The photoinjector laser system of Shenzhen Superconducting Soft X-Ray Free Electron Laser (S³FEL) is reported in this paper. This laser system operates at up to 1 MHz and produces more than 50 μJ infrared (IR) laser pulses. With a customized fourth harmonic generation (FHG) module, more than 2 μJ ultraviolet (UV) laser pulses were obtained. The power standard deviations of the IR laser and the UV laser are 0.093% and 0.395% respectively. While the pulse energy standard deviations are 1.087% and 1.746% correspondingly. We implemented the pulse stacking scheme to generate flat-top pulses. With four birefringent uniaxial crystals, the Gaussian pulses were converted to flat-top shape, featuring 10 ps pulse width and 0.5 ps rising and falling edges. A cut-Gaussian transverse profile with very sharp rising and falling edges can be produced after the spatial pulse shaper.

Various continuous-wave (CW) electron gun technologies are reviewed, including DC, superconducting radio frequency RF (SRF), hybrid DC-SRF and normal-conducting RF. Also, the SLAC Linac Coherent Light Source II (LCLS-II) normal-conducting RF gun and injector are described, and the performance to date, including the bunch emittance achieved and the dark current observed, is presented.