LHC Project Note 340 31 March 2004
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Executive summary of the workshop on LHC dipole geometry and stability and preparation for the implementation of changes M. Bajko, J.B Jeanneret, D. Missiaen, V. Parma, A. Poncet, W. Scandale, F. Seyvet and D. Tommasini Keywords: dipole,geometry
Summary A workshop was held 16th March 2004 in order to set a clear picture of the status of the geometry of the dipoles of LHC and to assess the need, or not, of a change of the hardware baseline method used to position and lock the cold masses in their cryostat vessel. This document summarizes the content of the presentations and spells the recommendations made at the workshop. An implementation of changes to the hardware baseline are outlined.
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Introduction
In the course of last year, it appeared that the transverse geometry of the main dipoles (MB) had not the expected stability, even for those MB’s which were not ’reshaped’. The reshaping action consisted in applying a mechanical action in order to obtain the nominal shape within the required tolerances for those cold masses which were out of tolerances after the welding action. This correction appeared to be inefficient and its use was stopped. The cold masses were migrating back to there original shape with time. This was bad in that not only the shape was inadequate in terms of aperture, but in addition, the corrector element were later misplaced by the return process. Last summer, the statistics of non-reshaped magnets was too small to draw conclusions, and was also biased because only MB’s with good initial shape were not reshaped. These intermediate conclusions were presented to the MARIC committee, accompanied by the recommendation to adapt the baseline assembly work in order to stabilize the MB. But the MARIC considered on the one side that the evidence for large instabilities was not sufficiently obvious to justify a change of the baseline, and on the other side that the proposed solution, namely to block the central post of the vacuum This is an internal CERN publication and does not necessarily reflect the views of the LHC project management.
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vessel was not sufficiently studied for being integrated into the baseline. The MARIC asked in addition to study the advantage of an end-align procedure, in order to solve the problem of out-of-tolerance extremities, see Ref. [1]. Presently a large number of non-reshaped MB’s data is at hand, and the blocking of the central post was the subject of several studies. It was therefore decided to organize a workshop, with the aim to review these issues and to express recommendations on both the need, or not, of new hardware steps, and on the usefulness and absence of risks of the proposed hardware solution. The program of the workshop is given in Annex 1, and the list of participants in Annex 2. Electronic copies of the presentations can be found in the WGA web site, see Ref. [2]. Most of this paper contains the summary and conclusions of the workshop, while the organization for the implementation of the changes to the baseline is discussed in Section 5.
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Summary of presentations Status of geometry
Geometry in industry, M. Bajko. The vertical geometry of the cold mass is well controlled. The most difficult parameter is the horizontal sagitta. Thanks to enlarged tolerances (H × V = ±1.5 × ±0.85 mm2 ), no cold masses had to be refused, but the tolerance is saturated. On the other hand, the tolerance of the position of the endflanges is nicely respected. Detailed statistics per company and with serial number show that after a trial period two firms seem to have reached an asymptotic regime, i.e,. the running average of the sagitta is stable. But, during a quite long period, the firm 1 exhibited a slow decay of the value of the sagitta (flatter cold masses). No obvious explanation was found until now. A recent severe correction of the welding press (12 to 13.5mm) indicates improvements (to be confirmed with a larger sample). Sagitta fluctuations remain large and difficult to explain. No clear correlation with lamination pack length was found. There is no substantial hope to obtain much better sagitta control. LHC Dipole Geometry at CERN; Stability Issues, E. Wildner. Sagitta analysis gives results similar to Marta’s talk at ITP20. Sagitta evolution from ITP20 to SMI2 through cold tests indicated further quite erratic changes (∼ 1mm r.m.s sagitta change from ITP20 to WP08 (half this value applies for aperture loss). The erratic behavior of sagitta changes is visible at all steps of measurement with no obvious saturation of the phenomenon toward the last steps measured. No correlation was found which may be used to build an indicator of stability. A sub-sample of 13 MB’s was fully measured at SMI2. These magnets were chosen for both good geometry (in particular small sagitta error) and for little changes between ITP20 and WP08. The geometry observed at SMI2 is not better (in fact slightly worse) that the one of the entire sample of 78 MB’s at WP08. It therefore seems that similar shapes at WP08 and ITP20 result from a stochastic chain of events rather than from particularly stable cold masses. No correlation was found which may be used to build an indicator of stability. 2
