Design and Analysis of a Magnetron Injection Gun for the Broadband-Tunable Gyrotron With a Pulsed Magnet

The continuous frequency tuning (CFT) of the gyrotron with pulsed magnets is a promising way to generate broadband high-power terahertz waves. However, the variation of electron beam parameters caused by magnetic tuning will severely affect the frequency tuning bandwidth and interaction efficiency. In this article, the design scheme of a magnetron injection gun (MIG) for a 400-GHz broadband-tunable gyrotron with a 15-T pulsed magnet is proposed based on the idea of constrained multiobjective optimization. The electron beam parameters are further controlled by the modulating voltage regulation as the magnetic field varying with time. The electron beams can maintain a pitch factor of 1.3 and a velocity spread lower than 10% within 2.7-ms pulsewidth near the peak of the magnetic field pulse. The stable oscillations of the fundamental harmonic TE1,4 and its high-order axial modes (HOAMs) were achieved with an output power of up to 1.74 kW and a frequency tuning bandwidth of 6.51 GHz around 385 GHz.