Development of a Displacement Sensor for the CERN-LHC Superconducting Cryodipoles

One of the main challenges of the Large Hadron Collider (LHC), the particle accelerator under construction at CERN (the European Organization for Nuclear Research) in Geneva, resides in the design and production of the superconducting dipoles used to steer the particles around a 27 km underground tunnel. These so-called cryodipoles are composed of an evacuated cryostat and a cold mass, that contains the particle tubes and the superconducting dipole magnet and is cooled by super uid Helium at 1.9 K. The particle beam must be centred within the dipole magnetic field with a sub-millimetre accuracy, this requires in turn that the relative displacements between the cryostat and the cold mass must be monitored with accuracy. Because of the extreme environmental conditions (the displacement measurements must be made in vacuum and between two points at a temperature difference of about 300 degrees) no adequate existing monitoring system was found for this application. It was therefore decided to develop an optical sensor suitable for this application. This contribution describes the development of this novel sensor and the first measurements performed on the LHC cryodipoles.