Cryogenic Test and High-Precision Temperature Regulation of a 50-Period Bulk HTS Undulator for SXFEL

A 50-period bulk high-temperature superconducting (HTS) undulator employing a staggered-array of RE-Ba-Cu-O bulks and magnetized by a 7 T solenoid is under development for the Shanghai Soft X-ray Free Electron Laser (SXFEL) facility. The system undergoes staged thermal control, including heating above 110 K to suppress superconductivity, followed by field-cooled magnetization at 10 K and flux freezing at 7∼8 K. Precise and uniform temperature control along the HTS insert is essential to achieve a homogeneous undulator field with minimal peak-to-peak error. In this study, we developed a Python-based high-precision temperature control system comprising a single temperature controller, 11 sensors, and two heaters mounted on the 2^nd-stage cold heads of the GM cryocoolers at both ends of the dummy HTS insert. Dynamic PID tuning allowed the control system to adapt to the temperature-dependent cooling capacity of the GM cryocoolers, enabling continuous, unidirectional temperature ramping and stabilization with better than ±0.01 K precision across the operation temperature range, 7∼10 K. The associated temperature variation along the 1.5 m-long dummy HTS insert remains within ±0.01K. The experimental setup and measurement results are explained in detail, along with a proposed updated cryogenic design for improved thermal performance.