The UE56 undulators at the storage ring BESSY II are elliptical polarizing insertion devices 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.
The UE56 undulators do not provide the antiparallel mode for the production of linearly polarized under arbitrary angles.
The Ue56-2 is a double undulator with two 30 period modules. Rotatable peramenent magnets at either end and in the middle of the device can be used for two different operation modes:
In the chicane mode the photon beams from both modules are separated in space by about ±200 μrad. When operating the two modules with opposite helicities a chopper in the beamline can be used for fast helicity switching (typically 100Hz).
In the modulator mode both modules emit the light in the same direction. By adapting the phase between the modules properly (using the center magnets) the light beams overlap coherently.
Fig. 3: Measurement of the Stokes Parameters S1 (linearity) and S3 (helicity) for different harmonics as a function of the shift of the magnetic rows. To change the polarization while staying at a fixed photon energy, both gap and shift have to be varied. (See graph for 1strd, 5th, 7th, ...) the intensity vanishes for S3 = 1. Elliptically polarized light (for example with S3 = 0.9) has to be used in order to have some flux. harmonic). In case of higher harmonics (3