Adli, E

Arnaud, M

Benoit, P

Berriaud, C

Broggi, F

Deront, L

Dudarev, A

Foussat, A

Humeau, M

Junker, S

Kopeykin, N

Leboeuf, R

Olesen, G

Olyunin, A

Pengo, R

Rabbers, J J

Ravat, S

Rey, J M

Sbrissa, E

Shugaev, I

Stepanov, V

ten Kate, H H J

Volpini, G

Védrine, P

CERN Document Server

508 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 16, NO. 2, JUNE 2006On-Surface Test of the ATLASBarrel Toroid Coils: OverviewA. Dudarev, C. Berriaud, J. J. Rabbers, F. Broggi, S. Junker, R. Pengo, E. Adli, S. Ravat, L. Deront, E. Sbrissa,G. Olesen, M. Arnaud, J.-M. Rey, P. Vedrine, G. Volpini, A. Foussat, P. Benoit, M. Humeau, R. Leboeuf,V. Stepanov, A. Olyunin, I. Shugaev, N. Kopeykin, and H. H. J. ten KateAbstract—The Barrel Toroid (BT) provides the magnetic fieldfor the muon detectors in the ATLAS experiment at CERN. TheToroid is built up from eight superconducting coils. Each coil con-sists of two 25 m 5 m racetrack shape double pancakes im-pregnated and pre-stressed inside an aluminum coil casing. The42-tons cold mass is cooled by forced-flow liquid helium circulatingin aluminum pipes glued to its surface. The coils are tested onsurface prior to their underground installation. The test programhas started in September 2004 and finished in June 2005. Thispaper describes the test set up and various commissioning tests per-formed at the ATLAS Magnet Test Facility. It includes the aspectsof test preparation, vacuum pumping, leak testing, cooling down,powering and warming up. The 8 coils have passed the tests suc-cessfully and have been assembled into the Toroid in the ATLAScavern. The testing completes the production of the so far largestracetrack coils in the world.Index Terms—Detector magnets, superconductors, toroid.I. INTRODUCTIONTHE Barrel Toroid [1] of the ATLAS magnet system [2]is currently under final stage of installation in the under-ground area of the ATLAS experiment at CERN. It consistsof eight very large racetrack coils (see Fig. 1). The coils wereacceptance tested on surface with a magnetic mirror which at-tracted them with a nominal force of the final toroidal assembly.The main parameters of the coil are given in Table I.The coils were assembled and tested in the same place and thetesting itself is the final step in the production line which havebeen organized in the West Area of CERN in 1999–2005. De-tails of the coil assembly are presented in [3]. Being unique andhuge in size, the coils could not be easily moved or stored out-side the assembly hall so one of the main constraints for the coiltest was the limited testing time. Any delay in the test schedulewould cause an interruption of assembly works, which were per-formed by external companies. The test program was shortenedto a minimum. In only a few cases some extra to the main pro-gram tests were performed to justify main parameters for thecoil acceptance.Manuscript received September 20, 2005.A. Dudarev, J. J. Rabbers, S. Junker, R. Pengo, E. Adli, S. Ravat, L. Deront,E. Sbrissa, G. Olesen, A. Foussat, P. Benoit, V. Stepanov, A. Olyunin,I. Shugaev, N. Kopeykin, and H. H. J. ten Kate are with CERN, CH-1211Genève 23, Switzerland (e-mail: Alexey.Dudarev@cern.ch).C. Berriaud, M. Arnaud, J.-M. Rey, P. Vedrine, M. Humeau, and R. Leboeufare with CEA-Saclay, 91191, France.F. Broggi and G. Volpini are with INFN-LASA, Via F.lli Cervi 201, 20090Segrate, Italy.Digital Object Identifier 10.1109/TASC.2005.869549Fig. 1. The individual Barrel Toroid coil has passed the on-surface test in theATLAS Magnet Test facility and is being prepared for the transport to the un-derground area of the ATLAS experiment.TABLE IMAIN PARAMETERS OF THE BT COILSIn order to save the time for mounting, dismounting andwarming up, two testing slots are constructed around themagnetic mirror. In the final toroid the liquid helium flow issupplied from the bottom and there is mirror symmetry be-tween the coils. The two slots were organized correspondinglyto accept so called X+ and X coils. A swap between them ispossible in principle but requires a rather complicated handlingand transport operation. Fortunately this option was never used.The operations needed for the testing and commissioning ofeach BT coil include the mechanical installation and alignment,mounting of the interfaces to cryogenic, vacuum, electrical andcontrol systems, leak tests and pumping, cooling down and en-ergizing, warming up and disassembly. It is similar to whathas to be done for a test of any other superconducting magnetbut the exceptional dimensions, time and space constraints havebrought some challenges to its realization. The experience of thetest campaign of the B0 model coil [4] helped to save time onall operations especially for the testing of the first BT coil [5].1051-8223/$20.00 © 2006 IEEEDUDAREV et al.: ON-SURFACE TEST OF THE ATLAS BARREL TOROID COILS: OVERVIEW 509Fig. 2. View on the assembly and test area at a certain moment of assembly inFebruary 2005. A few coils are already