Performance modeling of an elliptically polarizing undulator under magnetic loading

A 3.9-meter-long elliptically polarizing undulator recently has been design ed, constructed, and installed in the storage ring at Synchrotron Radiation Research Center (SRRC) to provide an ultra-brilliant polarizing light sour ce in the soft X-ray range. This undulator is the longest such polarizing d evice ever constructed, accounting for why its backing beams withstand an e normous magnetic force. Conventional deflection compensation scheme cannot be applied to improve the spectral performance owing to dependence of the m agnetic loading on the device's operating modes. This study systematically studies how magnetic loading affects the magnetic field errors of this elli ptically polarizing undulator, including numerical calculations and experim ental measurements. Ultimately improving the spectral performance of an ell iptically polarizing undulator depends on effectively minimizing the deflec tion sensitivity of the backing beams from the backlash of movable magnetic arrays.