For reasons of geometrical acceptance, 70 mm bore twin aperture quadrupoles are required in the LHC insertions. For an operating gradient of 160 T/m at 4.5 K, a design based on a four layer coil wound from two graded 8.2 mm NbTi conductors has been developed. Three 1 m single aperture quadrupoles of this design have been built and successfully tested. Thereafter, the magnets have been disassembled and the coils re-collared using self-supporting collars. In this paper, we describe the design features of the twin aperture quadrupole, and report on the initial collaring tests and procedures for collaring and final assembly of the 1 m magnet

Kirby, G A

Lucas, J

MacKay, R S

Nobes, S C

Ostojic, R

Treadgold, J R

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IEEE TRANSACI'IONS ON APPI.IUI> SUPBRCON~UCTIVI'I'Y, VOL. 10, NO. 1, MAKCH 200U 61 Design and Fabrication of a 1 m Model of the 70 mm Bore Twin Aperture Superconducting Quadrupole for the LHC Insertions G. A. Kirby, J. Lucas, R. OBtOjiC CERN, Geneva, Switzerland R. Machy, S. C. Nobos, J.R. Treadgold Oxford Instruments, Oxfordshire, England Abstract-Foy reasons of geometr ical  accep- tance, 70 mm bore twin aperture quadrupoles are required in the LHC insertions. For an operating gradient of 160 T/m at 4.5 K, a design based on a four layer coil wound from two graded 8.2 mm NbTi conductors  has been developed. Three 1 m single aperture quadrupoles of this design have been built and succcssfiilly tested. Thereafter, the  magnets have bocn disassembled and the coils re- collared using 6Clf-SUppOrthg collars. In  this pa- per, we describe the design features of the twin  aperture quadrupole, and report on the initial col- laring tests and procedures for collaring a n d  flnal assembly of the 1 m magnet. I ,  INTRODUCTION l 'he LI-IC insertion quadrupole prograin was started in the early 199O's, with the initial goal of developing a 70 mm aperture high gradient quadrupole operating: at 200 T/m in superfluid bcliurn for the LHC low-p triplcts [l]. The quadrupole i s  based on a four layer coil wound from hvo graded 8.2 mm NbTi conductors, and has also boon designed for a twin spertiire configu- ratio11 with a nominal operat,ing gradient of 160 T i m  at 4.5 K [Z] .  About 20 of these mfignets will bc! installed in the LHC experimentd and dump insertionb, notably in the regions of the injection and dump kickers, and in all other arcas where the 56 inm coil aperture of the LHC insertion qiiadrupolos may give rise to  aperture limita- tion. The expcricnce gained with t h o  8.2 mm cables was also essential for the development. of tho low current inser- tion qufidrupoles, which will bt! iiistallcd as individuaily powered magnets in the LHC difipcrsion suppressors and matching sections 131. In collaboration with Oxford Instru~nents, CERN has built and tested three 1 m singlc aperture quadrupoles with a 70 iiini bore (41. Thereafter, the last. two mag- nets have been disassemblcd, and the coils refurbished ancl prcparcd for assembly using self-supporting collars. Wc rccdl in this paper t.hc rriain design features of thc twin aperture qundrupolc, and describe in detail the ini- tial collaring tests ancl assembly proccdures developed for collaring and yoking of the 1 m twin aperture quadrupole. The magnet has been recently completed and is ready for testing. 11. DESIGN FEATURES Details of the conductor parameters and coil construc- tion have been described elsewhere [l]. To summarise, the coil has four layers made of two 8.2 mm NbTi graded conductors, with the transition between the cables in the middle of the second innermost layer. The nominal gra- dient of the quadrupole is 205 T i m  (1.9 K) and lG0 T/m (4.5 IC), and two magnets have been trained in a sin- gle aperture configuration up to the conductor limit of 260 T/m (1.9 K) and 200 T/m (4.5 K) [4]. YOKC TIE ROD . LAMINATIOR' Pig. 1. Transverse cross section of the quadrupole In contrnst with the single aperture configurations, where narrow, spacer-like collars were used, a self- supporting collar system is proposed for the twin aperture quadrupole, Fig. 1. The system consists of two 22 mm wide collars covering 180 degrees, and of two