Design of a Nondestructive Two-in-One Instrument for Measuring the Polarization and Energy Spectrum at an X-ray FEL Facility

摘要

The free electron laser (FEL), as a next-generation light source, is anattractive tool in scientific frontier research because of its advantages offull coherence, ultra-short pulse duration, and controllable polarization.Owing to the demand of real-time bunch diagnosis during FEL experiments,precise nondestructive measurements of the polarization and X-ray energyspectrum using one instrument are preferred. In this paper, such an instrumentbased on the electron time-of-flight technique is proposed. By considering thecomplexity and nonlinearity, a numerical model in the framework of Geant4 hasbeen developed for optimization. Taking the Shanghai Soft X-ray FEL userfacility as an example, its measurement performances' dependencies on thecritical parameters were studied systematically, and, finally, an optimaldesign was obtained, achieving resolutions of 0.5% for the polarization degreeand 0.3 eV for the X-ray energy spectrum.