The LHC accelerator will be equipped with more than 3500 superconducting spool corrector magnets. CERN has awarded the contract for the series production and testing of these corrector magnets to industry. Magnetic field measurements are done at the factory. Dedicated magnetic measurement benches have been built to test these corrector magnets in the resistive state at room temperature. The benches allow to measure the strength of the main field, normal and skew harmonics, the magnetic axis position and orientation of the main field with respect to the mechanical reference points of the magnet. This paper presents the objectives, a description and the performances obtained with the benches during first measurements at industry

Arn, A

Arshad, S A

Giloux, C

Patru, F

Reymond, H

Senis, R

Walckiers, L

CERN Document Server

1684 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 12, NO. 1, MARCH 2002The Measurement Bench for the LHC SpoolCorrector Magnets in IndustryA. Arn, S. A. Arshad, C. Giloux, F. Patru, H. Reymond, R. Senis, and L. WalckiersAbstract—The LHC accelerator will be equipped with morethan 3500 superconducting spool corrector magnets. CERN hasawarded the contract for the series production and testing of thesecorrector magnets to industry. Magnetic field measurements aredone at the factory.Dedicated magnetic measurement benches have been built to testthese corrector magnets in the resistive state at room tempera-ture. The benches allow to measure the strength of the main field,normal and skew harmonics, the magnetic axis position and ori-entation of the main field with respect to the mechanical referencepoints of the magnet.This paper presents the objectives, a description and the perfor-mances obtained with the benches during first measurements atindustry.Index Terms—Corrector, magnetic field measurement, super-conducting magnet.I. INTRODUCTIONTHE MAIN dipoles (MB) of LHC will be equipped withSextupole (MCS) and Decapole (MCD) “spool” correc-tors. Each decapole corrector has in addition an Octupole insert(MCO) and these together are designated as MCDO. In total2464 MCS and 1232 MCDO assemblies are required for theLHC. Half of these will be made in India in a collaboration be-tween CERN and CAT (Centre for Advanced Technology), theremainder will be built by two European firms, ANTEC, Spainand TESLA Engineering, UK. Pairs of MCS’s and MCDO’smagnets are mounted at each end of the MB’s to correct fielderrors in the dipole field caused mainly by persistent currents intheir superconducting cables (filament magnetization).The measurements performed so far on the model correctormagnets allow to define the following strategy for the seriesmeasurements of the LHC correctors [1]. A good correlation hasbeen obtained between measurements at nominal strength in su-perfluid helium and on a dedicated bench for measurements atroom temperature [2]. The bench described here has been de-signed to routinely control at room temperature the productionin the magnet manufacturer’s premises before and after system-atic tests of the quench performance of the magnets at 4.3 K. Weanalyze the performance reached to measure the field quality,the strength of the magnet, and the position of the magnetic axiswith respect to the mechanical reference points.Manuscript received September 24, 2001.The authors are with CERN, CH-1211 Geneva 23, Switzerland (e-mail:raphael.senis@cern.ch).Publisher Item Identifier S 1051-8223(02)04265-3.II. THE BENCH FOR ROOM TEMPERATURE MAGNETICMEASUREMENTSA. Functional DescriptionThe quality of the magnetic field and magnetic axis alignmentdepends on the precision of the magnet coils and their assemblyin the yoke and casing, difficult to measure on a finished magnetassembly. Therefore a relatively simple magnetic field measure-ment at room temperature is done by the manufacturer to obtainthese data and confirmation that the assembly is within toler-ance. This test also detects gross errors like inversely connectedcoils, coils with a missing turn or interturn short circuits. Thefield quality is recorded and an acceptance or out-of-toleranceindication obtained.The CERN-built room temperature industry bench (Fig. 1)includes a mechanical bench on which the magnet is installedwithout demounting the rotating measurement coil, and a rackcontaining the