Beam-based specifications compared to existing MB’s, B. Jeanneret. At WP08, spool piece positions are in tolerance vertically (mean,sigma) but close to the limit horizontally (both mean and sigma). Due to the change of sagitta after the cold test, one third of the MB’s have at least one aperture out of tolerance, but this can be corrected by an end-align procedure by transverse translation. As for aperture, late ITP20 production is slightly worse than before (a few MB’s are at the edge of the WP08 tolerance already at ITP20). This is partly related to the interruption of the reshaping process and maybe to the increase of the production rate. At WP08 and after end-align, 10% of the MB’s are out of the aperture tolerance. Further considering the observed changes between warm and cold conditions and the substantial erratic changes observed between WP08 and SMI2 (most likely a relaxation of stress accumulated during the warm-up after the cold test), this fraction grows to 17%, while the effective mid-cell offer is 16%. Considering in addition a possible asymptotic degradation (moving to the tunnel, further warm/cold transition, magnetic cycle) of .12mm mean and 0.24mm r.m.s of the horizontal shape (or 50% of the changes observed between ITP20 and WP08, added quadratically for r.m.s.), the fraction deserving mid-cell installation would grow to 22%. But the most unstable MB’s cannot be identified in advance, thus the choice of installing a MB near a MQ or in a mid-cell location may often appear later as a bad (or randomly made) decision. All these quantities are obtained by using the end-align procedure which improves both end positions and aperture. Even if the additional degradation factors are a bit speculative, their relative effect is worrying (5% more candidates for mid-cell locations with ∆x = 0.12 mm and σ(x) = 0.24 mm). In agreement with Elena’s conclusions, no useful correlation was found which may help to build an indicator of stability. Finally, dispersion suppressors must house ’golden dipoles’, with reduced aperture tolerances (tx × tz = ±0.8 × ±0.5 mm2 in the tunnel at installation). With WP08 to tunnel degradation of r.m.s horizontal dispersion of ∼ 0.5 mm, a large fraction of them will no more be golden already at day 1 with beam.
2.2
Hardware issues and options
Vibration of cold masses, B. Skoczen. A model of the cold mass which include the shrinking cylinder (elastic) and the laminations (non-linear, with jump of friction coefficient from dry to viscous and asymmetric elastic coefficient which is large in compression and small in elongation) allows to predict a hysteresis behavior of the assembly. The amplitude of the remanent deformation is in the range of millimeters, matching quite well with observations. As for using longitudinal vibrations to reduce accumulated internal stress energy, they may help to eliminate stored stress energy out of the welding press, but could not be used with a cryostated dipole (to discard stress energy stored after warm-up). In addition, much more testing work would be needed to qualify the techniques for the quite complicated and delicate cold mass. Accelerations during handling and transportations, C. Hauviller. Accelerations during transport of cryostated MB’s have been 3D-measured and compared to specifications. The worst case appears during the transport from Prevessin to SMI2 (at the round-about). The measured acceleration is smaller or equal to specifications longitu3
dinally and transversely while the vertical limit is slightly exceeded in a few cases. The critical case for the option of blocking the central post is the transverse acceleration which was always below a = 3 ms−2 , except for one case (a = 5 ms−2 ). The forces applied to the cold mass/feet assembly were discussed. The effective mass which must be used to compute the force F = meff a is not known precisely but shall be close to one third of the mass of cold mass or meff 10 tons, without transport restraints. In this case, a force F = 38 kN is quoted. With transport restraints, this value must be smaller, but is not precisely evaluated. If it is felt necessary, it may be envisaged to limit the specified truck speed in the round-about (presently 20km/h, like everywhere else). Dividing the speed by two would divide a by four. Cold mass displacement induced by quench, M. La China. Transient cold mass transverse movements were measured during quench events. The measurements were made at the extremities, but can be quite precisely converted in displacements at the center of the cold mass were the largest excursions. The largest of the two possible values are considered. The largest transient excursion is .12 mm. A third of the excursion is induced by magnetic forces and the rest, quite delayed (time scale hours) is induced by asymmetric warm-up. These excursions are marginal w.r.t. central post induced stress. Load carrying capacity of the cold foot, V. Parma. Half of the support posts is now produced. Out of this, 5% were tested under severe compression and bending. Test guaranteed load cases are 175kN under compression, 40kN under bending and 105kN/25kN under combined compression and bending. After a constant transverse load of 25kN in real conditions (i.e. under and attached to a cold mass) which lasted 6 weeks no visible creep was visible. Extrapolated to the LHC lifetime, the creep will remain below 0.3mm with a transverse offset of 5mm and below 0.1mm with 1mm of offset. After applying a force of > 70 kN to the post with a total deformation of 17mm (cold mass : 12,5 mm and post : 4.5 mm), no macrocospic rupture or damage were visible. A small ovalisation was visible at the height of the pushing device, probably due to a peak of local stress force which exceeded the 70kN which are valid for the rest of the post. The safe transient force of 25kN reported here must be compared to the potentially possible 38kN applied during the passage through the round-about on the way to SMI2, see the comments at the end of Claude’s talk, this value must be re-evaluated towrds a lower value.
