The UE112 undulator at the storage ring BESSY II is an elliptical polarizing insertion device of the APPLE-type. The difference to a standard planar undulator is the horizontally separated magnet rows. The shifting of these rows produces a helical field at the position of the electron beam causing an elliptical or even circular trajectory. Consequently, elliptically or circularly polarized radiation is emitted.
Additionally, the UE112 undulators can be operated in the antiparallel mode for the production of linearly polarized light with arbitrary angles.