Terahertz applications urgently require high performance and room temperature terahertz sources. The gyrotron based on the principle of electron cyclotron maser is able to generate watt-to-megawatt level terahertz radiation, and becomes an exceptional role in the frontiers of energy, security and biomedicine. However, in normal conditions, a terahertz gyrotron could generate terahertz radiation with high efficiency on a single frequency or with low efficiency in a relatively narrow tuning band. Here a frequency tuning scheme for the terahertz gyrotron utilizing sequentially switching among several whispering-gallery modes is proposed to reach high performance with broadband, coherence and high power simultaneously. Such mode-switching gyrotron has the potential of generating broadband radiation with 100-GHz-level bandwidth. Even wider bandwidth is limited by the frequency-dependent effective electrical length of the cavity. Preliminary investigation applies a pre-bunched circuit to the single-mode wide-band tuning. Then, more broadband sweeping is produced by mode switching in great-range magnetic tuning. The effect of mode competition, as well as critical engineering techniques on frequency tuning is discussed to confirm the feasibility for the case close to reality. This multi-mode-switching scheme could make gyrotron a promising device towards bridging the so-called terahertz gap.
The theoretical study of a step-tunable gyrotron controlled by successive excitation of multi-harmonic modes is presented in this paper. An axis-encircling electron beam is employed to eliminate the harmonic mode competition. Physics images are depicted to elaborate the multi-harmonic interaction mechanism in determining the operating parameters at which arbitrary harmonic tuning can be realized by magnetic field sweeping to achieve controlled multiband frequencies' radiation. An important principle is revealed that a weak coupling coefficient under a high-harmonic interaction can be compensated by a high Q-factor. To some extent, the complementation between the high Q-factor and weak coupling coefficient makes the high-harmonic mode potential to achieve high efficiency. Based on a previous optimized magnetic cusp gun, the multi-harmonic step-tunable gyrotron is feasible by using harmonic tuning of first-to-fourth harmonic modes. Multimode simulation shows that the multi-harmonic gyrotron can operate on the 34 GHz first-harmonic TE11 mode, 54 GHz second-harmonic TE21 mode, 74 GHz third-harmonic TE31 mode, and 94 GHz fourth-harmonic TE41 mode, corresponding to peak efficiencies of 28.6%, 35.7%, 17.1%, and 11.4%, respectively. The multi-harmonic step-tunable gyrotron provides new possibilities in millimeter–terahertz source development especially for advanced terahertz applications.
A minigyrotron scheme controlled by a compact pulse magnet to excite broadband terahertz (THz) radiation is presented here. In comparison to an open-cavity circuit, the adopted prebunched backward-wave interaction circuit can expand tuning bandwidth tenfold under the control of time-varying magnetic field strength, which also significantly extends the available duration time of the pulse magnet for gyrotron operation. A quasi-optical mode convertor and a Brewster window constitute the output system to transfer the broadband radiation from the circuit into free space. A systematic gyrotron design is also presented. Driven by a low-voltage electron beam, the minigyrotron is predicted to generate radiation with 10-GHz tuning bandwidth around 0.33 THz and a maximum peak power of 2.1 kW with 6-ms pulse duration, using a TE6,2 mode interaction. Such a THz gyrotron with broad tunable bandwidth, kilowatt level power, and with the unique advantage of a compact configuration is the key to high-power THz scientific and industrial applications.
Spoof surface plasmons (SSP) has become an active research topic in microwave and terahertz (THz) spectrum since its extraordinary optical and physical properties. The strong near field of SSP mode on the corrugated metal surfaces makes it especially attractive for developing a THz electronic source. A THz electronic source based on the efficient generation of SSP modes on the doubly corrugated metallic waveguide is proposed and studied in this paper. The analytical dispersion relations of SSP modes are obtained based on a modal expansion method and the field profiles of SSP modes are also presented by the finite integration method. Besides, the interaction between SSP and injected electron beam is modeled and implemented by particle-in-cell (PIC) simulation based on finite difference time domain algorithm. The gap size between the doubly corrugated metal surfaces can significantly influence the output power and PIC simulation results reveal that output power can be increased from 272 mW to 36.5 W when the gap size decreases from 90 to 40 μm at the frequency near 1 THz by the 19.55 kV, 1 A injected electron beam within 4.5-mm interaction length. The dependencies of the output performance on electron beam parameters are also investigated and we find that there is an optimized beam voltage for the given operation frequency. Various electron beams of pulse and direct current electron beam are studied and we find that half pulsewidth of periodical electron beam is more preferable than other emissive shape of injected electron beam for the given structure. Our studies on the efficient generation of SSP modes on the doubly corrugated metallic waveguide may provide a new way to develop THz electronic sources.
