Focusing and defocusing using mechanically corrected mirrors at the MX beamline at Alba

Abstract

A practical limitation of X-ray focusing mirrors is that, while providing nice gaussian photon distributions at the focal plane, they usually produce inhomogeneous beam profiles upon defocusing that are caused by the mirror polishing errors. This can become a limitation in order to match the beam size with the sample dimensions, as the sample is not uniformly illuminated. We outline a method to achieve well shaped unfocused beams that has been successfully proven at BL13-XALOC, the macromolecular crystallography beamline at the ALBA synchrotron. As received from the manufacturer, the mirrors had rms slope errors of 180 nrad rms (vertically focusing mirror, VFM) and 210 nrad rms (horizontally focusing mirror, HFM). Ray tracing simulations suggested that pronounced stripes arising from the surface waviness would appear upon defocusing. By using the elastic beam theory to model the deformations and calculating the necessary correcting forces exerted by a few mechanical actuators that were installed in the mirror mechanical holders, we were able to reduce the slope errors of the mirrors to 55 nrad rms (VFM) and 83 nrad rms (HFM). The corrected mirrors were installed at the beamline and they were tested with the X-ray beam from the undulator source.