A magnet sorting method for hybrid undulators using a magnetic field camera

[Background] Undulators usually need phase shimming to acquire better radiation spectrum, while phase shimming usually leads to decreased gap space and increased beam wake-field effects. Since the secondary magnetic field generated by the soft iron poles cannot be obtained directly before assembly, effective initial magnet sorting is required for the hybrid type undulator. [Purpose] This study aims to minimize the shimming distances of the hybrid undulators by sorting the magnet blocks with the help of a magnetic camera. [Methods] First of all, a new type of magnetic camera was developed, which could quickly characterize the magnetic field distribution of each magnetic block through a sensor array of 16 Hall probes that were calibrated by comparing their measured values on the same magnetic block. Then the error distribution in the magnet block was deduced from the field maps of each magnet block by least square fittings on those maps. Finally, the field signatures of these blocks were predicted by using a RADIA model with iron poles when they were installed on girder, and the magnets was sorted according to these signatures. [Results] The variation of the half-period integral of the optimized sample undulator decreased to 53% compared with the unoptimized situation. [Conclusions] This method effectively shortens the shimming distances and maintains the gap space, which is essential for the construction of small period undulators. (authors)