Search

Gyrotron performance is sensitive to cavity structure parameters, and the cavity shape is temperature dependent due to thermal deformation induced by temperature rise from ohmic loss power on finite-conductivity cavity wall. Accordingly, this paper studies a frequency-tuning scheme for terahertz gyrotron by properly controlling the cavity thermal deformation. By combining gyrotron nonlinear theory and finite-element method software, controllable thermal-frequency-tuning capability of a continuous-wave 263-GHz gyrotron is systematically investigated, toward maintaining gyrotron operating under gyromonotron condition in frequency-tuning band, and achieving high efficiency in broadband frequency-tuning range. After studying cavity thermal distribution, structure deformation, and electron beam–wave interaction, an optimized cavity structure with transition sections on both ends is proposed. Simulation predicts that with the two-transition-section cavity, via additional thermal tuning, the continuous-frequency-tuning band is capable of reaching 1.75 GHz, which is 5 times of the initial bandwidth. Furthermore, using the thermal-frequency-tuning technology, impressive high efficiency above 17% is obtainable in the whole frequency-tuning range.

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 low- $Q$ 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.

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% $\sim$ 25%. The broadband tuning pre-bunched ECM is promising for a new generation of broadband and high-power THz source.

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 ${\rm TE}_{01}$ 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 ${\sim}{\rm 8}~{\rm kW}$ 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.

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

In this paper, a low-voltage high-efficiency 330 GHz tunable gyrotron is investigated, which operates on the open-cavity TE 6,2 mode and employs an electron beam with voltage of 20 kV, current of 0.5A, and pitch factor of 1.5. Simulation achieves a competitive efficiency, more than 40%, under the assumption of using copper circuit wall and electron beam of 5% velocity spread. Besides, the investigation of the start-oscillation currents of the TE 6,2 and TE 1,4 modes indicates that the system is with reasonable broadband tunability.

The nonlinear analysis of a 330 GHz second-harmonic TE 94 mode low-voltage gyrotron is presented in this paper. With electron beam of voltage 20 kV and current 0.5 A, the interaction circuit is optimized to achieve efficiency up to 13.6%. Based on nonlinear analysis, mode competition of the gyrotron is preliminarily revealed. The gyrotron is promising to be applied in 500MHz dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP-NMR).

Dynamics of the axial mode transition process in a 0.33-THz second-harmonic gyrotron is investigated to reveal the physical mechanism of realizing broadband frequency tuning in an open cavity circuit. A new interaction mechanism about propagating waves, featured by wave competition and wave cooperation, is presented and provides a new insight into the beam-wave interaction. The two different features revealed in the two different operation regions of low-order axial modes (LOAMs) and high-order axial modes (HOAMs) respectively determine the characteristic of the overall performance of the device essentially. The device performance is obtained by the simulation based on the time-domain nonlinear theory and shows that using a 12-kV/150-mA electron beam and TE−3,4 mode, the second harmonic gyrotron can generate terahertz radiations with frequency-tuning ranges of about 0.85 GHz and 0.60 GHz via magnetic field and beam voltage tuning, respectively. Additionally, some non-stationary phenomena in the mode startup process are also analyzed. The investigation in this paper presents guidance for future developing high-performance frequency-tunable gyrotrons toward terahertz applications.