OPHELIE: a variable-polarization electromagnetic undulator optimized for a VUV beamline at Super-ACO

A planar/helical crossed overlapped undulator, called OPHELIE, is presented, composed of two identical ten-period (each 25 cm long) crossed undulators whose magnetic fields are produced by electromagnets. Such a design is quite simple, with only one mechanical translational motion, and versatile. After the presentation of the general concept of the crossed undulator and of its actual magnetic design, the expected performances in terms of flux and polarization capabilities are described. It appears that any state of polarization can be produced, at least on the first harmonic, with a large total polarization rate and a potentially high polarization switching frequency.