A terahertz electronic source based on the spoof surface plasmon (SSP) with 2-D subwavelength metallic grating is presented. The SSP dispersion relation of plasmonic grating is derived by a simplified modal expansion method, and the coupling and interaction between the SSP and the electron beam is studied by particle-in-cell simulation. The results reveal that the output performance highly depends on the location of electron beam from grating surface. For an injected electron beam with 19.15 kV and 0.5 A, the SSP output power can reach 22.7 W for the optimized distance of the beam from the grating surface at a frequency near 1 THz for the given structure. Besides, the influence of different electron beam parameters on output power is also investigated and we find that pulse electron beam is preferable than continuous electron beam for good performance. There is an optimized operation frequency for the given beam voltage. Furthermore, output performance can be improved by changing grating structure parameters. By decreasing the grating groove filling factor from 0.8 to 0.2, the SSP output power can be increased from 17.2 to 23.6 W. The SSP power can also be significantly enhanced from 14 to 28.6 W using shallow grating with a groove depth changing from 76 to 56 渭m for the optimized operation frequency with the same electron beam. The present work may provide a new avenue to obtain powerful THz electronic sources.
Theoretical investigation of a broadband quasi-optical mode converter for 330-GHz TE62 mode gyrotron application is presented. The converter consists of a Vlasov launcher and three reflector mirrors. Special considerations, including a Vlasov launcher with reasonably large aperture radius and optimized combination of two elliptic reflectors and a bifocal parabolic reflector, are the keys to achieve broadband mode converting. The optimized internal converter is well compatible with the gyrotron electron-optical system and generates Gaussian beam with efficiency higher than 80% in an extraordinary broadband range between 310 and 340 GHz. The principle of the broadband converter can also be applied to gyrotron amplifiers.
In a magnetic cusp gun, the canonical-angular-momentum (CAM) spread of the initially emitted electrons is crucial in generating substantial beam velocity spread. A new method called electron beam flow-feature compensation is proposed to build an axis-encircling electron beam with zero velocity spread by optimizing the cross-flow trajectories to compensate for the initial CAM spread. This method provides an effective solution to the velocity-spread problem in terahertz gyrotrons and increases the emission current to a level that is several times higher than the level that can be obtained using current technology.
Broadband continuous frequency tuning (CFT) in a terahertz gyrotron is promising for advanced terahertz applications. However, it is challenging to realize broadband CFT in a conventional open cavity, because a long cavity is helpful to expand the bandwidth but is generally difficult to suppress the high Q -factor gyromonotron competition. In this paper, a tapered cavity with a long effective interaction length is proposed to expand the CFT bandwidth. The tapered circuit can reduce the Q-factor of the first-order axial mode and accordingly suppress the gyromonotron competition. By selecting a reasonable Q-factor cavity, a gyrotron could generate effective radiation sequentially under gyromonotron and gyrobackward-wave oscillator (BWO) states during the magnetic field tuning. In gyromonotron range, the bandwidth is expanded because of the cutoff frequency shifting. On the other hand, in gyro-BWO range, the bandwidth is expanded because of the axial mode transition. The CFT bandwidth of 4 GHz is realized in a tapered 330-GHz TE12,4 mode low-voltage gyrotron. The principle is important for developing broadband CFT terahertz gyrotrons.
