A superconducting undulator prototype with the length of 4 m, the period length of 16 mm, and the gap of 5 mm for the Shanghai High Repetition Rate XFEL and Extreme light facility (SHINE) has been successfully integrated and delivered for acceptance test.

This study aims to propose a magnetic field measurement method tailored for small gap superconducting undulators and to optimize the phase error associated with the magnetic field.

A magnetic field measurement technique based on dual Hall probes without fixed guiding rails was put forward based on the calculating formulas derived for the position of the magnetic neutral plane and the magnetic field distribution on this plane using data from the dual Hall probes. Then, a phase error optimization method based on segmented powering of the superconducting undulator magnet was proposed. These methods were applied to measuring and optimizing the magnetic field in the middle section of the prototype, which was 2 m long and divided into two 1-m sections, each powered by separate power supplies.

The measurement results show that the repeatability of the effective peak magnetic field between two measurements at 100 A excitation current is better than 1.5 Gs, and the phase error repeatability is better than 0.2°. After fine-tuning the excitation current, the phase error of the 2 m segment is reduced to 4.6°, meeting the design requirements.

The results confirm that the magnetic field measurement and optimization method proposed of this study is practical and effective.