Coherence Control and Measurement at the CLS Using X-ray Interferometry

As synchrotron light source storage rings transition to multi-bend achromat (MBA) lattices for higher brightness and coherence, emerging challenges arise in beam diagnostic tools that accurately monitor and control the spatial degree of coherence. MBA lattice designs in modern fourth generation light sources significantly reduce the transverse beam size, posing challenges for beam monitoring due to diffraction effects at lower energies and technical limitations at higher energies. This work demonstrates that X‐ray interferometry can overcome diffraction limits by measuring the spatial degree of coherence. Proof‐of‐principle double‐slit interferometry experiments were conducted on the BXDS-IVU beamline at the Canadian Light Source (CLS) to quantify the modulus of the complex degree of coherence. The degree of coherence was measured as a function of the vertical source size. Systematic adjustments to the secondary source size and coupling revealed changes in degree of coherence. The method achieved sub-micron sensitivity in detecting source size variations, demonstrating that X‐ray interferometry can accurately measure small changes in degree of coherence and holds promise for measuring small beam sizes in fourth generation light sources. To validate the results, additional supporting experiments were conducted. The linearity of the detector was assessed as a function of incoming photon flux by gradually increasing the current of the electron beam in the storage ring. A refined model of the linear terms of the storage ring lattice was developed by using the Linear Optics for Closed Orbit (LOCO) algorithm, minimizing the difference of the measured and modeled response matrices. The LOCO-tuned model provides insights into the variations in the vertical beam size at different source points in the storage ring as the coupling factor is adjusted while the vertical source size was monitored by the X-ray synchrotron radiation (XSR) diagnostic beamline. Further coupling measurements were performed using the closest-tune approach with a bunch-by-bunch feedback system.