The Studies On High Brightness Ultrafast Electron Generation And Diffraction

With the development and breakup of ultrafast laser science and technology, it is now possible to determine many ultrafast dynamic processes in physics, chemistry and biology fields in the four dimensions with sub-Angstrom spatial and sub-ps temporal resolution. For ultrafast electron diffraction, ultrafast laser pulses are used to initiate ultrafast dynamic process, and ultrafast electron pulses are employed to probe ultrafast dynamic process. By precisely adjusting the time delay between ultrafast laser and ultrafast electron pulses,4D molecular movies can be achieved.Because of Coulomb repulsion, it is difficult for traditional ultrafast electron gun to simultaneously achieve high temporal resolution and high brightness. Therefore, traditional ultrafast electron diffraction needs enough long integral time to gather diffraction pattern with high signal-to-noise ratio. This limitation not only reduces efficiency of experiment, but also increases difficulty for the irreversible dynamics. So, it is crucial challenge for ultrafast electron diffraction to overcome the Coulomb repulsion of high brightness electron bunches. A phase-clocked high intense radio frequency cavity system is applied to compress longitudinally high brightness electron bunches into the sub-ps pulse-width. The main contents are presented as the following,Firstly, the development of ultrafast electron diffraction and its related theory research have been reviewed, and their recent researches are introduced. Several different ultrafast electron diffraction systems for overcoming the electron Coulomb repulsion are analyzed and compared.Secondly, a reliable, stable and phase-clocked radio frequency system is introduced and employed to compress ultrafast electron pulse duration. On the other hand, DC-RF electron diffraction system has been introduced and discussed.Finally, ultrafast electron bunch is characterized and studied. Its brightness is measured by the Faraday cup and electrometer, and its pulse duration is measured by the auto-accelerator. The zero time delay between the ultrafast electron bunches and femtosecond laser pulses is determined by laser induced plasma on the silver needle tip.