A Measurement System In Situ to Measure the Magnetic Field of an In-Vacuum Undulator

Two in-vacuum undulators (length 2 m) with period 22 mm (IU22) are constructed to be installed in the Taiwan Photon Source (TPS, 3 GeV) to provide a highly brilliant light source at hard X-ray energies. To verify ultimately the magnetic fields of these IU before installation in the storage ring, we have developed a system to measure the magnetic field in vacuum. To avoid contamination of the vacuum chambers, all components of the system are compatible with high vacuum. During actuation along the longitudinal direction, the positions of the Hall probe are monitored with optical devices and corrected with three two-axis stages. The deviations of position of the Hall probe with dynamic correction are less than 5 μm. The phase error reproducibility of this system is 0.15° and the first integral reproducibility is 13 G·cm. We applied this system to measure the magnetic performance of our in-vacuum undulators at varied gaps of the magnet array. The results of those measurements show no significant difference after assembly of the magnet arrays inside the chamber. The details of design and construction of this system are described, together with the results of measuring the magnetic field of the IU22.