2.3
Implications of changes to baseline
Blocking the central post in industry, F. Savary. Presently, the geometry of the cold mass is fixed on a bench with three feet. The two external ones are fixed while the central one is free transversely. The shape of the cold mass is thus its natural one. The geometrical axis (GA) is then measured. The end plates, the spool pieces and the end-cap are all centered on the GA and fixed. Blocking the central foot would be a simple operation if the geometry of the bench is fixed once for all to the nominal curvature of the cold mass. This option would increase neither the time of assembly nor the costs. But the audience expressed some fears about induced stress on the central 4
post in the cryostat deliberately. Allowing for adjustment of the curvature inside a window (e.g. slightly smaller than the tolerance) in order to minimize the stress would introduce more complications. On the other hand (see Marta’s talk) the geometry at ITP20 is very rarely out of tolerance, and the option to keep it as is in industry is workable. As for measurements, a rigid bench would no more allow to get the natural shape (presently ITP15). The ITP15 package would become useless (the shape would be identical at ITP15 and ITP20) and could be suppressed. But large deformations would not be noticed and could impose large stress in some cases on the central post later at CERN. Blocking the central foot at CERN, F. Seyvet. Blocking the central foot can be made with a simple additional aluminum cylinder introduced in the hollow central post. Two small half rings and four screws are needed to lock together the cylinder and the post. Retrofitting existing assemblies is feasible (no need to de-cryostat). The only drawback for already finished assemblies is the need to open the vacuum ring of the central post and thus imposing a second leak test of the vacuum vessel. But, we were told after the workshop that the vacuum seals must be replaced anyway, after finding a deficiency on some of them. Two baseline options are considered. A Blocking the central post at its natural position at the moment of the cryostating (i.e. the ITP20 geometry, with in addition the possible degradation during the travel to CERN). B Forcing the central post to a definite position (i.e. ITP20) The hardware is identical with both options. With option B, more work is needed and the target geometry must be at hand in a usable form. The transfer function ∆xCM /∆xfoot was measured and agrees quite precisely with calculations. A cost evaluation indicates 550 CHF per magnet (500/hardware and 50/manpower). The error margin of this value is presently ±30%. The total extra cost would therefore be of the order of 680 KSF. Some extra work (fiducialisation, control, measurements at cold) must be considered for an initial period of validation.
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Discussion
The discussion can be summarized as follows: • With the observed changes of the geometry after ITP20, the geometry is already marginally satisfying the tolerances at WP08. Identifying golden MB’s at that level is dangerous, because the sagitta can still change further, as it was observed on the 13 MB’s measured at SMI2. This would spoil the aperture in the Dispersion Suppressors (golden magnets) and even at some near-MQ locations. Considering the data in cold state, and a small budget for later changes of geometry (warm/cold in the tunnel, quenches, further relaxation of internal stress), the tolerances w.r.t. aperture cannot be satisfied even when using the end-align procedure after WP08.
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• The changes of the spool piece positions is quite large and almost saturates the tolerances in the horizontal plane. This is not very harmful but will complicate the operation of the machine with beam. • It must be envisaged to improve the hardware baseline. Blocking the central post is the most interesting approach, although there is no experimental evidence as yet that it will fulfill the needs. • Blocking the central post transversely is feasible and simple, thanks to its present design. Additional work and cost are not outstanding. It remains to do some tests and measurements, in particular during the cooling and warming transient states. Worries about stress induced during transportations were expressed, but fatigue limits of the central post are larger than the observed stress. If necessary, the specified speed limit can be lowered at the round-about on the way to SMI2. • As for the implementation, the best approach would be to lock the geometry at the nominal curvature at ITP20. But, fixing the end elements of the extremities on the geometrical axis on the cold mass ’as is’ (this is the present baseline) is adequate, provided that a careful follow-up of the curvature is made in the industry and used to correct the welding press. • At CERN, the central post can be blocked during the cryostating process. It is difficult to make a measurement of the geometry through the beam tubes on the cryostating bench. It is therefore proposed to keep with option A (block as is, see Fabien’s talk).
