Abstract
Harmonic operation could alleviate the requirement of strong magnetic field which is the major difficulty for terahertz gyrotrons. This work presents a theoretical investigation of a 4th harmonic 400 GHz gyrotron backward-wave oscillator (gyro-BWO) with relatively high efficiency and broadening bandwidth. An axis-encircling electron beam is employed to suppress the mode competition. The operating mode is the TE41 mode. The efficiency and bandwidth are optimized for the magnetic field tuning. Simulations suggest that the 4th harmonic circuit is capable of achieving highest interaction efficiency and most broadening bandwidth.