科研成果

2015
Qi X-B, Du C-H, Liu P-K. Broadband continuous frequency tuning in a terahertz gyrotron with tapered cavity. IEEE Transactions on Electron Devices [Internet]. 2015;62(12):4278-4284. 访问链接Abstract
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.
Du C-H, Qi X-B, Liu* P-K. Broadband tunable pre-bunched electron cyclotron maser for terahertz application. IEEE Transactions on Terahertz Science and Technology [Internet]. 2015;5(2):236-243. 访问链接Abstract
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.
Qi X-B, Du C-H, Liu P-K. High-efficiency excitation of a third harmonic gyrotron. IEEE Transactions on Electron Devices [Internet]. 2015;62(10):3399-3405. 访问链接Abstract
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.
Du C-H, Lee H, Qi X-B, Liu* P-K, Chang* T-H. Theoretical study of a fourth-harmonic 400-GHz gyrotron backward-wave oscillator. IEEE Transactions on Electron Devices [Internet]. 2015;62(1):207-212. 访问链接Abstract
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.
2014
Du C-H, Qi X-B, Liu P-K. Theory and experiment of a W-band tunable gyrotron oscillator. IEEE Transactions on Electron Devices [Internet]. 2014;61(6):1781-1788. 访问链接Abstract
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.
Du C-H, Liu P-K. Millimeter-Wave Gyrotron Traveling-Wave Tube Amplifiers. 1st ed. Berlin: Springer-Verlag Berlin Heidelberg; 2014 pp. 210. 访问链接Abstract
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.
2013
Du C-H, Chang T-H, Liu P-K. Design of a W-band gyro-TWT amplifier with a lossy ceramic-loaded circuit. IEEE Trans. Electron Devices [Internet]. 2013;60(7):2388-2393. 访问链接Abstract
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.
2012
Du C-H, Chang* TH, Liu* PK, Yuan CP, Yu SJ, Liu GF, Bratman VL, Glyavin MY, Kalynov YK. Development of a magnetic cusp gun for terahertz harmonic gyrodevices. IEEE Transactions on Electron Devices [Internet]. 2012;59(12):3635-3640. 访问链接Abstract
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.
2010
Du C-H, Liu P-K. Linear full-wave-interaction analysis of a gyrotron- traveling-wave-tube amplifier based on a lossy dielectric-lined circuit. IEEE Transactions on Plasma Science [Internet]. 2010;38(6):1219-1226. 访问链接Abstract
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.
2009
Du C-H, Liu P-K. A lossy dielectric-ring loaded waveguide with suppressed periodicity for gyro-TWTs applications. IEEE Transactions on Electron Devices [Internet]. 2009;56(10):2335-2342. 访问链接Abstract
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
Du C-H, Xue Q-Z, Liu P-K, Wang M-H. Modal transition and reduction in a lossy dielectric-coated waveguide for gyrotron-traveling-wave tube amplifier applications. IEEE Transactions on Electron Devices [Internet]. 2009;56(6):839-845. 访问链接Abstract
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.
2008
Du C-H, Liu* P-K. Loss-Induced Modal Transition in a Dielectric-Coated Metal Cylindrical Waveguide for Gyro-Traveling-Wave-Tube Applications. IEEE Electron Device Letters [Internet]. 2008;29(11):1256-1258. 访问链接Abstract
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.

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