Optimization Study And Application Exploration Of Mega-electron-Volt Ultrafast Electron Diffraction

Ultrafast Electron Diffraction(UED), as a probe for observing the ultrafast process, plays an important role in exploring the scientific frontiers of several fields such as physics, chemistry and material. In a Me V UED setup, the ultrashort electron bunchs are generated using a radio-frequency electron gun and decode structural evolution information with the diffraction patterns. Compared with the conventional ke V UED based on static electric accelerating field, the Me V UED has the advantages of higher charge and capability of probing thicker samples. This thesis studies the optimization of the Me V UED system, the designing and the construction of the system as well as the application exploration of the ultrafast physical processes.In the thesis, we conducted the optimization study focusing on two issues, the transverse spatial resolution and the longitudinal bunch compression. For the transverse spatial resolution issue, we employed the transfer matrix and proposed an bunch evolution model based on uniform three-dimensional ellipsoidal distribution to study the bunch dimensional evolution affected by factors such as the focusing field and the space charge effects. We also conducted the optimization study of the cathode laser using the particle tracking simulations. For the longitudinal bunch compression issue, we studied the velocity bunching scheme of the Me V UED bunch utilizing a linac and designed a coherenttransition-radiation based bunch length measurement method for the ultrashort bunch of low charge and low energy. To solve the problem that the transverse spot size reduces the radiation energy significantly, we conduct quantitatively study and import transverse focusing in the experiment. Based on the theoretical calculation, the experiment data implied bunch length shorter than 40 fs, which agreed the simulation results well.Guided by the theoretical optimization study, we designed and constructed a compact Me V UED setup. The setup is composed of several subsystems including a photocathode radio-frequency gun, a radio-frequency deflecting cavity, a laser system, a beam controlling and diagnostic system, a beamline controlling program and a data acquiring program.We tested the setup and obtained high-quality diffraction pattern of single crystalline gold in a single shot.Using the constructed Me V UED setup, we carried out pump-probe experiments to exploring ultrafast physics. Firstly, we studied the ultrafast process of laser-pumped metal needle using the electron shadowgraphy method and found the time zero of the pump-probe experiment. For the single crystalline gold sample, we conduct pump-probe experiment to resolving the structural evolution process in a 25-ps time window after laser pumping, we determined the decaying time constant of the diffraction pattern in the Debye-Waller effects. We also streaked the diffraction pattern after pumping using the deflecting cavity to study the continuously time-resolved mode of the Me V UED setup.These studies demonstrated the sub-picosecond timescale and atomic length resolving power of the Tsinghua compact Me V UED system in the pump-probe experiments.