A series of similar one meter long superconducting dipole models for the LHC is being manufactured and tested since 1995 for exploring design variants and assembly parameters.Until the end of 1997 all magnets of this series were based on a coil geometry subdividing the conductors in five distinctive winding blocks. In order to cope with new requirements of magnetic field distribution and coil design flexibility, one additional block has been added in the beginning of 1998. A significant number of models of both types have been built and tested, some of them re-built in a different version, adding up in more than 40 models tested so far. The paper reviews the performance of these two different coil designs in terms of manufacture, training behaviour and temperature margins as well as mechanical behaviour and magnetic field quality

Andreev, N

Artoos, K

Rodríguez-Mateos, F

Russenschuck, Stephan

Siegel, N

Siemko, A

Sonnemann, F

Tommasini, D

Vanenkov, I

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PERFORMANCE OF FIVE AND SIX BLOCK COIL GEOMETRIESIN SHORT SUPERCONDUCTING DIPOLE MODELS FOR THE LHCN.Andreev, K.Artoos, L.Bottura, F.Rodriguez-Mateos, S.Russenschuck, N.Siegel, A.Siemko,F.Sonnemann, D.Tommasini, I.Vanenkov CERN, Geneva, SwitzerlandAbstractA series of similar one meter long superconducting dipolemodels for the LHC is being manufactured and testedsince 1995 for exploring design variants and assemblyparameters. Until the end of 1997 all magnets of thisseries were based on a coil geometry subdividing theconductors in five distinctive winding blocks. In order tocope with new requirements of magnetic field distributionand coil design flexibility, one additional block has beenadded in the beginning of 1998.A significant number of models of both types have beenbuilt and tested, some of them re-built in a differentversion, adding up in more than 40 models tested so far.The paper reviews the performance of these two differentcoil designs in terms of manufacture, training behaviourand temperature margins as well as mechanical behaviourand magnetic field quality.1  THE MODELSThe regular CERN in-house model program for thedevelopment of the LHC dipoles was started in 1995 withthe fabrication of 1m-long single-aperture magnets, so-called MBSMS. The design of the MBSMS models,presented in previous conference papers [1] [2], is basedon circular collars of 197 mm outer diameter placedinside a vertically split yoke, held together by a boltedshrinking cylinder for easy re-assembly of the structure.2 COIL GEOMETRY:DESIGN FEATURESThe LHC dipole coils consist of two superposed layers,an internal layer and an external one. Each layer issubdivided in blocks of conductors separated by copperspacers. The two coil geometries tested on the models areshown in Fig.1 below.The first 13 magnets have been built with the 5-blockversion of the Yellow Book [3], consisting in 3 blocks ofconductors in the inner layer and two in the outer layer(fig.1a). As from MBSMS15, the coil design hasundergone a substantial evolution, being replaced by a 6-block one, consisting in four blocks of conductors in theinner layer and two in the outer layer (fig.1b). The last 6magnets have been built according to this 6-blockgeometry.2.1  Magnetic designThe original 5-block coil cross-section was optimizedusing deterministic techniques with the CERN programROXIE [4]. First a preliminary geometric modelling wasdone with a given choice on cable dimensions andconductor blocks, thereafter the model was iterated tofind an acceptable solution for field quality and peakfield/main field ratio. This coil design however did notensure sufficient tunability and flexibility for later fieldadjustments like compensating the persistent currentmultipoles at injection and fine-tuning of field quality. Inparticular a further compensation of the b3 term, ifrequired, would have been impossible because the copperwedges would become too small at the inner edge.Moreover the performance of the magnets with 5-blockcoils and laboratory tests indicate that the stressdistribution in the inner block of the inner layer is notfavourable. With the implementation of geneticalgorithms in ROXIE in 1997 [5] it was possible to makean extended the search for more appropriate designsconsidering all the constraints learned with the shortmodel program based on 5 blocks. The study gave twoalternative designs based on 6-block coils, all havingsimilar field quality and quench margin, however with adifferent number of turns and coil-block layouts. Thefinal choice of the geometry was based on mechanicalconsiderations (see paragraph 2.2) and sensitivity tomanufacturing