Seeding Free Electron Lasers with High Order Harmonics Generated in Gas

Abstract

Short wavelength Free Electron Lasers (FEL) may operate in the Self Amplified Spontaneous Emission (SASE) configuration, where the spontaneous radiation originated by the electron shot noise is gradually amplified. This generally leads to an output pulse consisting of uncorrelated temporal spikes with a limited longitudinal coherence length. When a FEL amplifier is seeded by a coherent light source, the coherence properties of the seed are transferred to the initial electron-beam modulation, allowing for an improved control of the longitudinal properties of the emitted radiation. The spiky structure is suppressed both in the temporal domain and spectral domain and the bunching process becomes more efficient. Associated intensity and jitter fluctuations are reduced, as well as the arrival-time jitter. In addition the use of an external seed reduces the saturation length and therefore the associated costs of the device. High order harmonics generated in gas permit to seed a FEL amplifier in the vacuum ultra-violet, enabling the direct generation of seeded-Free Electron Laser radiation directly at short wavelengths. We analyze here the basic requirements for the seed source in terms of seed level versus the electron shot noise, and the problems of coupling of the seed source to the FEL. We also review the most recent achievements with HHG seeding, in particular on SCSS Test Accelerator, SPARC and SFLASH.