As the output power of the relativistic backward-wave oscillator (RBWO) increases higher and higher, the problem of the power handling capacity has become more and more serious. In order to enhance the power handling capacity, the overmoded ratio of the slow-wave structure (SWS) should be increased to reduce the field intensity in RBWO. It is found that in the overmoded SWS, TM 01 performs as the surface wave, which supports lower power handling capability than the volumetric wave such as TM 02 or TM 03 . And the mode selection of TM 02 or TM 03 rather than TM 01 can be realized by optimizing the electrodynamic structure of SWS in order to increase their coupling impedances and group velocities. In RBWO operating in TM 02 or TM 03 the beam-to-microwave power conversion efficiency higher than 30% can be achieved. Besides, RBWO operating in TM 02 or TM 03 can performs very well in the low guiding magnetic field.
