Thanks to the use of seeding, modern high gain FELs can now produce high power pulses with a longitudinal coherence much higher than normally available from SASE FELs. This possibility of fully coherent FEL pulses in the X-ray spectral range has several benefits for user’s experiment and also it opens the door to new experimental possibilities such as coherent control experiments. The achievement of a full coherence in the FEL pulses does not only requires a coherent seed but also needs an electron beam whose properties does not change over the length of the final FEL pulse. For this reason, requirements for electron beam quality in seeded FELs are significantly higher than for SASE FEL. Starting from the FERMI experience we report in the talk examples of small electron beam perturbations that have a large impact on FEL properties.
"High quality electron beams for high quality FEL", Proc. SPIE 10237, Advances in X-ray Free-Electron Lasers Instrumentation IV, 102370F (23 May 2017); doi: 10.1117/12.2268337; https://doi.org/10.1117/12.2268337