The relativistic electron cyclotron maser (ECM) has been successfully applied to generating high-power THz wave. In order to realize the additional advantages of broadband tuning and high efficiency interaction, this paper is devoted to exploring the THz pre-bunched ECM. Other than a conventional open-cavity tunable gyrotron consecutively switching between axial modes to realize frequency tuning, a pre-bunched ECM system operates on the backward traveling-wave resonance to achieve broadband smooth tuning. Especially, an interaction circuit of specified axial profile of beam-wave detuning frequency is built to achieve high efficiency. An optimized 0.1 THz pre-bunched ECM system using an electron beam of 30 kV voltage and 3 A current is predicted to generate broad bandwidth of 10 GHz and efficiency between 10% ~ 25%. The broadband tuning pre-bunched ECM is promising for a new generation of broadband and high-power THz source.
High-harmonic gyrotrons are challenging to generate high interaction efficiency and simultaneously suppress the mode competition. Investigation in this paper reveals that a high-Q cavity is potential to realize high efficiency in a 94-GHz third-harmonic TE02 mode gyrotron. Unfortunately, the high-Q cavity exhibits severe mode competition from the lower harmonic modes. A start-up scenario of active parameter control is employed to suppress the mode competition. The third-harmonic TE02 mode gyrotron finally achieves the steady single-mode operation with efficiency up to 20%. The physical mechanism during the mode formation process is theoretically investigated according to frequency-domain and time-domain nonlinear theories. The theoretical investigation in this paper is of guidance for future developing high-harmonic gyrotrons, especially toward terahertz applications.
The requirement of strong magnetic field is one of the major difficulties for terahertz gyrotrons. A plausible solution is to operate at higher cyclotron harmonic denoted as s, in which the magnetic field strength is reduced to 1/s of the value for the fundamental harmonic operation. This paper presents a systematic theoretical investigation of a fourth-harmonic 400-GHz gyrotron backward-wave oscillator with relatively high efficiency. An axis-encircling electron beam is employed to suppress the mode competition. The operating mode is the TE41 mode. The efficiency and bandwidth are optimized for the magnetic field tuning. Simulations suggest that the fourth-harmonic circuit is capable of achieving highest interaction efficiency ~6.5%, and tunable bandwidth 2.8 GHz at 400 GHz. The weak beam-wave coupling and serious Ohm loss on the circuit wall limit the overall performance.
A gyrotron capable of both frequency and power tuning is a promising coherent millimeter-THz wave source. A self-consistent nonlinear theory is applied to investigate the electron cyclotron interaction between electron beam and wave modes of axial nonfixed profiles in an extended W-band TE01 mode cylindrical cavity. It is revealed that tuning the magnetic field strength can excite electron cyclotron resonances on forward wave, backward wave, and even simultaneous on both waves, which makes the system operate under distinctive states, namely the gyrotron backward wave oscillation state and the gyromonotron state. In this paper, a W-band prototype gyrotron oscillator based on an extended cylindrical waveguide cavity is built, and the experiment test indicates that the system starts oscillation in a relative wide range of the operation parameters. The measured frequency spectrum reveals the system iteratively switches between the lower order instability axial modes, and it operates under nonstationary oscillation states. The experimental measurement of highest output power ~8 kW is consistent with the theoretical predictions. An optimized gyrotron circuit with efficiency exceeding 20% and tunable bandwidth over 10 GHz is also presented. The free oscillation behaviors revealed in this paper provide interesting guidance for developing tunable gyrotrons in millimeter-THz wave range.
The first book to systematically introduce gyro-TWT theory, method and physics
A gyrotron traveling-wave amplifier (gyro-TWT) with the high-power and broad-band capabilities is considered as a turn-on key for next generation high-resolution radar. The book presents the most advanced theory, methods and physics in a gyro-TWT. The most challenging problem of instability competition has been for the first time addressed in a focused and systematic way and reported via concise states and vivid pictures. The book is likely to meet the interest of researchers and engineers in radar and microwave technology, who would like to study the gyro-TWTs and to promote its application in millimeter-wave radars.
