We report the main results of the magnetic field measurements performed on the full-size LHC superconducting dipoles tested at CERN since summer 1998. Main field strength and field errors are summarised. We discuss in detail the contributions related to the geometry of the collared coil, the assembled cold mass, cool-down effects, magnetisation of the superconducting cable and saturation effects at high field. Dynamic effects on field harmonics, such as the field decay during injection and field errors during current ramps, are assessed statistically

Aleksa, Martin

Amet, S

Ang, Z

Bottura, L

Buzio, M

Ferracin, P

Pagano, O

Remondino, Vittorio

Russenschuck, Stephan

Sanfilippo, S

Scandale, Walter

Todesco, Ezio

CERN Document Server

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 12, NO. 1, MARCH 2002 247Measurement and Analysis of the Field Quality ofLHC Prototype and Pre-Series SuperconductingDipolesM. Aleksa, S. Amet, Z. Ang, L. Bottura, M. Buzio, P. Ferracin, O. Pagano, V. Remondino, S. Russenschuck,S. Sanfilippo, W. Scandale, and E. TodescoAbstract—We report the main results of the magnetic fieldmeasurements performed on the full-size LHC superconductingdipoles tested at CERN since summer 1998. Main field strengthand field errors are summarized. We discuss in detail the contri-butions related to the geometry of the collared coil, the assembledcold mass, cool-down effects, magnetization of the supercon-ducting cable and saturation effects at high field. Dynamic effectson field harmonics, such as the field decay during injection andfield errors during current ramps, are assessed statistically.Index Terms—Accelerator magnets, field quality, magnetic mea-surements.I. INTRODUCTIONTHE MAGNETS for the LHC must satisfy strict require-ments regarding field quality. Harmonic errors in the su-perconducting magnets will affect important beam parameterssuch as orbit, tune, coupling and chromaticity. The requirementshave been expressed as a set of maximum field imperfectionsto be reached at operation [1]. A comparison of these require-ments to the field homogeneity realistically achievable duringseries production of dipole magnets shows that active correctionis necessary [2]. The correction system however can be efficientonly if the field errors are known at operating conditions. Thiscalls for systematic testing of dipoles in cold conditions and fora rigorous analysis of the correlation between the measured har-monics and the components of the cold mass, e.g., coil, collarsand iron, critical for field quality.As in warm electromagnets, in superconducting magnets twoimportant contributions to the field errors are generated by thegeometry of the coils and the saturation of the iron yoke. In addi-tion, three other contributions to the errors stem from the super-conducting properties of the cable and are especially importantat low fields: persistent currents in superconducting filaments,eddy currents in the strands and in the cable during ramps andfield decay and snap back at the change from constant currentoperation to the beginning of the ramp.We have used this as a frame to organize the resultsof field quality aspects in the long superconducting LHCManuscript received September 24, 2001.M. Aleksa, S. Amet, L. Bottura, M. Buzio, P. Ferracin, O. Pagano, V.Remondino, S. Russenschuck, S. Sanfilippo, W. Scandale, and E. Todescoare with CERN, CH-1211 Geneva 23, Switzerland (e-mail: stephane.sanfil-ippo@cern.ch).Z. Ang is with TRIUMPH, Vancouver, BC, Canada.Publisher Item Identifier S 1051-8223(02)03468-1.dipoles with austenitic steel collars tested so far. These arethe 15-m long prototypes MBP2N2, MBP2O1, MBP2O2and MBP2A2 and the first three of the 90 pre-series dipoles,HCLBB_001-CR001001, HCLBB_001-CR001 001 andHCLBB_001-CR002001, referred to as MBPSO01, MBPSO02and MBPSA01 respectively. A detailed description of thedesign, of the main parameters and of the main assemblyvariants can be found in [3] for the prototypes and in [4] for thepre-series dipoles. The systems for field measurement in warmand cold conditions are described elsewhere [5], [6].II. MAIN FIELDA. Transfer FunctionThe dipole transfer function measured in the straight part ofthe coil is plotted in Fig. 1 for selected prototypes and for thepre-series dipoles. Experimental data are compared to calcula-tions performed taking into account the different magnetic yokegeometry in prototypes and pre-series dipoles and in particularthe shape of the ferromagnetic inserts between yoke and col-lars [2]. For a given current, the transfer function varies for theprototypes and pre-series dipoles in relative range of 6 10 .The calculated values are in agreement within the same range.This is at the limit of the beam dynamics requirements (13 10 ). More statistics are needed