Research On Radio Frequency Control System Of HUST MeV Ultrafast Electron Diffraction Device

Ultrafast electron diffraction device(UED)is usually used as a special probe to detect the ultrafast structural evolution of substances and materials by means of ultrashort electron bunches.It is a very powerful tool in physics,chemistry,material and biological sciences.In UED,the femtosecond laser pumps excite the structural dynamics of the specimen,and then the Me V electron pulse is used to detect the corresponding structural changes.The device's temporal resolution should achieve about one hundred femtoseconds so as to detect the changes at the atomic scale.To explore the key technologies of ultrafast electron diffraction devices,Huazhong University of Science and Technology(HUST)launched the design and construction of the UED platform in 2018.To make the UED meet the technical demands of a hundred femtosecond resolution,it is necessary to pay great attention to the radio frequency(RF)system of the HUST UED because the RF system is the core of the UED and its performance is bound up with the beam quality.In this paper,the performance of the control system is presented.Based on the bean dynamics,the model and the control algorithm of the RF control system are created in MATLAB.Furthermore,the water-cooled system is designed.The research is mainly divided into three parts:(1)The main components and working process of the RF system of the UED are introduced.Based on the amplitude and phase of the acceleration field in the HUST UED RF system,the main influence on the phase stability of the RF system is analyzed.During the analysis of the stability of the RF system,the influence of the fluctuation of the high-voltage pulse modulator output on klystron is analyzed by means of the klystron model.(2)Based on the simulation results of SUPERFISH and ASTRA,the 1.4-cell electron gun is compared with the 1.6-cell electron gun in terms of bunch emission,cavity acceleration gradient,space charge effect and brightness.The 1.4-cell electron gun is selected and optimized.The water-cooling structure is designed according to the electron gun,and the water-cooling parameters are estimated according to the UED operating parameters.The SUPERFISH simulation determines the heat source,and then the ANSYS Fluent thermal analysis model of the cavity is completed to verify the necessity of the water-cooling system.The aperture of the water-cooling pipe and the water speed is changed separately to adjust the design of the watercooling system,and the corresponding thermal analysis results are presented.Finally,the watercooling system parameters are selected,and the design of the three-way water-cooling pipeline is completed according to the aforementioned analysis.(3)The low-level Radio Frequency(LLRF)control system is designed to achieve the resolution of 100 fs.Based on the beam dynamic analysis and the ASTRA simulation,the control accuracy is determined,the error of amplitude and phase should be no more than ±0.01% and ±0.05° respectively.The LLRF control system is modeled and analyzed by MATLAB/Simulink,then the control parameters of the PI controller are designed,and control accuracy of the LLRF control system is verified.The digital control algorithms used in FPGA,such as coordinate rotation digital computer(CORDIC),PI control and finite impulse response(FIR)filter are written by means of Verilog HDL to handle the sampled data and generate the control signal.The RF system project design is completed.