Optical design and preliminary commissioning results with a temporary undulator of the Sirius’ SAXS beamline SAPUCAIA

Abstract SAPUCAIA is the main SAXS (Small-Angle X-ray Scattering) beamline of Sirius, the Brazilian Synchrotron Light Source. Covering the energy range from 6 to 18 keV using an APU22 undulator source, SAPUCAIA aims to achieve high performance by minimizing parasitic scattering and beam divergence, and by providing high beam stability. The beamline consists of two main optical components, a High Dynamics Double Crystal Monochromator Lite (HD-DM-Lite [1]) with Si(111) and Si(311) crystal sets, and a side-bounce deflecting toroidal mirror, which focuses the beam in both directions. This design provides an approximately round beam at the sample position, with ~ 230 µm diameter (FWHM) and divergence of ~ 28 µrad (FWHM), with photon flux higher than 10 12 ph/s for almost the entire energy range (Si(111)). The beamline had its first beam in April 2024 and is currently in the commissioning phase. The commissioning was started with a provisional adjustable phase undulator, the PAPU, which allowed to advance the monochromator calibrations while the APU22 was not yet available. This work presents SAPUCAIA’s optical design and the main optical simulations performed to define the beamline optics. Furthermore, the initial beamline commissioning results, related to the HD-DCM-Lite calibrations, with the PAPU undulator, are presented, including the DCM-undulator tuning, rocking curve analysis and energy calibration. A comparison of the results obtained with those expected by simulations, performed using the ray-tracing code SHADOW [2] and the wave propagation code SRW [3], is also present.