Study Of Electon Bunch Longitudinal Profile Diagnosis Based On Terahertz Radiation From The Undulator

The measurement of longitudinal distribution of electron bunch especially the length of ultrashort bunch plays a crucial role in the operation of Thomson scattering based X-ray source,ultrafast electron diffraction(UED),short wavelength free electron laser(FEL),plasma wakefield acceleration,high power terahertz(THz)source,etc.At the present stage,the length of ultrashort bunch usually ranges from sub-ps-scale to fs-scale.Various bunch longitudinal distribution diagnostic methods have been developed based on multiple mechanisms while the measurement of the fs-scale ultrashort bunch still needs further development.To promote the diagnosis of electron bunch longitudinal distribution,this thesis systematically investigates the diagnositic method for ultrashort bunch and bunch train based on coherent undulator radiation both theoretically and experimentally.Regarding the single electron bunch,we infer the bunch length from the radiation energy curve from the widely tunable undulator.The measurement range and fitting error of Gaussian distribution bunch as a function of bunch energy is analysed from the parameters of the 8-periods permanent magnetic undulator.As the bunch energy increases,both the lower limit and the range of measurement decrease.For the 30 MeV Gaussian distribution bunch,the lower limit of the measurement is as short as 10 fs and the fitting error is less than 1 fs.We performed the experiments at Tsinghua Thomson X-ray linear accelerator(TTX).The bunch length varied from sub-ps-scale to tens of femtoseconds from the method based on coherent undulator radiation as the R56 of magnetic chicane changed,which agreed well with beam dynamics simulation and the method based on the fitting of the autocorrelation curve from coherent transition radiation(CTR)in time domain.We demonstrated the diagnosis of electron bunch length inferred from the coherent undulator radiation experimentally.We then devoted much efforts on the diagnosis of harmonics of bunch train form factor from the radiation energy curve experimentally.The high-intensity electron bunch train based on nonlinear longitudinal space charge oscillation was generated in TTX.The fundamental,the second and the third harmonics of bunch train form factor were obtained from the coherent undulator radiation energy curve,which agreed well with GPT simulation.The method based on the undulator was more sensitive for the measurement of thinner structure in electron bunch compared with the common CTR based method.In the thesis,we also developed a high power widely tunable THz radiation source based on the undulator.The radiation spectrum fully covered 0.4 THz to 10.0 THz with relative bandwidth about 10 %.The radiation pulse energy was proportional to the square of bunch charge from the measurement and pulse power reached as high as 10 MW.The high power widely tunable THz source has many potential applications in the frontier THz pumb and control researches.