Design,construction,and offline calibration of ARPolar prototype for SXFEL facility

Purpose The polarization characteristics of photoelectrons are critical for free-electron laser(FEL)experiments.Several FEL facilities have operated an eTOF-alike polarimeter based on the angular distribution of photoelectrons.The purpose of this paper is to introduce the design,construction and offline test of the prototype angular resolved polarimeter(ARPolar-CORE)for the Shanghai Soft X-ray Facility(SXFEL).This paper describes the overall design of this instrument,covering the main chamber,detector assemblies,and electronics.Methods To validate the performance of the instrument,the detectors and electronics were tested offline and calibrated.Moreover,based on the measured pulse shape obtained from the offline tests,a comprehensive numerical analysis is presented to evaluate the performance of the instrument.Results and conclusion The results show that the fluctuation in the electron charges is less than 0.8%under the SXFEL working conditions,corresponding to a linear polarity of 0.988±0.0175 for linearly and horizontally polarized FEL pulses.The analysis also indicates that a 0.16 eV mean square error is fully feasible for 645 eV FEL pulses.