Precise laser-beam synchronization for Shanghai Deep-Ultraviolet Free-Electron Laser

A general laser-beam synchronization method based on coherent radiation measurement is introduced. Fundamental principles and detailed steps to achieve spatial and temporal synchronization are described. Sub-picosecond level synchronization between the seed laser and the electron beam has been achieved with this method at the Shanghai Deep-Ultraviolet Free-Electron Laser (SDUV-FEL) test facility, which lays a solid foundation for seeding experiments such as high-gain harmonic-generation (HGHG), echo-enabled harmonic-generation (EEHG) and cascaded HGHG. Moreover, longitudinal profiles of the electron beam under different conditions are measured and analyzed.