inserts, lo- catcd in the poles not covered by the collars. The collars are mounted by turning each successive set by 90 degrees. In the end regions, round 180 degree collars are used. Thc collar assembly is locked using eight full length tapered kcys, which are finally fixcd to the collar end plates. The pole lends are connected in the connection box which is mounted on the lead end plate, so that each collared coil Submitted 011 the 26 Seglerribcr 1999 can be handled RS an independent unit, 105~-8223/00$10.00 @ 2000 IEEE 62 For the 1 m model, the collared apertures arc assem- bled in a twin aperture magnet using single piece yoke laminations. The mechanical link between the collared aperture and the yoke i s  made by four long keys at the poles, which centre the callw assemblies oncc the lamin& tions are in plncc and locked with dowel pins. The rigidity of the yoke assembly is provided by the inertia tube, to which the laminationti are locked in several positions, sim- ilar to the LKC arc quadrupole [5].  The main parameters of the quadrupole are given in.Table I. Mai TARTIE I parnmntars of the 70 mm bntc twin apcrture quadrupole Operating temperature Nominal gradient Nominal current Peak field in coil Quench field Porcentage of the  load l ine Storcd energy per aperture Inductance per npcrture Coil outer diarnotar (RT) Collar outor diameter (RI') Aperturc separation (RT) Yoke inner diameter (RT) Yoke outer diameter (RT) 4.5 K 16Q T/m 3600 h F.16 T 7.5 T 82. % 141 kJ/m 4.55 mII/m 139.2 min 186 mm 194.4 mm 188 mm 452 m m  111. SHORT MODEL COLLARING TESTS A 120 mm section of the quadrupole, Fig. 2, was a#- sembled to  verify the collar design and to confirm the functionality of tho tooling and collaring procedures. Cer- tain design options, in particular the feasibility of collar- ing with bare keys as opposed to using interposed sliding sheets, also needed to  be tested. Tests were also per- formed to iwsess whether the collar inserts could be re- moved from the design for case of assembly. Fig. 2. Short model in tho collaring press An assembly mandrel made from a steel rod and four nylon pads which collapse when the rod is pulled out, was used for the central support. The coil was mauniod around the mandrel with sections of quench heaters and ground insulation. Capacitance gauges for pressure mca- suremcnt (61 were inserted in the shim grooves and posi- tioned on the pole faces. Finally, the brass collaring shoes and the straight section collars were assembled. The collaring press, also shown in Fig. 2, consisted of four quadrupole and eight keying rams, all rated at 10 tons. The basic ides was to use the quadrupole prcss to squeeze the collars to the point when the tapcrcd keys enter the slots. The largest fraction of the coil pressure is then generated by pushing the keys fully in with the eight-way press. The exact sequence depends crucially on the comprcssive and frictional properties of the assembly. In the first short rnodcl assembly full width keys were used. The coil pressures were lower than anticipated with a noticeable imbalance between the different layers (aver- age pressure was 49, 16, 27,30 MPa in layers 1-4, respec- tively), The keys could be removed easily and the model disassembled without any damage. After severnl rebuilds designed to shim layer 2, the average pressures increased to  59, 68, 51 and 61 MPa, rcspectively. However, the rlis- assembly of the model became very difficult. In order to Extract the keys, tho four-way press had to  be used at its maximum force and the keys hammered out, The keys and the collars were plastically deformed with fragments of keys cold welded to  the key slots. It was also dificult to balancc the pressures in the four layers, since adding shim to layer 2 transfered the load from adjacent layers, recluc- ing their pressures. We found that the effective moduli of individual layers in thc collared assembly behaved nonlin- early and were different than those measured in the arch tests. In the second series of tests, the shims were sized with 80 MPa as target coil prcssure. However, thc 10-ton quadrupole press was not sufficient to get the kcys started. A 1 mm chamfer was machined in the leading