VME integrators, the bipolar power supply, aSun/Labview data acquisition system, and a printer to easilyrecord the main parameters of each measured magnets.The magnet, weighing about 8 kg, is lowered manually downonto the bench and positioned with the reference holes on themagnet’s mechanical casing. These holes are later used to posi-tion the magnets at the end of the MB dipoles within 0.1 mm. Abipolar DC power supply feeds the magnet under test (1 A forMCS and 0.5 A for MCDO), the nominal current of the magnetsbeing 550 A.Installation and removal of the magnet typically last oneminute and the automatic measurement procedure takes fourminutes. Three MCS and three MCDO benches were built, andhave now been installed in industry. The bench assembly andtraining of the operators was achieved in one day.B. Rotating Shaft Holding the Measuring CoilsThe measurement is based on the rotating coil principle [3].This method consists of taking the values of the flux linked withthe coil at (equally space) angular positions. The coil angularposition is read by an angular encoder. The signal (flux as afunction of angle) is obtained from the measured coil voltageduring the coil rotation by integration.The rotating shaft is equipped with a pair of symmetricallylocated single “outer” tangential coils, one serving as spare orfor diagnostic purposes. The rotation radius and opening angleof the coils are maximized to measure harmonics up to order 15.The bench is purposely open at the top to enable installationof the magnet without need to decouple the motor, encoder andshaft with respect to each other, so that the calibration of thebench is conserved. Slip rings add to robustness of operation.1051-8223/02$17.00 © 2002 IEEEARN et al.: THE MEASUREMENT BENCH FOR THE LHC SPOOL CORRECTOR MAGNETS IN INDUSTRY 1685Fig. 1. Bench description for the room temperature magnetic measurement.Due to the comfortable aspect ratio of the shaft (length/diameter) and design of the bearings, vibrations are reducedso that no bucking coils are needed. The bearings are in self-lubricatingbronze.Theirdiameterandvertical spacingguaranteea mechanical stability of the top of the rotating shaft within10 microns.C. Data Acquisition and AnalysisRotating coil based measurement [3] are repeated with bothdirection of rotation to correct for shaft twist and electronicdrifts, and with both current polarities to correct for permanentmagnetization and pick-up of external fields. Data are obtainedfrom averages of 10 such measurement sequences. The resultsare then analyzed to generate the levels of the harmonics up toorder 15, the position of the magnetic axis and orientation of thefield with respect to the reference holes. These measurementsare then compared with the acceptable levels, and if the toler-ance is exceeded anywhere it is highlighted on the screen. TheLabView/Sun based MMP program [4] used for the magneticmeasurement of all the LHC magnets has been tuned to reduceto a minimum the man machine interface.D. Locating the Magnetic Axis and Bench CalibrationRotating coils give the field multipoles in the reference frameof the rotation axis. The magnetic axis of a sextupole or de-capole is defined as the line where respectively the quadrupoleor octupole term vanishes [5]. LHC optics considerations re-quire the magnetic axis of the correctors to be aligned to within0.1 mm with respect to the ends of the MB main dipole magnets.The measurement gives the offset between two frames: the mea-Fig. 2. Position of the measuring coil axis with respect to the magnet referenceframe and the reference magnet’s mechanical frame.suring coil reference frame, – , with the origin at the rotationaxis passing through and the magnet reference frame, – ,with the origin at the magnetic axis passing through , locatedat ( ) in the – frame (Fig. 2). The measuredmultipoles translated by ( ) can readily be calcu-lated in the reference frame of the magnetic axis.The magnet is positioned on the bench using the referenceholes on the magnet’s flange welded on the external case andused to position the magnet at the MB ends. All magnet compo-nents (especially coils) are produced with reference dowel holessymmetrically located at 70 mm from the magnet’s axis. Thepositioning of the poles in the MCS or MCDO case is accu-rate enough such that mechanical