Frequency scanning interferometry based deformation monitoring system for the alignment of the FCC-ee machine detector interface

In a particle accelerator, two particle beams are accelerated and collided at one or several physics detectors. The machine detector interface (MDI) is the place where the accelerator ring is linked to the physics detector. It is the area where the alignment requirements are the most stringent, and the MDI of the lepton version of the future circular collider (FCC-ee) is no exception. It will implement a complex design and alignment requirement for typical components in the MDI are of the order of 30 μm. Until now, no working solutions have been proposed for the alignment of such MDI. In this contribution, two different systems are studied: the Surveillance d’Ouvrages par Fibre Optique (Structural Monitoring by Optical Fibers, SOFO) and the in-lined multiplexed and distributed frequency scanning interferometry (FSI). Both systems allow individual and simultaneous distance measurements of multiple portions along a fiber, and a new system using the FSI technology is proposed for the alignment monitoring. Using a network of helical shaped fiber allows the measurement of deformations if a sufficient amount of fibers is placed on the cylinder. With a sensor measurement accuracy of 1 μm, 3D coordinates of points at the end of a 4 m long and 0.25 m radius cylinder (FCC-ee MDI dimensions) can be determined at an accuracy under 5 μm. This system will provide a link between the alignment of the final focusing quadrupoles and the rest of the machine.