A cryogenically cooled, sagittal focusing scanning monochromator for ESRF

Abstract

A double crystal monochromator for fast scans over large energy ranges has been designed, constructed, and tested at ESRF. The instrument is based on a drive unit operated in air that rotates a large wheel sitting in UHV and supporting the two crystal assemblies. The link between the drive and the wheel is realized by a differentially pumped feedthrough. In order to withstand the full power of typical ESRF undulators without noticeable deformation, the first crystal is cooled by liquid nitrogen via pipes located in the hollow core of the differentially pumped feedthrough. The second crystal mount is based on an elastically deformable structure driven by piezoelectric and pneumatic actuators for precise angular adjustments. In its basic version the relative distance between the first and second crystal is kept constant (channel cut type of design), but a nearly fixed beam at the focal plane can be obtained, as the Bragg angle is changed, by the partial compensation of two different effects: the vertical displacement of the exit beam typical of channel cut designs and its divergence with respect to the incoming one due to the difference in lattice spacing between the first cooled crystal and the second one at room temperature. Horizontal focusing of the beam is achieved by sagittal bending of the second crystal. The major drawbacks of crystal focusing, the necessity of changing the radius of curvature while scanning, and the losses in throughput due to the anticlastic deformation experienced at high energy are circumvented by an appropriate choice of the crystal geometry and orientation.