Harmonic Generarion in X-Band RBWO Operating at Low Magnetic Field

Low magnetic field operarion of the backward wave oscillators is an active area of research. The possibility to use permanent magnets makes overall high power microwave system very compact and easy to handle. Low magnetic field operation suffers from many drawbacks which include low microwave conversion efficiency, mode competition and electron beam loss in the slow wave structure. We have found out that the harmonic generation of the output microwave frequency may hamper the efficiency of the device significantly espetially when it is desired to design a backward wave oscillator for a given fixed magnetic field value. In our case a backward wave oscillator has to be designed to operate efficiently at 0.74 T guiding B-field. The designed structure suffered from harmonic generation. The guiding weak magnetic field induces circular motion in the electron beam. The circularly rotating beam starts interacting with TE20 mode and second harmonic starts growing in the structure. This harmonic may become substantial in comparison to output in TM01 mode. In this paper we will present the design to suppress frequency harmonics and improve microwave conversion efficiency in TM01 mode.