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In recent years, high-power microwave (HPM) generator with a low working magnetic field has caught more attention as this device has the merit of low energy consumption and the potential of using a permanent magnet package in the HPM system. However, the conversion efficiency of this device is generally low because the electron beam will fluctuate transversely and the modulation depth will decline when the guiding magnetic field is low. To obtain concurrently high-power, high-efficiency, and low operating magnetic field, a transit-time oscillator (TTO) with two bunchers is designed and fabricated. Two bunchers are adopted in the TTO to modulate the electron beam twice. By introducing the second buncher, the modulation depth of the electron beam increases by 23%, and the conversion efficiency increases from 25% to 40.5% in simulations. A microwave with a power of 3.65 GW and a frequency of 4.31 GHz is obtained with a 600-kV, 15-kA electron beam under the guidance of a 0.5-T magnetic field in simulations. Furthermore, the corresponding experiments are carried out to verify the two-buncher TTO, and a microwave with a power of 3.3 GW and a frequency of 4.31 GHz is obtained. The experimental efficiency is 32% with an external magnetic field of 0.48 T. [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]

A high efficient relativistic backward wave oscillator with coaxial nonuniform slow-wave structures (SWSs) and depth-tunable extractor is presented. The physical mechanism to increase the power efficiency is investigated theoretically and experimentally. It is shown that the nonuniform SWSs, the guiding magnetic field distribution, and the coaxial extractor depth play key roles in the enhancement of the beam-wave power conversion efficiency. The experimental results show that a 1.609 GHz, 2.3 GW microwave can be generated when the diode voltage is 890 kV and the beam current is 7.7 kA. The corresponding power efficiency reaches 33.6%. [ABSTRACT FROM AUTHOR]