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Conclusions
It was concluded that going on with blocking the central post, with the options summarized here above, is a workable option. It was recommended to modify the hardware baseline accordingly. First experimental validation requires a period of one month, after what the decision will be validated. But considering the urgency of the situation (the installation of magnets in the tunnel must start 31st May), the implementation work must go on with the prejudice that the central post will be blocked for all dipoles. The decision might be reversed only if a show stopper or other new facts appear in the meantime.
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Further work for implementating blocking the central post
Three kind of dipoles must be considered, namely the 1130 cold masses (of which 1000 are not yet built) which are still to be assembled in their cryostat (case a), the 100 ones which are already fully assembled (case b), and a few ones which will be treated rapidly during the validation period (case c). Then, three categories of assembled MB’s must be distinguished for installation, with different tolerances, namely ’golden magnets’ for the dispersion suppressors, good magnets for near-MQ location, and mid-cell locations where relaxed tolerances are allowed. Thanks to a recent proposal by D. Missiaen, see Ref. [3], 6
the time consuming fiducialisation procedure at SMI2 (where the geometry of the two beam tubes is performed, called WP08b) can be avoided in many cases. The method consists in a precise cartography of the extremities (position and angles of the end-covers) which provides not only the position of the extremities, but also the sagitta of the cold mass to a r.m.s. precision of ∼ 0.1 mm, except for cold mass deformations which differ from a change of sagitta strictly speaking. In the latter case the error maybe larger (≤ 0.4 mm) but is still adequate. Additionally, methods to assess on possible additional deformations between WP08 (SMA18) and WP08b (SMI2) from measurements points on transport restraints and/or measurements of cryostat deformations are being worked out (F. Seyvet and D. Missiaen). The procedures for cases a,b and c is as follows : a - New magnets : The central post is blocked at WP02 (cryostating) ’as is’. At WP08, a non-conformity (or a holding point warning) is issued if the observed sagitta change is too large, i.e. ∆S = SWP08 − SITP20 > 1 mm, as it is already the case for other tolerances. At that point it may be decided to either end-align the MB or to adjust the central foot, depending on the overall quality of the current production (more good magnets might be temporarily needed for installation). As for when and how to make the adjustment, see below, case b. b - Existing assemblies : For already assembled magnets, it is not envisageable to bring them back to SMA18. This would disturb too much the already stretched work-flow. It is therefore proposed to do the job at SMI2. Here, the three kind of quality choices required for installation deserve a similar but slightly different treatment. Using the analysis made with the existing set of finished and measured dipoles, we must consider to fix the geometry of a large fraction of these MB’s to the geometry of ITP20, in order to respect the tolerances of aperture in Dispersion Suppressors (DS) and in nearMQ locations. Practice will say which fraction deserves this approach. The other fraction, which will contain in particular the heavily re-shaped magnets, may rather deserve a ’as is’ or WP08 blockage to avoid inducing too much stress uselessly. The degree of precision at which the ITP20 geometry must be reproduced depends on the precision at which the adjustment can be made. This in turn depends on the one hand on the precision of the measurements of geometry (with end-cartography the upper limit of error is of the order of δRend−cart = 0.4 mm, while a full fiducialisation offers nearly 0.1mm). As already said, for time saving and organizational aspects of the work in SMI2, as much work as possible will be made with end-cartography. With the geometry of ITP20, the aperture specification is in the shadow of the end-flange specification, i.e. ∆Rspec = 0.87 mm. Therefore the ITP20 target shall be met to ∆r = ∆Rspec − δRend−cart 0.4 mm. The validation process will help to make these values more precise. It remains also to understand what is meant exactly by ’ITP20 geometry’ in the absence of a full fiducialisation. Basically and within the precision of the method, the sagitta must be equal in both geometries and the end-flanges lie both within their tolerances and be close to the their ITP20 values.
