Investigation of high-order mode excitation in a terahertz second-harmonic gyro-BWO

As to gyrotron oscillators, operation at high harmonics is an effective solution to decrease the required magnetic field strengths and improve radiation frequencies towards the terahertz (THz) band. Unfortunately, significant challenges related to a harmonic gyrotron include weak interaction strengths and serious mode competition conditions. In this paper, we report on the excitation strategy and stable-state property of a THz second-harmonic (SH) gyro-BWO with the TE_24,3 mode. Such an extremely high-order mode interaction system is inherently capable of providing high power capacity and advanced mode selectivity. A competition-free parameter space is created by increasing the Doppler sensitivity of one fundamental-harmonic (FH) competing mode at low magnetic fields and simultaneously suppressing the Q factor of another FH competing mode in the near-cutoff region at high magnetic fields. The SH quasi-whispering-gallery mode can be stimulated with a medium output power at around 0.5 THz during the FH mode switching process. This work contributes to further exploiting high frequency steps in the high-order multi-mode frequency-tuning gyro-BWO.