Design And Optimization Of Low Velocity Speed Magnetron Injection Gun And Characteristic Analysis

In vacuum electronic devices,gyrotron travelling tube(gyro-TWT)is a fast-wave device based on the instability mechanism of electron cyclotron maser,it can radiate high frequency,broadband and high-power waves,and is widely used in radar,communication and electronic countermeasures.The magnetron injection gun(MIG)is the energy source of Gyro-TWT.The beam-wave interaction efficiency can be improved with low velocity spread MIG.Therefore,how to design the MIG with low velocity spread is the design goal.It is helpful to reduce the velocity spread by analyzing the characteristic of MIG.In the thesis,on the basis of the theory and practice of the Ku,Ka,Q,and W band gyro-TWTs,the MIG of the X,K,V and G band gyro-TWTs is designed,which can produce reliable electron beam for other series of gyro-TWTs.The main work and innovations of the thesis are as follows:1.The assumption of adiabatic compression theory is deduced,the trade-off equation of MIG is derived from no space charge effect and adiabatic compression assumption.The genetic algorithm and the basic principle of objective function are introduced,which is commonly used in the optimization of MIG.Taking the V-band TE₀₁ mode gyro-TWT MIG as an example,the initial design parameters of the MIG are calculated and then simulation is performed.However,both the pitch ratio and velocity spread cannot meet the goal.Then the MIG is optimized by the single-objective genetic algorithm,and the electric field and the magnetic field are compared and analyzed.Finally,the MIG with pitch ratio of 1.27 and velocity spread of 1.9%is obtained.In addition,the MIG of a K-band gyro-TWT with a pulse output of 250 k W is designed.According to the requirements,the initial design parameter is calculated.A problem relates to magnetic mirror phenomenon was encountered in the simulation.After the pitch ratio is reduced,the problem is solved.Then,the multi-objective genetic algorithm is applied in optimization program.Finally,the MIG with pitch ratio of 1.21 and velocity spread of 1.9%is obtained.2.The X-band TE₁₁-mode gyro-TWT works in the high-power continuous wave state.According to the design requirements,the traditional MIG is designed and optimized.Due to the influence of space charge effect,the velocity spread of the electron beam cannot be reduced.By increasing the chamfering amplitude of the front and rear electrodes,the mismatch between the electric field and the magnetic field on the cathode surface is reduced,and the quality of electron beam is improved.However,the designed X-band MIG is bulky to be compatible with existing equipment and used in vehicular transmitter.Then a compact MIG with low magnetic compression ratio is designed During the design process,it was found that the electrons are more sensitive of to the electric field and magnetic field on the cathode surface with low magnetic compression.The traditional nose-shaped cathode is unable to reduce the mismatch value and the velocity spread,so a new smooth arc surface cathode was designed.Finally,a compact MIG with a pitch ratio of 1.3 and a transverse velocity spread of 2.4%was obtained.In order to determine standard parameters of contact thermal resistance and thermal emissivity,thermal simulation is carried out to approximating experimental data.Finally,the influence of the thermal expansion of the MIG cathode on the electron beam is simulated and analyzed.The simulation results show that the deformation has little effect on the electron beam performance.3.G-band Gyro-TWT has high frequency and wide bandwidth.However,the structure of the MIG is too small,which causes some difficulties such as high simulation accuracy,long simulation time,and strong space charge effect.In order to improve the simulation accuracy and reduce the simulation time simultaneously,a method of MIG-performance-prediction simulation method is proposed.The feasibility and effectiveness of the method are verified by simulation.In order to reduce the length of the MIG,the magnetic field was re-fitted to the actual magnetic data.In the design process,the cathode shape is discussed.After optimization in MIG-truncation-simulation method,the MIG with a pitch ratio of 1.34 and transverse velocity spread of 1.7%is obtained.Increasing the magnetic compression ratio is beneficial to increase the radius and reduce the space charge effect.A MIG with a curved emitter is designed.Finally,the MIG with a pitch ratio of 1.2 and transverse velocity spread of 1.7%is obtained.In the thermal analysis,the experimental data and simulation data are in good agreement.