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The Field Quality Crisis Unit of the LHC main dipoles production

Systematic component of a4 in Firm2

November 2003, CERN. magnetic measurements of collared coils in Firm2 show an a4 systematic component of around 0.5 units since May 2003. Since the target range for a4 is very tight (0.15 units at most), if such a component would stay for all the production of Firm2, we would go out of the range.

17th December 2003, CERN. Crisis Unit meeting. Presents: Walter Scandale, Andrea Musso, Frederic Savary, Marta Bajko, Ezio Todesco, Christine Vollinger, Elena Wildner, Iouri Vanenkov, Per Hagen, Michele Modena. The situation of data is presented by Ezio Todesco (see ppt slides). The systematic component appears after collared coil 20, it is confirmed by data at cold, and is not associated to anomaly in a2. It could be due to a radial wave of period three with an amplitude of 0.05 to 0.1 mm.
- The connection with the additional insulation (cross-section 3) is excluded: in the other firms no effect is seen. Moreover, such an a4 should also give rise to a strong a2, if due to a shift in the midplane. Finally, high values of a4 are observed also in magnets with X-section 2.
- Andrea Musso has carried out a preliminary analysis showing no correlation of these large a4 with cross-section type or shim dimension.
- Iouri Vanenkov has carried out a preliminary analysis on correlation between multipoles, finding out some correlation with b2 and the collared coil dimension.
- The source of the asymmetry could be either the coil, or the coil assembly (insulation, protection sheets), or the collaring. It seems improbable that the coil is the source, since Firm2 does not use any special selection rule for upper and lower coil and it looks improbable that coils with similar features always end up in the upper (or lower) part. Anyway, in the case of 2032, that showed high a4 and has been disassembled for other problems (see web page), the coil has assembled the second time with the upper poles at the place of the lowers. A switch of the sign of a4 would imply that the coil is the source. Magnetic measurements will be carried out in the next weeks.
- A possibility could be that the tiny pieces of adhesive tape used during the assembly of insulation shims and quench heaters, if used only in the upper part, could produce a wave creating an a4.
- How to cure the problem by simmetrization: if the source is the coil or the assembly procedure, it should be enough to invert in half of the magnets the upper and lower part before putting the collars. If the problem comes from the collaring, one should explore the possibility of capsizing the collared coil when the iron yoke is assembled. This second possibility is less appealing since it would create problems in traceability (aperture 1 of collared coil would become aperture 2 in cold mass, and capsized). The increase in the random component should not be too critical.

(... to be continued ...)