Measurement of the CMS Magnetic Field
V. I. Klyukhin, A. Ball, F. Bergsma, D. Campi, B. Curé, A. Gaddi, H. Gerwig, A. Hervé, J. Korienek, F. Linde, C. Lindenmeyer, R. Loveless, M. Mulders, T. Nebel, R. P. Smith, D. Stickland, G. Teafoe, L. Veillet, J. K. Zimmerman
IIEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 18, NO. 2, JUNE 2008 395
Measurement of the CMS Magnetic Field
V. I. Klyukhin , Member, IEEE , A. Ball, F. Bergsma, D. Campi, B. Curé, A. Gaddi, H. Gerwig, A. Hervé,J. Korienek, F. Linde, C. Lindenmeyer, R. Loveless, M. Mulders, T. Nebel, R. P. Smith, D. Stickland, G. Teafoe,L. Veillet, and J. K. Zimmerman
Abstract—
The measurement of the magnetic field in the trackingvolume inside the superconducting coil of the Compact Muon So-lenoid (CMS) detector under construction at CERN is done with afieldmapper designed and produced at Fermilab. The fieldmapperuses 10 3-D B-sensors (Hall probes) developed at NIKHEF and cal-ibrated at CERN to precision 0.05% for a nominal 4 T field. Theprecise fieldmapper measurements are done in 33840 points insidea cylinder of 1.724 m radius and 7 m long at central fields of 2,3, 3.5, 3.8, and 4 T. Three components of the magnetic flux den-sity at the CMS coil maximum excitation and the remanent fieldson the steel-air interface after discharge of the coil are measuredin check-points with 95 3-D B-sensors located near the magneticflux return yoke elements. Voltages induced in 22 flux-loops madeof 405-turn installed on selected segments of the yoke are sampledonline during the entire fast discharge (190 s time-constant) of theCMS coil and integrated offline to provide a measurement of theinitial magnetic flux density in steel at the maximum field to an ac-curacy of a few percent. The results of the measurements made at4 T are reported and compared with a three-dimensional model ofthe CMS magnet system calculated with TOSCA.
Index Terms—
Flux-loops, Hall probes, magnetic field measure-ments, NMR probes, superconducting solenoid.
I. I
NTRODUCTION T HE Compact Muon Solenoid (CMS) is a general-purposedetector designed to run at the highest luminosity at theCERN Large Hadron Collider (LHC). Its distinctive features in-clude a 4 T superconducting solenoid with 6 m diameter by 12.5m long free bore, enclosed inside a 10 000-ton yoke made ofconstruction steel: five dodecagonal three-layered barrel wheelsand three end-cap disks at each end, comprised of steel platesup to 620 mm thick, which return the flux of the solenoid andserve as the absorber plates of the muon detection system [1],
Manuscript received August 24, 2007.V. I. Klyukhin is with the Skobeltsyn Institute of Nuclear Physics, MoscowState University, RU-119992, Moscow, Russia, and with CERN, Geneva 23,CH-1211, Switzerland (e-mail: [email protected]).A. Ball, F. Bergsma, D. Campi, B. Curé, A. Gaddi, H. Gerwig, A. Hervé,M. Mulders, and L. Veillet are with CERN, CH-1211, Geneva 23, Switzerland(e-mail: [email protected]).J. Korienek, C. Lindenmeyer, T. Nebel, R. P. Smith, G. Teafoe, and J. K.Zimmerman are with the Fermi National Accelerator Laboratory, Batavia, IL60510-0500, USA (e-mail: [email protected]).F. Linde is with the National Institute for Nuclear Physics and High EnergyPhysics, 1009 DB Amsterdam, The Netherlands (e-mail: [email protected]).R. Loveless is with the Department of Physics, University of Wisconsin,Madison, WI 53706, USA (e-mail: [email protected]).D. Stickland is with the Department of Physics, Princeton University,Princeton, NJ 08544, USA (e-mail: [email protected]).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.2008.921242 [2]. The yoke steel contains up to 0.17% C, up to 1.22% Mn,and also some Si, Cr, and Cu.A three-dimensional (3-D) model of the magnetic field ofthe CMS magnet has been prepared [3] for utilization duringthe engineering phase of the magnet system and early physicsstudies of the anticipated performance of the detector, as wellas for track parameter reconstruction when the detector beginsoperation.To reduce the uncertainty in utilization of the calculatedvalues for the magnetic field, which is used to determine themomenta of muons during detector operation, a direct measure-ment of the average magnetic flux density in selected regions ofthe yoke by an integration technique is done with 22 flux-loopsmade of 405-turn installed around selected CMS yoke plates.The areas enclosed by the flux-loops vary from 0.3 to 1.58on the barrel wheels and from 0.5 to 1.13 on the end-capdisks. The flux-loops measure the variations of the magnetic fluxinduced in the steel when the field in the solenoid is changedduring the “fast” (190 s time constant) discharge made possibleby the protection system provided to protect the magnet in theevent of major faults [4], [5]. The test of the protection systemduring the