Bergsma, F

Cataneo, F

Flegel, Wilfried

Gerwig, H

Giudici, P A

Panman, J

Petrucci, Guido

Rosset, C

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN–PPE/94–17610 November 1994THE HEXAGONAL TOROIDAL AIR-CORE MAGNETOF THE CHORUS DETECTORF. Bergsma, F. Cataneo, W. Flegel, H. Gerwig, P.-A. Giudici, J. Panman,G. Petrucci and C. RossetCERN, Geneva, SwitzerlandAbstractWe report on the hexagonally shaped toroidal air-core magnet used in the magneticspectrometer of the CHORUS experiment at CERN. The novel and unique features of thismagnet are the radially-constant field over its entire volume, with field lines straight and parallelto the hexagonal sides, and the negligibly small field-free region along its central axis. Adetailed description of its construction and performance is given.(Submitted to Nuclear Instruments and Methods) 11 INTRODUCTIONThe CHORUS Collaboration has constructed a new detector for the study of neutrinooscillations [1]. Part of this detector is a magnetic spectrometer for the charge determination oflow-energy particles of neutrino-induced events.The magnetic spectrometer consists of a hexagonal toroidal air-core magnet, developed,designed, and constructed at CERN. It is preceded and followed by scintillating fibre trackerplanes adapted to the special geometry of the magnetic field. The following gives a detaileddescription of this hexagonal magnet.2 GENERAL DESCRIPTION OF THE MAGNETThe magnetic spectrometer of the CHORUS detector needed to have the followingfeatures:• a toroidal magnet of large area, determined by the dimensions of the neutrino target• a negligibly small field-free region along the central axis• a minimal amount of material in the passage of traversing particles• easy particle tracking with tracker planes close to the magnetic volume.The above requirements were met by a hexagonal air-core magnet, composed of regulartriangular sectors. The windings in each of these sectors are equidistant and orthogonal to thepolygon side for the front and back faces, and parallel to the axis for the other planes. Figure 1shows schematically the winding arrangement.Fig. 1 Scheme of a toroidal sectorA toroidal magnet arranged in such sectors has novel and unique features:• its field is constant over the entire volume (no radial dependence)• the field lines in each sector are straight and parallel to the outer side of the polygon• along the axis  it has nearly no region with zero magnetic field• only small regions along the diagonals are filled with material.2These specific properties hold for any polygonal toroidal magnet with the same-typewinding arrangement. Its field can easily be calculated*.Figure 2 shows an overall view of the CHORUS hexagonal magnet.Fig. 2 The hexagonal magnetThe magnet is of a compact design. The conductors are made of Al-alloy 2.5 mm thick,representing just a few per cent radiation length for particles traversing the magnetized volume.Only the diagonal planes, where the conductors between the front and back faces are fixed tothe support sheets, are of material all along the axial depth of the magnet. Its azimuthalthickness is about 11 mm.The tracking in the target region of the CHORUS detector and at the magneticspectrometer is performed by planes with straight scintillating fibres of 500 m m diameter,viewed at one end by chains of image intensifiers. The compact arrangement of the magnetconductors prevents any field leakage to the outside of the front and back faces of the magnetthat could influence the nearby image intensifiers. A small leakage of 2 to 4 G at the hexagonsides arising from the cross conductors between the front and back faces is stopped by a3 mm-thick ARMCO sheet around the outer diameter of the support frame.-------------------------*Any sector-shaped winding of this type creates a uniform field if the radial planes limiting the sector arekept at the same magnetic potential, for instance by means of flat iron polar faces connected by a returnyoke. It is trivial in this case to verify that the acting ampere turns are increasing linearly from the sectorvertex towards the outer edge, corresponding to the equal increase of the air-path length between thepolar faces.3The neutrino beam is pulsed with two pulses of 6 ms length every 14.4 s. Operating themagnet in a pulsed mode, greatly reduces the average power dissipated in the windings andpermits simple air flow cooling instead of the more risky and complicated water cooling (seeSection 4).The main parameters of the magnet