Abstract

Self-seeding is a promising approach to significantly narrow the self-amplified spontaneous emission bandwidth of X-ray free-electron lasers (FELs) and hence to produce nearly transform-limited pulses. We study the radiation propagation through a grating monochromator installed at the Linac Coherent Light Source (LCLS). The monochromator design is based on a toroidal VLS grating working at a fixed incidence angle mounting without an entrance slit. It covers the spectral range from 500 eV to 1000 eV. The optical system was studied using a wave optics method to evaluate the performance of the self-seeding scheme. Our wave optics analysis takes into account the finite size of the coherent source, third-order aberrations and height error of the optical elements. Two propagation approaches are studied with time-dependent FEL simulations. In addition, the pulse-front tilt phenomenon effect is illustrated.

Highlights

  • Self-seeding is a promising approach to significantly narrow the self-amplified spontaneous emission (SASE) bandwidth and to produce nearly transform-limited pulses [1,2,3,4,5,6,7,8,9,10]

  • Considerable effort has been invested in theoretical investigation and R & D at the Linac Coherent Light Source (LCLS) leading to the implementation of a hard X-ray self-seeding (HXRSS) setup that relies on a diamond monochromator in the transmission geometry

  • Chromatic dispersion effects in the bypass chicane smear out the microbunching in the electron bunch produced by SASE lasing in the SASE undulator

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Summary

INTRODUCTION

Self-seeding is a promising approach to significantly narrow the self-amplified spontaneous emission (SASE) bandwidth and to produce nearly transform-limited pulses [1,2,3,4,5,6,7,8,9,10]. A self-seeding setup consists of two undulators separated by a photon monochromator and an electron bypass, normally a four-dipole chicane (see Fig. 1). Both undulators are resonant at the same radiation wavelength. A monochromatic pulse is created, which is used as a coherent seed in the second undulator (seeded undulator). Chromatic dispersion effects in the bypass chicane smear out the microbunching in the electron bunch produced by SASE lasing in the SASE undulator. The electrons and the monochromatized photon beam are recombined at the entrance of the seeded undulator, and the radiation is amplified by the electron bunch until saturation is reached. The required seed power at the beginning of the seeded undulator must dominate over the shot noise power within the gain bandpass, which is at the order of hundreds of Watts to one kilowatt in the soft X-ray range

THE LCLS SOFT X-RAY SELF-SEEDING SETUP LAYOUT
OPTICS MODELING
Grating modeling
Free space propagation
Focusing with M2 mirror
Mirror surface height errors simulation
SIMULATION DETAILS AND APPROACHES
Direct propagation approach
Phenomenological propagation approach
Z projection 6
COMPARISON OF APPROACHES
DISCUSSION AND CONCLUSIONS
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