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DS: Golden magnets require a good geometry at ITP20 and a precise adjustment of the central foot. Three steps are needed. • • • • •
Choose candidates by inspecting the ITP20 geometry Blockage, to allow for relative adjustment End-cartography or fiducialisation (further work will say) Adjustment to ITP20 if necessary, then blockage Fiducialisation
Near-MQ and Mid-cell: This category is the largest one and the simplest possible procedure must be used. • • • • •
Choose candidates by inspecting the ITP20 and the WP08 geometry Blockage, to allow for relative adjustment End-cartography Adjustment to ITP20 (Near-MQ) or WP08 (Mid-cell), then blockage End-cartography (done already anyway)
A sub-category consists of cryostated magnets which did not yet go through the cold test or WP08. The central post shall preferably be blocked before proceeding with further steps. as spelled-out here above. c - Validation period : For a period of one month, 10 MB’s will deserve a special treatment in order to validate the procedure of blocking the central post, and to perform some checks with measurements. A validation criteria is that the sagitta of the cryodipole with the central post blocked shall remain between definite tolerance (exact values and statistical approach to be made precise by the WGA, once the precision of the positionning process and of the measurements with end-cartography are better known). The control of these criteria will be obtained via fiducialisation at several check points as follows. • Consider as much as possible new cold masses • Make a fiducialization at WP01 • Block ’as is’ at WP02 • Make a fiducialisation at WP03 as often as possible (target: 3 per week) • Proceed to cold test immediately, see below • Do the stripping then transport • WP08 - fiducialisation and /or • WP08b - fiducialisation after some transport (not to SMI2, but back to SMA18) • In addition transient data are needed during the warm/cold transition, see below Sect. 5.1
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5.1
Measurements at cold
It is required to measure the geometry at cold and to compare it to the one at warm, as it was already done during the last few months, in order to check that indeed the assembly cold mass/central foot/cryostat envelope does not deform more than calculated. In addition, in-situ measurements of relative changes of geometry of the cold mass between warm and cold (and back to warm) must be made on one or two magnets with the capacitive gauge developed by EST-SU. Finally, and in parallel, relative motion of the external fiducials must be done with an optical system during the cool down time interval (between 300K and 70K). If good correlations (or absence of movement) are observed, only the third method would be used later. Agreement of AT-MTM on these steps is needed.
5.2
Additional work at SMI2
Blocking the central post on already finished cold masses requires more work at SMI2. Candidates for ’golden dipole’ class require fiducialisation. This operation must be made with the cold mass positioned at its nominal height, while blocking the post must be done at higher height. Further arrangements must be discussed with AT-VAC.
5.3
Coordination
All the above activities will be coordinated by F. Seyvet and D. Tommasini, who will inform the WGA and the MEB.
Acknowledgments We would like to thank P. Lebrun for his support to the organization of this workshop, all the speakers for the preparation of clear presentations, and the participants to the workshop who contributed to the discussions and allowed to reach a conclusion. We had the benefit of the advice of S. Fartoukh when preparing the implementation work discussed in Sect. 5.
References [1] LHC/MARIC-74-2003,28th July 2003. [2] http://lhc-proj-wga-wgr.web.cern.ch/lhc-proj-wga-wgr/default.htm [3] D. Missiaen, E. Claret and P. Winkes, LHC Project Note 339, March 2004.
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Annex 1: Program of the workshop Workshop on the LHC Main Dipole Geometry and Instability issues, 16th March 2004, Building 30, room 7-012 ———————————————————————— Session 1 - 9h00-10h30 1.1. Geometry in industry, M. Bajko, 15 1.2. LHC Dipole Geometry at CERN, stability Issues, E. Wildner, 20 1.3. Beam-based specifications compared to existing MB s, B. Jeanneret, 25 (Talks 60 +30 discussion = 90) ——————————————————— Session 2 - 10h50-12h15
Coffee 20
2.1. Vibration of cold masses, B. Skoczen, 10 2.2. Accelerations during handling and transportations of the cryo-dipoles, C. Hauviller, 15 2.3. Cold mass displacement induced by quench and consequences on the central foot, M. La China, 10 2.4. Load carrying capacity of the cold foot, V. Parma, 15 (Talks 50 + 35 discussion = 85) ——————————————————— Session 3 - 14h15-16h00
Lunch
3.1. Implications of blocking the central post in assembly and measurements of the cold mass, F. Savary, 15 3.2. Blocking the central foot, F. Seyvet, 15 3.3 Summary, B. Jeanneret, 15 3.4 Discussion and recommendations (Talks 45 + discussion 60 = 105) ——————————————————— Organising Committee: M. Bajko (Co-Chairman), B. Jeanneret (Chairman), A.Poncet, W. Scandale 10
Annex 2: List of participants Kurt Artoos Marta Bajko Jerome Beauquis Luca Bottura Oliver Bruning Ofelia Capatina Stephane Fartoukh Cedric Garion Marco la China Claude Hauviller Jean-Pierre Koutchouk Philippe Lebrun John Miles Dominique Missiaen Michele Modena Steve Myers Vittorio Parma Alain Poncet Jean-Pierre Quesnel Jean-Pierre Riunaud Frederic Savary Walter Scandale Fabien Seyvet Blazej Skoczen Lucio Rossi Raymond Veness Jos Vlogaert Elena Wildner
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