The 1-m model program for the main LHC dipoles is now mainly focussed on double-aperture magnets. In the past years an intensive program based on single-aperture dipoles allowed to select the series-design features among several variants for the coil cross section, the material of the collars and of the coil end spacers, the coil pre-stress and the cable insulation. The recent double-aperture models are dedicated to the fine-tuning of the baseline design and the manufacture of the coil ends. This paper reports about the fabrication and testing of these magnets and the results relevant for the series production of the 15-m long full-size dipole cold masses

Arteche, F

Sanfilippo, S

Siemko, A

Tommasini, D

Vanenkov, I

Wyss, C

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsSTATUS OF THE LHC SHORT DIPOLE MODEL PROGRAMMED.Tommasini, F.Arteche, S.Sanfilippo, A.Siemko, I.Vanenkov and C.WyssThe 1-m model program for the main LHC dipoles is now mainly focussed on double-aperture magnets.In the past years an intensive program based on single-aperture dipoles allowed to select theseries-design features among several variants for the coil cross section, the material of the collars and ofthe coil end spacers, the coil pre-stress and the cable insulation. The recent double-aperture models arededicated to the fine-tuning of the baseline design and the manufacture of the coil ends. This paperreports about the fabrication and testing of these magnets and the results relevant for the seriesproduction of the 15-m long full-size dipole cold masses.LHC DivisionPresented at the Seventh European Particle Accelerator Conference  (EPAC 2000)26-30 June 2000, Vienna, AustriaGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 427Abstract23 September 20001STATUS OF THE LHC SHORT DIPOLE MODEL PROGRAMMED.Tommasini, F.Arteche, S.Sanfilippo, A.Siemko, I.Vanenkov, C.WyssCERN, Geneva, SwitzerlandAbstractThe 1-m model program for the main LHC dipoles isnow mainly focussed on double-aperture magnets. In thepast years an intensive program based on single-aperturedipoles allowed to select the series-design featuresamong several variants for the coil cross section, thematerial of the collars and of the coil end spacers, thecoil pre-stress and the cable insulation. The recentdouble-aperture models are dedicated to the fine-tuningof the baseline design and the manufacture of the coilends. This paper reports about the fabrication and testingof these magnets and the results relevant for the seriesproduction of the 15-m long full-size dipole cold masses.1  THE MODEL PROGRAMMEThe CERN program of one-meter-long dipole modelsfor the LHC started in 1995 with single-aperturemagnets. In total 23 of such units were built until mid-1999: several of them were re-assembled into newvariants, totalling thus 39 single-aperture models testedat cold at a rate of about one per month. This programallowed to validate important choices for the main LHCdipoles, like the cable insulation, the coil cross section,the material and geometry of the coil end spacers, thematerial of the collars and finally the choice of assemblyparameters like the coil pre-stress and the relationbetween field harmonics and coil size [1,2,3,4].Since August 1999 the model program is focussed onthe fabrication of double-aperture models to study thetraining performance of the two-in-one structure. A nextand final step will be reproducibility and field qualityissues. 2  FEATURES OF RECENT MODELS The layout of the short dipole single-aperture modelswas already presented in previous conference papers [5].The same concept applies to the double-apertureversions, which feature the same collars and yokelaminations as the main dipoles, held together by abolted shrinking cylinder for easy re-assembly of thestructure. All the four double models and their differentversions presented in this paper, in total ten magnets,were made with most of the baseline components anddesign features set for the series production of the LHCdipoles. In particular the cable characteristics, the coilcross section, the material of the coil end spacers and thecollar geometry (with minor variants) were the same inall cases. Table 1 gives the main parameters.  Table 1: Main parameters of the short dipole models. Cable inner layer mid-thickness / keystone angle  1.90 mm / 1.25 deg Cable outer layer mid-thickness / keystone angle  1.48 mm / 0.90 deg Coil inner diameter  56 mm Coil length  1080 mm Number of turns inner/outer layer  15 / 25 Nominal magnetic field  8.33 T Nominal current  11.8 kA Short sample limit  9.8 T  The differences between magnets concerned mainlythe coil heads, which appeared in double aperturemodels to be weaker than in the single-aperture ones andrequire a tighter specification of assembly parameters.Table 2 summarises the specific features of thesemagnets with emphasis on the coil heads. All magnetsfeature austenitic steel collars, but in case of T4 the so-called "floating" collars (those which are not retained bythe collaring rods) where made in aluminium, and incase of T5 they were in plastic for versions 2 and 3. Incase of T6 and of T7 the material of all collars was thesame austenitic steel as foreseen for the LHC seriesdipoles.  