The potential use of two planar superconducting elliptical undulators—a vertically wound racetrack coil structure and a staggered array structure—to generate a circularly polarized hard x-ray source was investigated. The magnetic poles and wires of the up and down magnet arrays were rotated in alternating directions on the horizontal plane, an elliptical field is generated to provide circularly polarized light in the electron-storage ring and the energy-recovery linac accelerator. Rapid switching between right- and left-circularly polarized radiations is performed using two undulators with oppositely rotated wires and poles. Given a periodic length of 15 mm and a gap of 5 mm, the magnetic-flux densities in the elliptical undulator are $B_z=1.2\mathrm{T}$ ($B_x=0.6\mathrm{T}$) and $B_z=0.35\mathrm{T}$ ($B_x=0.15\mathrm{T}$) in the planar vertically wound racetrack coil and the staggered structure with poles rotated by $35°$ and $25°$, respectively. In maximizing the merit of the flux and the width of the effective field region in the two superconducting elliptical undulators, the trade-off rotation angles of the coils and poles are $20°$ and $5°$, for vertically wound racetrack coil and staggered undulators, respectively.

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