Xiang-Bo Qi

Broadband Tunable Pre-Bunched Electron Cyclotron Maser for Terahertz Application

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.

Theory and Experiment of a W-Band Tunable Gyrotron Oscillator

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.

Theoretical Study of a Fourth-Harmonic 400-GHz Gyrotron Backward-Wave Oscillator

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.

Terahertz Broadband-Tunable Minigyrotron With a Pulse Magnet

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.

Broadband terahertz-power extracting by using electron cyclotron maser

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.

Broadband Continuous Frequency Tuning in a Terahertz Gyrotron With Tapered Cavity

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.

Development of a 0.33THz pulse gyrotron

A 0.33THz pulse gyrotron is currently under development at Peking University, which is predicted to generate coherent broadband radiation in each pulse. The pulse gyrotron is designed based on the fundamental harmonic co-rotating TE62+(1) mode. The scheme of tuning magnetic field on backward-wave interaction is adopted to realize broadband radiation. The compact pulse gyrotron is promising in the applications of coherent imaging and detection in the medical science and industry.

High-Efficiency Excitation of a Third-Harmonic Gyrotron

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.

Conformal Cross-Flow Axis-Encircling Electron Beam for Driving THz Harmonic Gyrotron

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.

Terahertz Gyrotron Broadband Tuning Based on Local Field Shaping in a Low-

Terahertz (THz) gyrotron with broadband tuning capability is attractive for advanced applications, such as dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP-NMR). However, it is challenging to realize broadband continuous frequency tuning (CFT) in a conventional open cavity, in which the quality (Q) factor scaling law results in abrupt axial-mode Q-factor differences and narrow tuning range because of power switch off during higher order axial mode transition. In this paper, a novel cavity scheme based on formation of a local field is proposed to break the Q-factor scaling law and expand the CFT band. The field shaping with a local standing-wave formation is constructive in enhancing the Q factor of high-order axial modes in a lowQ open cavity, which obviously extends the CFT band and simultaneously reduces ohmic loss. Besides, a nonlinear up-tapered section of a parabolic-tapered waveguide is applied to reduce the power fluctuation in the axial mode transition process. In the preliminary design, a CFT band wider than 2 GHz with a minimum output power of 1 W is theoretically demonstrated in the TE8,2 mode 0.33-THz second-harmonic DNP-gyrotron with an ultralow voltage of 2.1 kV via magnetic field tuning.