The superconducting twin aperture main dipole magnets of the LHC accelerator are equipped with pairs of sextupole and decapole correctors at their ends. Similarly, octupole correctors are aligned at t he end of the main quadrupole magnets. Dedicated stations have been built for tests of these correctors at room temperature as well as superfluid helium temperature. Measurements of the training behav iour and of the magnetic field quality are routinely performed. The search for the magnetic axis and the transfer of its position to fiducials are performed at room temperature. A description and the performances obtained with these two benches are also presented

Ang, Z

Arn, A

Bottura, L

Giloux, C

Sievers, P

Smirnov, N

Vincent, S

Walckiers, L

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsLarge Hadron Collider Project LHC Project Report 201Z. Ang, A. Arn, L. Bottura, C. Giloux, P. Sievers, N. Smirnov, S. Vincent, L. WalckiersAbstractThe superconducting twin aperture main dipole magnets of the LHC accelerator are equipped with pairs ofsextupole and decapole correctors at their ends. Similarly, octupole correctors are aligned at the end of themain quadrupole magnets. Dedicated stations have been built for tests of these correctors at roomtemperature as well as superfluid helium temperature. Measurements of the training behaviour and of themagnetic field quality are routinely performed. The search for the magnetic axis and the transfer of itsposition to fiducials are performed at room temperature. A description and the performances obtained withthese two benches are also presented.*  LHC-MTAPresented at the European Particle Accelerator Conference, EPAC'98, 22-26 June 1998Administrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandGeneva, 10 July 1998Measurements of the LHC Corrector Magnets at Room and Cryogenic Temperatures2MEASUREMENTS OF THE LHC CORRECTOR MAGNETS AT ROOMAND CRYOGENIC TEMPERATURESZ. Ang, A. Arn, L. Bottura, C. Giloux, P. Sievers, N. Smirnov, S. Vincent, L. WalckiersCERN, CH-1211 Geneva 23, SwitzerlandAbstractThe superconducting twin aperture main dipolemagnets of the LHC accelerator are equipped with pairsof sextupole and decapole correctors at their ends.Similarly, octupole correctors are aligned at the end ofthe main quadrupole magnets. Dedicated stations havebeen built for tests of these correctors at roomtemperature as well as superfluid helium temperature.Measurements of the training behaviour and of themagnetic field quality are routinely performed. Thesearch for the magnetic axis and the transfer of itsposition to fiducials are performed at room temperature.A description and the performances obtained with thesetwo benches are also presented.1   INTRODUCTIONThis paper focuses on the description of the equipmentto evaluate the performances and the field quality ofthree types of correctors [1]. Pairs of MCS's sextupoleand MCD's decapole magnets are mounted at each endof the MB's, main LHC dipole bending magnets, tocorrect the sextupole and decapole harmonics duemainly to persistent currents in their superconductingcables. The MO octupole's are foreseen for Landaudamping and are mounted in the short straight sectionswith the main LHC quadrupoles.The reference radius to express the field quality of allLHC magnets has recently been increased to 17 mmwhich reflects better the larger good field region needed,particularly in the interaction region magnets. The fieldcharacteristics of the correctors analysed below will becompared with the integrated field errors of the MB's.Therefore the units used in this paper for the fieldharmonics are in Tm at the reference radius of 17 mm :x the MCS with a nominal strength of 0.055 Tm,x the MCD " " " 0.01 Tm,x the MO " " " 0.092 T m.2  THE CRYOGENIC TESTS STATIONThe measurement and power test equipment followsthe successful design of the stands used for the tests ofthe main LHC dipole short models.The cryostat (Fig. 1) is designed to test magnets aslong as 2.1 m and has a useful aperture of 0.3 m. Theenlarged part holds the heat exchanger below the lambdaplate, and four 2 kA current leads to feed the threemagnets either separately or in series. All types of LHCcorrectors that are slimmer than the inter-aperturedistance of 0.194 m can be tested in that cryostat.Dedicated suspension pieces allow to test three shortmagnets mounted on top of each other : 0.16 m longMCS or 0.11 m long MCD's. Eight hours are needed toreach liquid helium temperature, another four hours tocool down the 250 litres of helium to superfluidtemperature. Electrical heaters at the bottom of thecryostat warm up the assembly to room temperature inabout 20 hours.Fig.1 The Cryostat to test up to 3 corrector magnets.3  THE MAGNETIC MEASUREMENT'SEQUIPMENTThe measuring coils assemblies are made with 5identical coils (C1 to C5) adjacent to each