Advanced Scheme to Generate MHz, Fully Coherent FEL Pulses at nm Wavelength

Current FEL development efforts aim at improving the control of coherence high repetition rate while keeping wavelength tunability. Seeding schemes, like HGHG and EEHG, allow for generation fully coherent pulses, but powerful external seed laser required limits that can be achieved. In turn, this impacts average brightness amount statistics experiments do. order to solve issue, here we take a unique approach discuss use one or more optical cavities electron bunches accelerated in superconducting linac modulate their energy. Like standard seeding cavity is followed by dispersive section, which manipulates longitudinal phase space bunches, inducing density modulations with harmonic content undergo process an amplifier placed downstream. We will technical requirements implementing these setups operation range based on numerical simulations.