| Improving the conversion efficiency of high-power microwave(HPM)sources and reducing the guiding magnetic field are important ways to reduce the energy consumption of the HPM system.Improving the beam-wave efficiency of the HPM source can reduce the electrical power for the pulse drive source,thereby reducing its volume and weight.The excitation system and cooling system of the low guiding magnetic field have smaller energy consumption and volume,which can further realize the compactness of the HPM system.This paper studies a C-band low-magnetic field and high-efficiency relativistic klystron oscillator(RKO).It focuses on solving the physical and technical problem that the beam-wave interaction efficiency is difficult to improve under low magnetic fields.The device is systematically studied by means of theoretical analysis,particle simulation and experimental research,and the design principles of this kind of device are summarized.The main content of the paper is as follows:First,the theories of coaxial transit devices are studied.The influence of the structural parameters of the coaxial waveguide on its space limiting current and power capacity is analyzed.The results show that increasing the device radius can increase the space limiting current and power capacity.Based on the small signal hypothesis,the coupling coefficient and load conductance of any standing wave field are deduced,and the mode selection method of the multi-gap resonant cavity is given.The characteristic impedance of the coaxial resonant cavity is deduced and its radial distribution law is analyzed.The results show that the characteristic impedance is the largest and the ability to modulate the electron beam is the strongest at the radial center of the coaxial resonant cavity.Secondly,combined with theoretical analysis and simulation,a two-stage modulation cavity and external collector structure are designed.The modulation ability of double-gap buncher and three-gap buncher under low guiding magnetic field is analyzed and compared.The results show that the three-gap buncher can increase the modulation depth of the electron beam by 18.8% compared with the double gap cavity under 0.5T guiding magnetic field.Through cold cavity analysis and simulation,a single-gap secondary buncher is designed to avoid the mode competition caused by too many modes.By introducing the second buncher,the electron beam modulation depth is increased by 23%.In addition,the inner extraction-outer collection structure is designed,which is beneficial to the heat dissipation at the collector.In view of the reduction of power capacity caused by the reduced radius,a narrow output port-wide output waveguide structure is designed,which effectively reduces the surface of the output waveguide field strength.Then,based on the particle simulation method,the whole tube simulation of C-band low-magnetic field and high-efficiency coaxial RKO is carried out.The simulation results show that under the constraint of 0.5 T external magnetic field,the device can achieve 125% electron beam modulation depth through twice modulations.Under the conditions of a diode voltage of 600 kV and a current of 15 k A,the output microwave in simulation has a power of 3.65 GW and a frequency of 4.31 GHz,and the maximum RF electric field on the device surface is controlled within 900 kV/cm.Moreover,the asymmetric mode is not observed in the three-dimensional simulation,which shows that the device can work normally.In addition,using the design idea of two-stage modulation,a higher power level C-band RKO is studied.In the simulation,the C-band coaxial RKO outputs a microwave with a power of 6.4GW under the constraint of a 0.5T magnetic field,and the conversion efficiency exceeds 40%.Finally,the simulation model is processed and preliminary experimental research is carried out.When the guiding magnetic field is 0.48 T,the voltage is 612 kV,and the current is 16.85 k A,the microwave output with a power of 3.3GW and a frequency of4.31 GHz,a pulse width of 25 ns is obtained in experiments,and the power conversion efficiency is 32%.Simulation and experimental results show that RKO with two-level modulation structure has the advantages of low magnetic field and high efficiency. |
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