ENEA C. R. Frascati, Frascati (Roma)
SASE FELs represent a mature technology to achieve ultra high brightness X-ray radiation sources. Nevertheless seeded FEL schemes, combined with harmonic generation FEL cascades, may provide superior photon quality and improved performances. The basic element of a harmonic generation cascade is composed by two undulator sections, where the first is resonant at w1 and the second is resonant at w2 = n · w1 with n integer. In this layout, the first section modulates the current density while in the second section the emission process at the selected higher order harmonic takes place. The underlining idea consists in the fact that the current density modulation, which occurs during the FEL optical amplification, has higher order harmonic components which are responsible of the enhancement of the higher harmonics emission and which generate radiation when the beam is injected in a second undulator with the central frequency tuned to match the density modulation periodicity. In this communication a review of the basic concepts on challenges and perspectives of alternative (to SASE) FEL schemes will be presented.