are summarized in Table 1.Table 1Hexagon side lengthAxial lengthConductor materialConductor thickness at front & back faceConductor width at front & back faceRadius of front & back face curvatureMagnetic inductanceTotal number of turnsMaximum currentVoltage at maximum currentEffective pulse lengthFlat top lengthEffective power (2 pulses / 14.4 s)Ohmic resistanceInductanceStored energyCoolingConductor massTotal mass (including support frame,shielding, cooling ducts, etc.)1.5 m0.75 mE – Al Mg Si 0.5 (DIN)2.5 mm (total: 5.6 % rad. length)32 mm3550 mm1180 G2643200 A720 V~ 55 ms~ 12 ms~ 17 kW215 m W4.5 mH25 kJForced air circulation inside magnet~ 450 kg2300 kg3 CONSTRUCTION DETAILSThe  magnet is mounted in two large discs of Al-alloy of 3.6 m diameter with ahexagonal cut-out that act as a general support frame. Stainless steel sheets 2 mm thick and of890 mm axial width are fixed along the diagonals of the hexagon, and pre-stressed by means ofcalibrated spring washers to 7 t a sheet. They are constructed in three V-shaped sheets andwelded at the centre onto a small star-shaped rod, forming six identical triangular sectors. DetailA of Fig. 3 shows an enlargement of this region.4Fig. 3 Details of the conductor and winding arrangementBoth sides of the stainless steel sheets are covered with a 2 mm-thick glass-fibre epoxyplate to build up an insulating sandwich. The conductors of neighbouring triangles aresymmetric to the diagonal plane and fixed together onto this insulating sandwich with glass-fibre M8 screws. The conductors and winding arrangement of the magnet are enlarged inFig. 3.A conductor is made of a 2.5 mm-thick band of E–Al Mg Si 0.5 (DIN) alloy (tradename: Anticorodal 041), which combines high electric conductivity with good mechanicalproperties. It is bent in a symmetric U-shape and the two vertical arms of the U form the 32mm-wide conductor at the front and back face, while the base of the U is given by the 64 mm-wide and tilted part of the conductor, fixed to the insulating sandwich sheet at the diagonal plane(detail B of Fig. 3). Neighbouring U-shaped conductors of one triangular sector of the hexagonare separated by a gap of 2 mm and connected in a series by a solid cross conductor of50 ·  15 mm2 at the side of the triangle (detail C of Fig. 3). In this way, all conductors in atriangle form together a solenoid of varying cross section. Neighbouring triangle solenoids arein turn connected in a series to form the total toroid. At the front and back faces an H-shapedrubber profile was inserted into the 2 mm gap between neighbouring conductors in order to5avoid any electrical contact between them during pulsing, and to seal the front and back facestowards the air cooling flow (see Section 4).The electromagnetic forces which stress and deform the winding when it is pulsed havethree contributions. The largest contribution is from the action of the magnetic field on theconductors situated at the front and back faces. At the maximum current of 3200 A themagnetic forces act with a pressure of 0.58 N/cm2 and bend these conductors towards theoutside. The conductors hoop stresses  are proportional to the radius of their curvature and leadto a resultant radial force directed outwards, which is supported by the stainless-steel sheetplaced in the diagonal planes. The axial part of the U-shaped conductors is pushed by the fieldwith a force having a radial inward component plus a component perpendicular to, andtowards, the radial support plane. The total force resulting from these three contributions is upto about 4 t for each radial plane and is directed outwards. The forces acting on the solid crossconductors at the hexagon sides are also directed outwards and absorbed by the supportingstructure. Whereas these last forces are easily absorbed without any periodical deformation, theprevious forces, due to the thinness of the conductors, produce small elastic deformationsduring the pulses (see Section 5).To keep these movements reasonably small and to avoid any asymmetry, the diagonalstainless-steel sheets were spring loaded and pre-stressed with 7 t after compensation of theweight of the inner part of the magnet. For this purpose, the two upper hexagon corners areprovided with adjustment devices which permit the magnet geometry to be corrected prior tothe pre-stressing by compensating the gravity forces acting on it.4 PULSING, COOLING AND MONITORINGThe requirement of minimal material thickness for traversing particles at a relatively highfield