<title>Monochromators for small cross-section x-ray beams from high heat flux synchrotron sources</title>

Abstract

OtJ F -86 o 8 c18---j 3 l f o n o c h r u m a t o r s fur small cross-section x-ray beams from high heat flux synchrotron sources Gene Ice and Bernie Rienier Metals and Ceramics Diiisioii, Oak Ridge National Laboraton Oak Ridge TN 38931-6118Ali Khounsary Adumced Photon Source. Experimental Facilities DivisionArgonne National Labororor)!, 9700 S. Cass Al,eitue, Argonne I1 60439 ABSTRACTFor some x-ray experiments, only a fraction of the intense central cone of x-rays generated by high-power undulator sources can be used; the x-ray source emittance is larger than the useful emittance for the experiment, For example nith microfocusing optics, or for coherence expcriments, x-ray beams with cross sections less than 0.1 mm2 are desirable. With such small beams, the total thermal load is small even through the heat flux density is high. Analyses indicate that under these conditions, rather simple crystal cooling techniques can be used. We illustrate the advantages of a small beam monochromator, with a simple x-ray monochromator optimized for x-ray microdiffraction. This monochromator is designed to achieve negligible distortion when subjected to a narrow (0.1 mm wide) beam from an APS undulator A operating at 100 mA. It also allows for rapid and repeatable energy scans and rapid cycling between monochromatic and white beam conditions. INTRODUCTIONInsertion devices on third generation synchrotron sources provide ultra-brilliant (p/s.pm'*mrad'*eV) x-ray beams with kjlowatts of x-ray power into a small solid angle (Fig. 1). These intense x-ray beams challenge conventional x-ray monochromator optics and have led to the development of new high heat-load ray sources include, inclined Si crystals', diamond crystals', and cryogenically cooled Si.' These rnonochromators are designed to preserve x-ray flux while accepting hundreds of watts of power in the bright central cone of undulator radiation. Irr~onochromators.'~~ X-ray monochromators proposed for handling undulator radiation from 3rd generation x-I Although these new designs are useful for many experiments, they can be inefficient for experiments \vhere maximum beam brilliance is required in a very small beam emittance. For example. the bandpass of diamond crystals is about half that of Si, and it is difficult to get perfect diamond crystals with negligible mosaic spread. Similarly, inclined crystals are difficult to fabricate with negligible strain, and increase the required beam offset compared to symmetric Bragg geometries. With CQ ogenic Si monochromators, existing first crystal designs preclude small offsets and exhibit residual stresses from the cryogenic cooling manifold. For experiments bvhich can use only a small fraction of the beam central cone, the total useful power can be 1-3 orders of magnitude loner than for conventional experiments (Fig. 1). This allows much simpler first cqstals which tolerate high power density but only work nith small integrated power loads. DISCLAIMERPortions of this document may be illegible in electronic image products. Both x-ray m...