Thermal distortions of an inclined silicon crystal subjected to the high heat loads expected for a 2.5 m long undulator at the Advanced Photon Source were simulated, and the distortions were then used to calculate (111),(111) (+,(minus)) double crystal rocking curves. The inclination angle for all the simulations was either 80(degree) or 85(degree). The first crystal was assumed to be a slab that was uniformly cooled on its underside with liquid gallium. For some of the simulations, the undulator gap was set to 11.5 mm corresponding to K=2.2, and the crystals were oriented to diffract the first harmonic energy at 4.2 keV. For K=2.2, the total x-ray power incident on the crystal is 3.8 kW , and the peak power density transverse to the beam is 148 W/mm(sup 2). Rocking curve broadening in these cases was found to be no more than 0.42 arcsec. Simulations for the third harmonic at K=2.2 were also performed, and the broadening was found to be no more than 1.87 arcsec. In other simulations the undulator gap was widened, and rocking curves for the third harmonic at K=1.5 (19.9 keV) and the first harmonic at K=1.2 (8.2 keV) were obtained. Only weak broadening was found for these cases as well. Finally, rocking curves at 4.2 keV were not altered if a slit that passed only the central cone of the undulator was placed in front of the first crystal.
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