Ogitsu, T

Ohuchi, N

Ozaki, T

Sugahara, R

Tsuchiya, K

CERN Document Server

SUPERCONDUCTING FINAL FOCUSING SYSTEM FOR KEKBK. Tsuchiya, N. Ohuchi, T. Ogitsu, T. Ozaki, R. Sugahara, KEK, Tsukuba, Ibaraki, JapanAbstractA superconducting final focusing system for the KEKBB-Factory was constructed and has been operating since1998. It consists of two solenoid field compensationmagnets, two final focusing quadrupole magnets, andthree kinds of correction coils (normal and skew dipoles,and skew quadrupole). The system has been operatingwithout any serious problems and its total operating timehas reached 17,000 hours. This paper describes the detailsof the system and operating experiences gathered duringthe long-term operation.1 INTRODUCTIONKEKB is an asymmetric, two-ring electron-positroncollider, which circulates 8 GeV electron and 3.5 GeVpositron beams in opposite directions, and collides themat one interaction point (IP) with a finite crossing angle of±11 mrad. A schematic layout of the beam line near the IPis shown in Fig. 1. This figure shows the four super-conducting magnets (S-R, S-L, QCS-R, QCS-L) and twonormal-conducting quadrupole magnets (QC1RE andQC1LE). The final vertical focusing of two beams isprovided by a pair of superconducting quadrupoles (QCS-R and QCS-L). The solenoids (S-R and S-L) are forcompensating a solenoid field created by the Belledetector. The QCS and S magnets on each side of the IPare contained in a common cryostat. The vertical focusingstrength of QCS is tuned to the positron beam. Extravertical focusing for the electron beam is provided by apair of quadrupoles (QC1RE and QC1LE). Thegeometries of these quadrupoles were designed such thatonly the electron beam would see the quadrupole fieldswhen passing through them.Positrons enter the IP through QCS-R and leavethrough QCS-L. Electrons proceed in the oppositedirection. To minimize the synchrotron radiation back-ground near the IP, the magnetic axes of S-R and QCS-Rwere aligned on the same line, and the QCS-L axis wasPositronPositronQCS-L axis has35 mm offsetrelative to the detector axisQCS-RQCS-L S-L S-RQC1RESeptum QuadDetector axisElectronElectronQC1LEFull Quad θ=22mradIPFigure 1: Schematic layout of the beam line near IP.Subcooler Transfer  TubeScaleCold  BoxRecovery  TankCryostatCryostat QCS–RQCS–LS–RS–L0 1 2 3(m)Figure 2: Layout of the superconducting final focusingsystem for KEKB, together with the Belle detector facility.shifted by 35 mm with respect to the axis of the S-L [1].Figure 2 shows the layout of the superconducting finalfocusing system. As can be seen in this figure, thelocations of the cryostats containing QCS and S magnetsare completely inside the detector. Therefore, they mustbe compact and rigidly supported to withstand largeelectromagnetic forces applied on the coils from the 1.5 Tdetector solenoid. The magnet cryostats are held by rigidcantilever supports.A service duct, which comes out from the cryostat, isconnected to a service cryostat which is located outsidethe detector. Another multi-channel transfer line runsdown from here and extends horizontally towards asubcooler-cold box unit, which sits at the side of the Belledetector.2 COMPENSATION MAGNETTwo compensation solenoids (S-R and S-L) wereinstalled at the closest locations to the IP. Their magneticfield direction is opposite to the detector solenoid, andminimizes the effect of the detector solenoid field on theaccelerator beams. Table 1 summarizes the mainparameters of the magnets. The design parameters ofthese solenoids were selected to satisfy a condition that anintegral of Bz along the beam line becomes zero.Because of the presence of the detector solenoidal field,the peak fields on the conductors and the central fields innormal operation are reduced by 1.5 T from the valueslisted in Table 1, and the axial repulsive forces work onthe solenoid. The calculated forces on S-L and S-R are 22kN and 2.8 kN, respectively. Since