Effects of transverse electron beam motion in a relativistic backward wave oscillator operating at low guiding magnetic field

We found that the electron drift length, L1, between a diode area and a resonant reflector in a relativistic backward wave oscillator operating at a low guiding magnetic field has a periodical influence on device power, which is a special characteristic due to periodical transverse motions of electron beams. In this paper, electron motion characteristics under both TM02 and TM01 fields are investigated using single-particle simulation, and the result shows that the condition of beam expansion is dependent on the beam state that spatially meets the TM02 standing-wave field. A larger beam expansion will lead to a larger axial energy spread in the modulation area, which is unfavorable for efficient beam-wave interaction. Furthermore, rapid transverse expansion of electron beams can be observed under a TM01 travelling-wave field, which is the main cause for the obvious decline in the current collected by the collector as the output power reaches saturation.