Reflective Optics for the ESRF Beamline ID 26

Abstract
The ESRF beamline ID26 is dedicated to XAFS studies on ultra-diluted samples in the energy range 2.5-25 KeV using the excitation of X-ray fluorescence. The source consists of three phasable planar undulators, inserted in an high β section of the storage ring. The optical design, based on grazing incidence mirrors, aims at obtaining a highly stable clean beam with efficient harmonic rejection together with small vertical and horizontal focal spot sizes. The heat load associated with unwanted high energies is dump in the less critical component, i.e. the first mirror. The latter is a flat Si mirror which acts as a heat sink, introduces a cut-off at high energy and steers the beam out of the brehmsstrahlung emission cone of the straight section. Horizontal and vertical focusing are performed separately by a couple of piezoelectric (PZT) mirrors in a classical Kirkpatrick-Baez configuration. To overcome the limitation due to the maximum length of the commercially available PZT ceramics (150 mm), several plates have been assembled side to side thus allowing a much wider angular acceptance. A wavefront sensor will provide in-situ monitoring of the mirrors shape. Ray-tracing simulations (using the computer code Shadow) show that theoretical focal spot sizes as small as 106x8 µm² FWHM could in principle be obtained.