commissioning of the CMS magnet provided the op-portunity for the flux-loop measurements. The system of 80 3-DHall probes (out of 95 probes installed) measured the remanentfield on the disk steel-air interface to be added to the flux-loopmeasurements after full discharge of the CMS coil.To investigate if the measurements of the average magneticflux density in the CMS yoke plates could be done with accuracyof a few percent using flux-loops, a special R&D program wasperformed with sample disks made of the CMS yoke steel fromdifferent melts [6], [7].Those studies showed that the contribution of eddy currentsto the voltages induced in the test flux-loop is negligible. Thecontribution of eddy currents to the voltages induced in theflux-loops installed on the CMS magnet yoke when the “fast”discharge of the CMS coil occurs was investigated also [8] withVector Fields’ program ELEKTRA [9].The precise measurement of the magnetic field in the trackingvolume inside the CMS coil is done with a fieldmapper de-signed and produced at Fermilab. The fieldmapper uses 10 3-DB-sensors (Hall probes) developed at NIKHEF and calibrated atCERN to precision at 4.5 T field.Monitoring the CMS magnetic field is done with 4 NMRprobes (Model 1062-R) of the METROLAB PT 2025 high pre-cision Teslameter [10]. The probes are installed near the innerwall of the superconducting coil cryostat. Two probes of thesame type were also used in the fieldmapping to measure the
96 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 18, NO. 2, JUNE 2008
Fig. 1. The CMS fieldmapper mounted inside the hadronic barrel calorimeterinner volume. field along the coil axis, and at the largest radius of the measuredvolume. II. T HE F IELDMAPPER D ESCRIPTION
The volume mapped by the fieldmapper inside the CMS coilis a cylinder of 1.724 m radius and 7 m long. The fieldmappershown in Fig. 1 inside the measured volume moves alongthe rails installed along the coil axis in the hadronic barrelcalorimeter, stopping at predefined points where two sensorarms can be rotated through 360 , stopping at predefined angleswhere the magnetic field is sampled. The azimuth steps are 7.5in magnitude. Steps along the coil axis are fixed to 50 mm by atensioned toothed Kevlar belt.Each arm of the fieldmapper contains 5 3-D B-sensors locatedat radii 92, 500, 908, 1316, and 1724 mm off the coil axis. Thedistance between the negative and positive arm B-sensors alongthe coil axis that coincides with the CMS Z-axis, is 950 mm.Therefore, making 19 steps along the coil axis in positive direc-tion, the fieldmapper delivers the B-sensors of negative arm inthe same Z-position where B-sensors of positive arm were be-fore and vice versa.Made of nonmagnetic materials, the fieldmapper uses pneu-matic power, the gas flow is controlled with 24-V piezoelectricvalves, the remote operation is performed via a ProgrammableLogic Controller and operator’s LabVIEW [11] console, and thelaser ranger is used for absolute Z-coordinate reference after un-scheduled stop.The alignment of the fieldmapper azimuth axle with respectto the CMS coil axis is performed with a precision better than1.9 mrad. The read-out of the B-sensors is performed via theCANopen protocol [12], [13].
Fig. 2. Magnetic flux density measured at radius 92 mm along the coil axisin the range of (cid:6)
III. M
EASURED AND M ODELED
CMS M
AGNETIC F IELD C OMPARISON
A. Mapping the Inner Coil Volume
Fieldmapping the inner coil volume is done at five differentvalues of the magnetic flux density in the center of the coil: 2, 3,3.5, 3.8, and 4 T. The dependence of the central magnetic fluxdensity measured with the NMR probes on the coil currentis linear in the range of the coil current from 4 to 19.14 kA thatcorresponds to the range of from 0.85 to 4 T.At each of these values the magnetic flux density ismapped in 141 azimuth planes, at 48 azimuth angles, thus thefull number of the points mapped with 5 B-sensors is 33840.The central part of the volume in Z-range of 2.55 m withrespect to the coil middle plane is mapped twice in the samepass of the fieldmapper through the volume with B-sensors ofpositive and negative arms. The difference between magneticflux density B measured in the same point with the B-sensor ofpositive and negative arm does not exceed 1 mT.In Fig. 2 the magnetic flux density B measured at 4 T centralfield near the coil axis with B-sensors located at radii 92 mmis displayed without any corrections for the B-sensor misalign-ment. This plot demonstrates the high quality of the measure-ments and shows no variations of B with the azimuth angle. Thegeneral precision of measurements is 0.07%.The measurements discovered a very small asymmetry ofthe magnetic flux density with respect to the coil middle planecaused by one missing turn out of 2180 designed turns.
