The Study Of Ultrashort Electron Pulse Generation And Structural Dynamics Based On The RF Compression Ultrafst Electron Diffraction

Combing the ultrashort electron pulse and ultrafast laser techniques,Ultrafast Electron Diffraction(UED)is well developed recently for directly observing the atom motions in space and time.In UED,the femtosecond laser excites the structural dynamics and then after a variable time delay,the keV-MeV electron pulse diffractes the corresponding structural change.With sub-picometre spatial resolution and few hundred femtosecond temporal resolution,UED has been extensively explored in all kinds of physical,chemical and biological studies.The Coulomb interaction dominated keV electron pulse based UED can work as a substitution for X-ray free-electron lasers(X-FEL)diffraction with lower price,smaller apparatuses and sub-milliangstrom spatial resolution,however,the temporal resolution reported in literatures is>200 fs due to the Coulob interaction among electrons and the laser-RF synchronization jitter,which needs to be improved.In this dissertation,we have home-built the RF compression based keV high brightness electron source UED,and then analyzed the temporal resolution of our system and the temporal resolution limit of this technique.A series of experiments on structural dynamics on phase change material have been carried out with the UED in our lab.The main contents are summaried as follows:(1)We have finished the building,the performance testing and partial optimization of the RF compression based keV high brightness electron source UED in our lab.The synchronization jitter in our system is 1.2 ps FWHM,and after compression,the electron pulse duration is 300-400 fs FWHM,then the temporal resolution is 2.3 ps FWHM for 105 electrons per pulse.Considering the influence of the electrode field emission and dielectric surface flashover induced breakthrough,we have designed the structure optimized DC high voltage electron gun.After processing,the electron gun can work at 70-80 kV and field strength 7-8 kV/mm.We have precisely simulated the electron pulse propagation and controle in the system with the softwares of GPT and POISSION SUPERFISH,and these simulations are uitilized to analyze the experiment results and system optimization.(2)We have found and demonstrated the Coulomb interaction induced jitter amplification effect for the first time,and pointing that the Coulomb interaction in mulitielectron pulse makes the arrival timing jitter at the sample position larger than the laser-RF synchronization jitter(the relative timing jitter).We provide the clear mechanism analysis and the optimization method for this jitter amplification effect,and we also present the unavoidable and the general existence of this effect in mulitielectron pulse where laser-electron pulse synchronization is needed.(3)We have explored the temporal resolution limit in RF compression based keV high brightness electron source UED through simulation.The influence from transverse coherence length,pulse duration,synchronization jitter and Coulomb interaction to the electron pulse compression are collected,and novel jitter correction method is provided.Base on the compression and jitter correction methods in this simulation,the temporal resolution can reach sub-10 fs.(4)We have presented and demonstrated utilizing the laser pulse shaping technique to generat self-compression ultrashort electron pulse for the first time.Combining the tuning of the electorn band structure in novel heterostructure photocathode and the direct optical transition model of the photoelectric effect,the initially chirped femtosecond laser pulse will produce energy chirped photo-electron pulse,then the chirped electron pulse will generate self-compression.This self-compression can be used for high temporal resolution UED.(5)We carried out the structural dynamics in phase change material Ge2Sb2Te5 with the UED system in our lab,including the femtosecond laser induced crystallization,amorphization and transient structural dynamics.The detailed analysis have been made for the femtosecond laser precisely controlled crystallization process,the textured structure in the crystalline phase during amorphization and the transient thermalization dynamics which violates the Debye-Waller effect.