Study Of Mega-volt Electron Diffraction And Imaging Facility With High Space-time Resolution

Ultrafast electron diffraction and imaging facility is a powerful tool to probe the structural changes at the atomic scale,which has played an important role in physics,chemistry,material science,life science and other fields.The time resolution of kilo-volt ultrafast electron diffraction is still larger than 100 femtoseconds,the basic time scale of atom motion,which is mainly limited by the serious space charge effect.Mega-volt electron diffraction based on photocathode microwave electron gun can overcome the above limitation and improve the temporal resolution.Because of the higher charge density in electron beam,mega-volt electron imaging facility holds great potential to realize single-shot imaging.This dissertation is devoted to simulation optimization and experimental study of mega-volt ultrafast electron diffraction and imaging with high performance and it also explores the related advanced beam manipulation techniques.Based on the fact that the diffraction pattern had low signal-to-noise ratio and the diffraction peak intensity had large fluctuations for Mega-volt electron diffraction prototype facility built in2014,we optimized the dark current,electron detection efficiency,electron source stability,and spatial overlap between the sample and the electron beam to achieve better performance.With these efforts,high quality diffraction patterns were obtained and the laser-induced structural change dynamics of single crystal gold were recorded with high signal-to-noise ratio.In addition,we used the buncher cavity to compress the electron beam and utilized microwave deflector with the 1 femtosecond /pixel time resolution to successfully measure the ultrashort electron bunch with pulse width below 10 fs.The mega-volt ultrafast electron imaging simulation was performed by particle tracking code and the requirement on electron source was theoretically quantified for ultrafast electron imaging in a single shot.Based on the theory and simulation,we developed the first acceleratorbased mega-volt ultrafast electron imaging prototype facility in China.We obtained high quality image of nanostructures and thickness contrast of the carbon film.The spatial resolution of single and multiple imaging was 50 nm(standard deviation).As the first application of megavolt ultrafast electron imaging,we observed electron spatial density modulation at nanometer scale.Our results may have a strong impact on emerging mega-volt ultrafast electron imaging facility and compact intense x-ray sources.In view of the problem of the large timing jitter caused by the microwave compression,we proposed and demonstrated a novel mechanism of using space charge force to compress relativistic electron beam without introducing additional time jitter to improve temporal resolution of mega-volt electron diffraction.In order to obtain the picosecond electron beam with small energy spread,the nonlinear energy chirp of picosecond electron beam was compensated by a harmonic cavity in an active way.In the experiment of compensation of nonlinear energy dispersion with a passive wakefield structure,the quadrupole wakefield was measured and found to be in good agreement with theory.The research will promote the application of corrugated structures in many accelerator-based scientific facilities.