Session 4: Procedures to Follow-up Production and Preliminary Analysis

Session Chair: Jean-Bernard Jeanneret, Scientific Secretary: Massimo Giovannozzi

Marta Bajko: Assembly Procedures and Follow-up of Dipole Shape at 300  K

Marta Bajko discussed the assembly procedures and follow up of the dipole shape at warm conditions. Constraints derived from beam dynamics considerations impose limits to the mechanical tolerance of the order of 2 mm. Furthermore, to minimise feed down error generated by spool pieces, admissible alignment errors are defined to be ± 0.3 mm systematic and 0.5 mm random in both horizontal and vertical planes. Cylinders are bent (one half) to with approximately 9 mm sagitta. The whole magnet is bent under the press, during welding. About 23 % of its curvature is lost due to elastic energy. Therefore, a slightly higher sagitta has to be generated to compensate for the spring back. Each manufacturer follows the same procedure. An iterative procedure was applied until the value of 9 mm for the sagitta was achieved. After several attempts it was found that a sagitta of about 12 mm during the welding combined with half cylinders of the same sagitta gives the right final value. Re-shaping was applied in the industry on several magnets during the optimisation of the initial parameters to correct out of tolerance sagitta after the welding process in the press. The re-shaped magnets show signs of instability, i.e. their shape tends to go back to the initial state that it had before re-shaping. Presently, six out of eleven considered dipoles show such an unstable behaviour. There are ten more dipoles with high probability of this type of behaviour. For these magnets spool pieces might be displaced transversally by as much as 1-1.5 mm. Concerning this point, it was mentioned that, from beam dynamics considerations, horizontal or vertical misalignment of the spool pieces are not equivalent. In fact, vertical misalignment of a decapole spool piece generates skew components (e.g. a₄) for which no corrector is available. No indications concerning the stability over longer periods (LHC lifetime) are available. Following also a suggestion endorsed by the Machine Advisory Committee, no more dipoles will be re-shaped, while waiting for a better solution to this problem.

Juan Garcia: Follow-up and Checkpoints of Magnetic Axis at 300  and 1.9  K

Juan Garcia presented measurements of the magnetic axis that are carried out both at warm and at cold conditions. These measurements are performed with different instrumentations at CERN and at manufacturers premises. A comparison between the results taken with the two systems could be observed and is currently investigated. Concerning the magnetic axis, a deviation of the horizontal and vertical offsets with respect to the theoretical geometry could be observed before and after cold tests. Measurement examples were shown for different manufacturers illustrating this problem. Further tests on the warm-cold correlation are planned to start in May 2003. Then, measurements of the magnetic axis compared to the theoretical geometry axis for both apertures are presented, showing a fair agreement. A detailed analysis on the mechanical stability of the dipole shape before and after cold test is in progress and results are expected in the next months.

Walter Scandale: Geometry: Analysis and Trend

Walter Scandale reviewed the present status and trends of dipole geometry. The spread in dipole shape, for the set of magnets under consideration, is rather large, certainly out of the mechanical tolerances. Three situations were considered, i.e. after welding, after manufacturing, and after cold test. The spread is minimum after manufacturing, while it goes back to its initial state after cold test. It is confirmed that re-shaping is not a stable solution to cure problems related with dipole shape. The goal now is to find better solutions within the first six months of the year 2003, and then to take a final decision concerning the strategy to follow. Another crucial point is that rather large movements are observed on the magnet ends, which might be a critical issue for the positioning of spool pieces. Four out of six dipoles follow this behaviour: intermediate stages show certain variability, not the initial and final stages. One dipole shows a different behaviour, i.e. after cold test it goes back to its state after re-shaping. Presently, no measurements have been performed to check whether the magnet continues moving after each cool-down: this might be worth investigating. However, it is already clear that quenches do not seem to have a significant impact on shape.

Minutes by Massimo Giovannozzi