with periodically acting forces led to the choice of E–Al Mg Si 0.5 as the conductormaterial; it has good mechanical properties and its specific resistivity is 30 mW  mm2/m. Toassure the quality of the numerous contacts (two for each turn), as is usual for Al conductors allcontact surfaces between the U-shaped conductors and the solid cross-bars were gold plated.The resulting total ohmic resistance of the winding is 215 mW .As stated earlier, the magnet is pulsed in accordance with the neutrino beam, with twopulses spaced by 2.8 s within 14.4 s. The maximum magnetic field has to be stable during the6-ms period of the neutrino ejection. This was achieved by choosing a flat top of 12 ms for themagnet. Furthermore, to keep the ohmic heat losses as small as possible, the rectifier is pulsedwith an over-voltage in order to quickly reach the flat top of the pulse. Thus the pulse overallrise time could be reduced to 50 ms. The fall time is given by the discharge time with a timeconstant of 30 ms. The effective duty cycle of the magnet is therefore ~ 0.8% and thecorresponding ohmic heat losses are ~17 kW.Owing to the symmetry the magnetic field is equal in all magnet sectors. It is monitoredin three of the sectors by Hall probes read out together with the neutrino data. Figure 4 showsthe pulsed magnetic field as seen by one of the probes.6Fig. 4  Hall probe signal of magnetic field pulse,horizontal scale: 20 ms/div.,vertical scale: 400 Gauss/div.The ohmic losses are mainly produced in the thin U-shaped conductors, i.e. in the frontand back faces of the magnet and in its radial planes. Owing to the immediate proximity of thefibre tracker planes at both sides of the magnet the cooling by air of the front and back faces canbe done only from inside the magnet. Outside these faces, 2 mm-thick polycarbonate discs arefixed to the diagonals and sealed at the ARMCO shielding in order to fully separate thermallythe magnet from the detector planes, as indicated in Fig. 3.The cooling is achieved by a dedicated cooling unit, equipped with a pulsion andextraction fan. It creates a total air flow of ~ 2 m3/s. This air flow is divided into six equalstreams, one for each triangular sector. As can be seen from Fig. 3, for each sector the flow ofcold air passes the cross conductors with the help of a thin walled polycarbonate tube, is led toits centre and guided with deflector sheets along the inside surfaces. The warm air leaves asector by passing between the cross conductors in the area not occupied by the inlet tube. It isextracted above each of the six diagonal planes, re-collected, and fed back into the cooling unitwithout heating up any other part of the detector. To allow for the easy passage of air, the solidcross conductors connecting the thin U-shaped conductors are only 15 mm wide for a centre-to-centre spacing of 34 mm.When the magnet is pulsed the thin front and back face conductors move under the actionof the electromagnetic forces. At a maximum current of 3200 A the centre of the longestconductors will swing outwards by 2 to 2.5 mm. The swing of the shorter conductors isaccordingly smaller. The magnet thereby acts as a sound generator, similar to a big drum. Fornormal operation the measured sound level is 75 dB(A) at a distance of 30 cm from themagnets front face. Its frequency spectrum ranges from about 30 to 250 Hz with a maximumat around 80 Hz. The noise level of the cooling system itself is about 73 dB(A).To reduce the ageing of nearby CHORUS detector elements (nuclear emulsions andscintillating fibres) and to run the image intensifiers at a lower noise level, the hexagonalmagnet is placed together with these elements in a cool box and kept permanently at 5 ° C. Thetemperature of the air blown onto the hexagonal magnet is 4° C. Under pulsing conditions theoutlet air has a maximum temperature of 10 ° C.7The maximum instantaneous ohmic loss at full magnet current amounts to ~ 2.2 MW. Itis therefore necessary to carefully control the value of the magnet duty cycle and to monitor themagnet temperature. Consequently, thermo-switches and thermistors are fixed onto some ofthe conductors at the front and back faces of each magnet sector and interlocked with the powersupply. The temperature of the longest of these conductors rises under normal pulsingconditions to ~ 25 ° C. Two series of thermo-switches are provided that open respectively at60° C and 80° C. On reaching the limiting temperature, the 60° C switches stop the rectifier frompulsing, and