Andreev, N.

Apollinari, G.

Auchmann, B.

Barzi, E.

Bossert, R.

Chlachidze, G.

Karppinen, M.

Nobrega, F.

Novitski, I.

Rossi, L.

Smekens, D.

Turrioni, D.

Yamada, R.

Zlobin, A. V.

IEEE Transactions on Applied Superconductivity

English

Open Access Repository

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013 4000904Development and Test of a Single-Aperture 11 TNb3Sn Demonstrator Dipole for LHC UpgradesA. V. Zlobin, N. Andreev, G. Apollinari, B. Auchmann, E. Barzi, R. Bossert, G. Chlachidze, M. Karppinen,F. Nobrega, I. Novitski, L. Rossi, D. Smekens, D. Turrioni, and R. YamadaAbstract—The upgrade of the LHC collimation system foreseesinstallation of additional collimators around the LHC ring. Thelongitudinal space for the collimators could be provided by re-placing some 8.33 T NbTi LHC main dipoles with shorter 11 TNb3Sn dipoles compatible with the LHC lattice and main systems.To demonstrate this possibility, FNAL and CERN have started ajoint program with the goal of building a 5.5 m long twin-aperturedipole prototype suitable for installation in the LHC. The first stepof this program is the development of a 2 m long single-aperturedemonstrator dipole with a nominal field of 11 T at the LHCnominal current of 11.85 kA and ∼20% margin. This paperdescribes the design, construction, and test results of the firstsingle-aperture Nb3Sn demonstrator dipole model.Index Terms—Accelerator magnets, large hadron collider,magnet test, superconducting coils.I. INTRODUCTIONTHE LHC operation plans include an upgrade of the LHCcollimation system [1]. Additional cold collimators willbe installed in the dispersion suppression (DS) areas aroundpoints 2, 3 and 7, and high luminosity interaction regionsat points 1 and 5. The required space of ∼3.5 m for theadditional collimators could be provided by using 11 T dipolesas replacements for several 8.33 T LHC main dipoles (MB).These dipoles, operating at 1.9 K and powered in series with theMBs, will deliver the same integrated strength at the nominalLHC current. The operating field level requires using Nb3Snsuperconductor. Recent advances in the development of high-field Nb3Sn accelerator magnets suggest that this technology isready for this application.To demonstrate feasibility, CERN and FNAL have starteda R&D program to build and test a 5.5 m long twin-apertureNb3Sn dipole for the LHC upgrade. Two such cold masses witha cold collimator in between will replace one 14.3 m long LHCMB dipole. The program started in 2011 with the design andconstruction of a 2 m long single-aperture Nb3Sn demonstratorManuscript received October 9, 2012; accepted November 13, 2012. Dateof publication December 24, 2012; date of current version January 28, 2013.This work was supported by Fermi Research Alliance, LLC, under ContractDE-AC02-07CH11359 with the U.S. Department of Energy and EuropeanCommission under FP7 Project HiLumi LHC, GA no. 284404.A. V. Zlobin, N. Andreev, G. Apollinari, E. Barzi, R. Bossert, G. Chlachidze,F. Nobrega, I. Novitski, D. Turrioni, and R. Yamada are with Fermi NationalAccelerator Laboratory, Batavia, IL 60510 USA (e-mail: zlobin@fnal.gov;andreev@fnal.gov; apollina@fnal.gov; barzi@fnal.gov; bossert@fnal.gov;guram@fnal.gov; nobrega@fnal.gov; novitski@fnal.gov; turrioni@fnal.gov;yamada@fnal.gov).B. Auchmann, M. Karppinen, L. Rossi, and D. Smekens are with theEuropean Organization for Nuclear Research, CERN CH-1211, Genève 23,Switzerland (e-mail: bernhard.auchmann@cern.ch; Mikko.Karppinen@cern.ch; Lucio.Rossi@cern.ch; david.smekens@cern.ch).Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TASC.2012.2236138Fig. 1. Cross-sections of the cold mass.dipole magnet with a 60 mm bore, an 11 T field at the LHCnominal current of 11.85 kA and 20% margin [2].This paper summarizes details of the design and assemblyof the single-aperture Nb3Sn demonstrator dipole and discussesits quench performance. Results of magnetic measurements andheater studies are reported elsewhere [3], [4].II. MAGNET DESIGNThe design concept of the single-aperture 11 T Nb3Sndemonstrator dipole is described in [5]. To allow for the beamsagitta in straight Nb3Sn magnets, the coil aperture was in-creased to 60 mm. The coil cross-section was optimized toprovide a dipole field of 11 T at the 11.85 kA current with 20%margin, and geometrical field errors below the 10−4 level.The coil cross-section was designed using 15.1 mm wide and1.29 mm thick Rutherford cable and 0.10 mm thick insulation.The 6-block coil consists of 22 turns in the inner layer and34 turns in the outer layer separated by 0.506 mm thick in-terlayer insulation. The mid-plane insulation is 0.125 mm percoil. Coil end blocks were designed to reduce the field leveland minimize integrated low-order field harmonics.The cross-section of the demonstrator dipole cold mass isshown in Fig. 1. Coil pre-stress and support is provided bystainless steel collars, a vertically split iron yoke, Aluminumclamps and a thick stainless steel skin. Two thick stainless steelend plates restrict the axial coil motion.The coil pre-stress has to be sufficient to compensate for thedifferences in the coil and structure thermal contractions duringcool-down, and for the Lorentz forces during magnet excitation.The design of the mechanical structure and the coil pre-stresswere optimized to maintain the coils under compression up tothe maximum design field of 12 T and keep the coil stress below165 MPa during magnet assembly and operation.Magnet quench protection is provided by internal stripheaters installed between the ground insulation layers.1051-8223/$31.00 © 2012 IEEE4000904 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013Fig. 2. Keystoned 40-strand cable.Fig. 3. Coil after curing (left) and reaction (right).III. MAGNET CONSTRUCTIONA. Strand and CableThe cable was made of 0.7 mm in diameter Nb3Sn RRP-108/127 strand with ∼55 μm sub-elements, the nominalJc(12 T, 4.2 K) ∼ 2750 A/mm2, the ∼53% Cu fraction andRRR > 60 [6]. The Rutherford cable has 40 Nb3Sn strands, a15 degree transposition angle and an 86% packing factor. TheIc degradation after cabling based on the extracted strand testsis ∼4% [7].A 440 m long piece of cable was fabricated at FNAL provid-ing two ∼210 m long unit lengths for the demonstrator dipolecoils and a ∼25 m piece for short sample measurements. Thecable was made in two steps: first with rectangular cross-sectionand then after annealing, it was re-rolled into the final keystonedcross-section (Fig. 2). A ∼220 m cable piece for a spare coilwas fabricated at CERN using a one step procedure.B. Coils, Ground Insulation, Protection HeatersEach coil consists of two layers, 6 blocks and 56 turns. Bothlayers were wound from a single piece of cable insulated withtwo layers of 0.075 mm thick and 12.7 mm wide E-glass tape.The coil poles were made of Ti-6Al-4V alloy and wedges ofstainless steel. The end spacers were also made of stainlesssteel using the selective laser sintering (SLS) process. The cablelayer jump is integrated into the lead end spacers.Coils were made using the wind-and-react method. Duringwinding each coil layer was impregnated with CTD1202xliquid ceramic binder and cured under low pressure at 150 ◦Cfor 0.5 hour. During curing both coil layers were shimmedazimuthally to the same size of 1 mm smaller than the nominalcoil size to prevent cable over-compression due to the Nb3Sncable expansion during reaction [8]. Coils were reacted sepa-rately in Argon using a 3-step cycle with Tmax = 640 ◦C for48 hours. Coil pictures after curing and reaction are shown inFig. 3.Before impregnation the Nb3Sn coil leads were splicedto flexible NbTi cables and the coil was wrapped with a0.125 mm thick layer of S2-glass cloth. Coils were impregnatedwith CTD101K epoxy and cured at 125 ◦C for 21 hours. Theradial and azimuthal coil sizes were measured in the freecondition using a 3D Cordax machine. The lengths of thecoils, MBH02 and MBH03, used in the demonstrator dipolewere 1965 and 1971 mm respectively including saddles andsplice box.The coil ground insulation consists of 5 layers of 0.127 mmthick Kapton film. Two quench protection heaters composed ofFig. 