axis, located by definition at70 mm from one of the dowel holes in the magnet’s case, andmagnetic axis coincide within 0.1 mm [2].The axis of rotation of the coils is not perfectly centeredwith respect to the dowel pins positioning the magnets on thebench. A calibration corrects the resulting systematic error.A reference magnet is measured in three different positions:rotated by 180 with respect to the normal position to measure– offset ( ), and turned end to end tomeasure the angular offset ( ). In the reference magnet,we know precisely (metrology data) the length between thetwo dowel holes and thus the deviation , from the nominalvalue (140 mm). The LabView application calculates thethree values ( ) by combiningthis mechanical deviation , with the difference between coilrotation axis and magnetic axis. Now by this procedure theright deviation of the magnetic axis with respect to the me-chanical axis ( ;; ) can begenerated.E. Accuracy of the Transfer Function MeasurementSeveral systematic or random errors degrade the absolutemeasurement of the field/current absolute relationship (alsocalled transfer function). A calibration is needed to know forinstance the error induced in the transfer function due to theradial or the angular position and the opening angle of the coil.All the benches are calibrated with respect to each other inorder to agree within about 0.6%.1686 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 12, NO. 1, MARCH 2002Fig. 3. Main field strength for 9 magnets measured at Antec and at CERN. The figure also shows the 1% range centered on the average of the main field strength.F. Measurement ProcedureDeviations of the magnetic axis of more than 0.1 mm fromthe mechanical axis and/or angular deviations of more than 1.5,2, 2.5 mrad twist of the magnetic field, respectively for MCS,MCD and MCO magnets, will cause the magnet to be rejected.The magnet field acceptance criteria specifies that all harmonics(skew and normal) must be below 1% of the main field at 17 mmradius corresponding to an axis offset of 0.1 mm and to a twistangle of 1.5, 2 and 2.5 mrad following MCS, MCD and MCOmagnet.The system prints the two coordinates of the magnetic axisposition and orientation of the main field with respect to thereference dowel holes in the mechanical case of the magnet, andmultipoles calculated in the reference frame of that magneticaxis. All harmonics up to order 15 are displayed in Tm/A andreported at a reference radius of 17 mm.The automatic analysis provides verifications of the mainmagnet parameters discussed above, i.e., it prints and storesthese results on-line.III. MAGNETIC MEASUREMENT RESULTSIn this section we present a comparison of the first mea-surements, performed using the CERN-built room temperaturebench at ANTEC (Spain), on 9 pre-series MCS magnets, withthose measurements on the same magnets made with an iden-tical bench maintained at CERN.A. Main Field ComponentFig. 3 shows the results for the main field (B3-MCS mag-nets). The difference in the average transfer function betweenthese two sets of measurements is 0.3%. Exchanging the twoelectronic racks on the same bench affects the main field at alevel of 0.2%. Further, there have been some indications thatthe coil calibration also drifts in the 0.1% range. Together, thesetwo errors could account for the 0.3% difference in main fieldbetween the two benches. The differences between racks werenot affected by swapping integrators. Part of the error may comefrom calibration of the current measurement, but in practice itFig. 4. Field orientation measured at Antec and CERN. The dotted lineindicates the more pessimistic expected measurement error.is difficult to adjust this to much better than 0.1%. Also the sta-bility of this adjustment is not known. The average transfer func-tion in the two sets of measurements is measured at 1 A withthe same standard deviation of 0.3% and approximately thesame maximum range around the average of 0.64%, 0.24%(ANTEC), and 0.63%, 0.20% (CERN). Regarding magneticfield quality, the acceptable limit for harmonics other than is10 Tm/A, well above the difference between the two benchesfor harmonics up to order 15 (Maximum difference forTm/A).B. Field Orientation and Magnet’s Axis PositionThe field orientation results are shown in Fig. 4, and the axisposition