Magnet block arrangements for the APPLE-II elliptically polarized undulator

The uniform field region (magnetic field roll-off) of the distributions of horizontal and vertical fields in the elliptically polarized undulator (EPU) of the APPLE-II structure is too short. Hence magnet block arrangements such as (1) the magnet block magnetized with a tilt angle, (2) a step shim of thickness 0.5 mm placed at the magnet edge, and (3) shortening the transverse dimension of the magnet block have been investigated to enlarge the uniform field region. The advantages and disadvantages of these block arrangements are discussed. The pure and hybrid structure of the EPU with an optimized end-pole design and the merit of flux as a function of varied phase have also been studied.