Development of a displacement sensor for the CERN-LHC superconducting cryodipoles

One of the main challenges of the Large Hadron Collider (LHC), a new particle accelerator currently 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 consist of an evacuated cryostat and a cold mass containing the particle tubes and the superconducting dipole magnet. The latter is cooled by superfluid helium at 1.9 K. The particle beams must be centred in the dipole magnetic field with a sub-millimetre accuracy. This requires that the relative displacements between the cryostat and the cold mass must be monitored with great 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 based on low-coherence double interferometry, which measures with micrometer precision the distance between a mirror welded to the dipole cold mass and an optical head attached in the inner wall of the cryostat. This contribution describes the development of this novel sensor and the first measurements performed on the LHC cryodipoles.