MECHANICAL COUPLING BETWEEN THE LHC CRYOGENIC DISTRIBUTION LINE AND THE SHORT STRAIGHT SECTION HOUSING THE SUPERCONDUCTING QUADRUPOLE - THEORETICAL ANALYSIS AND EXPERIMENTAL VALIDATION

Liquid and Gaseous Helium are supplied to the superconducting magnets of the Large Hadron Collider (LHC), presently being assembled at CERN, by a cryogenic distribution line (QRL). The cryogenic module of the twin-quadrupole Short Straight Section (SSS) is supplied with these cryogens through a jumper connection linking the service module of the QRL and the SSS. The internal as well as external features of the jumper construction allow for relative displacements between the QRL and SSS with sufficient flexibility to reduce the reaction forces responsible for elastic deformations when the SSS is moved for alignment. The SSS is composed of a cold mass and of a vacuum vessel equipped with fiducials posted on the external vacuum vessel reinforcement rings allowing the precise alignment of the machine when the cryostat is finally closed. A deformation of the structure linking the cold mass and the external vacuum vessel resulting from reaction forces induced by relative displacements of the SSS and the QRL, if unpredictable, would result in an unacceptable misalignment of the quadrupole magnetic axis. A unified FE model was generated at RRCAT to study the elastic behavior of the SSS under the conditions of alignment. This model was validated using a 40 meter long dedicated test setup at CERN. Correlations in the matrix form were generated so that all displacements of the cold mass can be related to the corresponding movements done for alignment. This transfer function, linking the action on the SSS external alignment jacks and the position of the cold mass will be used to properly align the machine in operation.