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A high efficiency relativistic backward wave oscillator working at a low guiding magnetic field is designed and simulated. A trapezoidal resonant reflector is used to reduce the modulation field in the resonant reflector to avoid overmodulation of the electron beam which will lead to a large momentum spread and then low conversion efficiency. The envelope of the inner radius of the slow wave structure (SWS) increases stepwise to keep conformal to the trajectory of the electron beam which will alleviate the bombardment of the electron on the surface of the SWS. The length of period of the SWS is reduced gradually to make a better match between phase velocity and electron beam, which decelerates continually and improves the RF current distribution. Meanwhile the modulation field is reduced by the introduction of nonuniform SWS also. The particle in cell simulation results reveal that a microwave with a power of 1.8 GW and a frequency of 14.7 GHz is generated with an efficiency of 47% when the diode voltage is 620 kV, the beam current 6.1 kA, and the guiding magnetic field 0.95 T.

A low magnetic field relativistic backward wave oscillator with single mode structure is presented. Particle-in-cell simulation results show that 1.25 GW output power with 37% efficiency is generated under 0.88 T. The mode purity of the output signal is high because higher modes are cut off by the structure. According to the analytical results, the influence of bombardment of electrons on the surface of the slow wave structures is minor. A modulation cavity is adopted to enhance beam-wave interaction and realize mechanical frequency tunability. The power capacity is increased though redistribution of electric field. The computational results indicate that the device with a single mode structure is a competitive candidate for devices working at low magnetic field especially for devices focused with permanent magnet.