Abstract
The echo-enabled harmonic generation (EEHG) scheme holds promising prospects for efficiently generating intense coherent radiation at very high harmonics of a conventional ultraviolet seed laser. We report the lasing of the EEHG free-electron laser (FEL) at an extreme ultraviolet (EUV) wavelength with a seeded FEL facility, the Shanghai soft x-ray FEL. For the first time, we have benchmarked the basic theory of EEHG by measuring the bunching factor distributions over one octave down to the EUV region. Our results demonstrated the key advantages of the EEHG FEL, i.e., generation of very high harmonics with a small laser-induced energy spread and insensitivity to beam imperfections, and marks a great step towards fully coherent x rays with the EEHG scheme.
Highlights
Free-electron lasers (FELs) that are able to provide tunable high-power coherent radiation have a wide array of applications in biology, chemistry, physics, and material science [1]
The accelerator of the soft x-ray FEL facility (SXFEL) consists of a photoinjector, a linac with S-band and C-band accelerator structures, and a magnetic bunch compressor
We have reported the first lasing of an extreme ultraviolet (EUV) FEL at 24 nm and the observation of coherent emission down to 8.9 nm with the enabled harmonic generation (EEHG) technique at the SXFEL
Summary
Free-electron lasers (FELs) that are able to provide tunable high-power coherent radiation have a wide array of applications in biology, chemistry, physics, and material science [1]. A HGHG FEL are sensitive to beam imperfections, in particular, linear and nonlinear energy chirps [29,30] These limitations may be overcome with the echoenabled harmonic generation (EEHG) technique [20,21], which employs two modulator-chicane modules to imprint strong bunching on the electron beam. The highly nonlinear phase space manipulation process inherent to the EEHG technique efficiently damps initial linear and nonlinear correlations in beam longitudinal phase space, leading to an enhanced insensitivity to beam imperfections These advantages have stimulated worldwide efforts in exploring the potential of the EEHG technique for producing fully coherent x rays in FELs. Initial proof-ofprinciple experiments demonstrated bunching up to the 15th harmonic [24,25,26] and lasing at the 3rd harmonic (∼350 nm) [27] with a seed laser in the far-infrared (FIR) wavelength. Comparing with HGHG, our experimental results clearly show the higher-frequency up-conversion efficiency and less sensitivity to beam imperfections of EEHG and pave the way towards coherent, intense, and stable x rays
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