| Bunch length is one of the most critical indicators of beam parameters of high-performance electron accelerators,and it has an essential influence on bunch stability and energy spread.Especially in the field of free electron laser(FEL),it is often necessary to obtain picosecond or even sub-picosecond ultra-short electron beams to produce strong coherence and high brightness laser.Therefore,the bunch length measurement is of great significance for understanding the operating status of such machines,and for optimizing the operating parameters.The accelerator team of Huazhong University of Science and Technology(HUST)set up a free electron laser terahertz device(HUST THz-FEL)in 2014 and successfully debugged the beam.Since then,a lot of experimental researches have been carried out.According to the experimental results,the beam tail generated from the electron gun is too long,and the loss rate of the beam current in the linac is about 20%.This not only reduces the power utilization but also brings hidden dangers to the safe and stable operation of the device.Simultaneously,the device lacks an effective bunch length measurement method,which brings great difficulties to machine research and beam optimization.This project intends to design a set of deflector systems for the downstream of the electron gun and the downstream of the linac:(1)The former is used to measure the longitudinal distribution of the beam from electron gun.The measurement results guide the microwave power adjustment of the two focusing cavities.At the same time,this system can truncate the beam tail to improve the efficiency of the downstream linac;(2)The latter is used to measure the longitudinal distribution of the beam from the linac,and cooperate with the undulator to produce high quality laser.Due to the symmetry of its own structure,the traditional cylindrical deflector leads to the appearance of degenerate modes.And it needs to additionally design dielectric rods or asymmetric holes to achieve mode separation.In order to solve this problem,the rectangular deflector is selected for design in this thesis.Due to the non-axisymmetrical of the transverse cavity section,the mode separation can be achieved naturally when the aspect ratio is not 1.The design process is greatly simplified.This thesis' s main contents include the design of rectangular cavity structure,beam dynamics simulation,test cavity machining and cold test.In the cavity structure design,singlecell deflector and 2-cell deflector are designed for the low-energy segment and high-energy segment,respectively.Comprehensive considering the impact of beam transmission pipes,couplers and other key devices to optimize the cavity parameters.The simulation shows that when the feed power is 1 MW,the deflecting voltages of the single-cell cavity and 2-cell cavities are 1.20 MV and 1.65 MV,respectively.In the beam dynamics simulation,the process of the beam passing through the deflector,scraper and linac is simulated.The simulation results show that the bunch length and energy spread are optimized after the beam chopper,and the measurement error of bunch length in the high energy section does not exceed 5%.In the test cavity machining and cold test experiments,key devices such as test cavity,perturbation body and bracket were designed,and an experimental platform for measuring the field distribution of the perturbation method was built.Taking the deflecting voltage as the standard,the error between the measurement result and the simulation result is less than 1.3%. |
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