Polarizers are widely used to change the polarization of millimeter waves on the electron cyclotron resonance heating and current drive (ECRH and CD) systems. A new method based on the coordinate transformation and the Fourier expansion (the so-called C-method) has been developed for design of polarizers on the HL-2A ECRH and CD systems. This method transforms the grating problem to an eigenvalue problem, making it easy and clear to understand and solve. The comparison between the C-method, the integral method, and the low power test results is presented. It indicates that the C-method can be considered as a rigorous numerical method for the design of polarizers. Finally, two polarizers were designed based on the C-method which can be used together to achieve almost arbitrary polarization.

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.

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.