Table 2: Main specific features of recent short dipoles.Magnet Specific featuresT4 V1V2V3V4V5T5 V1V2V3T6 V1T7 V1collared with high pre-stress gradient in the coil headsre-collared with aluminium collars in the coil headsre-collared with a longitudinal clamping structure in the headsferromagnetic yoke more far away from non-connection sidere-collared, longitudinal clamping structure not tightenedmore uniform azimuthal pre-stress on the coil headscollared with different collars (austenitic steel + plastic)new assembly of non-connection side major end spacerslower pre-stress in the non-connection side headslower pre-stress in connection and non-connection side heads3  FABRICATION AND COOL DOWN2.1  CoilsAll coils have been wound in the same way withcables of similar characteristics. In all cases the cablewas wrapped on line with the baseline all-polyimideinsulation [4] during the winding process, its tension wasof about 700 N for the inner layer and 500 N for theouter layer. All coils were built with the same type ofend spacers, of the isoperimetric type and made in G-11with the exception of the smallest ones (filled black infig.1) which were in ULTEMTM for magnets T4 and T5,in IXEFTM for T6 and in G-11 for T7. The bonding-sizingcycle was the baseline foreseen for the series LHCdipoles, based on heating to 135°C for coil sizing under2pressure (between 70 and 100 MPa), and furtherwarming up to 185°C for turn-to-turn bonding. Duringthis thermal cycle the coil extremities were blockedagainst stoppers.The main coil fabrication detail of the modelsdiscussed in this paper is the assembly of the major endspacer of the outer layer (fig.1).Figure 1: Layout of inner and outer layer ends andazimuthal pressure distribution.The usual way of coil fabrication is to start windingaround a metallic end spacer, which is, after thebonding-sizing cycle, replaced by a G-11 spacer gluedonto the coil with epoxy resin.In case of T5.V3 the major end-spacer of the previousversion was re-glued with a 0.2 mm thick B-stage epoxypre-preg tape placed between the spacer and the cable.This operation had two purposes :1. provide a better matching between cable-end andspacer;2. match the end-spacer size to the coil geometry ofstraight section In case of T7 the G-11 major end-spacers of bothinner and outer layer were mounted already duringwinding and, as for T5.V3, a pre-preg tape was insertedbetween the spacers and the cable. 2.2  Assembly parameters The main assembly parameters explored in thesemagnets concern the coil heads, the confirmation of thegood performance of mixed-material collar-packs andthe effect of moving the ferromagnetic part of the yokemore far away from the non-connection coil end.1. Pre-stress profile in the coil heads.To confirm the results obtained on single aperturemodels [4] the four magnets have been assembled withdifferent pre-stress profiles in the coil ends. T4.V1 hasthe typical profile of single aperture magnets, T5.V1 amore uniform pre-stress distribution like the singleaperture magnet S23.V3 [4], T6.V1 a base pre-stress inthe non-connection head of 40 MPa instead of the usual65 MPa, and T7 only 35 MPa this time in bothconnection and non-connection heads (fig.1).2. Use of different clamping structures in the heads.A longitudinal clamping structure, referred as "end-cage", was used for pulling the coil ends against themagnet end plates by tie bolts acting through the collarsbetween the major end-spacers and the end-plate. Thisend-cage was used for T4.V3 and T4.V4, thereafterreleased for T4.V5.3. Ferromagnetic yoke further away from coil heads.The ferromagnetic yoke of the LHC dipoles does notcover the coil ends, but is at about 80 mm from thebeginning of the major end-spacers. On T4.V4 andT4.V5 the above distance was increased by 110 mm.4. Use of mixed-material collar-packs.This concept was already explored on single aperturemagnets [4].  T4.V1 was collared with the main collarsin austenitic steel and the so-called "floating" collars inaluminium, T5.V2 and its subsequent versions werecollared with the floating collars made out of plasticmaterial. 2.3  Coil pre-stress at cold & during excitationThe inner and outer layer azimuthal stress, measuredat different assembly phases and at cold with capacitivegauges [7], is reported in Table 3. The design pre-stressvalues were chosen based on single aperture modelresults : between 50 and 60 MPa for the inner layer andbetween 60 and 80 MPa for the outer layer.  