An $S$ -Band Long-Pulse Relativistic Backward-Wave Oscillator With Coaxial Extractor

The physical mechanism, simulation results, and main experimental results of an S-band long-pulse relativistic backward-wave oscillator (RBWO) with a coaxial extractor are presented. The central ideas of the device include: 1) dual resonant cavities replacing the cutoff neck are used to enhance the efficiency and to decrease the electric field; 2) nonuniform slow-wave structures (SWSs) are introduced in the device to improve the efficiency; 3) a coaxial extractor is well designed, which is beneficial to the extraction of generated microwaves; and 4) the diode and the collector are reasonably designed to reduce the influence of cathode and collector plasmas. In simulation, microwaves centered at 3.77 GHz are generated, with the power of 3.3 GW and efficiency of 43%. However, the pulse duration of the device does not exceed 70 ns owing to the pulse-shortening phenomenon in the initial experiment. It was found that the plasma caused by an intense radio frequency electric field on the surface of the coaxial extractor played a main role in pulse shortening. To suppress the pulse shortening, we focus on two aspects. One was to optimize the electrodynamic structures to reduce the electric field on the surfaces of the coaxial extractor while maintaining relatively high efficiency. The other was to improve the surface roughness of SWSs thereby increasing the breakdown threshold of the RF field. In the experiment after these measures were taken, microwaves centered at 3.752 GHz are generated, with the power of 2.3 GW, efficiency of 35%, repetition rate of 20 Hz, and pulse duration above 110 ns.