Abstract
Free-electron lasers (FELs) are fourth-generation light sources that deliver extremely high intensity, ultrashort light pulses over a broad wavelength range from far-infrared to hard x ray. FELs based on the self-amplified spontaneous emission principle have been successfully operated with ultrahigh brightness and a broad wavelengths tuning range with good transverse coherence but poor temporal coherence. In contrast, the laser-seeded FELs have provided full coherence but at selected central wavelengths, usually the harmonics of the laser seeds, with relatively narrower tuning range. We report the experimental demonstration of a high-gain harmonic-generation (HGHG) FEL that is continuously tunable over a wide range using the combination of optical parametrical amplification, variable-gap undulator, and harmonic selection, where the temporal coherence is preserved as confirmed with the Michelson interferometry. In order to achieve higher photon energies, the first try of cascaded HGHG with a fresh-bunch technique is also made at the Shanghai Deep Ultraviolet Free-electron Laser test facility. DOI: 10.1103/PhysRevSTAB.16.020704
Highlights
High-gain free-electron lasers (FELs), serving as high-intensity coherent light sources, are being actively developed around the world
Free-electron lasers (FELs) based on the self-amplified spontaneous emission principle have been successfully operated with ultrahigh brightness and a broad wavelengths tuning range with good transverse coherence but poor temporal coherence
We report the experimental demonstration of a high-gain harmonic-generation (HGHG) FEL that is continuously tunable over a wide range using the combination of optical parametrical amplification, variable-gap undulator, and harmonic selection, where the temporal coherence is preserved as confirmed with the Michelson interferometry
Summary
High-gain free-electron lasers (FELs), serving as high-intensity coherent light sources, are being actively developed around the world. To improve the temporal coherence of SASE-FEL radiation, various seeded FEL schemes have been proposed and demonstrated, including external seeding [15,16,17,18,19,20,21,22,23,24,25,26] or self-seeding [27,28,29] Another significant feature of an FEL, especially important for serving different users or element-specific. One weakness of this method is that dealing with energy chirp of the electron beam will change beam-related optics and many other parameters Another method by utilizing very wide-bandwidth laser seed is proposed and demonstrated [31,32,33,34,35]. Combining advanced OPA technology with variable-gap undulators and HGHG technique, a fully-tunable seeded FEL becomes possible and has been recently demonstrated at the Shanghai Deep Ultraviolet Free-electron Laser test facility (SDUV-FEL) [38]
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