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

## TL;DR

This paper presents a nondestructive, dual-purpose instrument for real-time measurement of polarization and energy spectrum of X-ray FELs, utilizing a Geant4-based model for optimization.

## Contribution

It introduces a novel electron time-of-flight instrument design for simultaneous polarization and energy spectrum measurement in X-ray FELs.

## Key findings

- Achieved 0.5% polarization degree resolution.
- Achieved 0.3 eV energy spectrum resolution.
- Optimized instrument design based on systematic parameter study.

## Abstract

The free electron laser (FEL), as a next-generation light source, is an attractive tool in scientific frontier research because of its advantages of full 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 energy spectrum using one instrument are preferred. In this paper, such an instrument based on the electron time-of-flight technique is proposed. By considering the complexity and nonlinearity, a numerical model in the framework of Geant4 has been developed for optimization. Taking the Shanghai Soft X-ray FEL user facility as an example, its measurement performances' dependencies on the critical parameters were studied systematically, and, finally, an optimal design was obtained, achieving resolutions of 0.5% for the polarization degree and 0.3 eV for the X-ray energy spectrum.