Development of a Magnetic Field Measurement System for a REBCO Bulk Superconducting Undulator at SXFEL

The Shanghai Soft X-ray Free-Electron Laser (SXFEL) facility is currently developing a high-temperature superconducting (HTS) undulator employing a staggered-array configuration of RE-Ba-Cu-O (REBCO) bulks. This prototype device, designated HTSU12, consists of 50 periods with a period length of 12 mm and a clear vacuum aperture of 4.3 mm. This ultra-narrow aperture imposes significant challenges in performing accurate magnetic characterization. To address this challenge and enable precise quantification of key performance parameters, specifically the phase error and field integral errors, a multifunctional high-precision magnetic field measurement system has been developed for in-vacuum characterization. It enables magnetic field scanning with a Hall probe and integral field measurements using a stretched wire. This paper presents a comprehensive description of the magnetic field measurement system, including its overall design, mechanical implementation, and integration of measurement and control functionalities.