Study On Millimeter-wave Gigawatt-level Relativistic Klystron Amplifier

The relativistic klystron amplifier?RKA?has significant applications,such as high energy accelerator,plasma heating and other fields,due to its unique characteristics,such as high peak power,high efficiency,and stable amplitude and phase.With the continu-ous development of high power microwave technology,the higher requirements are put forward to the output power and frequency of the RKA.In order to obtain a higher Pf²,the investigation of the millimeter-wave gigawatt-level RKA is significant.In order to extend the operating frequency of the RKA to Ka band,a coaxial high frequency structure is adopted to transmit high electron beam power.The oversized coaxial high frequency structure is proposed to enlarge the power capacity of the device.Therefore,the thesis proposed a Ka band oversized coaxial relativistic klystron amplifier?OCRKA?.The key techniques such as theoretical analysis,physical design of the high frequency structures and the input and output couplers,suppression of self-oscillation of the axial mode cou-pling and the angular high-order mode are investigated.The research results give a new way for the development of millimeter-wave high power source.The main research con-tents and conclusions of the thesis are as follows:1.Based on the formula of electronic coupling coefficient without gate clearance,the electron coupling coefficient of coaxial resonator gap is established by considering the electric field distribution and the boundary conditions of the coaxial resonator.The electron coupling coefficients of coaxial and cylindrical resonator are compared.Based on the one-dimensional small signal and the single electron motion theory,we adopt some assumes as:the coaxial resonator is simplified to one-dimensional without gate clearance and the beam-wave interaction is a perfect process in the gap.The analytic formula of the electron load conductivity for different axial electric field distributions in the three-gap and the four-gap resonators are obtained.We analyze the characteristics of the electronic load conductivity for the three-gap and the four-gap coaxial resonators,respectively.Thereby,we obtain the gap transit angle of the optimal beam interaction of each mode,the compet-itive relationship between the modes and the basic conditions of the single-mode work.2.We studied the basic characteristics of the coaxial resonator and general disci-plinary conclusions of it in the millimeter-wave band.The dimensional sensitivity of the coaxial resonator structure is studied,which provides the data for the error control and the experimental debugging.The physical design and particle simulation optimization of diode,input cavity,middle cavity and output cavity are carried out,respectively.The structure of diode is optimized to improve the efficiency of electron beam introduction to drift tube and reduce the transmission instability of electron beam,and the efficiency of electron beam transmission is greater than 90%when the length is about 300mm.Accord-ing to the coupling matching condition of the input cavity,the optimum coupling extenal quality factor is about 224 by theoretical analysis and simulation,which realizes the match-ing between input cavity and electron beam and the absorption rate of input microwave power is greater than 96%.The effect of the idler cavities with different gap numbers,such as one-gap,three-gap and four-gap idler cavity on electron beam modulation,is ana-lyzed.The research results show that the modulation ratio of the electron beam can reach125%after passing through the input and two idler cavities.A non-uniform periodic out-put structure is proposed to solve the problem of high efficiency beam-wave interaction,by which the output power of 1.18GW is realized.The influences of the distance between the resonant reflector and the resonator on the reflection efficiency of the resonant reflec-tor and on the resonant frequency of the resonant cavity are studied.A mode converter(the coaxial TEM mode to circular waveguide TM₀₁mode)and a high power cone horn antenna at Ka band are designed for next experiment.3.The two-dimensional?2D?particle simulation?PIC?method is used to simulate the main parameters affecting the performance of the device,and the parameters are opti-mized to realize the best performance of the OCRKA.Under the condition of the electron beam voltage of 525kV,the beam current of 6kA,the input frequency of 30.5GHz and the input power of 1.4kW,the 2D PIC simulation obtains the output power of 1.17 GW,the electronic efficiency of 37.1%,and the gain of 57dB.According to the optimized struc-ture,the whole tube simulation model with the actual input/output couplers is established.The 3D simulation is used to the 2D and the results show that the output power is up to880MW and the saturation gain is 58dB.4.The self-excitation oscillation mechanism of the coupling between the cavities is studied,and the inhibition method is proposed.The results show that the introduction of attenuating materials between the cavities can effectively suppress the self-oscillation phenomenon of the coupling between the cavities.The self-excited oscillation of angular asymmetric mode can be suppressed by the excitation angular uniform operating mode in the input cavity.In order to realize the angular uniform mode excitation of the input cavity,an internal coaxial input system is proposed,by which the angular amplitude uniformity of the input cavity reaches 98.86%,and the phase fluctuation is less than 0.06°.In order to provide a channel for the input waveguide in the coaxial device,a novel mode converter?TEM mode-sector TE mode-TEM mode?is proposed.5.The preliminary "cold" and "hot" test were carried out.Firstly,the resonant fre-quency of each cavity is debugged to meet the requirements,and the input/output couplers are cold tested and debugged.Secondly,the electron beam generation and transmission experiments are carried out,in which the electron beam flux rate reaches 91.3%with the beam current of 5.02kA,the diode voltage of 478kV and the focusing magnetic field of0.8T.Finally,we made a preliminary hot test and the results verify that the device operates in the amplifier state with the output power of the OCRKA of 775.6 kW.