the 80° C ones switch off the rectifier supply voltage. Finally, the pulse frequencyand the pulse lengths of the rectifier are limited electronically.5 MATERIAL FATIGUEThe CHORUS experiment will take data during two years of 200 running days each. Themagnet pulsed with the neutrino beam will therefore ideally undergo approximately5 ·  106 cycles.The pulsed magnetic field inside the toroid will periodically deform the thin conductors atthe front and back faces. This can lead to rupture of the conductors due to material fatigue. Theproblem of conductor fatigue has been studied and optimized by finite element analysis andverified experimentally on a number of conductors of shape similar to that in the magnet. Thetest conductors have been pulsed with current in an external constant magnetic field in order toproduce approximately the same stress conditions as in the real pulsed magnet*. The highestrisk for material fatigue is in the bent corners of the U-shaped conductors. To reduce this risk,the corners are bent with a inner radius of R = 6 mm, and the front and back part of theconductors are pre-formed with a radius of R = 3550 mm, as indicated in Fig. 3. This valuewas chosen as a compromise between the need to limit the stress and the wish to maximize theB·l value in the restricted space. With the above chosen parameters the maximum effectivestress at the critical corners can be reduced to ~ 32 N/mm2, which is about half the value of70 N/mm2 for 107 alternating traction cycles given for this Al-alloy in the material propertiestable under the ideal conditions of a test specimen.The hoop stress created by the electromagnetic forces is the same for all front and backface conductors. But in the critical region it produces an elastic deformation proportional to theconductor length — mainly at the bend and its fixation to the diagonal sheet. As a result, theconductors will swing outwards under the magnetic pressure with an amplitude proportional totheir length. This swing is 2 to 2.5 mm for the longest conductors. In addition, micro cracksdue to the bending, with R = 6 mm and opening up under the periodic action of the magneticpressure cycle, could reduce the theoretical maximum stress value considerably. It is thereforeimpossible to give a confident number for the probable lifetime under running conditions dueto several uncertainties that include: precise fatigue fracture values for this Al-alloy, residualstresses after the bending process, and the elastic behaviour of the entire magnet. One can onlysay that its lifetime is limited.------------------------* In the real case the stresses are proportional to I2, where I is the current in the conductor; in the fatiguetests they were only proportional to I. The maximum stress in the tests was however higher than in thereal case.8In order to keep the magnet cycles to a strict minimum the magnet is pulsed only whendata are recorded in the CHORUS detector. The magnet has undergone about 1.2 ·  106 cyclesin the first year of the CHORUS experiment running.AcknowledgementsThe hexagonal magnet is an integral part of the CHORUS detector. We gratefullyacknowledge the support of, and the many discussions we had with, our colleagues of theCHORUS Collaboration, and especially with its spokesman K. Winter and its technical co-ordinator J.P. Fabre.The realization of the magnet was possible thanks to the skill and dedication of the manypeople who contributed to its various aspects. In particular we wish to thank K. Mühlemann forhelp in performing the fatigue test and S. Fahnauer, M. Löbe (both from Humboldt UniversityBerlin) and D. Rotil for their care in preparing the mechanical pieces and in the magnetassembly. B. Danner and his collaborators H. Graskamp, C. Valentini and O. Bohner adaptedthe magnet current supply for these special pulsing conditions and took care of its functioning.P. Pepinster and P.A. Rochat designed and prepared the magnet's cooling system, andG. Frémont and R. Rey-Mermier supplied the Hall probes for the field measurements andelectronics for reading the magnet currents. R. Ferreira, M. de Jong and K. van der Poelincorporated the magnet into the CHORUS trigger and general data acquisition system.REFERENCES[1] Proposal of the CHORUS Collaboration for a new search for neutrino oscillations: CERN-SPSC/90–42 (1990).

The hexagonal toroidal air-core magnet of the Chorus detector

The hexagonal toroidal air-core magnet of the Chorus detector

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