CoSAXS is a multipurpose SAXS instrument located at the 3 GeV ring of MAX IV Laboratory in Sweden. This instrument provides a versatile platform for conducting Small-Angle X-ray scattering (SAXS) experiments on a wide range of research fields. With an extensive pool of sample environments, CoSAXS enables the application of multiple techniques and complex experiments on solid and solution samples. To accomodate the high demand and facilitate the rapid exchange of sample setups, a standardized mounting system has been implemented and additive manufacturing techniques are utilized for fast and efficient prototyping and production of customized sample holders. Furthermore, CoSAXS is equipped with advanced sample environments, such as the setup for milliseconds Time-Resolved SAXS-WAXS experiments in solution (TR-XSS). Among other studies it has been used in non-reversible protein reactions after laser activation of caged compounds.

CoSAXS is a versatile SAXS/WAXS beamline at the 3 GeV diffraction-limited ring of MAX IV Laboratory in Sweden. The optical design [$*$ ,$**$] delivers X-ray beams from 4–20 keV with 0.01% bandwidth and photon flux of 10¹²–10¹³ ph/s, with up to 10% coherent flux at 7.1 keV. Beam sizes at the sample range from 250 × 250 μm² to 30 × 15 μm² (FWHM). The SAXS detector (Eiger2 4M) moves longitudinally and transversely inside a 15 m vacuum vessel. The fixed WAXS detector (Pilatus3) is positioned at the vessel entrance, and a Mythen2 1K in air provides 1D WAXS. The q-range spans ~6 × 10⁻⁴ to 3 Å⁻¹ (d-spacings: 1 μm–2 Å). Supported techniques include solution and solid SAXS/WAXS, SEC/AF4-SAXS [$*******$], USAXS, TRSS in the ms range [$***$, $****$], and coherent scattering [$*****$]. Sample environments include magnetic fields, rheology, biaxial stretching [$******$], and microfluidics [$********$]. Control and data systems are described in [$****$]. After nearly 5 years of operation, CoSAXS has completed 190 proposals, including 19 proprietary research projects. The beamline has a high demand and has contributed to 47 publications.

We report a novel concept of hybrid semitransparent beamstops for small-angle X-ray scattering (SAXS) instruments, removing the need for a separate photodiode to monitor the transmitted X-ray intensity. A beamstop is used to block the unscattered primary X-ray beam after it passes through the sample, protecting the detector while enabling measurement of the scattered signal. The design combines a semitransparent aluminum core with a highly absorbing steel cover to suppress parasitic scattering from the beamstop itself. The aluminum thickness is tailored to match the desired X-ray energy range, allowing sufficient transmission for beam monitoring while maintaining beam attenuation. Thanks to its modular architecture, the beamstop can be easily adapted to different beamline configurations, X-ray energies, and flux conditions.

The CoSAXS beamline at MAX IV Laboratory has incorporated an in-line sample viewer. This new feature allows users to visually monitor and optimize the sample position within the beam path directly, which is crucial for experiments requiring precise micrometric alignment. The in-line viewer is designed to support intricate experimental setups such as microfluidics, where precise control of fluid flow and sample positioning is essential. The viewer is particularly helpful for TR-XSS, where experiments involve triggering structural changes with a laser and then rapidly collecting X-ray scattering data. Precise alignment is needed to ensure accurate measurements. The primary goal of the in-line viewer is to enable users to achieve and maintain micrometric precision in sample positioning, which is often necessary for advanced experiments.