Field shape variation induced by thermal and electromagnetic forces in the 5-block 10 m long LHC dipole models

The field shape of three 5-block 10m long two-in-one LHC dipole models has been measured before and after the first training cycle. A Systematic variation is observed in the normal sextupole and decapole components in all coils. We show that this effect is induced by outward radial movements of four blocks of the inner coil. Tests with Fuji paper confirm our interpretation. Other random variation of the multipoles, mainly normal and skew quadrupole, are shown to be compatible with random displacement of the coil characterised by an r.m.s. value of 12-25µm. Scaling laws for multipoles induced by random displacements are worked out.