The present status and perspectives of seeded x-ray FELs

25.09.2012, 14:00

Dr Luca Giannessi, FERMI@Elettra, Sincrotrone Trieste S.C.p.A, Trieste, Italy

Free electron lasers in Self Amplified Spontaneous Emissions (SASE) represent a mature technology to achieve ultra high brightness X-ray radiation. Neverthelessseeded schemes, eventually combined with harmonic generation cascades (HGHG), may provide superior photon quality and improved performances. As in other amplified spontaneous emission lasers (ASE), the seeding technique allows to improve the longitudinal coherence properties of an FEL amplifier, mitigating the lack of the longitudinal modes selection associated to an optical resonator.

Seeding an FEL amplifier introduces a number of advantages beyond the natural reduction in saturation length and the improvement in longitudinal coherence. It is indeed possible to control the amplified pulse shape and length and to exploit the harmonic generation process in extending to higher frequencies the effects of the seed. Even in spectral ranges where seed sources are not available, "self" seeding scheems have been tested where the seed is obtained by passively filtering a fraction of the ASE generated by the laser.