A NOVEL UNDULATOR-BASED PGM BEAMLINE FOR CIRCULARLY-POLARIZED SYNCHROTRON-RADIATION AT BESSY-II

We present the design of a beamline for circularly polarised radiation from an elliptical double undulator to be built at BESSY II. The UE56 undulator of Sasaki type will emit simultaneously two angularly separ ated circularly polarised beams of opposite helicities. The undulator will, through emissions in first, third and fifth harmonics, provide c ircularly polarised radiation in the energy range 89-1328 eV. The beam line essentially consists of a plane grating monochromator (PGM) worki ng with collimated light and employing only one set of optical element s for steering and monochromatising the two beams. A high energy resol ution from 6000 to 13 000 can be achieved at a flux between about 10(1 0) and 10(12) photon/s, respectively. The helicity of circular polaris ation can be switched dynamically in the beamline at a high rate (>100 Hz) by use of a suitable chopper. The degree of circular polarisation of the source ranging from 78 to 100% is well preserved by the beamli ne. In the worst case - at the lowest energy - the degree of polarisat ion is reduced by 5%. In the higher energy range above 357 eV the inpu t degree of circular polarisation can even be increased by the beamlin e.