The continuous frequency tuning of a second-harmonic (SH) 800-GHz gyrotron is experimentally studied. The influence of the electron pitch factor on harmonic mode competition and frequency tunability has been thoroughly examined. By controlling the pitch factor, the competition from the fundamental parasitic mode can be effectively suppressed. The stable oscillations of the SH mode were achieved with an output power of up to 150 W and a continuous frequency tuning range of 0.8 GHz around 800 GHz. Similarly, the gyrotron operating in the fundamental mode can achieve wideband continuous frequency tuning range of 8.71 GHz around 385 GHz.

In harmonic gyrotrons, the narrow operating region of the harmonic mode is significantly restricted by the severe mode competition from the fundamental parasitic modes, which makes it important to study the nonlinear effects of beam-wave interaction in the case of a nonideal electron beam. In this article, the effect of the electron velocity spread and the beam radius spread on the mode excitation, interaction efficiency as well as mode competition are analyzed for the desired harmonic mode and the parasitic fundamental mode operating far from the cut-off frequency. The effect of velocity spread is interpreted via the electron cyclotron resonance condition as well as the electron beam susceptibility with respect to the resonator field. The results show that the velocity spread can greatly broaden the effective operating region of the harmonic mode and the efficiency degradation can be contained at a reasonable level by proper design consideration of the interaction circuit. Whereas, the beam radius spread is the main factor that accounts for the efficiency degradation and can aggregate the mode competition. The results are referential to the design and analysis of harmonic gyrotrons as well as continuously frequency tunable gyrotrons.

We report the design and initial experimental results of an 800-GHz second harmonic gyrotron prototype, in which a 15-T pulsed magnet is applied. This gyrotron prototype is developed at the Wuhan National High Magnetic Field Center (WHMFC). An in-house software for analyzing and designing terahertz gyrotrons and an experimental research platform are developed. In addition, alignment adjustments, eddy current effects, and radiation tests are discussed. The initial experimental results show that the prototype gyrotron can irradiate ~40 W at fundamental harmonic 383.7-GHz (TE _1,4 ) waves at 14.14 T and a few milliwatts at second harmonic 0.8-THz (TE _8,5 ) waves at 14.86 T.

The misalignment is an important factor that accounts for the deviation of the experimental results from the theoretical design of high-frequency gyrotrons. This article investigates the effect of the electron beam misalignment on the operation of a second harmonic 0.8 THz gyrotron through theoretical analysis and experiments. The rise of the starting current, efficiency degradation, continuous frequency tunability, and harmonic mode competition of the 0.8 THz gyrotron under the effect of different types of beam misalignment is analyzed comprehensively using a self-consistent model, providing a general picture for the potential effect of beam misalignment on high-frequency harmonic gyrotrons. The alignment process and experimental results of the 0.8 THz gyrotron tube installed in a 15 T magnet with XY-tables and optical probes are also discussed. The results are referential to the development of high-frequency gyrotrons for high-power THz applications.

The key point of gyrotron design is the analysis of the radio frequency (RF) behavior in gyrotron resonators. This article proposes a comprehensive, user-friendly and effective gyrotron design toolbox (GYROCOMPU) based on the MATLAB platform. GYROCOMPU associates a relatively complete set of codes for gyrotron analysis and integrates them together into a graphical user interface (GUI). The solving algorithm of the cold cavity equations and self-consistent equations is improved. The solving method improvement in these calculation algorithms renders the analysis more efficient and accurate. Three typical examples of gyrotrons working in the 140-GHz TE₀₃ mode, 42-GHz TE₀₃ mode, and 140-GHz TE10,4+ mode are analyzed. All main codes, including the cold cavity calculation, starting current calculation, self-consistent calculation, and mode competition calculation, are compared with the classical code package (COAXIAL) or with results from the literature. The simulation results show that GYROCOMPU can accurately and effectively complete the analysis of gyrotrons.

The operation of gyrotrons highly depends on the alignment between the magnet and the tube, which will cause instability problems or degrade the output efficiency, especially for high-frequency gyrotrons with a small cavity size. In this letter, the influence of misalignment, including parallel shift, arbitrary tilt, and their co-existence, on the behavior of the electron beam was studied. A prototype of an 800 GHz gyrotron with a sapphire window was developed to study the correlation between the misalignment and the beam trajectory. A method to determine the tilt angle and shift distance was proposed. The feasibility of the misalignment correction method was verified in the experiment.