these solenoids are0-7803-7191-7/01/$10.00 ©2001 IEEE. 181Proceedings of the 2001 Particle Accelerator Conference, ChicagoTable 1: Main Parameters of Compensation Magnets.S-L S-RCentral field (T) 4.53 5.80Current (A) 487 603I/Ic @ 4.5 K (%) 59 75Bmax on the conductor (T) 4.59 5.83Stored energy (kJ) 121 258Coil length (mm) 461 616        inner dia. (mm) 190 190        outer dia. (mm) 230 230        No. of turns 3749 4981excited in a field of 1.5 T, the current dependence of theradial bursting forces on the coil is quite differentcompared with that of ordinary solenoids [2]. A ratherthick bobbin (helium inner cylinder) of 7 mm thicknesswas selected and the windings were impregnated withepoxy resin in order to manage such forces.The coils were made of monolithic NbTi super-conducting wire with a cross section of 1.1 mm x 1.9 mm(Cu/SC ratio 1.0). The insulation is provided bypolyimide group resin. The coils were impregnated withepoxy resin under pressure after being wound on a heliuminner cylinder, and enclosed in helium vessels while axialcompression forces were applied. The expected axialcompression forces on the assembled coils were 516 MNand 250 MN for S-R and S-L, respectively.During the first test in a vertical cryostat, the S-Rmagnet required a number of training quenches to reachthe design current. However, the S-L magnet reached thedesign current without quenching. After installation intothe horizontal cryostat, both magnets showed goodperformance; no training quenches were required to reachtheir design currents with and without the 1.5 T detectorsolenoid field.The Bz field profile along the positron beam line wasmeasured with a Hall probe. The results are shown in Fig.3 when the S-L, S-R and the detector solenoid were allexcited.3 QUADRUPOLE MAGNET WITHCORRECTION COILSThe quadrupole magnets are iron-free superconductingmagnets [2],[3]. They produce a field gradient of 21.7T/m within a coil aperture diameter of 260 mm. The coil-6.0-4.0-2.00.02.0-2000 -1000 0 1000 2000 3000meas.cal.Bz (T)Z-position (mm)Figure 3: Bz distribution along the beam line.Table 2: Main Parameters of Quadrupole Magnets.QCS-L QCS-RField gradient     (T/m) 21.66 (21.84) 21.73 (21.84)Current                (A) 2963 2963Effective length  (mm) 483 (486.1) 385 (387.7)I/Ic @ 4.5K         (%) 70 70Bmax on the cable (T) 4.3 4.3Stored energy      (kJ) 87.5 69.7Coil length          (mm) 617 521       inner dia.      (mm) 260 260       outer dia.      (mm) 289.8 289.8Collar outer dia.  (mm) 340 340Numbers within parentheses are measured values.design is based on a set of cos 2θ windings that areclamped by stainless-steel collars. The two-dimensionalstructure for both QCS-L and QCS-R are the same, whiletheir lengths are slightly different. Since thesequadrupoles are rather short and have a large aperture, weperformed three-dimensional field calculations in order tooptimize the shape of the coil end. Table 2 lists the mainparameters of the quadrupoles and Fig. 4 shows thetransverse cross section of the QCS-L magnet. The innerdiameter of the warm bore is 160 mm and the outerdiameter of the cryostat is 480 mm. From the innermostpart, the main components are: a warm bore, innerthermal radiation shield, inner wall of the helium vessel,correction coils, main quadrupole coil, 316LN stainless-steel collar, outer wall of the helium vessel, outer thermalradiation shield and vacuum vessel. The small holes onthe collar are for the bus lines and the instrumentationwires of the S magnet.We used a NbTi/Cu Rutherford cable for these magnets.It consists of 24 multifilamentary strands of 0.59 mmdiameter (Cu/SC ratio 1.8) twisted with a pitch of about60 mm (filament size 6 µm). The residual resistivity ratioof the stabilizing copper is about 160. The width and themid-thickness of the bare cable are 7.000 ± 0.025 mm and1.090 ± 0.006 mm, respectively. The cable is insulatedwith two kinds of Upilex tape: 25 µm tape