4. Assembly of two dipole coils with protection heaters and groundinsulation.Fig. 5. Installation of collar blocks around coils (left), collared coil inside ironyoke (right).0.025 mm thick stainless steel strips were installed on each sideof the outer coil surface. One heater was placed between the 1stand 2nd Kapton layers and the other between the 2nd and 3rdlayers, covering the outer-layer coil blocks. The correspondingstrips on each side of each coil were connected in serious,forming two independent heaters [4].C. Magnet Assembly and InstrumentationDetails of the magnet assembly are reported in [9].Two coils surrounded by the ground insulation and 316Lstainless steel protection shells were placed inside the laminatedcollars made of Nirosta high-Mn stainless steel. Assemblyof two coils with protection heaters and ground insulation isshown in Fig. 4. The collar block installation around the coilsis shown in Fig. 5 (left).The collared coil is installed inside a vertically split yokewith a 400 mm outer diameter made of SAE 1045 iron andfixed with Al clamps. The yoke length is 1950 mm which prac-tically covers the entire coil and the Nb3Sn/NbTi lead splices.The 12.7 mm thick 304L stainless steel shell is pre-tensionedduring welding to provide the coil final pre-compression. Two50 mm thick 304L stainless steel end plates welded to theshell restrict the axial coil motion. Picture of the collared coilassembly inside the iron yoke and the collar-yoke shim is shownFig. 5 (right).The magnet was instrumented to monitor its parametersduring assembly and test. Instrumentation includes 10 voltagetaps (VT) on the inner layer and 9 voltage taps on the outerlayer of each coil, 64 strain gauges (SG) on the coils, magnetshell and end bullets, and 4 resistive temperature sensors (RTD)along the magnet shell. A schematic of VT positions is shownin Fig. 6.The coil prestress is provided by the coil midplane and radialshims installed inside the collared coil as well as the collar-yokeshims placed near the coil midplane area (see Fig. 5, right). Theshim positions and sizes were defined using the FEA modelZLOBIN et al.: SINGLE-APERTURE 11 T Nb3Sn DEMONSTRATOR DIPOLE FOR LHC UPGRADES 4000904Fig. 6. Voltage tap position in inner and outer layers of the 11 T demonstratordipole coil.Fig. 7. Quench history and origin in TC1 and TC2.to achieve the target coil prestress [5] and then experimentallyverified and optimized by testing magnet mechanical models[10]. During magnet assembly the prestress was controlledby strain gauges installed on the coil in two cross-sectionsalong the magnet straight section. Based on the strain gaugedata the coil prestress at room temperature was close to thedesign values. The used instrumentation did not provide theinformation on coil preload after cool-down.IV. TEST RESULTSThe 11 T demonstrator dipole (MBHSP01) was tested atFermilab’s Vertical Magnet Test Facility (VMTF) [11]. The testprogram included quench training and ramp rate dependencestudies, magnetic measurements, and quench protection studiesboth at 4.5 K and 1.9 K, as well as temperature dependence andcoil RRR measurements.The quench current limit for the demonstrator dipole wasestimated based on measured witness sample data. It is 13.0 kAat 4.5 K and 15.0 kA at 1.9 K, which corresponds to the borefields of 12.3 T and 13.4 T respectively.The magnet quench history with quench locations is sum-marized in Fig. 7. Magnet training started at 4.5 K with thenominal current ramp rate of 20 A/s. The first quench occurredat 8.8 kA, or 68% of the short sample limit at 4.5 K. It wasoriginated in the coil pole turn (segment A9_A10 in Fig. 6).However, after 5 consecutive quenches in the high field area,the quench locations moved to the outer-layer mid-plane blocksof both coils (segment B2_B3).The magnet exhibited “reversed” ramp rate dependence atcurrent ramp rates below ∼60 A/s. Above this level the ramprate dependence had a regular behavior with quenches origi-nated practically simultaneously in the mid-plane blocks of theinner and outer coil layers (segments A2_A3 and B2_B3). Sim-Fig. 8. Ramp rate dependence of magnet quench current at 4.5 K and 1.9 K.Fig. 9. Temperature dependence of magnet quench current (dI/dt =50 A/s). The top dashed line represents the dependence of magnet SSL versustemperature.ilar performance was also observed at 1.9 K. Based on that themagnet quenching was continued with different current ramprates between 40 and 70 A/s in order to reach the maximumpossible quench current at both temperatures.Ramp rate dependences of magnet quench current at 4.5 Kand 1.9 K are shown in Fig. 8. At both temperatures the magnetquench current reached its maximum at dI/dt ∼ 50− 60 A/s.During the test an unplanned magnet warm-up to roomtemperature took place to eliminate contamination in the heatexchanger at VMTF. The magnet showed short retraining inthe second thermal cycle (TC) at 1.9 K (Fig. 7). The maximummagnetic field of 10.4 T was reached at 1.9 K and a ramp rateof 50 A/s, corresponding to 78% of magnet short sample limit(SSL) at dI/dt = 0.The temperature dependences of the quench current expectedbased on SSL calculation and measured at a ramp rate of 50 A/s(in all quenches but one at ∼4.5 K) are shown in Fig. 9. Themagnet demonstrates the regular temperature dependence ofquench current, but also exhibits its substantial degradation andsome erratic quench behavior typical of flux jump instability.All quenches at intermediate temperatures were initiated in theMBH03 outer-layer mid-plane block (segment B2_B3).The observed limited quench performance could be causedby several reasons including Jc degradation and/or flux jumpsin the coil outer-layer leads. Coil lead damage during magnetassembly and poor Nb3Sn/NbTi lead splice could also explainthe observed quench performance. To verify the above hypoth-esis several special tests were performed.Fig. 10 shows quench current vs. holding time at currentplateau at 4.5 K and 1.9 K. Magnet current was ramped to apre-set current at 20 A/s and held until quench. Data with zero4000904 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013Fig. 10. Quench current at current plateau vs. holding time at 4.5 K and1.9 K.Fig. 11. Maximum spike voltage at a different current intervals.holding time corresponds to a regular quench at 4.5 or 1.9 Kat the chosen ramp rate in the range of 35–50 A/s. All holdingquenches were initiated in the MBH02 outer-layer mid-planeblock (segment B2_B3 in Fig. 6). Reproducibility tests and testsat different ramp rates were not performed. Exponential fit tothe experimental data suggests that the stable magnet current isabout 2.5 kA and 2 kA lower than the maximum quench currentreached at 50 A/s at 1.9 K and 4.5 K respectively, which isconsistent with currents extrapolated to dI/dt = 0 on the ramprate dependence shown in Fig. 8.The Voltage Spike Detection System (VSDS) [12] was usedto record the voltage across one coil balanced with the voltageacross the opposite coil at a sampling rate of 100 kHz. Thehistogram in Fig. 11 shows the maximum voltage spike am-plitude at different current intervals recorded during ramp up at4.5 K and 1.9 K. Comparing these data with the data collectedfor other Nb3Sn magnets made of similar RRP-108/127 strand[13], [14] one can conclude that their level is similar andmuch lower than in magnets made of more unstable RRP-54/61strand [15].A special instability suppression test, similar to that de-scribed in [14], was performed by reducing Jc in the outer-layer mid-plane area by locally heating them with protectionheaters powered with a small DC current. A reduction ofmagnet quench current was observed. This experiment as wellas modest voltage spike activity do not support the hypothesisthat flux jumps are limiting quench current in the outer coillayer.The residual resistivity ratio (RRR) of cable was measuredfor all segments of both coils during magnet warm-up. Theaverage RRR value was ∼100 (lowest 80, highest 118). Thesenumbers are smaller than ones measured in well-performingcoils made of similar strand [14]. The splice resistance was alsomeasured and found smaller than 2 nOhms.V. CONCLUSIONThe first 2 m long, single-aperture demonstrator dipole hasbeen built and tested at Fermilab. The magnet reached 10.4 T or78% of SSL at 1.9 K and a current ramp rate of 50 