to confirm this value forthe pre-series. The observed spread may be due to non nominalpole shims used during collaring.B. Magnetic LengthThe nominal magnetic length of the LHC dipoles is 14.3 m.All dipoles measured (with an accuracy of 10 ) exceededthis value (see Fig. 2), apart from the prototype MBP2O2 thathas a magnetic length 0.2% smaller than the other dipoles.The reasons are related to two assembly parameter differences.MBP2O2 is the first 15-m-long dipole having a longitudinalsymmetric yoke with a reduced length (the other prototypeshad a longer asymmetric yoke) and is the last dipole equippedwith non magnetic laminations in the ends of the magnet. Onthe other hand the pre-series dipoles have nested laminations,i.e., non magnetic laminations with reduced diameter comple-mented by an outer ferromagnetic ring. These two differencesresult in a reduction of the end contribution to the integratedfield and thus in a shorter magnetic length of MBP2O2 with re-spect either to the other previous prototypes or to the following1051-8223/02$17.00 © 2002 IEEE248 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 12, NO. 1, MARCH 2002Fig. 1. Transfer functions measured in the prototypes and the pre-seriesdipoles compared to calculations. The different yoke geometry of prototypeand pre-series dipoles was taken into account for the calculations.pre-series. The difference in length among the two aperturesof a dipole ( in Fig. 2) is at most 3 mm (2 10 ). Notethat in most cases, the first aperture has systematically a highermagnetic length than the second one. Finally for the pre-seriesdipoles, the standard deviation of the relative spread over thesix measured apertures 2.7 10 , is in agreement with thetolerance of 3 10 given by beam dynamics [1].III. GEOMETRIC ERRORSA. Warm–Cold CorrelationWarm measurements (i.e., at 300 K) can give a good eval-uation of the geometric field errors. This is demonstrated inFig. 3 where we have plotted the correlation between the low-order allowed harmonics measured in warm conditions (com-pleted cold mass) and the geometric value in cold conditions,both averaged in the straight part of the coil. Warm measure-ments were performed at 12 A current with positive and neg-ative current to eliminate the residual magnetization. The geo-metric harmonics at cold have been computed as the average ofthe measured values on the ramp up and ramp down poweringbranches at 5 kA. The correlation with the slope one is verygood and offsets small. Table I reports the standard deviationof the warm–cold correlation for normal and skewlow-order harmonics up to the decapole. The standard deviationdefined as in [7], corresponds to the spread around the ideal cor-relation line. It provides an estimate of the range of field qualitycontrol that can be achieved during series production in the casethat warm measurements only are used to provide feed back onthe coil geometry. is about three times lower than thenatural spread of the harmonics from magnet to magnet [2]. Thisconfirms our expectation that measurements at 300 K are an ef-fective means to control the geometric harmonics of a dipole inoperating conditions. The small offset observed could beinterpreted as an effect of the different thermal contractions ofthe dipole components after cool-down.B. Coil Size and Azimuthal ShimsThe coil used for the prototypes and the first pre-series dipolesdo not have the same azimuthal size because of small variationsof cable size or insulation thickness. In addition the collar di-mensions were adjusted in the final iteration toward pre-seriesFig. 2. Magnetic length at nominal current for the prototypes and the pre-seriesdipoles. The nominal magnetic length is 14.3 m. Each bar describes the lengthof one aperture (two lengths per magnet).Fig. 3. Scatter plot of the normal sextupole (left) and decapole (right)measured in warm and cold conditions (geometric part only). Experimentalpoints are compared with a linear fit with a slope 1 for the ideal correlation.TABLE ISTANDARD DEVIATION AND OFFSET OF THE CORRELATION BETWEENMEASUREMENTS OF GEOMETRIC HARMONICS AT 300 K AND AT 1.9K (IN UNITS AT 17 mm)production. To compensate for the dimensional errors, supple-mentary pole shims were added between the coils and the col-lars. The difference with respect to the nominal pole shim wasat most 0.3 mm for the first pre-series MBPSO01. The influenceof the azimuthal coil size on the allowed harmonics was ana-lyzed, compensating the collar deformation using the methoddescribed in [7]. The variation of the azimuthal size is computedfor each pole and layer from the additional, known shim thick-ness reduced by 10%.Fig. 4 reports the correlation between the harmonic variationreconstructed using the above procedure and the geometric har-monics measured in cold