Extreme ultraviolet free electron laser seeded by high-order harmonic

• The 13th harmonic of a Ti:sapphire laser has been significantly amplified with the SCSS test accelerator.
• The temporal and spatial overlap of the electron beam and the high harmonic radiation were precisely tuned for achieving the seeded free electron laser operation.
• With numerical simulation the present HH-seeded FEL scheme could readily be applied to the wavelength region down to ~10 nm.

The 13th harmonic of a Ti:sapphire (TiS) laser in the extreme ultraviolet region, which was efficiently generated with a loose focusing geometry, was injected as a seeding source to a 250-MeV free-electron-laser (FEL) amplifier. When the amplification conditions were fulfilled, strong enhancement of the radiation intensity by a factor of 650 was observed. The random and uncontrollable spikes, which appeared in the spectra of the Self-Amplified Spontaneous Emission (SASE) based FEL radiation without the seeding source, were found to be suppressed drastically to form to a narrow-band, single peak profile at 61.2 nm. The properties of the seeded FEL radiation were well reproduced by numerical simulations. We discuss the future precept of the seeded FEL scheme to the shorter wavelength region.