A/s showinglimited quench performance. Most quenches at low ramp rates,all holding quenches and quenches at intermediate temperaturesinitiated in the mid-plane block of the outer coil layer. Only fewtraining quenches occurred in the high field area at the verybeginning of the test at 4.5 K and 1.9 K. Quench locations,ramp rate and temperature dependence studies, and additionaltests point to the outer-layer leads in both coils limiting themagnet quench performance. Possible conductor damage inthe mid-plane area during coil fabrication or magnet assemblycould cause the observed degradation. The investigation of allpossible causes continues, including a magnet autopsy. Allappropriate changes will be implemented in the fabricationprocess of MBHSP02, a 1 m long dipole model of the samedesign, and tested.Experimental data related to magnet quench protection andfield quality were also collected during this test [3], [4] pro-viding an important input to magnet design and performanceoptimization.ACKNOWLEDGMENTThe authors thank the technical staff of FNAL TechnicalDivision for contributions to magnet design and fabrication.REFERENCES[1] L. Bottura et al., “Advanced accelerator magnets for upgrading the LHC,”IEEE Trans. Appl. Supercond., vol. 22, no. 3, p. 4002008, 2012.[2] A. V. Zlobin et al., “Development of Nb3Sn 11 T single aperture demon-strator dipole for LHC upgrades,” in Proc. PAC, New York, 2011, p. 1460.[3] G. Chlachidze et al., “Quench protection study of a single-aperture 11 TNb3Sn demonstrator dipole for LHC,” presented at the Appl. Supercond.Conf., 2012.[4] N. Andreev et al., “Field quality measurements in a single-aperture 11 TNb3Sn demonstrator dipole for LHC upgrades,” presented at the Appl.Supercond. Conf., 2012.[5] A. V. Zlobin et al., “Design and fabrication of a single-aperture 11 TNb3Sn dipole model for LHC upgrades,” IEEE Trans. Appl. Supercond.,vol. 22, no. 3, p. 4001705, 2012.[6] M. B. Field et al., “Internal tin Nb3Sn conductors for particle acceleratorand fusion applications,” Adv. Cryogenic Eng., vol. 54, p. 237, 2008.[7] E. Barzi et al., “Development and fabrication of Nb3Sn rutherford cablefor the 11 T DS dipole demonstration model,” IEEE Trans. Appl. Super-cond., vol. 22, no. 3, p. 6000805, 2012.[8] N. Andreev et al., “Volume expansion of Nb3Sn strands and cables duringheat treatment,” Adv. Cryogenic Eng., vol. 48, no. AIP, p. 941, 2012.[9] A. V. Zlobin et al., “Status of a single-aperture 11 T Nb3Sn demonstratordipole for LHC upgrades,” in Proc. IPAC, New Orleans, 2012, p. 1098.[10] I. Novitski et al., “Study of mechanical models of a single-aperture 11 TNb3Sn dipole,” presented at the Appl. Supercond. Conf., 2012.[11] M. J. Lamm et al., “A new facility to test superconducting acceleratormagnets,” in Proc. PAC, Vancouver, Canada, 1997, p. 3395.[12] D. F. Orris et al., “Voltage spike detection in high field superconduct-ing accelerator magnets,” IEEE Trans. Appl. Supercond., vol. 15, no. 2,pp. 1205–1208, Jun. 2005.[13] R. Bossert et al., “Fabrication and Test of 4 m long Nb3Sn QuadrupoleCoil Made of 114/127 RRP Strand,” presented at the CEC/ICMC,Spokane, WA, Jun. 2011.[14] G. Chlachidze et al., “The study of single Nb3Sn quadrupole coils using amagnetic mirror structure,” IEEE Trans. Appl. Supercond., vol. 21, no. 3,pp. 1692–1695, Jun. 2011.[15] G. Ambrosio et al., “Test results of the first 3.7 m long Nb3Sn quadrupoleby LARP and future plans,” IEEE Trans. Appl. Supercond., vol. 21, no. 3,pp. 1858–1862, Jun. 2011.

Development and Test of a Single-Aperture 11 T Nb3Sn Demonstrator Dipole for LHC Upgrades

A. V. Zlobin

N. Andreev

G. Apollinari

B. Auchmann

E. Barzi

R. Bossert

G. Chlachidze

M. Karppinen

F. Nobrega

I. Novitski

L. Rossi

D. Smekens

D. Turrioni

R. Yamada

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Development and Test of a Single-Aperture 11 T $ \hbox{Nb}_{3}\hbox{Sn}$ Demonstrator Dipole for LHC Upgrades

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Development and Test of a Single-Aperture 11 T Nb3Sn Demonstrator Dipole for LHC Upgrades

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