conditions. The shape of the collar wasmodified from prototype to pre-series dipoles to reduce the ef-fect of collar deformation [8], hence the prototype and pre-se-ries magnets should be considered as two separate families. Thedata of each family were fitted with a line of fixed slope one. InTable II we report the value of the offset and the spreadaround the correlation line . As shown in the Fig. 4, theeffect of a variation of the azimuthal coil size is in expected di-rection and magnitude. A rather larger spread is observed (seeALEKSA et al.: MEASUREMENT AND ANALYSIS OF THE FIELD QUALITY OF LHC PROTOTYPE 249Table II) but it is compatible with the constraints given by beamdynamics [1]. The prototypes and the pre-series dipoles do notshow an evident difference in this respect. The spread can be dueto variation in coil geometry, assembly procedures and manu-facturers. The presence of an offset reflects a non zero value ofodd multipoles foreseen in the nominal design. The differencebetween this offset and the nominal value is mainly due to de-formation of the collars during the assembly, not taken into ac-count in the reconstruction. A complete analysis of the differentbehaviors between prototypes, pre-series and nominal design isgiven in [9].IV. SATURATION AT HIGH FIELDA. Transfer FunctionAs shown in Fig. 1, the saturation of the ferromagnetic yokeresults in a reduction of the transfer function at nominal fieldby approximately 56 units with respect to the geometric value.This reduction is in good agreement with the prediction. Oncethe measured values are shifted vertically to the match the cal-culation at 5 kA, the error between calculation and measurementat nominal field is at most 6 units.B. Higher Order HarmonicsIn Fig. 5, we display the measured high field behavior ofquadrupole , sextupole , octupole and decapolein the pre-series dipoles as compared to the variation ex-pected based on the saturation of the ferromagnetic yoke [10].The measured values have been shifted vertically to match thecomputed value at 5 kA, thus removing the effect of differentgeometry among the magnets. The calculation is in satisfactoryagreement with the measurements for even multipoles. Forand , a significant discrepancy is visible at high current. Inparticular for the measured average high field variation isabout 0.15 units, while a negligible variation is expected. Wehave found that the difference between the measured and com-puted values varies linearly with the square of the current. Thisindicates that the high field variation of the allowed harmonicsmay be caused by elastic compression of the coil under the elec-tromagnetic loads.V. CONTRIBUTIONS OF THE SUPERCONDUCTING CABLEA. Persistent CurrentsThe magnetization associated with persistent currents in theNbTi superconducting filaments affects the allowed harmonics,especially at low fields. This contribution is therefore impor-tant at injection field (0.54 T). Fig. 6 summarizes the values ofthe field contribution from persistent currents measured at in-jection in the prototypes and in the pre-series dipoles. The per-sistent current effect is computed as the difference between theharmonic value at injection current and its geometric value. Forcomparison, the upper and lower bounds of the expected con-tribution are reported in the same graph.The order of magnitude measured matches roughly the expec-tation, except for MBPSO02 that displays an unusually smallercontribution ( 16%).Fig. 4. Scatter plot of the variation of the geometric normal sextupole anddecapole (ends excluded), as measured and reconstructed by calculation (takinginto account as build geometry).TABLE IIOFFSET AND STANDARD DEVIATION OF THE CORRELATION BETWEENGEOMETRIC HARMONICS AND RECONSTRUCTION (IN UNIT AT 17 mm)Fig. 5. Variation of the normal quadrupole, sextupole, octupole and decapoleas a function of the current and comparison to the computed values.Fig. 6. Contribution of the persistent current to the field errors for theprototypes and the pre-series dipoles. The measured values are compared tothe expected ones (upper and lower 1 bound). The different apertures of thedipoles are numbered from 1 to 14 to help the understanding.B. Eddy CurrentsDuring the acceleration of the injected particles, the dipolemagnet is ramped at a maximum ramp-rate of 10 A/s, cor-250 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 12, NO. 1, MARCH 2002Fig. 7. Contribution of the coupling currents to the field errors for theprototypes and the pre-series dipoles. The measured values are compared tothe expected ones (upper and lower 1 bound).responding to a field change rate of approximately 7 mT/s.Coupling currents in the strands and cables induce field errorsproportional to the ramp-rate. The main contribution originatesfrom cable coupling