First coherent photons from a double stage cascaded FEL with fresh bunch injection technique.

On October 11, 2012 the beam line FERMI FEL-2 has generated its first coherent photons. This is the first experimental demonstration of a seeded free electron laser configured as a two stages cascade operating in the "fresh bunch injection”  mode, where the second stage is seeded by the light produced by the first stage. In this configuration the photons in the second stage are generated from a fresh portion of the electron bunch, which has not been heated by the seed of the first stage, overcoming the problem of the induced energy spread which limits the order of the harmonic conversion in a high-gain harmonic-generation seeded FEL.
The first commissioning experiments were done at a final wavelength of 14.4 nm in planar polarization. This wavelength is the result of an harmonic conversion to the 6^th harmonic (43.3 nm) in the first stage and the 3^rd harmonic in the second (i.e., 18^th harmonic of the seed laser). The FEL pulse energy has been optimized to increase the flux to about 20-30uJ. Further tests at the final wavelengths of 10.8nm (4^th harmonic conversion in the second stage) and 8.7 nm (5^th harmonic conversion in the second stage) are in progress. The figure shows one of the single mode spectra obtained from FEL-2 at 14.4 nm.