D2AM, a beamline with a high-intensity point-focusing fixed-exit monochromator for multiwavelength anomalous diffraction experiments

D2AM is a french CRG beamline installed at the ESRF (European Synchrotron R adiation Facility) in Grenoble, with half of the time dedicated to biologic al macromolecule crystallography and half to materials science studies (dif fraction, wide-angle and small-angle scattering). It is constructed at the front-end BM02 of the ESRF storage ring, using the X-ray beam from a 0.8 T bending magnet. D2AM entered into routine operation at the end of 1994, and is used either for single-wavelength or for multiwavelength anomalous diff raction studies. The beam is monochromated by an Si[111] two-crystal monoch romator with a resolution of about 2 x 10(-4). The first crystal is water c ooled. The X-ray photon energy covers the range between 6.5 keV (lambda sim ilar or equal to 1.9 Angstrom) and 17 keV (lambda similar or equal to 0.7 A ngstrom), a domain of energy with many K or L absorption edges of heavy ato ms of interest for biological macromolecules studies and in materials scien ce. The X-ray beam is focused in the vertical plane by two long curved mirr ors and in the horizontal plane by the second crystal of the monochromator which is given an adjustable sagittal curvature. A spot size of 0.3 x 0.1 m m (FWHM) is measured at the sample position. Both mirrors are cut out of a 6 "-diameter 1.1 m-long Si single crystal, perished and coated with a 400 A ngstrom Pt thin film. The rugosity is better than 4 Angstrom r.m.s. and the longitudinal slope error is better than 5 x 10(-6) rad r.m.s. The first mi rror is water cooled, the second is not. The beam intensity on the sample i s about 10(11) photon s(-1) on a 0.3 x 0.3 mm focus area at 100 mA. in the storage ring of the ESRF. The harmonic rejection ratio obtained with the tw o mirrors is better than 10(-5) for lambda/3. The combined optical system, mirror/monochromating-crystals/mirror, used on D2AM constitutes altogether a high-intensity point-focusing fixed-exit monochromator, which has the add itional property that the energy resolution is not dependent on the beam di vergence in use. Its stability and resolution are perfectly adapted to mult iwavelength anomalous diffraction studies. The alignment of the mirrors and the monochromator is fully automated, taking 5 min, with the exception of the adjustment of the sagittal focusing. During multiwavelength diffraction experiments the wavelength is changed by a fast single monochromator rotat ion. Neither realignment of the mirrors nor readjustment of the beam focusi ng are necessary. The stability and reproducibility of the selected X-ray p hoton energy is better than 0.5 eV.