currents closing through the interstrandresistance . The value of is controlled during cableproduction and is expected to be above 20 . Fig. 7 showsthe average contribution of coupling currents measured in theprototypes and pre-series dipoles, compared to the expectedvalues for a of 20 . The measured values are referred to aramp starting at injection with the maximum rate of 10 A/s. Asshown in the Fig. 7, this contribution is small for all harmonicsand has significantly lower values than the expected ones. Thisimplies that the average is well above 20 in the dipolestested so far [11].C. Decay and SnapbackSuperconducting dipoles are characterized by a significantdrift of the magnetic field (decay) when the current is constantwith typical time scales in the order of several minutes to sev-eral hours [12], [13]. The decay contribution to the field errors,measured over a simulated injection plateau of 1000 s, is sum-marized in Fig. 8. Only the envelope of the values measured onall magnets tested so far has been plotted.The same pre-cycling conditions have been used for all mea-surements, starting with a quench and a current cycle with aflat top of 30 min. at the nominal current of 11 850 A (8.4 T).The maximum decay expected is estimated as roughly one thirdof the persistent current contribution to the field. In Fig. 8, theupper and lower expected bound are also reported. The mea-surements are in accordance with these upper and lower boundestimates. In particular for the normal sextupole, the upper limitappears conservative and for the decapole the is large.VI. CONCLUSIONOur understanding of the field quality in the 15-m long LHCdipoles progresses steadily as more magnets are being produced.We are presently confident that the geometric harmonics mea-sured at warm are well correlated to the values under oper-Fig. 8. Expected (1 bound) and measured decay contribution to the fielderrors in the prototypes and in the pre-series dipoles. The two solid linesrepresent the envelope of the measured values.ating conditions. Fine control of the geometric harmonics canbe achieved acting on assembly features such as pole shims.Similarly the yoke saturation effect appears to be broadly un-derstood, although the measurements at high field indicate anelastic deformation of the coil originally not foreseen in thefield errors budget. Among the contributions related to super-conducting effects, the field errors generated by persistent cur-rents are the strongest and show the largest absolute variationfrom magnet to magnet. This contribution will therefore requireattention and careful characterization. At present both the fielderrors caused by coupling currents in the cables and the fielddecay at injection are smaller than initially allocated in the errorbudget.REFERENCES[1] O. Brüning and S. Fartoukh, “Field quality specifications for the LHCmain dipole magnets,”, LHC Project Report, 2001, to be published.[2] P. Ferracin et al., “Control of the field quality for the production of themain LHC dipole,”, LHC Project Report 467, 2001.[3] M. Modena et al., “Manufacture and performance of the LHC maindipole final prototypes,” in Proc. 2000 Part. Acc. Conf., Vienna, Aus-tria, 2000, pp. 2144–2148.[4] M. Modena et al., “Final prototypes, first pre-series units and steps to-ward series production of the main LHC dipoles,” in Proc. IEEE Part.Acc. Conf., Chicago, IL, USA, 2001.[5] J. Billan et al., “Ambient temperature field measuring system forLHC superconducting dipoles,” Proc. IEEE Trans. Magn., vol. 32, pp.3073–3076, 1996.[6] J. Billan et al., “Twin rotating coils for cold magnetic measurements of15-m long LHC dipoles,” IEEE Trans. Appl. Superconduct., vol. 10, pp.1422–1426, Mar. 2000.[7] Z. Ang et al., “Coil size and geometric field quality in short modeldipoles for LHC,” Proc. IEEE Trans. Appl. Superconduct., vol. 10, pp.53–56, Mar. 2000.[8] D. Perini, CERN, LHC Division, CH-1211 Geneva 23, Switzerland, per-sonal communication.[9] P. Ferracin et al., “Impact of coil deformations on field quality in thelarge hadron collider main dipole,”, submitted for publication.[10] S. Russenschuck, 1st Int. Roxie Users Meeting and Workshop, S.Russenschuck, Ed. Geneva, Switzerland: CERN, 1999.[11] A. Akmethov et al., “Loss and dynamic magnetic field measurements inLHC dipoles,”, submitted for publication.[12] M. Haverkamp et al., “Studies of decay and snapback effects on LHCdipole magnets,” in EUCAS’99, Barcelona, Spain, 1999, Proc. Inst.Phys. Conf. Ser. 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Measurement and Analysis of the Field Quality of LHC Prototype and Pre-series Superconducting Dipoles

Measurement and Analysis of the Field Quality of LHC Prototype and Pre-series Superconducting Dipoles

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