other, thecentral one , C3, being centred on the rotating axis. Thisallows to reject the main harmonic for both quadrupoleand sextupole magnets :Quadrupole term rejection : V1 - (V2+V3) + V4 ,Sextupole rejection : V1 - 3V2 + 3V3 - V4,with V1 = voltage across C1, etc ...Easy checks of the accuracy of the mounting for thestrengths measurements or of the quality of the rotationof the shaft are performed due to its symmetrical feature :comparing values measured by C1 to those by C5. Adetailed comparison between measurements of theharmonic orders higher than the sextupole in the MCSshows that bucking the main harmonic is not necessaryfor the warm measuring bench but clearly improves themeasurements with the cold bench where the the qualityof the shaft rotation is much more difficult to ensure.11005001800630DBAFCE30011005001800300630 A : 2kA gas cooled currentleadsB : Joule - Thomson valveC : Heat exchangerD : Encoder & motor for themagnetic measurementE : Magnetic measurementshaftF : Corrector magnetsDBCFE33.1  The equipment for cryogenic measurementThe measuring shaft used for the cold measurements[2] comprises four coil assemblies each with an effectivelengths of 0.2 m and measure individually the magnets.Since it is impossible to precisely center themeasuring shaft onto the magnet's axis, themeasurements are systematically corrected for thisoffset, based on the suppression of the skew term of themain harmonic and the lower harmonic order measuredat the nominal strength.3.2  The bench for room temperaturemeasurementThe bench measuring the magnet at room temperature(Fig. 2) is transportable and set up for measurements in ashort time. Moreover it has proven to be a very usefultool for training staff in magnetic measurement and as atest bed for the software and hardware needed for allmagnetic measurement systems to be developed for theLHC project. The equipment and procedure for axismeasurement are described in 5.2.The voltage signal across the 400 turns measuring coil[3] amounts to 16 mV for a MCS corrector powered with1 A. The measurements must be taken with bothpolarities to correctly subtract the effect of the earthmagnetic field and of the magnetisation of the iron yokeafter cold tests.Figure 2: Bench for the magnetic measurement and axispositioning of the different types of correctors at roomtemperature.4  POWER TESTS PERFORMEDThe power tests at cryogenic temperature follow theprogram applied to the 1 m long LHC model dipoles [4].Ramp rates as high as 1 kAs-1 induce quenches muchabove the nominal field strength (nominal rate =10 As-1).The measured resistance due to the inter-poleconnections range between 15 and 300 n:. Themeasurements resolution is 3 n:. These resistance willhave to be reduced since the allowed loss is 10 mW at600 A.The hot spot temperature never exceeds 80 K ifmagnets are quenched alone. A dedicated test wasperformed to analyse whether a quenching sextupolemagnet can absorb the stored energy of several othermagnets in series with the power supply intentionallykept running during the duration of the quench.Although the quenching magnet has dissipated 900 J, 6times the stored energy at nominal current, a maximumof 18 kA2*s was reached in the conductor, correspondingto 80 K and the voltage across the magnet stayed below50 V.5  MEASUREMENT AND TOLERANCESOF THE MAGNETIC FIELD QUALITY5.1  Main field componentFigure 3 shows, as a function of current, the differencebetween the measured strength of a MCS sextupolemagnet and a straight line to enlighten the hysteresis dueto the persistent currents in the superconducting cable.The width of this hysteresis ( 1.710-4 Tm  @ 17 mm ofradius) corresponds to 0.2 unit of sextupolar error in aMB (main LHC 15 m long dipole) at the injection fieldof 0.54 T. The MCS's will correct the sextupolecomponent of the MB's including the time dependenteffects. The slope of the current has therefore to changesign to first compensate the decay during injection thenthe snap-back at the start of acceleration. The hysteresisloop will be crossed and fine tuning of the current willcontrol the resulting effect of 5 units of chromaticity.Similarly the MO octupole correctors have a width ofthe hysteresis equal to 1.5*10-4 Tm : 3 times thetolerance requested for the control of the footprint in thetune diagram at LHC injection. The hysteresis width ofthe MCD's is expected to have a small but noticeableeffect on the dynamic aperture if not taken into account.Figure 3 : Field strength of a MCD corrector :difference between the strength and straight line givingthe average to enlighten the hysteresis due to persistentcurrents.DC motorSafety clutchAngular encoderTable for azimutal adjustmentRotating measuring shaftCorrector magnetx-y adjustment tableTable for azimutal adjustment-0.0010.0000.001-1 -0.5 0 0.5 1Current (kA)B3 - (0.122*I)  [ Tm @ 17 mm , I in kA ]45.2  