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The Field Quality Crisis Unit of the LHC main dipoles production

A missing shim in magnet 1027

29 october 2002, Manufacturer 1. Warm magnetic measurements of collared coil 1027 (see file) show a spike in b2 (red alarm, i.e. more than 8 sigma) and a yellow alarm in b3, a2 (yellow alarm, i.e. between 4 and 8 sigma) in position 8 of aperture 1. Position 7 is in between - this seems to indicate that the problem is not related to the measure, but that something wrong is in the magnet. The error is in all multipoles, i.e. no symmetry is followed: this means that it should be in one quadrant only. Moreover, only the low order multipoles are affected: this means that the source of error should be far from the aperture (i.e. outer layer). Simulations by E. Todesco on November 1 show that a missing shim of 0.8 mm fits well the numerical data; A. Devred confirms these results using a different code (see file)

6 november 2002, Manufacturer 1. The measurement is repeated, giving similar results.

7 november 2002, CERN. Meeting of the crisis unit: J. Vlogaert, W. Scandale, M. Cornelis, J. Rinn, C. Vollinger, E. Todesco, P. Fessia, J. Miles, A. Devred. The case is illustrated by E. Todesco and A. Devred. Even though the effect is much weaker than for 2002, the numerical evidence of a missing shim is strong. Foreseen location of the missing shim: aperture 1, right side, lower pole, outer layer. The decision of decollaring the magnet is taken. Wide discussions about the necessity of replacing the mass insulation and quench heaters, as indicated in the specification. W. Scandale asks for a double measure of field quality if the cold bore will have to be re-worked due to deformations.

12 november 2002, Manufacturer 1. Presents: M. Cornelis, J. Miles, J. Vlogaert. Collared coil 1027 is decollared. A missing shim along 1 m is found where foreseen by the analysis of magnetic measurements. All the insulation and quench heaters, are replaced.

18 december 2002, Manufacturer 1. magnetic field of collared coil 1027 is measured (see file); the spike in aperture 1 due to the missing shims has disappeared. This second measurement is performed with the new system. A spike in the main field of about 8 units in one position of aperture 2 is visible. For this reason the measurement is repeated (see file); the spike in main field is disappeared as it was related to a faulty measurement.

In the positions not affected by the missing shim, field harmonics showed a variation between the first and second decollaring similar to what already observed in Firm 2 (see related web page).

It is interesting to note that this collared coils has been refused for field harmonics variations much smaller than the ones in 1006 (see file). In the case of 1006, we had indications that the origin of the field variations were a coil waviness in all the poles of the order of 0.1 mm. This was considered not dangerous from the point of view of quench performances. Indeed, the magnet had no quenches in the straight part. In the case of 1027 the harmonic variations were traced back to a problem on one side of a pole only, i.e. a missing shim of 0.8 mm, and therefore it was clear that a decollaring was necessary. This example shows the impossibility of having a specification with ranges for multipole variations along the axis for collared coil acceptance: indeed, each case must be treated separately and the collared coil is refused only when there is a strong evidence of a faulty assembly procedure or component.