edgc of the keys so that they could start with a smaller kcy slot, in- creasing thc collar movement. A trial with full width keys showed, however, that the chamfer plastically dcformed the collar slots and made deep dents in the keys. The design of the collars and keys allowed for a reduc- tion of key width by 2 mm and introduction of 1 min thick sliding sheets on both sides of a key. The sheets provide a flat smooth surface, while spreading the inter- nal forces. With this assembly, the keys were pushed in easily using only half the press force. On disassembly, we found no traces of cold wclding. The key chamfer, how- ever, reduced the area for transmitting thc force, so that the sliding sheets showed signs of shearing. Also, some of the sliding sheets bent at the root of the key slots, preventing the keys to  fully enter. An averagc pressure of 70 MPa was recorded with the keys in place. When the quadrupolc press was relcamd, the pressure dropped by 15 MPa, and due to  creep after 16 hours, by another 10 MPa. ARer cooldown to  77 K, the average pressure reduced to 30 MPa. On a second thermal cycle, the pres- sures in the coil at room temperature and 77 K were re- produced within zk2 MPa. The assembly was subsequently re-collared without the 'T' shaped collar inserts. For identical shims sizes, the pressure in the coil decreased by about 10 MPa. Such a large reduction is explained by the fact that  the small as- pect ratio of the collar (width to inner radius), determined by the available spacing between the apertures, results in their considerable ovalisation a t  high pressures. The in- serts tend to keep the collar wsembly round, and thus enhance the prestress in the coils. The short model tests clearly demonstrated that the collar inserts and sliding sheets are essential features of the keying system. We also observed that due to nonlinear behaviour of the assembly, larger collar movement than expected was required. We concluded that the necessary movement could not be achieved with chamfered keys, and that the LO-ton quadrupole press was insufficient. A 9O-ton four-way press was built and the original eight-way keying press incorporated in the new frame. Its operation wm verified on a new assembly of the short model. IV. COIL SIZE MEASUREMENTS AND SHIMMING The coil size was measured in four locations along the straight section of the coil. A fixed volume system, cali- brated with a precision cut steel base, was used to measure the change in shim size required to achieve the design pres- sure in the coil. The measurements showed that the coils were undersized on the average by 0.27, 0.55, 0.32, and 0.31 mm for layers 1-4, respectively, with an rms variation of 0.04 mm. The average layer moduli were 9.9, 10.0, 10.9 and 8,4 GPa, respectively, with variations of 10 and 20 % for layers 1 and 2, and 5 % for layers 3 and 4. The coils were sorted according to the their size to help balance the pressures between the poles. For each layer, shims were calculated corresponding to the size of each pole and ter- get prestress. of 80 MPa after collaring, and the largest shim was manufactured and installed in a11 four polcs. In this way we expect that none of the layers will be below 40 MPa after cooldown. In order to shim the coil ends, the assembled four layer pole was placed in a measurement cavity of the nominal size, Fig 3. The cavity was closed and the pressures in the four layers measured using a set of four capacitance gauges. The pressure in each layer was iteratively ad- justed by adding shims to the mid-plane. The final shim in the coil ends was divided into three sections. In the section covering the ramp box, a shim size corresponding to  the pressure in the magnet straight section was used. In the region of the end turns, the shims were tapered from the ramp box size to aero. Finally, no shims were placed in the coil heads. Fig. 3. Measurement of coil size in the ends V. MAGNET ASSEMBLY It  was forcsecn that the collaring of thc quadrupole apertures wtm to be done using full length keys. This as- pect of the dcsign could not be tcstcd on the short model, and presented the greatest unknown in the assembly of the magnet. Furthermore, ns