Sub-picosecond Electron Bunch Length Measurements Using Coherent Transition Radiation At SXFEL

As a fourth-generation light source,Shanghai X-ray Free Electron Laser(SXFEL)facility promises to generate ultra-short pulses of intense X-ray radiation,which are fully-coherent and widely-tunable.These incomparable features will make SXFEL a powerful tool in solid state physics,material science,bioscience,and many other elementary scientific research.Electron beam peak current is one of the most demanding parameters for excellent FEL performance.In practical experiments,electron bunches will pass a series of bunch compressors before they enter the radiating undulator.Thus a high-resolution measurement of sub-picosecond electron bunch length is necessary for both beam characterization and bunch compressor parameter settings.Transverse RF deflecting structure(TDS),Electro-optical(EO)sampling and coherent radiation measurement are three standard diagnostics tools for FEL electron bunch length measurement.Coherent radiation measurement is based on the detection of the radiation emitted by electron bunches under different conditions.Coherent transition radiation(CTR)and coherent synchrotron radiation(CSR)are the most used radiation sources.Compared with other time domain measurements,CTR-based measurement is a frequency domain technique,which can achieve a high time resolution in an easy and economical way.In this thesis,the fundamentals of CTR-based bunch length measurement are systematically studied and a measuring system for SXFEL was designed.The spectral analysis showed that this measuring system could measure the electron bunch length from 20 fs to 3ps effectively.By detecting the interferogram of CTR generated by electron bunches with a THz interferometer and a Golay Cell(spectral range: 0.02-20THz)at SXFEL,longitudinal electron bunch length was successfully measured.Detailed measuring process and data analysis were discussed.In order to check the reliability of measured results,the electron bunch length was also estimated based on dispersive strength of the bunch compressor and the energy spread of electron bunches that were measured in the experiments.The comparison showed that the measured bunch length met very well with the estimated bunch length.Furthermore,the reconstruction of longitudinal beam profile based on CTR experiments was also studied.With a standard infrared light source,the frequency response of the measuring system could be obtained and then the bunch form factor could be determined.Longitudinal beam profile could be reconstructed by recovering the phase information by Kramers-Kronig analysis of the form factor.Since CTR-based measurement was a multi-bunch detection,a novel wavelength-domain technique that could fulfill both requirements of single shot and high resolution reaching to the femtoseconds range was introduced in the end.