The elliptically polarized undulator beamlines at BESSY II

During 1998/99 three beamlines of the UE56-PGM type were built at BESSY-II to meet the increased need for synchrotron radiation of tuneable, i.e. linear to fully circular, polarization. We report on the results of characterization measurements, which were focused on spectral resolution, photon flux and polarization properties. The spectral resolution for all three beamlines. exceeds E/DeltaE =100 000 at 64.1 eV as obtained from absorption spectra of doubly excited He, The flux measurements resulted in values that closely follow the theoretical prediction of Up to 1 x 10(14) photons/(s 100 mA 0.1%) at 160 eV. The degree of polarization turned out to be close to 100% with circular and linear content, each of up to 100% depending on the undulator shift value, as predicted by theory. We also give first results on the double beam mode in which both undulator modules produce horizontally separated beams. This mode was used for fast switching of the polarization, utilizing a motorized chopper in the beamline. (C) 2001 Published by Elsevier Science B.V.