Metrology of the LHC dipole cold masses

In order to provide the largest possible mechanical aperture for the LHC beam, the dipole cold masses have to match the circular trajectory of the particle beam. The requirements on the dipole cold mass geometry are dictated by the beam optics of the LHC machine and by the mechanical deformation limits of the interconnection zone. The geometry of the approximately 15 m long, 0.57 m diameter, and 30 t weight dipole cold mass is verified by the measurement of the axes of the cold bore tubes. The tight tolerances imposed, necessitate the use of a high accuracy 3-D measuring system based on optical methods. During the last 2 years, 6 prototypes and 4 pre-series magnets have been assembled at CERN. The summary of the results obtained on these cold masses is presented, as well as the evolution of the tooling and the measuring method.