THERMAL BILAYER MONOCHROMATOR CRYSTAL FOR A HIGH-RESOLUTION MULTIWAVELENGTH ANOMALOUS DIFFRACTION BEAMLINE AT THE EUROPEAN SYNCHROTRON-RADIATION FACILITY

High intensity for diffraction experiments with high energy resolution, on an intense source like bending magnet at the European Synchrotron Radiation Facility, requires a strict control of the curvature of the optical elements placed in the beam for geometrical focusing and for wavelength monochromatization. Unwanted curvature of the first crystal of the monochromator can arise from the thermal load. Indeed, due to the size of the crystal, only cooling from the rear is conceivable. This induces a front-to-rear thermal gradient and, as a consequence, a strong spherical curvature. A new technique was developed for the CRG/D2AM beamline in order to control this effect. It was shown by calculation that this curvature can be exactly compensated, whatever the heat load power is, by the thermal expansion of a metallic layer coated at the rear of the crystal. First results confirm the predicted behavior and show how sensitive the technical problem of the fixation of this bilayer is on its cooling device. © 1995 American Institute of Physics.