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Photocathode laser pulse shaping is a crucial technology for enhancing the performance of X-ray free-electron lasers by optimizing the quality of electron beams generated from photocathodes within high-gradient radio frequency guns. By precisely shaping these laser pulses, it is possible to significantly reduce the transverse emittance of produced electron bunches. The optimal pulse shape is an ellipsoidal distribution, commonly referred to as the Kapchinskij–Vladimirskij profile. A pulse-shaping scheme utilizing a commercial Yb:KGW laser operating at 514 nm with a repetition rate of 1 MHz and duration of 260 fs has been developed for generating electron bunches with high peak and average power. This study presents the experimental realization of ellipsoidal pulses via three-dimensional amplitude shaping using spatial light modulators at 514 nm, followed by conversion to UV (257 nm) suitable for Cs 2Te photocathodes. The preservation of pulse shape and a high conversion efficiency during this process are investigated and our experiments pave the way for future emittance minimization for X-ray free-electron lasers.

A key issue of X-ray free-electron lasers is the quality of the photoelectron beams generated from a photocathode by laser pulses in the high-gradient RF gun. Controlling the shape of these laser pulses can strongly reduce the transverse emittance of the generated electron bunch. For this purpose, a laser pulseshaper at 515 nm is presented that can be used directly with alkali antimonide photocathodes for photoemission. The first results regarding generation and measurement of flattop and parabolic pulses as well as introduction of modulations for THz generation are presented and show the potential for emittance optimization with 3D ellipsoidal pulses with the proposed pulseshaper. The experiments are carried out for Cs2Te photocathodes, which require second harmonic generation of the shaped pulses and thus allow investigation of pulseshape preservation in nonlinear frequency conversion processes.