and 50 µm tapewith B-stage epoxy resin.0 100mmHe  Vessel   Correction  Coils  CollarQuadrupole  Coil80K  ShieldVacuum  VesselMultilayer  InsulationFigure 4: Cross section of the quadrupole magnet.182Proceedings of the 2001 Particle Accelerator Conference, ChicagoThe final field measurements of QCS magnets wereperformed after they were installed into the beam line. Wemeasured the field distribution along the magnetic axisand the integral field using two kinds of harmonic coils,one with 50 mm long and other with 800 mm long. Theradii of the harmonic coils are 40 mm. The multipole fieldof the QCS-R was measured by setting the harmonic coilaxis on the magnet center. However, that of the QCS-Lwas measured around the axis of the S-L magnet that isshifted by 35 mm from the QCS-L field center. Figure 5shows the integral multipole coefficients of these magnets[4].Three correction coils (normal and skew dipole coils,and skew quadrupole coil) are embedded inside thequadrupole winding. The field strength of the dipole isabout 0.05 T at 50 A and the gradient of the skewquadrupole is about 0.4 T/m at 50 A. The effectivelengths are about 440 and 500mm for the dipoles and theskew quadrupole, respectively. With these strengths, wecan rotate the quadrupole field by ±10 mrad and shift itscenter by ±3 mm. The fabrication method was similar to that of thecorrector magnets for RHIC [5]. First, one-layer coilswere wound on an epoxy-coated flat substrate by using amulti-wiring technique. These flat coils were made atBNL using a 0.33 mm diameter superconducting wirecovered with 25 µm thick Kapton insulation. Then, thecoils were assembled onto the helium inner cylinder by aJapanese company. The total thickness for threecorrection coils, including the insulation and the bindingsof the coils, was 6.6 mm. Since these coils are-0.6-0.4-0.200.20.40.60.813 4 5 6 7 8 9 10QCSR  bnQCSR  anMultipole coefficients (x 10-4)Multipole order-1.2-1-0.8-0.6-0.4-0.200.23 4 5 6 7 8 9 10QCSL bnQCSL anMultipole coefficients (x 10-4)Multipole orderFigure 5: Integral multipole coefficients of QCS magnetsat 3000A.placed inside the quadrupole and solenoid, various forcesand torques are exerted on each coil [2]. We used arelatively thick stainless-steel cylinder (8 mm) for thehelium inner vessel in order to manage these forces.4 CRYOSTATSevere spatial constraints were imposed on the cryostatdesign and very compact cryostats had to be built [6],because they are placed almost inside the detector. Inaddition, in the case of the left-side cryostat, thequadrupole axis had to be shifted horizontally by 35 mmfrom the solenoid axis. This introduced significantdifficulty in the fabrication and assembly. Figure 6 showsa cross sectional view of the left-side cryostat.The cryostat system consists of three parts: a horizontalannular vessel (magnet cryostat) containing the magnets,a service cryostat, and a service duct connecting bothcryostats. The helium vessel is made of 316L stainlesssteel. The inner cylinder works not only as a part of thepressure vessel, but also as a mechanical support of thecoils. The helium vessel is surrounded by a thermal-radiation shield. The outer shield consists of a stainless-steel shell with cooling pipe, while the inner shieldconsists of a 7 mm thick 316L stainless-steel cylinderwith helical grooves for liquid nitrogen. About 30 layersof aluminized Kapton were arranged between the vacuumvessel and the thermal shield. However, since the spatialseparation between the helium vessel and the thermalshield is limited (6-7 mm gap), no superinsulation wasinstalled.The helium vessel containing the solenoid and thequadrupole was suspended in the vacuum vessel by eightsupport rods. The rods were designed to withstand largemagnetic forces which arise from various Lorentz forcesdue to combined operation of the magnets. They are madeof titanium alloy, Ti-6Al-4V [ELI], which has a highultimate tensile strength and