Design And Experimental Research Of High Quality Electron Beams In Terahertz Free Electron Laser Source

Terahertz free electron laser(THz-FEL)source utilizes high energy electron beams wiggling in an undulator to produce terahertz radiation.Compared to those traditional terahertz sources that base on gas lasing or quantum cascade lasing,the radiation power of THz-FEL source is usually 6 orders higher.Its average power can reach the level of watt or kilowatt and pulser power is usually beyond megawatt.The lasing medium of THz-FEL source is free electrons that get rid of nucleus,which makes its radiation wavelength tunable in a wide range.The merit of tunable wavelength and extremely high radiation power makes THz-FEL source great potential in the area of molecular biological detection,national defense security and astronomical observation etc.THz-FEL source is mainly composed of injector,undulator and optical cavity.Injector is responsible for providing electron beams for THz radiation.As FEL requires high quality electron beams in terms of emittance,energy spread,bunch charge etc.,the generation of high quality electron beams is one of the greatest challenges in the whole system.This thesis does both theoretical and experimental research on the design and realization of high quality electron beams in the THz-FEL facility built by Huazhong University of Science and Technology(HUST).The main contents of this thesis consist of four parts.The first part,including Chapter 2,is the theoretical basis of this article and introduces beam dynamic theory of the injector system.The second part,including Chapter 3 and Chapter 4,makes detailed analysis and optimization of beam dynamic process in the linac and the transport line.The third part,including Chapter 5,introduces beam-commissioning scheme of the THz-FEL facility and displays some important experimental results.The fourth part,including Chapter 6,is about the design of a RF chopper and proposes to deploy a RF chopper system after the electron gun to achieve short electron bunches.Longitudinal and transverse beam dynamics in the linac were discussed separately in Chapter 3.For the longitudinal beam dynamics,two novel algorithms,named current iteration and approaching(CIA)and macro particle tracking(MPT),were proposed to rigorously analyze beam loading effects in the linac cavity.Several energy test experiments have all shown that the difference between the results predicted by the novel algorithms and measured in the experiments was less than 1.4%,while the conventional method would give a value of 4%～5%.For the transverse beam dynamics,the effect of space charge force on the transverse motion was analyzed and a reduced beam envelope equation was proposed according to those related parameters of the THz-FEL facility.Based on the reduced equation,physical and technical design and off-line testing of the focusing solenoid was carried out.Testing results showed the difference between the designed and the measured distribution was less than 1% in the main region,and the uniformity was better than 0.5% in the paraxial region.Beam dynamics design of the transport line was presented in Chapter 4.Two important issues,beam matching and achromaticity,were analyzed in detail and some general equations referring to matching of Twiss parameters and double bend achromat(DBA)were derived.According to the requirements of the facility for Twiss parameters,the MAD code was used to calculate the matching results and parameters of the magnets.Besides,the influence of interference of quadrupoles’ fringe field and deviation of reference orbit on beam qualities were discussed.Experimental research referring to beam commissioning and diagnostics was discussed in Chapter 5.General ideas of beam commissioning were introduced,as well as principles and methods for measuring key beam parameters.Besides,some important beam diagnostic results were demonstrated based on the present commissioning level.It was shown that some parameters,such as energy,bunch length and bunch charge,were satisfied while energy spread and emittance still needed further optimization.Based on the theoretical and experimental results that had shown long bunch tail,several schemes were discussed to find a solution for short electron bunches.Finally,RF chopper was found to be the best choice according to the requirements and characteristics of the THz-FEL facility.Physical design referring to layout of the scheme and dimensions of the RF cavity was performed.Then both CST and Parmela code were used to simulate the effect of the RF chopper on beams.The simulation results showed that more than 78%of the beam tail could be removed effectively while preserving the head portion in acceptable level,which verified the validity of the RF chopper scheme.