Various polarization features of a variably polarized undulator with different phasing modes

This article reviews various schemes for extending the novel polarization light of insertion devices. According to this review, the “Apple-II” device associated with the Elliptically Polarized Undulator (EPU) structure can be used in the polarization switching from a horizontally linear direction, to right/left circular, to a vertically linear or from a horizontally linear direction, to linear at 45°/135°, to a vertically linear direction. Meanwhile, half period length (λu/2) of the phasing distance is sufficient to fulfil the various polarization modes. Also discussed herein is the optimization between the polarization rate and photon flux when tuning the energy in the different polarization modes. The derived equations predict which conditions will satisfy the corresponding polarization mode. The condition will give us how to move the phase position to obtain the various polarization modes. Additionally, the field distribution of a prototype “Apple-II” device with EPU structure is constructed and measured, verifying that the prediction correlates well with the measured results.