Positioning of the magnet's axisA MCD corrector misaligned with respect to thedecapolar component of the MB will result in aproportional octupolar field applied to the beam, theaverage of it over octants of the LHC will degrade thedynamic aperture. A tolerance of 0.1 mm is needed forthe magnetic axis survey of the correctors. The mostdemanding tolerance is to centre these correctors withrespect to the measured MB's curved axis to better than0.3 mm in average. On the other hand, the accuracy ofpositioning the poles in the MCS or MCD correctorshardly ascertains that mechanical and magnetic axescoincide within 0.1 mm [1]. The "warm" measuringbench is therefore equipped with all the facilities neededto mark the correctors with survey points.The magnet is first oriented by the angle andpositioning tables in order to cancel respectively the skewmain term and the component one order lower than themain harmonic. A precise arm locks the lateral referencesurface of the rotating measuring shaft into areproducible angular position in order to first rotate theencoder case at the reference position, then the magnet.The measuring shaft is then replaced by a reference jighaving the same rotation axis and equipped with thesame reference surfaces. This jig is equipped with fingersin hard metal able to punch survey marks in the magnet'scase. Both the measurement sensitivity and themechanics including the accuracy between these fingersand the rotation axis or the reference surfaces allow apositioning of the survey marks within 25 Pm.The MCS will correct a sextupolar component of0.0087 Tm at 17 mm due to persistent current effects inthe MB's at injection field, giving a change ofchromaticity of about 550 units. The control of thechromatic coupling requests a skew sextupole term lowerthan 4*10-4 Tm at 17 mm. The systematic error allowedin the field direction adjustment is therefore 5 mrad. Averification of the parallelism between the referencesurface of the measuring shaft and the coils effectivesurfaces is possible by swopping the shaft top to bottom.The actually measured error of 2.8 mrad can be takeninto account in the alignment software.5.3  Field qualityA dipole field component of 4*10-7 Tm measured onthe "warm bench" was traced to be due to the currentleads of the MCD correctors : it would induce a fieldequal to 5 ppm of the main dipole at injection. Timedependent effects have been measured in the MCS to benegligible : the 18-pole term is below 1*10-8 Tm for awidth of the persistent current amounting to 6*10-6 Tm.Figure 4 compares the quality of the measurements ofthe harmonic content of a MO octupole corrector madewith both the warm and the cold benches to typical fieldcomponents and specifications. All the values arereferred to the nominal strength of the magnets. Theratio between measurement quality and values toleratedfor the beam is similar for the other types of correctors.Figure 4 : Accuracy of the measurements for the"Warm measurement", for the "Cold measurement",Field error "Harmonic" measured on a MO corrector,"Tolerance" for the beam.6  CONCLUSIONS FOR THE SERIESMEASUREMENTSThe measurements performed so far on the modelcorrector magnets allow to define the following strategyfor the series measurements of the LHC correctors.A simple bench measures at room temperature thefield quality : a good correlation has been obtained withmeasurements at nominal strength in superfluid helium.The accuracy of positioning the magnetic axis is 25 Pm.It is planned that the magnet manufacturers will makequench tests at 4.3 K on the series since the three types ofcorrectors analysed here reach their nominal strength innormal boiling helium.The cold test station described here would have toverify statistically some of their characteristics : jointresistance, field strength and hysteresis due to persistentcurrent.REFERENCES[1] J. Salminen et al., "Experience with the fabricationand testing of the sextupole superconductingcorrector magnets for the LHC", MT15, Bejing,Oct 1997.[2] "A. Ijspeert et al., "Test results of the prototypecombined sextupole-dipole corrector magnet forLHC", IEEE Trans on Applied Superconductivity,Vol. 3, No 1, pp 773-776, March 1993. [3] J. Billan et al., "New technique of fabrication ofsearch coil for magnetic field measurement byharmonic analysis", AT-MA Int. Note 117, CERN,May 1995. [4] D. Leroy et al., "Test results on 10 T LHCsuperconducting one meter long dipole models",IEEE Trans on Applied Superconductivity, Vol. 3,No 1, pp 773-776, March 1993.1.E-081.E-071.E-061.E-051.E-041.E-031.E-021.E-01B1 A1 B2 A2 B4 B5 A5 B12 A12Warm meas Cold Meas Harmonic Tolerance[ T.m @ 17 mm ]

Measurements of the LHC Corrector Magnets at Room and Cryogenic Temperatures

Measurements of the LHC Corrector Magnets at Room and Cryogenic Temperatures

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