Generation of a train of short pulses by means of FEL emission of a combed electron beam.

We present the experimental demonstration of a novel scheme for the generation of ultrashort pulse trains based on the FEL lasing from a multi-peaked eletron energy distribution. At SPARC we generated two electron beamlets with relative energy difference larger than the FEL parameter ? by illuminating the cathode with a combed laser, followed by a manipulation of the longitudinal phase space by velocity bunching in the linac. The SASE FEL radiation obtained by sending such beam in the undulator is analyzed by a FROG diagnostic revealing the double-peaked spectrum and temporally modulated pulse structure.