Synchrotron Radiation Research Center (SRRC) is constructing a 3.9 meter long Sasaki type elliptically polarizing undulator (EPU) to emit a higher photon flux with various linear, elliptical and circular polarization lights. After evaluating the EPU5.6 prototype's testing, the magnet block is redesigned with shims to increase the good field region of vertical field up to 20 mm. In addition, an end pole scheme is adopted for the orbit displacement free termination; the corresponding electron trajectory is calculated as well. In this work, we describe the mechanical design considerations to achieve the field performance under considerable magnetic forces in light of the gap width variation and the phase shifting. The field error's analysis and correcting methods to satisfy the stringent magnetic field requirements are also presented.
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