A SAGITTAL FOCUSING AND SCANNING MONOCHROMATOR FOR 8-80 KeV SYNCHROTRON RADIATION AT DORIS III. R.G. van Silfhout and S. van Dijken, EMBL, c/o DESY, Notkestrasse 85, D-22603 Hamburg, Germany

A fixed exit, double crystal Si(111) X-ray monochromator for fast energy scans over a large energy range has been designed and commissioned. Heart of the instrument is a large rotation stage operated in air that drives a platform placed in a high vacuum environment supporting both crystal assemblies. The rotation stage is linked to the monochromator crystals by a differentially pumped feedthrough hence providing a fast and accurate setting of the Bragg angle. In order to withstand the full power of a typical DORIS bypass wiggler without severe distortion the first flat crystal is cooled by water flowing through microchannels cut very close to the diffracting surface. The second crystal is mounted on an elastically deformable structure driven by piezoelectric motors. Horizontal focusing of the 60 mm wide beam is accomplished by sagittally bending the second crystal with piezoelectric based actuators equipped with precise vacuum compatible encoders. These actuators circumvent a major drawback for crystal focusing, ie. the necessity to change the radius of curvature while scanning, and provide important corrections of crystal twist and parallelism of X-ray beam path and cylinder axis made by the bent crystal. The design was improved in terms of throughput for a wide energy range through exhaustive finite element analyses.