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Session 2: Historical Overview and SpecificationsSession Chair: Jean-Pierre Riunaud, Scientific Secretary: Massimo GiovannozziJean-Pierre Koutchouk reviewed the different stages in the specification of the field quality of the LHC main dipoles starting from a beam dynamics point of view. In late eighties, beam dynamics was dominated by the b3 (random and systematic) multipole. The dynamic aperture was computed mainly by using fast indicators (smear and amplitude detuning) due to a limitation in computing resources. Also, experiments performed on existing machines (e.g. Tevatron) were used as guidelines in the design of the LHC. In early nineties, error tables (issued from numerical simulations of LHC dipoles, scaling from HERA dipoles etc.) started to be available, thus requiring the computation of the corresponding dynamic aperture. The simulations were performed based on simplified assumptions, e.g. all the dipoles follow the same Gaussian distribution (as long as systematic effects from different manufacturers can be neglected) and the lattice is super-symmetric. The target dynamic aperture is between 6-8.5 s. As a result of these studies, the cell length was set to the maximum allowed value and b3, b5 correctors were introduced at each dipole end. In addition, the inner coil aperture was increased from 50 mm to 56 mm. In the late nineties, the value of the target dynamic aperture was increased to 12 s to allow for a safety margin of a factor of two. This safety margin was deduced from the analysis of the limits of the tracking model as well as the experience of HERA, were simulated results showed the dynamic aperture being twice as large as the measured one. The increased computer power allowed to improve the computation of the dynamic aperture: additional criteria, such as nonlinear resonances, chromatic coupling, chromo-geometric detuning terms etc., were introduced. Error distributions which differ for each octant, non super-symmetric lattice, tune split to help the correction of coupling effects, additional a3 and b4 correctors, and a reduction in the number of b5 spool pieces by a factor of two, were the new ingredients in the numerical simulations. An important achievement is the definition of a target error table (always for injection energy) for both the main dipoles and the main quadrupoles producing a target dynamic aperture. Presently, the actual reference is the LHC Project Report 501, in which the values of the normal and skew harmonics for the main dipole are derived based on criteria such as the control of the mechanical aperture and the preservation of the dynamic aperture. With respect to the error table 9901, the new target values represent a consolidation. The influence of the multipoles a1 and B1 is considered for the first time, including an impact on the closed-orbit correction system. For some harmonics, the tolerances were relaxed (a1, a2, a3, b4, b5, b7), while for others (B1, b2) they were tightened. Finally, b3, a4, and a11 are unchanged. The requirement on the b3 at high field has been stable (with one exception), namely it should be positive and as large as is allowed by the b3-correction system of the dipoles. In this way, the b3 at injection is minimized. It is stressed that the impact of the increase of b5 tolerances on off-momentum dynamic aperture should be carefully studied. No clear mechanism has been proposed to explain the asymmetry in the tolerance bands for b7, which might be worth testing against small variations of the parameters used in numerical simulations. Stephan Russenschuck reviewed the evolution of the design of the cross-section of the LHC main dipole. Starting from the Yellow Book design (1995) that featured a five-block coil design, a beam separation distance of 194 mm, and combined aluminium collars with a ferromagnetic insert (MBP1), the design of the main dipole for the LHC has undergone a considerable evolution. The five-block coil was originally designed for a magnet with separated collars and a beam separation distance of 180 mm. The main advantage of the five-block coil is that it provides the highest possible average quench margin (of both inner and outer layer). Design changes on the five-block coil, which were carried out in 1996, made it very inflexible to even small adjustments. These changes were motivated mainly by a request from SL-AP for a partial compensation of the persistent currents, namely a reduction of multipole b3 at injection field from -4.8 to -4.0 units, calculated at 10 mm, which is equivalent to Db3 of 2.3 units computed at 17 mm. Additionally, the thickness of the ground plane insulation, the conductor insulation, adjustments at the cable's narrow edge, and the ferromagnetic insert in the combined collars had made the five-block coil very inflexible. However, flexibility is needed to compensate the lower order (odd) field harmonics that arise due to deformations during manufacturing and cool-down. Additional objectives that were taken into account for the coil re-optimisation included a lower b11 field component, an increase in the quench margin (inner layer coil), a better mechanical support (conductors placed as radial as possible) and lower sensitivity with respect to manufacturing tolerances. The coil design was found by using genetic optimization algorithms and a detailed study of three different design options. This led to the so-called V6-1 coil design (six-block coil with 40 turns; one turn less than the original five-block coil version). The V6-1 coil remained unchanged since autumn 1998 and a final adjustment was foreseen, as soon as sufficient data from the prototype phase would have been gained. A re-design of the iron yoke was triggered in 1999 from mechanical considerations, for example, manufacturing difficulties concerning the ferromagnetic insert part. Additional objectives were a lower variation of the b2 and b3 field components versus excitation and a reduction of the b3 component at injection field level. The MBP2 yoke design had subsequently undergone engineering changes to improve tooling and manufacturing and to enhance the rigidity of the structure. A re-optimisation of the shape of the iron yoke or the coil block configuration was not performed. Changes include an increased “nose” in the insert, cut-offs for the compensation of b2 and b4 drifts due to this nose, and the change of collar material to stainless steel with a relative permeability of 1.0022. As the design of the magnets is now frozen and the computational tool allows the modelling of very fine geometrical details, a refined numerical ROXIE model was created taking into account the modified shape of the iron yoke and the stainless steel collars. Also, the influence of the beam screen is now considered. Detailed analysis and comparison between simulation of beam screen effect and direct measurements showed a good agreement. Although the integrated design process is well established, it was not really used during the various iterations. This resulted in a field quality of the pre-series magnets that do not meet the SL-AP target error tables. Michele Modena: Final Design of the LHC Dipole for Pre-series and Series Contracts Michele Modena first described the process of the main dipoles production, the different components participating to the field quality achievement, their tolerances, and the specifications given to the manufacturers. On the individual components, he pointed out how CERN has delivered both the end-spacers and the inter-layers directly to the companies only for the “Pre-Series” Contract. He emphasised that the magnetic length should be controlled within a range of ± 15 mm according to the specifications, thus resulting in a stacking factor in the range of 98.5 % ± 0.25 %. Concerning the acceptance tests, it should be noted that “CERN takes full responsibility for the magnet field quality…”, this means that a control of the multipoles is not foreseen as a measure to reject a magnet, provided all the assembly procedures are correctly followed by the manufacturers. During the production, two warm magnetic measurements will be performed on the collared coils and after yoke assembly on the cold mass. The first measurement is an official holding point on the production. It is expected that out of tolerance components or incorrect assembly will be revealed by these measurements (the latter being already occurred twice up to now). Fine-tuning of the magnetic length will be carried out symmetrically, by adding or subtracting special laminations to both ends. This is to avoid any movement of the magnetic centre of the main dipole. It should be noted that the magnet end effects would be different if the end-packs length change. Tests have been carried out introducing more iron-lamination in the end-packs and the results are in agreement with the expectations. It seems that the systematic differences between factories are mainly due to production tooling, rather than due to a variation of the materials. Oliver
Brüning: Criteria Used for Field Error Specifications Oliver Brüning reviewed the criteria that are used to define the target field quality of the LHC magnets. The mechanical aperture should be enough to accommodate closed-orbit distortion plus ten times the beam size. This condition imposes constraints on the closed orbit, parasitic dispersion, momentum spread, momentum offset, and b-beating, both at injection and at high energy and hence on multiple errors responsible for these effects. Alignment errors should be controlled so that the feed down errors from the short straight sections and the spool piece alignment should be smaller than the corresponding errors in the main dipoles. The correctors strength should allow correcting the various errors under most stringent conditions, i.e. top energy for both injection and collision optics, as well as for ultimate performance. Concerning the beam dynamics, it is said that upper and lower bounds on detuning as a function of the amplitude and/or the momentum offset and momentum spread are required to avoid resonance crossing (or at least to avoid low-order resonances). The target value of the dynamic aperture is 12 s. Tolerance bands are computed by varying the strength of each multipole until the dynamic aperture is reduced by 0.5 s (estimated accuracy of the numerical simulations). As a result, the target values for B1 and a1 are determined by the strength of the orbit correctors, b2 is bounded by b-beating, a2 by the correctors’ strength (skew quadrupoles); b3 and a3 are limited by correctors’ strength; b4, a4, b5 are limited by detuning considerations and dynamic aperture; b7 and the higher-order components are limited by the dynamic aperture.
Minutes by Massimo Giovannozzi |
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Last update on 07-04-2003 08:30:00
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