Investigation On Virtual Cathode Oscillator With Electron Beam Modulation

Among numbers of high power microwave (HPM) sources, the virtual cathode oscillators (VCO) have many special merits, such as, its simple structure, high peak power ability and the allowance of rather poor electron-beam (e-beam) quality and so on. These characters make it easy for them to be developed into a convenient, reliable and compact HPM sources. The research of VCO attracts extensive attention of scientists home and abroad, especially scientists working in the military science-technology field. Kinds of virtual cathode oscillators-, reflex triodes, reditrons, coaxial vircators, resonant cavity frequency locking vircators, multy-vircator phase locking devices, etc., appeared with some improved performance during the last two decades. Nevertheless, due to the beam-microwave interaction space is general asymmetric and the processes of beam-wave interaction are quite complicated with high-order nonlinearities, up to now, a conclusive theory which could describe the physical processes in detail and could give out a universal scaling law of the VCO devices has not been obtained. The pretty low beam-microwave transmission efficiency the frequency shift from shot to shot the uncertainty of the output microwave power are still the main drawbacks of the VCO type HPM sources. Recently VCO researchers around the world made great efforts to understand the physical mechanism of the beam-wave interactions further and to think out of ways to make the interaction processes controllable. As an effort of studying the inherent physical laws and improving the performance of the VCO type devices, two kinds of vircator with e-beam modulation was studied theoretically, experimentally and by particle simulation in detail in this dissertation.In theory, based on the time changing characters of the ratio of the penetrated electron current density to the injected current density changed with time, the main microwave generation mechanism in the virtual cathode oscillator was analyzed. The oscillating electric field modulates the injected beam and extracts energy from it, the reflected electrons are propitious to the gain of microwave, the transmitted electrons are unfavourable to gain of microwave in general. Microwave could develop in two or more frequency bands; the main frequency is in the low frequency band. On the base of analysis of the factors which affect the beam-wave interaction, it was found that modulation of the injected electron beam could improve the performance of the VCO type devices, therefore two kinds ofvircators with injecting beam modulation were brought forward.By employing a 2.5D particle-in-cell code, which involves the space-charge-effect and the nonlinear beam-wave interactions, the microwave develop processes in VCO were simulated. According to the phase relation of electron beam to electric field in different regions and the electron beam distribution, it was found that microwave could gain only in the region between anode and virtual cathode, the reflected electrons gave energy to microwave and the transmitted electrons obsorbed energy from microwave. A coaxial extraction VCO with two gaps was simulated; the physical process of the beam-wave interaction and the dependence of the microwave radiation on the structure parameters were obtained. The results of simulation showed that at the beam parameters of 520kV and 13kA, a micowave output with peak power of 2.2GW, average power of 1.03GW could be got, the average efficiency reached to 15.2%, and the frequency was about 3.66GHz.A transverse extraction VCO with two gaps was simulated by a 3D particle-in-cell code. The physical picture of the beam-wave interaction and the dependence of the microwave radiation on the structure paramenters and the electron beam paramenters have been obtained. The results of simulation showed that at the beam paraments of 720kV and 13.8kA, a microwave output with peak power of 2.8GW, average power of 1.15GW was obtained; the average efficiency could reach 11.6%. The mode in the output window was TE10, and the frequency was in S band.