In normal planar undulators the electrons typically are deflected in the storage ring plane. This leads to a linear polarisation of the light emitted on axis with the polarisation vector lying in the deflection plane. Polarisation sensitive methods like magnetic dichroism require a variable linear and/or circular polarisation of the incoming radiation. This can be achieved by special undulators capable of generating two orthogonal transverse field components. Each of these can be employed for generating linearly polarised radiation. If both field components have equal amplitude and are phase-shifted by 90° with respect to each other the resulting field is helical, leading to circular polarisation of the emitted light.
A possible design principle for elliptically polarising superconductive undulators is scetched in fig. .. By tilting the coil packs in the way shown in the figure, two orthogonal field components of nearly equal amplitude and a fixed phase difference of 90° are generated. A purely horizontal field or a field with reversed helicity can be achieved by nesting such a helical undulator with a planar undulator which can be used to compensate or over-compensate the vertical field component of the helical undulator, respectively.
This concept was developed at the Forschungszentrum Karlsruhe in collaboration with the University of Erlangen in 2004. Other designs were proposed later by the Lawrence Berkeley National Laboratory and the Brookhaven National Laboratory. The further investigations of LAS on this topic aim at carefully comparing these different design approaches, optimising and experimentally testing elliptically polarising superconductive undulators.