Vertical measurement and on-line correction of the magnetic field for a 1.5-m-long superconducting undulator

The 1.5-m-long superconducting undulator (SCU) must have its magnetic field measured in a vertical Dewar to verify the field quality before the SCU is integrated into a horizontal cryostat. The magnetic field measurement mechanism must ensure that the trajectory of the Hall probe scanning the magnetic field is in the center of the magnetic gap, that is, the trajectory of the beam passing through the SCU. The good field region of the SCU’s magnetic field and the position of the Hall probe are given by Opera and the magnetic field distribution formula. Precise machining technology assists the positioning of the Hall probe. Due to the existence of mechanical errors, there is no guarantee that the peak magnetic field is also uniformly distributed under a uniform gap. The adjustment of the gap helps the SCU magnetic field distribution to be improved. The adjustment amount of the gap is obtained by comparing the measured data with the theoretical value. After the SCU returns to room temperature, keep the electrical connection, and adjust the gap on-line. Both peak magnetic field uniformity and phase error of the adjusted magnetic field are improved. This greatly reduces the difficulty of SCU’s magnetic field correction and makes the subsequent SCU horizontal test meaningful.