Two color experiments at SPARC: external and intra-bunch seeding

The FEL-SPARC operation with two bunches at different energies has been characterized using different tools. The effect of the e-beam matching on the SASE operation has been explored by controlling the transport with an ad hoc developed control system, allowing either the transport in matching conditions and in different configurations. Since the two bunches have, at the entrance of the undulator, different phase space characteristics we used this effect as a knob to enhance or suppress one or the other FEL mode. We investigated the effect of seeding on both frequencies and observed beating waves, furthermore we observed effects of intra-bunch seeding when only one of the modes is seeded.