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Compact is a significant research direction of low-frequency high-power microwave devices. In this paper, a compact S-band relativistic backward wave oscillator is presented. The radial dimension of the device is reduced by utilizing the characteristic that the cutoff frequency of the fundamental mode (quasi-TEM mode) of the coaxial Slow-Wave Structure (SWS) is zero. Moreover, a coaxial extraction structure is designed to enhance its resonance characteristics to reduce the number of slow-wave structures. In addition, a reflective structure with the phase difference between inner and outer conductors is used, which replaces the pre-reflection cavity and shortens the axial size of the device. A relativistic backward oscillator with a 2.5-period coaxial slow-wave structure is proposed through theoretical analysis, simulation calculation, and experiment. Its radial dimension is 160 mm (∼1.28λ), and its axial dimension is 320 mm (∼2.56λ). In the experiment, the results show that the output microwave has a center frequency of 2.41 GHz, a power of 1.5 GW, a pulse width of 92 ns, and continuous operation for 20 s at a repetition of 10 Hz. [ABSTRACT FROM AUTHOR]

The energy consumption of high-power microwave sources has attracted much attention in the research field of high-power microwaves. One way of decreasing the energy consumption is to reduce the guiding magnetic field. In this work, a high-efficiency transit-time oscillator is proposed with a low guiding magnetic field. In order to enhance the modulation depth of electrons and increase the conversion efficiency under a low guiding magnetic field, two bunchers are adopted to modulate the electron beam. 2.5D simulation results show that the proposed device generates microwaves with a power of 3.6 GW and a frequency of 4.31 GHz. The power conversion efficiency is 40% and the modulation depth is 125% when the guiding magnetic field is only 0.5 T. In addition, a 3D simulation verifies the results. [ABSTRACT FROM AUTHOR]