two high-current electron accelerators of the Radan series that were set to operate synchronously to within ~300 ps. The pump wave was generated by a relativistic Ka-band backward-wave oscillator with a power of ~100 MW. To direct this wave into the scattering region, a quasioptical system was used. The scattered radiation detected in the experiment was a short (~200 ps) pulse with a spectrum-integrated power of up to 1 MW. Due to the relativistic Doppler shift, the scattered radiation spectrum covered frequencies up to 150 GHz

A new millimeter-wave resonant ring using a traveling TE/sub 01/ wave in an oversized cylindrical waveguide has been developed for high-power tests of accelerator components. Novel low-loss miter bends with flat mirrors that utilize mode mixing to minimize losses were used in the resonant ring. Low-power measurements show a maximum effective power gain factor that exceeds 35:1 at the operating frequency of 34.272 GHz. Total quality factor is approximately 21400, and the reflection coefficient from the input to the ring resonator is less than 1%.

A helically corrugated waveguide was used for a gyrotron backward-wave oscillator (gyro-BWO) experiment. A thermionic cathode was used to produce an electron beam of 90-215 keV in energy, 2-3 A in current, and pitch alpha of up to 1.6. The oscillator achieved high-efficiency frequency-tunable operation. At a fixed beam voltage of 185 kV and a current of 2 A, the output frequency was tuned by adjusting the magnetic field in the interaction cavity. A maximum power of 62 kW and a 3-dB frequency-tuning band of 8.0-9.5 GHz (17% relative tuning range) with a maximum electronic efficiency of 16.5% were measured. In addition, the interaction frequency could be tuned by varying the electron beam energy. At a fixed cavity magnetic field of 0.195 T, the output frequency and power from the gyro-BWO were measured as a function of tuning electron beam energy while the beam current was maintained at 2.5 A. A 3-dB relative frequency tuning range of 8% was measured when the electron beam voltage was changed from 215 to 110 kV.

During the last decade, a line of gyrotrons ranging in frequency from 24 to 84 GHz with the output power from 3 to 35 kW continuous wave, and a series of gyrotron-based systems have been developed by the Institute of Applied Physics, Nizhny Novgorod, Russia, jointly with GYCOM, Ltd., Nizhny Novgorod. Main technical characteristics of the gyrotrons, as well as the architecture of the gyrotron-based millimeter-wave power sources for applications are presented. The purposely developed transmission lines, applicators, and power control make gyrotron systems flexible and easily adaptable tools for research and development. A number of millimeter-wave technologies are extensively studied today using gyrotron systems; ceramics sintering and joining (including nanoceramics), functionally graded coatings, rapid annealing of semiconductors, microwave plasma assisted chemical vapor deposition, multiply charged ion production are among the most advanced development.

A free electron maser (FEM) has been built as a pilot experiment for a mm-wave source for applications on future fusion research devices such as ITER, the International Tokamak Experimental Reactor. A unique feature of the Dutch Fusion-FEM is the possibility to tune the frequency over the entire range from 130 to 260 GHz at an output power exceeding 1 MW. In the first phase of the project, the so-called inverse set-up is used. The electron gun is mounted inside the high-voltage terminal. The entire beam line was tested successfully with extremely low loss current, lower than 0.05%. First generation of mm-waves was achieved in October 1997. Up to now the highest peak power measured is 700 kW at 200 GHz. This was achieved with a beam current of 8 A and an accelerator voltage of 1.77 MV. Output power, start-up time and frequency correspond well with simulation results. Parameter scans for the longitudinal undulator gap, accelerator voltage and reflection coefficient have given a wide range of interesting data of which a few highlights are given.

A helical corrugation of the inner surface of an oversized cylindrical waveguide provides, for certain parameters, an almost constant value of group velocity and close to zero longitudinal wavenumber of an eigenwave for a very broad frequency band. The use of such a helical waveguide as an operating section of a gyrotron traveling wave tube (gyro-TWT) allows significant widening of its bandwidth and an increase in the efficiency at very large particle velocity spreads. In this paper, the new concept is confirmed by theoretical analysis and "cold" measurements of the helical waveguide dispersion. Results of a linear and nonlinear theory of the helical gyro-TWT as well as two designs for subrelativistic (80 keV, 20 A) and relativistic (300 keV, 80 A) electron beams are also presented. For both designs, parameters providing a very broad frequency band (about 20%) and high efficiency (above 30%) have been found. When the transverse velocity spread is increased from zero up to a very high value of 40%, simulations showed only a 20%-30% narrowing in the frequency band and a 20% decrease in electron efficiency. The theoretical analysis demonstrates important advantages of the helical gyro-TWT over the "smooth" one in frequency bandwidth, sensitivity to electron velocity spread, and stability to parasitic self-excitation.

A novel electrodynamic system to realize high efficiency and a broad frequency band in a gyro-TWT amplifier at millimeter and microwave frequencies is described. A cylindrical waveguide having a helically grooved wall selectively couples an electromagnetic waveguide mode at a frequency close to cutoff to the forward travelling wave of a lower mode, resulting in an eigenwave with a constant positive group velocity and approximately zero axial wavenumber over a wide frequency band. For a travelling-wave gyroamplifier, such a dispersion characteristic permits operation with significantly increased bandwidth and reduced sensitivity to electron-beam axial velocity spread. Numerical calculations will be presented to show the beam-coupled dispersion characteristics, stability, and gain profiles of a simple amplifier, based upon a linearized coupled-wave theory.

Operability of the wide-band reflector based on the effect of microwave beams multiplication was tested in the "hot" experiment. A microwave system of the new type including such a reflector was used in a free electron maser (FEM)-oscillator with a guide magnetic field. The high-current accelerator "Sinus-6" forming a 500 keV/120 A electron beam was used as the FEM driver. The double-resonance regime, when both the undulator and the cyclotron resonance conditions were fulfilled simultaneously, provided maximum output power 7 MW and electron efficiency 12% at the designed frequency 45 GHz.

A review of the experimental study of single-mode oscillators based on stimulated bremsstrahlung and Cerenkov radiation of high-current relativistic electron beams is given. Three types of Cerenkov oscillators are investigated in detail: orotrons, surface wave oscillators and a flimatron (free electron maser (FEM) based on Smith-Purcell radiation). The bremsstrahlung oscillators studied are gyrotrons with TM modes, a ubitron operating at a quasi-critical frequency and cyclotron autoresonance masers. Electrodynamic and electron methods of mode selection provide stable radiation with a reproducible space structure of radiation in all oscillators under study. The radiation power attained 50-100 MW for long and 10-30 MW for short millimeter wavelengths at the efficiency up to 5-10 percent. Various types of oscillators are compared. Promising methods for increasing power and radiation frequency are discussed.

FEL's based on the stimulated undulator radiation (ubitrons) are compared with those based on the stimulated cyclotron radiation [cyclotron autoresonance masers (CARM's)]. If the high-current accelerators are used as electron injectors, then from the viewpoint of simplicity of oscillatory electron energy pumping, criticality with respect to electron velocity dispersion, and efficiency, CARM's seem to be more effective than ubitrons at mm and sbmm waves. For such HF generators, resonators based on selective Bragg reflection of electromagnetic waves in corrugated metallic tubes are most atractive. CARM's of this type yield 6 MW at a 4 mm wavelength and 10 MW at a 2 mm wavelength in the single-mode regime.