Electro-optic Methods Of Longitudinal Bunch Characterizing For High-brightness Electron Beam

The measurement of longitudinal bunch characteristics is one of the key diagnostics for the accelerator facilities driven by high-brightness electron beam.In short wavelength free electron lasers,the measurements of longitudinal profile for ultrashort bunches with a duration of 100 fs or less are the essential requirements to confirm the peak current and lasing process.Meanwhile,the jitter of beam arrival time affects the time resolution of pump-probe technique,the collision efficiency in a linear collider,and the stability of photon yield in Thomson scattering X-ray source.Electro-optic(EO)diagnostic technique can measure bunch longitudinal profile and arrival time in a single-shot and nondestructive way with a high temporal resolution.Based on the Tsinghua Thomson scattering X-ray source(TTX),this dissertation carries out extensive and in-depth research on EO method for longitudinal bunch characterizing both theoretically and experimentally.In the thesis,the spectrally resolved detection has been studied at first.We design and construct a compact,online EO diagnostic system based on spectral decoding,and realize the measurements of longitudinal bunch profile and beam arrival time in a single-shot and nondestructive way.The measurement results of bunch length and arrival time jitter are527 fs rms and 471 fs rms,respectively.We discuss the temporal resolution,the amplitude and stability of EO signal during the measuring process.The spectral width and chirp of the probe laser is found to be the major limitation of temporal resolution and improvement schemes are proposed,such as introducing the spatially resolved EO detection,employing probe laser with wider spectrum,using EO crystal with higher response frequency,and decreasing the distance between probe laser and electron beam.Spatially resolved detection is the second EO diagnostic technique studied in the thesis.Firstly,we analyze the relationship of signal amplitude and detection distance for the intrinsic disadvantage in the spatial decoding because of oblique incidence,which provides the basis on the correction of longitudinal profile.Then,we compare the measuring results and differences of zinc telluride and gallium phosphide,and the obtained bunch lengths are 106 fs rms and 82 fs rms,respectively.The response frequency cutoff of the EO crystal and the Coulomb field broadening of low-energy bunches are found to be the main limitations of temporal resolution in spatially resolved detection.Finally,we propose a method to eliminate the influence of field broadening and acquire the real bunch length,which receives a result of 60 fs rms.The accuracy of beam arrival time measurements in the spatially resolved detection is approximately 10 fs.By analyzing the Fourier spectrum of the time jitter and environmental conditions,we discriminate the influence of the temperature and humidity in the laser room,which are proved to be the main sources of the slow drift.Based on the results of arrival time in the spatially resolved detection,we accomplish a verifying experiment of feedback control to remove the slow drift,which decreases the jitter from 430 fs rms to320 fs rms,and the slow drift from 335 fs rms to 110 fs rms.