a low thermal conductivity.The service cryostat functions as an interface betweenthe cryostat and the multi-channel transfer line comingfrom the subcooler box. On the top flange of the servicecryostat there are bayonet joints for transfer lines, currentleads for solenoid and correction coils, two control valves,and some service ports. The current leads for QCSmagnets are not on the service cryostat. They are on thesubcooler box, and the current is supplied to the QCSmagnets via superconducting bus lines in the multi-channel transfer line.During a performance test, the heat load of eachcryostat system was evaluated from the mass flow rateand the temperature increase in the cryostat. The valueswere 12.1 and 10.2 W for the right and left side cryostat.These are slightly less than the design values.5 ELECTRICAL SYSTEMA schematic diagram for the electrical circuit is shownin Fig. 7. The SCR power supply for the QCS magnets iscomposed of a main power unit (3500A, 15V) and an183Proceedings of the 2001 Particle Accelerator Conference, ChicagoSupport rodQuadrupole magnetCorrection coilsThermal shieldSolenoidSupport rodThermal shieldφ185φ160φ116.2172070070035.113.2Figure 6: Cross section of the L-side cryostat.auxiliary power unit (40A, 15V). Two QCS magnets areexcited in series by this power supply, while othermagnets have their own power supplies.The quench protection circuit of QCS is composed of ahigh-speed SCR switch and two energy dump resistors.The values of the resistors for QCS-R and QCS-L wereset at 0.1 Ω and 0.12 Ω, respectively, in order to optimizethe time constants of the current decay of QCS-R andQCS-L. For quench protection of other magnets, we usedDC circuit breakers since they did not require rapid decayof the currents in case of quenching. The selectedresistors were 0.71 Ω and 2 Ω for the solenoid and thecorrection coil, respectively. The typical quench thresholdis set at 1V for 10 ms or more.6 COOLING SYSTEMThe main components of the cooling system are of theTRISTAN mini-beta insertion quadrupole magnet system[7], although the cryostats and some parts of the transferlines were renewed. The cooling capacity of this system is150 W + 30 L/h. The magnets are cooled by a single-phase LHe of 4.5 K at a flow rate of 21 g/sec duringsteady state operation.Cool-down is achieved by forced circulation of coldgaseous helium. The gas temperature is gradually loweredin order to follow the magnet temperature. Cool-downtakes about 27 hours, during which time the peak(3 units) (3 units)±20V,±50A P.S.±15V±40A ±20V,±50A P.S.QCS–RQCS–L30V,650A30V,650A15V,3500AZHRZVRQKSRZHL ZVL QKSL S–RS–LFigure 7: Electric circuit of the magnet system.temperature difference between the magnets and gas is50K.Figure 8 shows a typical cool-down curve of the magnetsystem. The cooling system performed well even duringand after quenching at full current excitations. The heatloads of the system evaluated from the redundant coolingpower of the refrigerator was about 75 W + 30 L/h.7 OPERATIONAL EXPERIENCEFive superconducting coils in each cryostat andpresence of the 1.5 T external field generate large netLorentz forces and torques on the magnet as a whole. Wemeasured the movement of each cryostat duringexcitation. A large effect can be seen when the S magnetor the dipole correction coil is excited when the detectorsolenoid is on. The front ends of the L-side and R-sidecryostat moved upward by 0.58 mm and downward by0.1mm, respectively, at their operating currents. When thedipole correction coils are excited, the cryostat moved by0.2 to 0.3 mm either vertically or horizontally dependingon the kinds of dipole coil (normal or skew). However,these movements are not so serious, since they are stableand reproducible effects.The cooling system is controlled by a computerized andautomated system. Since the KEKB was commissioned,the system was cooled down eight times. Figure 9 shows050100150200250300-200204060801000 