In theseconditions the inner layer loses pre-stress at a magneticfield of about 7 T and the outer layer not before 9 T.In all cases the pre-stress loss between ambienttemperature (after collaring) and cold followsapproximately the rule (in MPa):σcold≈ 0.5⋅(σcollaring  - 15)Table 3: Coil stress (MPa) in inner/outer layers.At coldagnet AfterollaringAfteryoking B = 0 T B = 9 T Unloadingfield (T)4   V1      V2      V3      V4      V55   V16   V17   V156 / 8550 / 7650 / 74- / -50 / 7450 / 6244 / 8759 / 8360 / 8755 / 8255 / 8055 / 8055 / 8052 / 6450 / 9765 / 9018 / 3218 / 3118 / 3017 / 2817 / 2816 / 2413 / 3622 / 320 / 40 / 40 / 40 / 30 / 30 / 00 / 70 / 67.1 / 9.57.1 / 9.57.1 / 9.57.0 / 9.37.0 / 9.37.0 / 8.87.0 / ~9.87.0 / ~9.6OUTER LAYERINNER LAYERPre-stress (MPa)20406080major end spacer of outer layerT5T4T734  TRAINING PERFORMANCE In all double-aperture models, practically no quenchesbelow 9 T occurred in the straight part. However,compared to single-aperture magnets, in the double-aperture design, because of the cross talk between thetwo apertures, the peak magnetic field in the outer layerheads is about 0.5 T higher and the central coil sides areexposed to a field higher than that seen by the lateralcoil sides. This difference is reflected in the trainingperformance: the optimization of the straight sectiongiven by single-aperture models in terms of coil cross-section and pre-stress parameters is still valid for thedouble-aperture magnets, but the coil heads appear to beweaker. Fig. 2 : number of training quenches in the coil heads.In fact, as shown in fig. 2, all the double-aperturemodels still show training quenches in the ends below9 T. From the good results obtained with T5.V3, itappears that a particular care is required in the assemblyof the major end spacers and in providing a sufficientlyhigh and relatively uniform azimuthal pre-stress in thecoil heads with tighter tolerances than what was allowedon single-aperture magnets. The importance of theseparameters seems to be confirmed by the badperformance of T6 and T7, which were assembled withlow pre-stress in the coil ends. To verify this assessmentT7 will be reassembled with higher pre-stress.Other options, like the use of longitudinal clampingstructures or the use of aluminium collar packs in theends, do not seem to bring an advantage in terms oftraining performance. The effect of shifting theferromagnetic yoke further away from the coil ends isstill under investigation. Preliminary results on T4.V4and on T4.V5 suggest a gain of about 0.2 T in thetraining performance.In reality most important for the LHC is the quenchlevel after a thermal cycle, and from this point themodels with sufficient high pre-stress in the coil heads,like T4 and T5, confirm the robustness of the presentdesign. In fact the first quench level after thermal cyclewas for example 9.15 T for T4.V4 and 9.4 T for T5.V3(these magnets reached a maximum field of about9.7 T).Finally the two magnets made with mixed-materialcollar-packs showed again, as in the case of single-aperture models, the same performance as that ofmagnets collared with all-austenitic steel collars. 5  CONCLUSIONS AND NEXT STEPS The excellent training performance of the straightsection of all the ten models confirm the robustness ofthe coil cross section and the correct choice of the designpre-stress levels with austenitic steel collars. Concerning the training performance of the coil ends,three outcomes can be derived from these models :1. first quench levels after the thermal cycle above 9.0T could be obtained with a particular care in theassembly of the major end spacers and a tight controlof the pre-stress profiles in the ends;2. first results obtained on T4 with the ferromagneticyoke longitudinally shifted 190 mm away for the coilends are encouraging and  are to be confirmed after athermal cycle and on other models;3. the use of a longitudinal clamping structure with thepresent design of end spacers does not seem toimprove the training performance of the coil ends. At present, even if performances acceptable for theLHC machine can be already achieved, the coil endsremain the weak point of these magnets. Model work isstill under way to understand and solve these lastproblems during this year. Finally, the short dipole model program will come toan end next year with the fabrication of three identicalmagnets for studying the reproducibility of trainingperformance and of magnetic field quality. 6  ACKNOWLEDGMENTSThe authors wish to express their thanks to L.Evans forhis support and active contribution to the short modelactivity, and to all the laboratory teams involved inmagnet fabrication and testing for the excellent work.REFERENCES[1] N.Andreev et al :" Mechanical behaviour of theshort models of LHC Main Dipole magnets", MT-15, Beijing, China, October 1997.[2] N.Andreev et al : " Performance of Five and SixBlock Coil Geometries in Short SuperconductingDipole Models for the LHC", PAC-99, New York,USA, March 1999.[3] Z Ang, L.Bottura, S.Russeschuck, A.Siemko,D.Tommasini, L.Walckieers : "Coil Size andGeometric Field Quality in Short Model Dipoles forLHC", MT-16, USA, 1999.[4] K.Artoos et al : " Status of the Short Dipole ModelProgram for the LHC MT-16, USA, 1999.[5] N.Andreev et al: " The 1m Long Single ApertureCoil test Program for LHC", EPAC-96, Sitges,Spain, June 1996.[6] N.Siegel, D.Tommasini, I.Vanenkov : "Design anduse of capacitive force transducers forsuperconducting magnet models for the LHC", MT-15, Beijing, China, October 1997.0246810121416T4.V1 T4.V2 T4.V3 T4.V5 T5.V1 T5.V3 T6.V1 T7.V1Below 9TBelow 8.33T

Status of the LHC Short Dipole Model Programme

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