Abstract
We study the dependence of the peak power of a 1.5 A Terawatt (TW), tapered x-ray free-electron laser (FEL) on the transverse electron density distribution. Multidimensional optimization schemes for TW hard x-ray free-electron lasers are applied to the cases of transversely uniform and parabolic electron beam distributions and compared to a Gaussian distribution. The optimizations are performed for a 200 m undulator and a resonant wavelength of λr ¼ 1.5 A using the fully three-dimensional FEL particle code GENESIS. The study shows that the flatter transverse electron distributions enhance optical guiding in the tapered section of the undulator and increase the maximum radiation power from a maximum of 1.56 TW for a transversely Gaussian beam to 2.26 TW for the parabolic case and 2.63 TW for the uniform case. Spectral data also shows a 30%–70% reduction in energy deposited in the sidebands for the uniform and parabolic beams compared with a Gaussian. An analysis of the transverse coherence of the radiation shows the coherence area to be much larger than the beam spotsize for all three distributions, making coherent diffraction imaging experiments possible.
Highlights
Self-amplified spontaneous emission x-ray free-electron lasers (SASE X-FELs) [1,2,3] have given us the ability to study structures and dynamical processes at unprecedented spatiotemporal scales, with a simultaneous resolution in space and time of 1 Å and 1 fs
We evaluated the effect of changing the transverse electron distribution in an optimized tapered free-electron laser
The tapering strategy applied is the multidimensional optimization method described in Ref. [12] based on maximizing the output power by improving the optical guiding and mitigating the detrimental effects of diffraction of the radiation throughout the tapered section of the undulator
Summary
Self-amplified spontaneous emission x-ray free-electron lasers (SASE X-FELs) [1,2,3] have given us the ability to study structures and dynamical processes at unprecedented spatiotemporal scales, with a simultaneous resolution in space and time of 1 Å and 1 fs. X-FELs are having a great impact on the field of bioimaging [5,6,7] Research in this field will benefit from a larger number of coherent photons/pulse, a factor of 10 to 100 larger within a pulse duration of 10–20 fs. Seeding or self-seeding an FEL amplifier leads to much larger output power [11] In light of these promising results many recent efforts have been devoted to optimize the tapered section of a self-seeded X-FEL to reach power levels of one TW or larger [12]. We discuss the impact of time dependent effects by presenting results from multiple frequency simulations and examining the change in maximum output power, bunching and radiation size as IV we study the transverse coherence properties of the output radiation from the optimized tapered X-FEL
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