the coil ends were found do be quite sok, the transition regioii hetweeii the straight section and the coil ends was expected to he particularly delicate. Fig. 4. Collaring of the quadrupole using fill1 length koya The four poles were msscinblcd around a collapsible mandrel, and the assembly introduced in the collaring M press, as shown in Fig. 4. In the first step, the coils were compressed over 100 mm with 50 ton force in the quadrupole press, and a set of fGUr 100 mm full width keys wcre inserted without sliding sheets in every second slot. As the keys were pushed in successively over the whole length of the magnet, the adjacent slots opened sufficiently for inserting a set of 1.5 min undersized short keys and full length sliding sheets. These were subse- quently pushed in, and the previously filled slots opened for a set of still larger keys. At this stage, the mandrel. wm extracted. After three further steps, reaching an un- dersize of 0.25 mm, it was possible to introduce nominal full length keys in one of the quadrant slots. These were pushed 2/3  of the way, and the second set of full length keys introduced. In the final stage, all eight full length keys were pushed fully in and fixed to the end plates. The complete collaring operation requircd eight passes and five sets of 100 mm intermediate width keys, As shown in Fig. 5, the prestress in the coils progressively increased and after releasing the press averaged 100 MPa, with a minimum of 90 MPa in layer 3. 1dC i z c  1W p m  ,c 40 211 a 2 4 e 8 I O  Ksylng step Fig. 5. Avorage prestrcaa in the coil. Steps 1 -7  progressive compres- sion with 100 mm long undersized keye. Full lougth keys inserted in step 8 rind pushed fully in (9). Collaring press released (10); rolaxation alter one day (11). The assembly of the magnet proceeded with jointing of the poles and completing the instrumentation. In the last phase, the two collared apertures were placed vertically in the yokc end plate with the connection boxes at tho bottom. Four full lcngth centring keys were fitted into the slots in the collar poles, and yoke laminations lowercd over the apertures one at a time. A small press wm used t o  align a 5 cm pack of laminations, Fig. 6, which was locked with staggered dowel pins. The yoke was finally longitudinally secured with four tie rods, and the inertia tube fitted over the yoke. Fig. 6. Assembly of tho collared apertures in the two-in-one yokc! laminations. VI. CONCLUSIONS A 1 m model of the 70 mm bore twin aperture quadrupole for the LHC insertions has been designed and built. The four layer quadrupole coils, previously tested in single aperture configuration, were re-shimmed and assembled using self-supporting collars. Full length keys were used for locking the collars which were pro- gressively compressed with undersized short keys. Thc collared apertures were assembled in the twin aperture quadrupole using single piece laminations and an inertia ‘cube, and the magnet made ready for testing. REFERENCES [l] R. Ostojic, T.M. Taylor, G.A. Kirby, ”Design and Conslructlon of n One-Melre Model of the 70 mm Aperture Quadrupolo for tho LHC Low-p hortions“,  MT-13, IEEE Trans. Magn, Vo1.30, No.4, July 1994, pp. 1750-1753 [2] G.A. Kirby, R. Ostojic, T.M. Taylor, G. Winquart, ”Dwigrr of the 70 mm Aperture Supermnducting Qundrupole for llic LHC Dump Insertions”, IEEE nans. App. Supsrcanductivity, 7 (199?), No. 2, pp.862-566. [3) G. A. Kirby et al., ”Design and Construction of a 1-m Model of EL Low Current Superconducting Quadrupole for LHC Insertions” this conferencc [4] M. Lamm et al., “Teats of the 70 m m  Quadrupole for the LHC Low-,8 Insertions”, ASC’98, IEEE ?’ranS. hppl. Suporconduc- tivity, Vol. 8, No. 2, pp. 455-458. [5] J M. Rifflet et al. “Status of the Cold Mass of the Short Straight for tho LHC”, MT-15, Beijing, LHC Projwt Report 182. 161 N.Siegel, D. Tomarsini, 1. Vanenkov, “Design and Use of Cnpm itiva Force Tkansducers for Superconducting Magnet Models for the LHC”, MT-15, Beljing, LHC Project Report 173. 

Design and Fabrication of a 1 m Model of the 70 mm Bore Twin Aperture Superconducting Quadrupole for the LHC Insertions

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