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J Synchrotron Radiat. 2017 Sep 1;24(Pt 5):906-911. doi: 10.1107/S1600577517008402. Epub 2017 Jul 29.

Phase-merging enhanced harmonic generation free-electron laser with a normal modulator.

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National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, People's Republic of China.


A phase-merging enhanced harmonic generation free-electron laser (FEL) was proposed to increase the harmonic conversion efficiency of seeded FELs and promote the radiation wavelength towards the X-ray spectral region. However, this requires a specially designed transverse gradient undulator (TGU) as the modulator to couple the transverse and longitudinal phase space of the electron beam. In this paper, the generation of the phase-merging effect is explored using the natural field gradient of a normal planar undulator. In this method, a vertical dispersion on the electron beam is introduced and then the dispersed beam travels through a normal modulator in a vertical off-axis orbit where the vertical field gradient is selected properly in terms of the vertical dispersion strength and modulation amplitude. The phase-merging effect will be generated after passing through the dispersive chicane. Theoretical analysis and numerical simulations for a seeded soft X-ray FEL based on parameters of the Shanghai Soft X-ray FEL project are presented. Compared with a TGU modulator, using the natural gradient of a normal planar modulator has the distinct advantage that the gradient can be conveniently tuned in quite a large range by adjusting the beam orbit offset.


beam orbit offset; free-electron laser; natural transverse gradient; phase-merging effect


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