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Abstract
Abstract High-gain harmonic generation (HGHG) is effective to produce fully coherent free-electron laser (FEL) pulses for various scientific applications. Due to the limitation of seed lasers, HGHG typically operates at a low repetition rate. In this paper, a harmonic-enhanced HGHG scheme is proposed to relax the peak power requirement for the seed laser, which can therefore operate at megahertz and a higher repetition rate. Moreover, the setup of the scheme is compact and can be adopted in an existing single-stage HGHG facility to extend the shortest achievable wavelength. Simulations show that FEL emission at 13.5 nm (20th harmonic) can be obtained with a 270 nm, 1 MW (peak power) seed laser.
Highlights
High-brightness, highly stable, and fully coherent pulses from high repetition rate free-electron lasers (FELs) in the extreme ultraviolet (EUV) and X-ray spectral regimes are capable of driving scientific applications [1,2] such as timeresolved coherent spectroscopy [3], photon scattering [4], and coherent control [5,6]
We study a compact harmonic-enhanced high-gain harmonic generation (HGHG) scheme to further extend the highest harmonic for FEL emission in a high repetition rate, single-stage HGHG
Another scheme with a similar layout is the double-stage harmonic cascade HGHG demonstrated at the FERMI FEL to extend the wavelength coverage of HGHG [34]
Summary
High-brightness, highly stable, and fully coherent pulses from high repetition rate free-electron lasers (FELs) in the extreme ultraviolet (EUV) and X-ray spectral regimes are capable of driving scientific applications [1,2] such as timeresolved coherent spectroscopy [3], photon scattering [4], and coherent control [5,6]. Compared with self-seeding schemes, external seeding schemes, including high-gain harmonic generation (HGHG) [15,16,17] and echo-enabled harmonic generation (EEHG) [18,19,20,21], can generate FEL pulses with lower energy fluctuation They provide extra capabilities to be highly synchronized with the external laser and to control the coherence in multi-color, multi-pulse implementations [22,23]. Compared to a regular HGHG, an additional modulation undulator resonant at a high-harmonic wavelength is introduced, which essentially causes efficient electron density modulation up to the 20th harmonic wavelength, but with relaxed peak power requirement for the seed laser and reduced electron energy spread growth. Since the electrons are bunched at a high harmonic of the seed laser wavelength, the density modulation can be achieved via a short dispersion section This ensures the compactness of the proposed scheme.
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