A pulse prototype of a W-band TE01 mode gyrotron traveling-wave tube (gyro-TWT) amplifier is designed, and it features high gain and broadband capabilities. The TE01 mode input coupler is constructed by mounting a sapphire pill-box window onto a Y-type mode converter. The high power output window will employ a triple-sapphire-disc configuration to achieve return loss lower than -30 dB over a bandwidth of 8 GHz. To suppress the spurious oscillations and realize high-average power potential, a new lossy ceramic material with weak electric conductivity is loaded in the TE01 mode cylindrical interaction waveguide. The loss-free output taper is carefully optimized to suppress oscillations and maintain broadband amplification. Employing a magnetic injection gun of beam voltage 70 kV, beam current 3 A, pitch factor 1.5, and axial-velocity spread 5%, theoretical investigation predicts that the gyro-TWT amplifier is of excellent performance, which includes being driven to saturation with input power Pin <; 0.4 W, highest efficiency of 32.4%, and the bandwidth of 4.2 GHz with output power exceeding 50 kW.
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.
A lossy dielectric-lined (DL) waveguide is inherent with excellent mode-selective-propagation ability. A millimeter-wave gyrotron-traveling-wave (gyro-TWT) amplifier based on such kind of waveguide is characterized with high stability. In this paper, the analytical expressions of the field components of the operating modes in the DL waveguide are obtained from the eigenequation, and the linear theory of electron-cyclotron-maser (ECM) instability in the DL waveguide is developed by employing the full-wave-interaction method. This linear theory takes the waveguide structure and the characteristics of the lossy dielectric material into consideration. It is capable of accurately calculating the ECM instability between a cyclotron harmonic and a circular polarized mode, as well as effectively predicting the linear stability of the DL-waveguide-based interaction system. The validity of the linear theory is verified via comparing with results obtained using a coherently developed self-consistent nonlinear theory. Numerical calculation reveals a series of interesting results. This paper provides specific guidance for future designs of millimeter-wave lossy dielectric-loaded gyro-TWTs.
A dielectric-loaded (DL) waveguide is an attractive possibility for interaction circuits with high-power sources in the millimeter-wave regime down to tenths of millimeters, particularly for gyrotron-traveling-wave-tube amplifiers (gyro-TWTs). We present results on a systematic investigation of the influence of the periodically loaded lossy dielectric on the propagation characteristics of the operating modes, which reveals that a complex mode in the periodic system can be mapped to a corresponding mode in an empty waveguide or a uniform DL waveguide. Dielectric losses not only induce modal transitions between different modes with similar field structures and close phase velocities in the uniform system but also unify the discrete mode spectrum into a continuous spectrum in the periodic system. Since the lossy dielectric functions as a power sink, the higher order Bloch harmonic components arising from the structural periodicity are suppressed, and the mode spectrum of the lossy periodic system degenerates into that of an empty waveguide. This alleviates the potential danger of spurious oscillations induced by the higher order harmonic components, making the periodic lossy DL waveguide promising in a high-power millimeter-wave gyro-TWT
A metal cylindrical waveguide coated with an inside layer of lossy dielectric which affects the propagation characteristics of a guided electromagnetic mode is investigated for gyrotron-traveling-wave tube (gyro-TWT) amplifier applications. This paper reveals a series of novel phenomena. The dispersion curve of a higher order mode has a turning point during its evolvement from the fast wave region to the slow wave region. An electromagnetic mode in the lossy dielectric-coated waveguide exhibits a transverse partial-standing-wave distribution. The dielectric loss induces modal transition which results in the dispersion curves of a pair of nearby modes crossing each other and interchanging mode structures. Modal reduction caused by strong dielectric loss merges a pair of nearby modes into one. In this one merged mode, the dielectric-coated waveguide is equivalent to a conventional cylindrical waveguide with imperfect conducting wall. This improved understanding of lossy dielectric-coated metal cylindrical waveguide is of value and usefulness for application toward gyro-TWTs capable of high-power and wide bandwidth.
The mode identification principles, mode structures, and propagation characteristics of electromagnetic modes in a metal cylindrical waveguide coated with an inside layer of lossy dielectric have been investigated for gyro-traveling-wave-tube applications. For the first time, the loss-induced modal transition is revealed, in which the dispersion curves of a pair of nearby modes cross each other, and their mode structures interchange. The relations among the dispersion curves, mode structures, and propagation attenuations are also presented. The distinctive discriminations of propagation properties between different modes enable us to explore many promising applications using lossy dielectric-coated waveguides.