tolerances [6]. Table 1 shows theoptimized parameters of five and six-block geometries.In the 6-block version the number of turns decreasedby one (i.e. 40 per pole), but the margin to short samplelimit increases by 0.11T, which is explained by the lowerratio of peak field to central field. Further, geometricmultipoles were systematically introduced to partiallycompensate the persistent current effect at injection fieldlevel to ease correction scheme requirements.Fig. 1a : 5-block geometry     Fig. 1b : 6-block geometry0-7803-5573-3/99/$10.00@1999 IEEE. 154Proceedings of the 1999 Particle Accelerator Conference, New York, 1999Table 1: Design parameters of single aperture 5 and 6 block5 block 6 blockTurns inner layer 15 15Turns outer layer 26 25%load line (inner layer) 86.5 85.64%load line (outer layer) 82.5 84.92Peak/main field inner 1.05 1.03Peak/main field outer 0.87 0.89Current @ 8.36T 11.5 kA 11.8 kAMaximum central field 9.65 T 9.76 TForce parallel to cablebroad side inner layer34kN/m 17kN/mb3 geometric @ 17 mm + 0.3 + 4.1b3 geo + pers @ 17 mm -11.8 - 6.5Finally the electromagnetic force parallel to the broadface of the cable has been considerably reduced. The first two turns of the inner layer are now alignedparallel to the field direction, reducing considerably theshear stress to which they are submitted during excitation.2.2 Radial supporting of inner layer.In addition to a lower radial electromagnetic force on thefirst turns of the inner layer, the 6-block geometryprovides a better radial support of these turns. This can beseen in fig 2a and 2b which show the measureddistribution of radial pressure of a 5-block inner layer anda 6-block inner layer.The imprint was obtained with a pressure sensitive tapeinserted between the inner layer coil and a mouldcompressing the coil in the azimuthal direction with apressure of about 50MPa. The first upper turns of the fiveblock coil in fig. 2a are not in good radial contact with themould, whilst the six block geometry in fig. 2b provides amore uniform radial support to all the conductors. 3  CONSTRUCTIONAL VARIANTS Variants explored through the model fabrication concerncable types, material of collars and coil pre-stress.3.1 Cable types. One 5-block magnet and all 6-block are made with the15.1mm wide cable corresponding to the present LHCspecification, which is slightly less compacted and withmore rounded corners compared to the previous 15.0mmwide cable used for all other 5-block magnets [2].3.2 Material of collars.The MBSMS base design relied on aluminium collars, ofsimilar rigidity as those of the twin aperture dipoles.Later, a number of magnets have been collared withaustenitic steel collars, which is the present baseline forthe LHC dipoles. In general, the use of austenitic steelimproves the training behaviour of 5-block coils [2], butthis effect is not noticeable in the 6-block coils whichappear to have a more stable structure.3.3 Coil pre-stress.The coil stress of inner and outer layers has beenmeasured on all models with specially developed straingauge transducers and capacitive pressure transducers[7][8]. In terms of coil pre-stress the models made so farcan be divided into two groups: one of high pre-stress inwhich coils are compressed by the collars up to themaximum excitation fields and the other of lower pre-stress in which the inner layer unloads from the collarsbefore the magnet reaches 9T. To the first group belongmost of 5-block magnets, and to the second group most ofthe 6-block magnets. Cold tests have shown that magnetsof the second group have higher initial quenching fieldsthan magnets of the first group. This effect appears to beless important for 6-block magnets. For simplicity thedata presented later will not be grouped by coil pre-stress. 4  PERFORMANCE The advantages foreseen for the 6-block coil geometryversus 5-block appear especially for the trainingbehaviour. The dynamic behaviour and magnet protectionare similar for both designs and linked to cable properties.4.1 Training. The 6-block magnets, both with aluminium and austeniticsteel collars, show better performance compared to 5-block magnets. Fig. 3 shows the average quenching fieldof the 5 and 6-block magnets built so far, withoutdistinction between collar material.Fig. 2a : 5 blocks     Fig. 2b : 6 blocks7.588.599.5101st 2nd 15th after th. cycleQuenching field (T)5 block6 blockFig.3 : average quenching field of 5 and 6-block magnets155Proceedings of the 1999 Particle Accelerator Conference, New York, 1999Six-block magnets allow to gain in average about 0.5T interms of quench performance. One possible reason forthis better performance of 6-block can be found