Nonlinear harmonic generation in a free-electron laser seeded with high harmonic radiation

Seeding a Free-Electron Laser (FEL) drastically improves the temporal coherence of the self-amplified spontaneous emission and reduces the saturation length, even with very low-intensity seed. We report here on the achievement of single-shot nonlinear harmonics from the 2nd to the 7th order at 23 nm, on the FEL of the SCSS Test Accelerator operated at 150 MeV and seeded at 160 nm with harmonics generated in gas. In particular, a clear intensity enhancement leading to saturation effect observations and temporal coherence improvement for both odd and even harmonics are highlighted due to the seeding. Single-shot measurements both in spatial (vertical) and spectral domains are compared to analytical theory and numerical simulations.