| At present,the application of high-power microwave technology is limited by the strong magnetic field system commonly used to guide high-power microwave(HPM)generator.Relativistic backward-wave oscillator(RBWO),as a typical representative of HPM generators,has the advantages of high power and high-repetitive operation,but the work efficiency is always low under low magnetic field,which becomes the bottleneck that restricts its development.In this paper,based on the basic configuration of Klystron-like RBWO,a high-efficiency RBWO with low-magnetic-field operation is proposed and studied in view of the low-magnetic-operation characteristics such as the wide beam envelope,low bunch current and insufficient energy extraction.Through theoretical research,numerical simulation and experimental study,the conversion efficiency of C-band RBWO with low-magnetic-field operation is remarkably raised.One of the focus of this thesis is to understand the law of low-magnetic-field beam and to control it effectively.On the one hand,from the view of the beam oscillation phase,the mechanism of the beam phase affecting the envelope expansion state in the strong microwave field is revealed through theoretical analysis.By controlling the beam phase entering the resonant reflector,the beam energy spread in RBWO is reduced,and the device efficiency is increased by 8 percentage points.On the other hand,from the view of the beam amplitude,the physical connotation of the amplitude of beam envelope affecting the efficiency of RBWO is given through three-dimensional nonlinear theory,and a tube head structure with anode cavity is put forward.The amplitude of beam envelope is reduced effectively by locally enhanced radial electric field,and the beam bunching is promoted,which enhanced conversion efficiency by 5 percentage points.Another focus of this thesis is the local and global control of RF field.On the one hand,the resonant field in the extraction cavity is enhanced by introducing the waveguide cavity structure which provides partial microwave reflection,and the concentrated transit radiation is enhanced,and the efficiency is increased by 5 percentage points.On the other hand,the RF power distribution in the device is controlled by non-uniform slow wave structure and internal reflector,which makes the energy extraction more concentrated in the extraction cavity and gives full play to the advantage of klystron-like RBWO energy concentration extraction.Combined with the above-mentioned design idea,the device design and particle-in-cell simulation of high efficiency RBWO with low magnetic field in C band are carried out.With a guiding magnetic field of 0.32 T,when the diode voltage is 820 kV and the current is 15.5 kA,a microwave with frequency of 4.36 GHz,power of 5.3 GW is generated in the simulation,indicating a conversion efficiency of 42%.Based on TPG 2000 pulse power generator,the relevant experimental research was carried out.After intense optimization of experimental structures,a microwave with frequency of 4.4 GHz,power of 5.0 GW is generated in the experiment,with a guiding magnetic field of 0.42 T,a diode voltage of 815 kV and a current current 18.5 kA,indicating a conversion efficiency of 33%.This is a record of high power and high efficiency of HPM generator with low magnetic field in the same frequency band.At last,based on the experimental results,a permanent magnet for the RBWO operation is designed,and the system scheme of a 5 GW-level permanent magnet packaging HPM source is given. |
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