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- S.D. Chen, K. Fang, X. Huang, C. Pellegrini, J. Wu
SLAC, Menlo Park, California, USA
- S.D. Chen, C.-S. Hwang
NCTU, Hsinchu, Taiwan
- C. Emma, C. Pellegrini
UCLA, Los Angeles, California, USA
- K. Fang, S.-Y. Lee
Indiana University, Bloomington, Indiana, USA
- C.-S. Hwang
NSRRC, Hsinchu, Taiwan
- S. Serkez
DESY, Hamburg, Germany
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For the ultimate use for the scientific experiments, the free electron laser (FEL) will propagate for long distance, much longer than the Rayleigh range, after exiting the undu- lator. To characterize the FEL for this purpose, we study the electromagnetic field mode components of the FEL photon beam. With the mode decomposition, the transverse coher- ence can be analyzed all along. The FEL here in this paper is a highly tapered one evolving through the exponential growth and then the post-saturation taper. Modes contents are analyzed for electron bunch with three different types of transverse distribution: flattop, Gaussian, and parabolic. The tapered FEL simulation is performed with Genesis code. The FEL photon beam transverse electric field is decom- posed with Gaussian-Laguerre polynomials. The evolutions of spot size, source location, and the portion of the power in the fundamental mode are discussed here. The approach can be applicable to various kind scheme of FEL.
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