Influence of Azimuthal Coil Size on Skew Multipoles in the LHC Dipoles

The field quality in superconducting accelerator magnets is strongly influenced by the azimuthal dimension of the superconducting coil. Asymmetries between the upper and lower poles steer skew harmonics that can endanger the beam dynamics stability. We present dimensional measurements of a large number of coils that have been carried out in one of the three manufacturers of the main LHC dipoles. A magneto-static model is then used to work out the influence of coil non-nominalites on field harmonics. Comparison to magnetic measurements carried out at room temperature shows that skew harmonics can be partly traced back to azimuthal coil dimensions. We focus on harmonics, which are more critical with respect to the beam dynamics limits (a2 and a4). Finally some strategies are presented, like the sorting of individual coil poles in order to reduce the detrimental effects on these multipoles.