The use of seeding allows modern high gain FELs producing high power pulses with a longitudinal coherence higher than normally available from SASE FELs. The 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.
Full coherence of FEL pulses does not only requires a coherent seed but also needs an electron beam whose properties do 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 electron beam perturbation that have a large impact on FEL properties.