5 10 15 20 25 30 35R-cryostatL-cryostatSubcoolerTemperature (K)Liq. He Level (%) Time (hour)Figure 8: Cool-down curve of the system.184Proceedings of the 2001 Particle Accelerator Conference, Chicago01020304050Nov/21 Jan/5 May/18Jun/23 Oct/7 Jan/5 Oct/2 Jan/5step 3 (Liq.)step 2 (120K- 4.5K)step 1 (300K-120K)Cool-down time (hour)1998 1999 2000 2001Figure 9: Cool-down time of the system.the elapsed time of each cool-down process. A relativelystable cool-down can be seen in the figure, although in thelast two cool-down slightly longer times were required.This was caused by an incorrectly opened valve in theLN2 supply line in the initial step of cool-down.When the quench protection circuit is triggered due to amagnet quench or power supply trouble, the cooling ofthe cryostats stops automatically and the cryostats areisolated from the cooling system. After settling thetemperature rise of the magnet, the cryostat is re-cooledand recovered to the normal operating condition. Theaverage recovery time is 3.5 hours.Another concern in a long-term operation of thesuperconducting magnet system is failures of thecomponents. In Fig. 10 a summary of failures thatoccurred during past three years is shown. We classifiedthem in five categories: cryogenic plant (compressors,cryogenic equipments which include valves and monitors),utility (cooling water system, air compressor), othersources (accelerator safety interlock, mis-operation, etc.),electric power system (by thunderstorm, earthquake, etc.),and quench (beam induced quench, quench detectionsystem and power supply trouble). Although the totalnumber of failures is not so small, in many cases thecontinuous operation of the system was not interrupted.The number of those that interrupted normal operationand required an operator’s assistance to return to normalis listed in the parentheses. Most of these problems in1999 and 2000 were due to thunderstorm, earthquake, andpower supply problems. Therefore, we improved thepower supplies to be less sensitive to the AC line voltagechange in October 2000.8 SUMMARYA superconducting final focusing system for KEKBwas constructed and has been operated during past threeyears. Its total operating time has reached about 17,000hours.02468101214161998 1999 2000 2001Cryo plantUtility (cooling water etc.)OthersElectric powerQuench Number of failuresYear(2) (11) (15) (0)Figure 10: Summary of failures.Number of failures that interrupt normal operation of thesystem is listed in parentheses.In spite of its complexity, this system of ten super-conducting magnets and cooling system has been workingvery stably without experiencing any serious problems.By improving the interlock system of the power supplies,we would like to reduce the downtime of the system evenmore.9 REFERENCES[1] KEKB B-Factory Design Report, KEK Report 95-7(1995).[2] K. Tsuchiya et al., "Superconducting magnet Systemfor KEKB B-Factory," Proc. of the 5th EuropeanParticle Accelerator Conference (EPAC'96), p. 2287.[3] K. Tsuchiya et al., "Superconducting magnets for theInteraction Region of KEKB," IEEE Trans. Appl.Superconductivity, Vol. 9, No. 2 (1999), p.1045.[4] N. Ohuchi et al., “The performance of the super-conducting magnet system in the KEKB interactionregion,” Proc. of e+e- Factories’99, p.217, KEK Pro.99-24.[5] A. Morgillo et al., “Superconducting 8 cm CorrectorMagnets for the Relativistic Heavy Ion Collider(RHIC),’ Proc. of the 1995 PAC and Int. Conf. onHigh-Energy Accelerators, p.1393.[6] N. Ohuchi et al., “Cryostats for the KEKB IR Super-conducting Magnets,” Adv. in Cryo. Eng., Vol. 45(2000), p.787.[7] K. Tsuchiya et al., “Helium Cryogenic Systems for theSuperconducting Insertion Quadrupole Magnets of theTRISTAN Storage Ring,” Adv. in Cryo. Eng., Vol.37A (1992), p.667.185Proceedings of the 2001 Particle Accelerator Conference, Chicago

Superconducting Final Focusing System For Kekb

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