errors are given in Figs. 5 and 6.Also shown in these figures are the differences between thetwo benches. Note that in all these figures the full scale of theplots represents the tolerances specified for MCS magnets. Theresidual differences between the two sets of measurementsshould be compared with the uncertainty in the measurementwhich originates predominantly from the reproducibility inmechanical positioning of the magnet on the bench. For bothbenches the standard deviations in the reproducibility on theARN et al.: THE MEASUREMENT BENCH FOR THE LHC SPOOL CORRECTOR MAGNETS IN INDUSTRY 1687Fig. 5. Axis dx offset measured at Antec and CERN. The dotted line indicatesthe more pessimistic expected measurement error.Fig. 6. Axis dy offset measured at Antec and CERN. The dotted line indicatesthe more pessimistic expected measurement error.magnetic axis position and field orientation measurements,taking repetitive mounting of the magnet on the bench intoaccount, are evaluated at 5 m and 0.05 mrad respectively,with reference holes drilled within tolerance ( ). Neverthe-less we noticed that the standard deviations in the axis positionand field orientation could be spread more largely (up to10 m, 7 m, 0.1 mrad) depending on the deviationof the dowel holes diameter from this tolerance; i.e., accuratecalibration of the bench and precise measurement becomedifficult for the following version. The reference holes drilledin the pre-series magnets cases were not precise enough due todifficulties introduced by the nickel plating. This uncertaintycontributes to errors in the measurements via two channels.During calibration of the bench the magnet is installed inthree different positions. Errors of up to twice the magnetpositioning uncertainty can therefore enter the calibration, andsubsequently appear as a systematic error in the measurement.The positioning uncertainty appears once again as a randomerror during the actual measurement. The final errors canincrease by up to twice the systematic uncertainty plus oncethe random, for each bench. The differences between the twobenches shown in Figs. 4–6 are therefore in reasonable linewith expectations associated with the more pessimistic error inthe measurement (dotted line in Figs. 4–6).Comparison between measurements performed at CAT(India) and at CERN on MCS and MCDO magnets give similarresults. The bench now at industry is producing results of thesame quality as prior to delivery, and these results are adequatefor testing the magnets to the specifications.IV. CONCLUSIONSGood agreement is found between measurements made atmagnet manufacturers and repeated at CERN with the CERN-built room temperature Industry bench. The results confirm suc-cessful installation and operation of the bench in industry.The quality of measurements performed on this bench so faron the spool corrector magnets allows to maintain the basis ofthe previous test station so as to define the new generation ofbench to measure the other corrector magnets (Lattice and Orbitcorrectors, Correctors for inner triplets …) longer and biggerand with different mechanical reference points than the spoolcorrector magnets. These new benches are being manufacturedat CERN.ACKNOWLEDGMENTThe authors wish to acknowledge the advice of M. Allitt andA. Ijspeert during the prototyping phase of these corrector mag-nets, P. Viret and M. Ky for commissioning the measurementbenches, and the encouragement of P. Sievers (CERN-LHC-MTA).REFERENCES[1] Z. Ang et al., “Measurements of the LHC corrector magnets at room andcryogenic temperatures,” in EPAC-98, Stockholm, Sweden, June 1998.[2] J. Salminen et al., “Experience with the fabrication and testing of thesextupole superconducting corrector magnets for the LHC,” in MT15,Bejing, Oct. 1997.[3] L. Bottura, “Standard analysis procedures for field quality measurementof the LHC magnets—Part I: Harmonics,” LHC Engineering Specifica-tion, LHC-XMT-ES-0001 rev 0.2, 2001.[4] L. Madaro et al., “A VME-based Labview system for the magnet mea-surements of the LHC prototype dipoles,” in EPAC-96, Sitges, Spain,June 1996.[5] A. K. Jain, “Harmonic coils,” in Proc. CERN Accelerator School (CAS),S. Turner, Ed. Geneva, 1998, pp. 175–218.

The Measurement Bench for the LHC Spool Corrector Magnets in Industry

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