B. Measured and Calculated Magnetic Field Comparison
To explain the observed magnetic field asymmetry a full CMSlength 3-D model is calculated with Vector Fields’ programTOSCA [14]. The model is based on a half-volume of the CMS
LYUKHIN et al. : MEASUREMENT OF THE CMS MAGNETIC FIELD 397
Fig. 3. Magnetic flux density (left scale) measured at the coil axis with NMRprobe (rhombs) in the range from (cid:0) B value 4.0124 T. Different colors mark four different sets of the measure-ments when the fieldmapper traveled in both axial directions. steel yoke and contains 21 coil conductors to model the missingturn and 1922958 nodes.For each set of the measurements the current in the model isnormalized to get the value measured with NMR probe oraveraged B value measured with B-sensors in the coil middleplane at radius 92 mm. Then the measured and calculated Bvalues are compared along Z-direction.The NMR probe measurements are done on the axis at twovalues: 3 and 4 T. The NMR probe measurement is also done inhorizontal plane at maximum radius 1.724 m at the central fieldof 4 T. The field gradient and noise conditions allowed mea-suring B values on the axis in Z-range from 1.675 to 3.025 mand measuring B values at the maximum radius in Z-range from1.767 to 2.583 m with NMR-probes. At the maximum ra-dius both negative and positive arm B-sensors covered the fullZ-range of 3.5 m.In Fig. 3 the comparison between the NMR measurementson the coil axis and the calculated values is displayed for thecentral field 4.0124 T. The triangles of different color show thedifference between the measured and calculated B values: themaximum difference reaches 5 mT when the measured field is3.8178 T. Four sets of the NMR measurements do not differmore than by 0.5 mT.In Fig. 4 the comparison between the NMR and B-sensorsmeasurements done at the maximum radius and the calculatedvalues is also displayed for the central field 4.0124 T. The blacktriangles show the difference between the NMR measurementsand calculated B values: the maximum difference reaches2 mT when the measured field is 3.9152 T. The slanted andright crosses show the difference between the B-sensors mea-surements and calculated B values: the maximum differencereaches 5 mT when the measured field is 3.7974 T. Both NMRand B-sensors measurements consist well. Thus, the CMS3-D model with one missing turn explains well the very smallmagnetic field asymmetry observed in the measurements. Noaxial shift of the coil with respect to the yoke is required to fitthe measurements and the calculations. Fig. 4. Magnetic flux density (left scale) measured along the coil axis in hor-izontal plane at radius 1.724 m in the range of (cid:6) (cid:0) B value 4.0124 T. This conclusion is confirmed by the coil alignment and thecoil cryostat position measurements.IV. T HE F LUX -L OOP M EASUREMENTS
Measuring the averaged magnetic flux density inside the steelblocks of the CMS yoke is performed with the system of 22flux-loops made of 405-turns mounted around the selected el-ements of the yoke. During the coil “fast” discharge (190 stime-constant) from the full current to zero, voltages with am-plitudes up to 3–4.5 V were induced in the loops.The digitization of the flux-loop voltages is done withseven Measurement Computing USB-based DAQ modulesUSB-1208LS with 4 differential 12-bit analog inputs each [15].The off-line integration of the voltages and adding the rema-nent fields reconstruct the maximum magnetic flux density insteel blocks at the CMS magnet full excitation. The values ofthe remanent fields are below 10 mT.In Fig. 5 the induced voltages and reconstructed magneticflux densities are shown for three flux-loops mounted in threeiron layers between the muon drift chambers in the lower az-imuth sector of the next to the central barrel wheel of the yoke.The areas enclosed by the flux-loops are 0.38 (inner layer),1.29 (middle layer), and 1.58 (outer layer). The min-imum area enclosed by the flux-loop on the barrel wheels ofthe yoke is 0.3 , the areas enclosed by the flux-loops on theend-cap disks of the yoke vary from 0.5 to 1.13 .At the central field 4.0124 T the magnetic flux densities re-constructed with the flux-loops in the barrel wheels of the yokevary from 0.62 to 1.97 T. The magnetic flux density measuredwith the flux-loops in the end-cap disks of the yoke vary from1.66 to 2.62 T. The precision of measurements is expected to bea few per cent.
98 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 18, NO. 2, JUNE 2008
Fig. 5. Voltages (curves 1, 2, and 3) induced in steel blocks of the barrel nextto central wheel in the inner (1), middle (2), and outer (3) layers during the coil“fast” discharge from full current of 19140 A to zero and integrated magneticflux densities (curves 4, 5, and 6) in the inner (4), middle (5), and outer (6)layers.
V. C
ONCLUSION
The first measurements of the magnetic flux density in thevolume of more than 65 inside the superconducting solenoidcoil at 4 T central field is done with precision of 0.07%.The developed 3-D model of the CMS magnet describes wellthe magnetic flux distribution over all the magnet volume and isused outside the CMS coil to provide the magnetic field for thesimulation and tracking reconstruction programs.The developed flux-loops technique permits to measure theaverage magnetic flux densities in steel elements of the yokewith precision of a few per cent.The response of the CMS coil central field to the coil currentis linear within a current range from 4 to 19.14 kA.Monitoring the magnetic flux density in different regions ofthe magnet yoke with 3-D B-sensors developed at NIKHEF isstable and provides the magnetic field values with a high preci-sion. Monitoring the magnetic flux density inside the coil with NMR probes of three different types gives the reference mag-netic field values in all the range required by the CMS detectorperformance for the particle physics.R
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