AN ESRF BEAMLINE DEDICATED TO POLARIZATION-SENSITIVE XAS AT LOW EXCITATION-ENERGIES

The growing demand for spectroscopic studies requiring high linear or circular polarization rates has stimulated the design of exotic sources and specialized beamlines. At the ESRF, it has been proposed to build a new type of helical undulator (HELIOS). The first prototype will be installed on beamline No. 6 which is a high priority international beamline dedicated to X-ray spectroscopy or dichroism at low excitation energy (0.5-10 keV). HELIOS will be segmented into two identical modules with slightly deflected injection axes: the output will thus consist of two well separated beams with fully independent control of polarization. Two types of optics should be available: (i) low energy experiments will be carried out on a side branch equipped with grating optics: (ii) Bragg optics will be developed on the straight branch and used for experiments at higher energies. This paper deals with preliminary design considerations related to the high energy branch only. Special attention was paid to minimize the energy difference between the two separated excitation channels and to preserve the excellent polarization characteristics of the source. This led us to propose the construction of a four-mirror collimation device making the two incident beams rigorously parallel. In principle, the highest circular polarization rates should be obtained with a modular ''twisted'' [2 + 2]-crystal monochromator, but at the expense of a significant loss of intensity.