Analysis of spectral quality of OK-HGHG seeded FEL

Seeded FELs offer excellent spectral brightness and stability. However, these benefits come at the cost of requiring a high seed laser power, which is hardly compatible with next-generation FELs operating at MHz repetition rates. To overcome this limitation a modified seeded scheme called Optical-Klystron High-Gain Harmonic Generation (OK-HGHG) has been recently proposed. Contrary to standard HGHG mode, where a strong seed laser modulation of the electron beam energy is required, OK-HGHG relies on the use an additional modulator undulator that combined to a second magnetic chicane can significantly reduce the requirements in terms of seed laser power. We experimentally studied the scheme at FERMI showing that efficient FEL generation can be achieved with a seed power reduction down to 3 orders of magnitude. In this work we report abut detailed studies on the spectral properties of the FEL when operated in this regime.