The SCSS test accelerator Free-Electron Laser seeded by harmonics produced in gas

Today, single-pass Free-Electron Lasers (FEL) produce a highly bright radiation, the Self Amplified Spontaneous Emission (SASE), which spectral and temporal profiles are composed of a series of spikes. We demonstrate here the strong and coherent amplification of the 5th harmonic of a Ti: Sa laser (800 nm, 10 Hz, 100 fs) generated in a gas cell, i.e. 160 nm, by the SCSS (SPring-8 Compact SASE Source, Japan) Test Accelerator FEL. This is obtained by overlapping transversally, spectrally and temporally the external harmonic source in the in-vacuum undulator with the electron beam (150 MeV, 10 Hz, 1 ps). With only one undulator section, the 160 nm seeded emission achieves three orders of magnitude higher intensity than the unseeded one, and presents a quasi perfect Gaussian shape in the spectral distribution. With two undulator sections, the seeded FEL spectrum reveals first effects of saturation. Those spectacular phenomena are associated to the generation of intense and coherent Non Linear Harmonics (NLH) at 54 nm and 32 nm. Finally, in view of the low seed level required, such amplification associated to NLH schemes would allow the generation of fully coherent soft X-ray radiations down to the water window