Instrumental Uncertainty in Measuring the Geometry of the LHC Main Dipoles

In the Large Hadron Collider 1232 superconducting dipoles will bend the two 7 TeV energy beams along a 27 km-long circular trajectory. The series production (assigned to three European firms) requires a well-defined procedure to check, in every magnet, the respect of the dimensional specifications. To verify tolerance of some tenths of millimeter over the 15-meter length in each cold mass, a laser tracker is necessarily used. To access the two beam apertures and to increase the measurement accuracies, the laser tracker is placed in different stations around the dipole defining a ’multi-station measuring procedure’. The noise affecting all the data taken so far suggested a careful analysis of the procedure itself. Through the computer modeling (based on a Montecarlo algorithm), the statistical error was quantified and compared to the experimental error. From this comparison the critical aspects of accuracy limitations from the multi-station procedure were better understood.