Magnet sorting algorithms for a prototype of the SRRC EPU

The Synchrotron Radiation Research Center (SRRC) is constructing a prototype of Sasaki-type elliptically-polarizing undulator (EPU) characterized by its four longitudinal movable magnet rows, whose shifting determines the polarization of synchrotron radiation. A total of 280 pure NdFeB blocks with square cross-section of 40 mm×40 mm and thickness of 14 mm are magnetized into two types identified by the main field orientation. To minimize field errors, the blocks are sorted and assigned appropriate locations on the rows. Due to the special symmetry of the blocks, the 40 mm×40 mm square face, and rows structure, the orientation and locations of blocks can be assigned freely. However the cutting on one pair of opposite angles on each square face for rigid clamping reduces the freedom of permutation. Moreover, the perturbation of the field from the shifting of rows must be taken into account. The authors would like to present the sorting algorithm and address its contribution to field performance