DIAMOND PHASE PLATES FOR X-RAY CIRCULAR AND LINEAR POLARIMETRY AT THE ESRF. C. Giles^1,2, C. Malgrange³, J. Goulon¹, F. de Bergevin⁴, C. Vettier¹, A. Fontaine⁵, E. Dartyge⁶, S. Pizzini⁵, F. Baudelet⁶, G. Grubel¹, L. Varga³, A. Freund¹. ¹European Synchrotron Radiation Facility, B.P.220, F-38043, Grenoble, France; ²Laboratorio Nacional de Luz Sincrotron, Cx.P. 6192, 13081-970, Campinas, Brazil; ³Laboratoire de Mineralogie-Cristallographie, Universites de, Paris 6 et 7, Tour 16 case 115, 4 Place Jussieu, 75252 Paris, Cedex 05, France; ⁴Laboratoire de Cristallographie, CNRS, BP 166X, F-38042, Grenoble, France; ⁵Laboratoire de Magnetisme Louis Neel, CNRS, F-38042 Grenoble, France; ⁶LURE, Universite Paris Sud, Batiment 209D, 91405 Orsay Cedex, France.

Low emittance third-generation storage rings, like the ESRF, provide an optimum condition for the use of perfect crystal phase plates combined with ondulator sources. Their low angular and spatial distributions in both the horizontal and vertical directions are suitable to an excellent performance of diamond crystals as quarter-wave plates (QWP) and half-wave plates (HWP). Recent experiments performed at the ESRF (Troika Beamline) confirm 99% and 97% efficiencies of diamond phase plates in the transformation of horizontal linear polarization into circular polarization and linear polarization in the vertical plane respectively. A diamond QWP has been used to investigate the magnetic domains in a spiral antiferromagnet like holmium via magnetic x-ray scattering. Furthermore, the successive use of two QWP allowed the production of linear polarization in any desired direction and the complete characterization of the polarization state of the circularly polarized beam within 1%. The diamond phase plate set in an asymetric Laue transmission geometry at 9 keV, allowed a transmission efficiency of 40%. Diamond phase plates have also been tested in the energy dispersive absorption spectrometer at DCI ring (LURE, Orsay) in order to record X-ray Magnetic Circular Dichroism (XMCD) spectra at different rare-earths absorption edges from Pr L2 (6440 eV) to Tm L3 (8648 eV). The efficiency of the phase plate has been measured comparing such XMCD spectra with those obtained in the standard technique (using the elliptically polarized beam below the orbit plane). The polarization rate and the intensity on the sample where higher with the QWP technique. Finally, good XMCD spectra have also been obtained using beryllium phase plates with a mosaicity of the order of 80 arcsec. The good efficiency of mosaic crystals as phase plates far from the Bragg peak can be interpreted.