Abstract
The beam energy spread at the entrance of an undulator system is of paramount importance for efficient density modulation in high-gain seeded free-electron lasers (FELs). In this paper, the dependences of high harmonic bunching efficiency in high-gain harmonic generation (HGHG), echo-enabled harmonic generation (EEHG) and phase-merging enhanced harmonic generation (PEHG) schemes on the electron beam energy spread distribution are studied. Theoretical investigations and multidimensional numerical simulations are applied to the cases of uniform and saddle beam energy distributions and compared to a traditional Gaussian distribution. It shows that the uniform and saddle electron energy distributions significantly enhance the bunching performance of HGHG FELs, while they almost have no influence on EEHG and PEHG schemes. A further start-to-end simulation example demonstrated that, with the saddle distribution of sliced beam energy spread controlled by a laser heater, the 30th harmonic can be directly generated by a single-stage HGHG scheme for a soft x-ray FEL facility.
Highlights
In recent years, enormous progress has been achieved in the seeded free-electron lasers (FELs), which hold great potential to deliver high brilliance radiation pulses with excellent longitudinal coherence in the extreme ultraviolet and even x-ray regions
III demonstrated that the saddle distribution of sliced beam energy spread controlled by a laser heater can be maintained in the following accelerations of LINAC, and the saddle beam energy distribution is capable of driving a 30th harmonic upconversion in a single-stage highgain harmonic generation (HGHG) operation of soft x-ray free-electron laser facility (SXFEL), even though it has a larger sliced beam energy spread than a Gaussian case
In order to obtain a comprehensible idea of the energy spread distribution effects on different seeded configurations, by using the nominal parameters of SXFEL, uniform and saddle energy spread distributions are investigated for the density modulation process and compared to the previous Gaussian distribution case under the same rms deviation, i.e., beam energy spread
Summary
Enormous progress has been achieved in the seeded free-electron lasers (FELs), which hold great potential to deliver high brilliance radiation pulses with excellent longitudinal coherence in the extreme ultraviolet and even x-ray regions. II, by using a set of nominal parameters of the Shanghai soft x-ray free-electron laser facility (SXFEL) [28], the bunching efficiencies in HGHG, EEHG and PEHG schemes with different electron beam energy spread distribution are theoretically derived and numerically simulated, which shows that the uniform and saddle cases may significantly enhance the bunching performance of HGHG It indicates that the beam energy distribution is of great importance for the HGHG scheme, the frequency up-conversion number of a single-stage HGHG can be improved to 30 or even higher with a uniform or saddle electron energy distribution.
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