Speckle interferometry measurements of the conductor displacements in the CERN-LHC main dipole

Magnetic field quality in superconducting magnets strongly depends on conductor position in operational conditions. Simulations based on finite-element models (FEM) provide the field of stresses and displacements inside the magnet. Due to the complex mechanical behavior of the coil and to the different materials composing the magnet, it is not trivial to build a reliable mechanical model. We present an experimental method based on optical measurements to validate the accuracy of the FEM in evaluating differential displacements at room temperature. A short sample of dipole coils is loaded through a small press and speckle interferometry measurements detect differential displacements with an accuracy of 1.5 /spl mu/m. Comparison with the numerical results allows the testing of the most critical features of the model, i.e., a pressure-dependent coil elasticity and the interfaces between different materials. A good agreement between measurements and deformed geometry foreseen by the FEM is found.