Generation of a multipulse comb beam and a relaive twin pulse fel

A radiofrequency electron gun driven by a train of laser pulses joined to a compressor generates trains of THz- subpicosecond e- pulses. Assuming a prompt electron emission, the laser train generates a train of electron disks at the cathode, then the disk train evolves towards a slug with a slight density modulation but also with a peculiar sawtooth energy modulation. This kind of energy modulation is transformed into a density modulation by a buncher recovering at a good extent the initial intensity beam profile. The THz electron beam formation and the FEL interaction are studied, from start-to-end, by simulations for the SPARC machine, with two electron pulses generated through that mechanism.