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The Field Quality Crisis Unit of the LHC main dipoles production

Inner radial shift of conductor block in collared coil 3135

27th January 2004, Manufacturer 3. Warm magnetic measurements of collared coil 3135 (see file) show various red and yellow alarms in measurement positions 19 and 20  in aperture 2. The observed pattern is confirmed by a short mole measurement with a coil of a pick-up length of 12.5 cm. From the standard measurement, we see in position 19 on the normal multipoles: b2 of 7.5 sigma, b4 of 4.9 sigma, b6 of 5.5 sigma, b8 of 5.8 sigma, b12 of 4.7 sigma, b14 of 7.3 sigma and on the skew multipoles: a2 of 5.5 sigma, a4 of 6.8 sigma, a5 of 5.5 sigma, a7 of 5.2 sigma, a13 of 6.8 sigma, and a14 of 8.2 sigma. In measurement position 20, we see on the normal multipoles b2 of 5.0 sigma, b6 of 8.1 sigma, b8 of 5.0 sigma, and b12 of 5.0 sigma and on the skew multipoles: a5 of 4.4 sigma, a7 of 5 sigma, a9 of 4.4 sigma, a13 of 7.4 sigma, and a14 of 7.9 sigma.

The analysis of the inverse problem shows that the field anomaly of aperture 2, pos. 19 is compatible for all multipoles with an inward radial movement of block no. 6 of at least 0.55 mm (best fit) and of block 5 of at least 0.275 mm in quadrant 1 (see Figure below).

                Fig.: Expected defect in CC 3135 is a combined conductor movement of blocks no. 5 and 6.

17th February 04, Firm 3:  The collared coil 3135 undergoes the procedure of disassembling in presence of P. Fessia, E.Todesco and C. Vollinger from CERN. After the removal of the collar packages, it could be seen that the pole shim of the outer layer was pushed inwards and that it slid on the outer contour of the conductors of the blocks no. 6 and the copper wedge between the blocks 5 and 6 and the collars (see picture 2 and illustration in picture 1). The thickness of the pole shim is 0.8 mm (i.e., the nominal value) and due to the sliding the shim was folded.

Picture 1: Illustration of the defect found in collared coil 3135: the outer shim (thick solid line) slides from the nominal position (left part) toward the inner layer copper wedge (centre) and finally towards the collar pole (right part), pushing down the upper part of the inner layer (arrows)

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Picture 2:  Ill-positioned pole shim of the outer layer, sliding onto the inner layer and causing a movement of the inner layer blocks 5 and 6.

The ill-positioned pole shim left a weak indentation mark on the top cable of the outer layer (see picture 3). The observed defect caused a radial movement of the conductors of the block no. 6 of 0.8 mm inwards. Moreover, it is likely that block 5 was partially following the inward radial movement of block 6, since the glue between the conductors of the inner layer and the adjacent copper wedge was fully intact. The observed movement is exactly in agreement with the expectations from the simulation.

Picture 3: Indentation mark on the top cable of the outer layer resulting from the ill-positioned pole shim.

It was decided to proceed with a further coil disassembly of CC 3135 and the collared coil was fully de-collared by Noell personnel. The poles have been separated, later also the two layers of the lower pole of the second aperture have been partially separated on a length of about 2.5 m from the non-connection side coil end in order to check for further damage. We could not see any further marks on the conductors, neither was a permanent conductor movement visible.