Challenges for the FCC-ee machin detector interface alignment

The Future Circular Collider (FCC), a 93 km long circular collider, is one of the projects under study at CERN for the post Large Hadron Collider (LHC) era. Its goal will be to search for new particles while confirming and refining measurements on known ones, such as the Higgs Boson. In order to reach the designed luminosity and the requested accuracy of the collider, the Machine Detector Interface (MDI), and more precisely the components inside, such as the final focusing quadrupoles, the Luminosity Calorimeter (LumiCal), screening and compensation solenoids, will need to be extremely precisely aligned and monitored. The alignment of the accelerator components of the MDI is always difficult due to the detector components around the interaction point. This non continuity in the accelerator also creates an inevitable hole in the alignment system. One needs to design a system around the detector in order to align both sides of the accelerator as no space is available through the detector and therefore no line of sight through the detector is allowed. Though, the MDI currently designed is an innovative and elegant solution, having the final focus components of the accelerator, such as quadrupoles or Beam Position Monitor (BPM) and the LumiCal supported by a skeleton to hang in cantilever configuration inside the detector. This design raises additional challenges as it will limit the type of sensors usable in these confined conditions due to the lack of space, radiations, cryogenic cold and magnetic fields. Today, preliminary known alignment requirements for the inner components are extremely tight and current alignment systems cannot reach these values. This paper will underline these complexities and show why they represent challenges for the FCC-ee MDI alignment. Design, sensors, technology, and alignment tolerances will be discussed.