Double multilayer monochromator to tailor bending magnet radiation spectrum Thierry Bigault ♦ , Eric Ziegler, Christian Morawe, Robert Hustache, Jean-Yves Massonnat, Gerard Rostaing European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France ABSTRACT A double-reflection multilayer monochromator is being developed at BM5 in order to fulfill two different functions. As a primary monochromator, it provides higher bandpass and higher photon flux than the Si(111) Bragg crystal monochromator. In combination with the crystal monochromator, it rejects the harmonics and the beam exit can be kept fixed. An additional aim is to preserve the beam coherence. Design issues and performances evaluated on the beamline are presented. Keywords: multilayer monochromator, bending magnet beamline, coherence, harmonic rejection, fixed-exit. 1. INTRODUCTION Hard x-ray reflecting multilayers, generally consisting in a periodic stack of strongly absorbing layers separated by weakly absorbing spacer layers, are being developed for an increasing number of applications using synchrotron radiation. In the Bragg condition, multilayers may provide a high reflectivity with an energy bandpass of the order of 10 -2 , to be compared with 10 -4 in the case of perfect crystals 1 . The aim of the present work is to develop and implement a pair of multilayer optics that is integrated into the layout of the BM5 beamline at ESRF, in order to be used routinely. This beamline is a test and development station for x-ray optics, innovative instrumentation, methods and techniques 2 . At the moment, a double-Bragg Si(111) fixed-exit monochromator is in operation on the beamline. For many experiments a high spectral purity of the beam is needed, and in some cases a higher flux would be an advantage while a lower energy resolution would still be acceptable. To overcome the limits of the present crystal monochromator in the above cited cases, the multilayers has to fulfill two optical functions: the first one is to monochromatize the beam from the bending magnet with a higher bandpass, and the second one is to reject the harmonics from the Si(111) monochromator. Other concerns of the design are the reflectivity over the lower part of the beamline energy range (8 – 22 keV), the size of beam accepted, the stability of the multilayers under white beam radiation, and the preservation of the transverse spatial coherence of the beam. The choice was made to deposit the multilayers on simple flat substrates, and to achieve only the above-mentioned optical functions, other functions like concentrating of focusing the beam being achieved by distinct optical devices. For example the second Si(111) crystal in the crystal monochromator is sometimes replaced with a the a sagittally bent crystal 3 , thus concentrating the beam in horizontal. This device may be combined with the multilayer monochromator for harmonic rejection. Another example is the combination of the multilayer monochromator with a Kirkpatrick-Baez optics for 2-dimensional microfocusing 2 . More importance was set on the surface quality of the multilayer mirrors and in particular the effect on the spatial coherence of the beam. This is also a reason for choosing flat crystals, since better surface quality is usually achieved with flat mirrors than with figured or bent substrates. The width of the substrate is 45 mm, and 25 mm only are covered by the present multilayer stripe. The deposition of two other multilayer stripes is planned for different applications, requiring for example higher energies or higher resolution 1 . ♦ bigault@esrf.fr, phone 334-76-88-29-39; fax 334-76-88-23-25; esrf.fr