in a morestable radial support of the conductors of the inner layerclose to the collar pole as suggested from the test in fig. 2.This effect is confirmed by the smaller variation of theradial pressure between the collars and the first turns ofthe inner layer measured at zero current during the cool-down and the training tests (fig.4).4.2 Magnetic field.The six block geometry, having one more block ofconductors in the inner layer, allows more freedom forpossible adjustements of the high order field harmonics.However, in terms of sensitivity of magnetic field qualityto manufacturing tolerances (in particular final coil sizeafter collaring), the two coil geometries are identical. Intable2 are shown the change of geometric field harmonicsfor an increase of 0.1mm in the inner layer coil size aftercollaring, extrapolated from measurements on 13 five-block magnets and 11 six-block magnets. Concerning theouter layer, in both cases its size has an important effectonly on b3, of about 1.3 units per 0.1mm.Table 2: Change of geometric field harmonics (units 10-4at a reference radius of 17mm) versus a changeof 0.10mm in the inner layer size,measured data.b3 b5 b7 b95 block magnets -1.7 +0.4 -0.1 0.056 block magnets -1.6 +0.4 -0.2 **range of b9 for all magnets between 0.02 and 0.10 units.Finally, the measured non allowed multipoles weresimilar in both cases, showing that the introduction of onemore spacer does not affect the quality of assembly.4.3 Magnet protection.In terms of magnet protection no substantial differencewas observed between 5 and 6-block design. The hot spottemperatures (fig.5) at 9T range from 200K to 270K anddepend in both cases on cable parameters like RRR andCu/Sc ratio [9].Finally, at nominal current, the time between firing thequench heaters and start of quench (heater delay) was thesame for 5 and 6-block magnet (about 50ms).5 CONCLUSIONSThe advantages foreseen for the 6-block coil cross sectionof the LHC main superconducting dipoles have beenconfirmed experimentally on short models. The 6-blockgeometry has proven to be better performing than the 5-block in terms of mechanical stability of the conductorblocks and of quench behaviour. Concerning magneticfield reproducibility and sensitivity to coil size tolerancesthe two coil geometries are equivalent. Finally magnetprotection parameters are very similar for the twodesigns, and are dominated by the cable characteristics.6  ACKNOWLEDGEMENTSThe authors would like to thank C.Wyss and L.Evans fortheir support to the short dipole model program, and allthe teams involved in magnet fabrication and testing.7 REFERENCES[1] N.Andreev et al, "The one meter long singleaperture dipole coil test program for LHC", proc. ofthe 5th EPAC, Sitges, Spain, 10-14 June 1996.[2] N.Andreev et al, "State of the short dipole modelprogram for the LHC", Proc. VI European ParticleAccelerator Conference,Stockholm, June 1998.[3] "The Large Hadron Collider - Conceptual Design"CERN/AC/95-05, October 1995.[4] S. Russenschuck et. al.: "Integrated Design ofSuperconducting Accelerator Magnets - A casestudy of the main quadrupole, The EuropeanPhysical Journal, Applied Physics, 1, 1998.[5] S. Ramberger, S. Russenschuck: Genetic algorithmsfor the optimal design of superconductingaccelerator magnets, EPAC 98, Stockholm, Sweden.[6] S.Russenschuck :"Comparative study of differentcoils for the LHC main dipoles", LHC ProjectReport 159, December 1997.[7] I.Vanenkov et al.“Mechanical performance of a twinaperture 56mm bore 1m long dipole model madewith SSC type cables”, MT-14, June 1995.[8] N.Siegel, D.Tommasini,I.Vanenkov“Design and useof capacitive force transducers for superconductingmagnet models for the LHC”, MT-15, 1997.[9] F.Sonnemann, et al. “Quench process and protectionof LHC Dipole Magnets”, LHC Project Note, 1999.0510152025 assembly cooldown discharge 1kA Cycle 9kA Cycle 10kA Cycle 11 kA  Quench1 Quench 2 Quench3 Quench 4 Quench 5 Quench 6 Quench 7 Quench 8 Quench 9 Quench 10 Quench 11 Quench 12 Quench 13 Quench 14 Quench 15 Quench 16Estimated Stress (MPa)Typical 5 block coil magnetTypical 6 block coil magnetFig.4: radial pressure between collars and inner layer conductors.18020022024026028030011000 11500 12000 12500 13000 13500 14000 14500quench currentT(K)5 block magnets6 block RRR=1806 block RRR=114Fig. 5: hot spot temperature for 5 and 6-block magnets156Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

Performance of Five and Six Block Coil Geometries in Short Superconducting Dipole Models for the LHC

Performance of Five and Six Block Coil Geometries in Short Superconducting Dipole Models for the LHC

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