Single-block measurement for the cryogenic permanent magnet undulator sorting

The magnet sorting is a standard step in the undulator fabrication procedure. The shortest undulators period used in the High Energy Photon Source (HEPS) is only 12 mm. To the short period undulator, the sorting may be more important than the long-period undulator. Normally, the Helmholtz measurement is used as the input for the work. It is the averaged orthogonal magnetization of each block. In order to investigate whether the Helmholtz coil measurement is enough for the sorting, a careful study has been made. Firstly, a magnetic camera was used to scan the 3D surface field of a magnet. Afterward, its field integral is measured by the stretched wire. In the measurement, the magnet was placed at different statuses. The results were checked to see whether they are consistent with expected features supposing a homogeneous magnetized block. Finally, the results measured by the Helmholtz coil and the stretched wire were compared. The surface field scan demonstrates that the field over a magnet block is inhomogeneous. Moreover, the field integral measurement by the stretched wire also shows big difference when the different magnet sides toward the wire. The comparison between the stretched wire and the Helmholtz coil measurement shows no correlation. The study presented in this paper reveals that the homogeneity of the magnetization is imperfect. Therefore, the Helmholtz coil data are insufficient to the short-period undulators sorting.