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The Field Quality Crisis Unit of the LHC main dipoles production

Inner radial movement of a conductor block in collared coil 2084

6th May 2004, Manufacturer 2. Warm magnetic measurements of collared coil 2084 (see file) show a problem in measurement position 7 of aperture 1. 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 on the normal multipoles: b5 of 5.8 sigma, b8 of 6.6 sigma, b12 of 5.8 sigma, b13 of 5.0 sigma, and on the skew multipoles: a4 of 5.3 sigma, a6 of 7.2 sigma, a7 of 4.2 sigma, a12 of 4.4 sigma and a14 of 5.6 sigma.

First analysis shows that the defect is not following any symmetry, i.e. it is likely to be in one quadrant only. The analysis of the inverse problem applied to the position 7 of the short mole measurement makes us expect to see an inward radial movement of block 6 exceeding 0.3 mm in the second quadrant of aperture 1 (see Figure below). Indeed, this movement leaves a very small anomaly in b14 of 4.3 sigma which cannot be explained by this conductor movement.

           Fig.: Expected defect in CC 2084 is a conductor movement of block no. 6.

9th July 04, Firm 2: The collared coil 2084 undergoes the procedure of disassembling in presence of  M. Bajko, F. Savary and E. Todesco from CERN. The removal of the rods was unproblematic. It has been decided to directly separate the poles of aperture 1 and not to open the collared coil from the top, since the defect is very localized.

After separation of the poles, a radial movement of block no. 6 of only about 0.15 mm inwards could be seen at the indicated spot. It is remarkable that block no. 6 was well glued to the copper wedge and fixed in this displaced position. Also, the glue between the two conductors of block no. 6 was fully intact and no conductor detachments could be observed along the coil length.