Abstract
The requirement of strong magnetic field is one of the major difficulties for terahertz gyrotrons. A plausible solution is to operate at higher cyclotron harmonic denoted as s, in which the magnetic field strength is reduced to 1/s of the value for the fundamental harmonic operation. This paper presents a systematic theoretical investigation of a fourth-harmonic 400-GHz gyrotron backward-wave oscillator with relatively high efficiency. An axis-encircling electron beam is employed to suppress the mode competition. The operating mode is the TE 41 mode. The efficiency and bandwidth are optimized for the magnetic field tuning. Simulations suggest that the fourth-harmonic circuit is capable of achieving highest interaction efficiency ~6.5%, and tunable bandwidth 2.8 GHz at 400 GHz. The weak beam-wave coupling and serious Ohm loss on the circuit wall limit the overall performance.