A plane-grating monochromator for circularly polarized undulator radiation at BESSY II

At BESSY II, a third-generation 1.7 GeV storage ring is under construction. A planar elliptical undulator will be installed as a source of X-rays that have a high degree of circular polarization. Radiation in the first, third and fifth harmonics will cover the energy range 87-1330 eV. The beamline will essentially consist of a plane-grating monochromator working with collimated light in the dispersion plane. A single set of optical elements can be used to handle the two angularly separated beams of opposite helicity from the double undulator. The degree of circular polarization ranges from 73 to 100%, and a flux of up to 5 × 10¹³ photons s^-1 (100 mA)^-1 can be achieved. A maximum spectral resolution of about 13 000 will be possible at 100 eV using a 20 µm slit.