installed into the vacuum vessels. Onecold mass is prepared for integration. One coil is in the X+ test bench. Beingpart of the production schedule a delay in the might affect the overall planning.Particular acceptance criteria are used to characterize the coilsduring on-surface testing. The detailed analysis of the test re-sults with respect to the acceptance criteria is presented and dis-cussed in [6]. The main steps in the test program includes rampup to nominal current, test of quench heaters, fast dischargeand monitoring of the mechanical and cryogenic behavior. Thispaper describes the BT coil testing in general with focus on thetest set up and various commissioning tests.II. GOALS & CONSTRAINTSVarious commercial and technical problems have caused sig-nificant delay in the construction of the ATLAS Barrel Toroidalmagnet and it started to slow down the construction of the wholeATLAS experiment in the underground cavern. At the earlystages of the project the West area of CERN was planed to beused only for the acceptance test of the individual BT coils.Later on it was decided to assemble the coils in this area aswell. The area, see Fig. 2, was reorganized to allow to do theassembly works on the cold masses (Integration 1) and with asmall shift in time the assembly of the vacuum vessels (Integra-tion 2). In these activities a few companies and institutes wereinvolved under main supervision of the CERN-CEA team. Asa result the on-surface test of individual coils was included inthe production schedule. The main constraint in this schedule isrelated to the exceptional dimensions of the constructed objectswhich can not be easily moved or stored elsewhere. It meant thatany delay in the test program would affect the whole produc-tion schedule. From the other point of view even the single BTcoil is one of the biggest superconducting magnets ever tested inthe world and it had to pass the acceptance test to guarantee itsreliability for many years of continuous operation in the LHCexperiment.The test station is built only for purpose of testing of theATLAS coils and the first big object, which is tested there, wasthe B0 9-meter model coil. The successful test of this prototypehas confirmed the design concept of the BT coils and at the sametime includes the commissioning of the test facility itself. Themajor difference between the prototype and the BT coil is theirFig. 3. The difference between B0 and BT coils is mainly in length. Mechan-ical, power and cryogenic installations are identical for both coils.length of 9 and 25 m respectively (Fig. 3) so the test prepara-tion procedures are quite similar for the BT and the B0 coils.The B0 testing also has shown the critical issues in the test setup and has helped to understand the time required for the var-ious test items. The time needed to prepare the B0 for the testwas a few months and the test program had taken a few weeksuntil the maximum current was reached. Such a long time wasnot acceptable for the coil production. One of the main tasks ofthe BT coil test team was to minimize drastically the time of theacceptance test but to ensure, at the same time, the highest levelof all required controls on these huge superconducting coils.Specific acceptance criteria have been developed and usedto verify the parameters the BT coils during their on-surfacetesting. The criteria are reflecting the principle parameters re-quired for the safe operation of the ATLAS Toroid during twentyyears. They can be presented in a few groups:Electrical criteria: maximum current up to 22 kA withouttraining steps below 21 kA; ground insulation more than 1at 2 kV and 5 K; slow discharge (where dI/dt is higher than atramp up) without quench; fast discharge without damage anda maximum temperature of the quenched double pancake lessthan 100 K; double pancake resistances below ; stabilityat steady state conditions at 20.5 kA for 10 h;Vacuum and cryogenic criteria: vacuum is better thanunder pumping; leak rate of thermal shield andcold mass cooling lines less than at lowertemperatures; average cold mass temperature less than 5.2 K;thermal gradients less than 0.5 K; heat load to cold mass lessthan 150 W; heat load to thermal shield less than 1500 W;Mechanical criteria: sliding of cold mass on cryostops duringthermal cycling; maximum load on single tie-rod less than 225 t;flexion of tie-rod with ;Criteria on internal diagnostic: no failures in sensors for themagnet safety system; not more than one failure per group ofsensors for the control system; initiation of normal zone by pro-tection heaters within 10 s at 5 kA (or 5 s at 22 kA).Based on these criteria a compressed test program was pro-posed and realized. In only a few cases some additional testswere required to justify the coil performance.III. TEST SET UP & MAIN OPERATIONSThe test station itself comprises two test slots around themagnetic mirror, a cryogenic plant, a 24-kA power supply,510 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 16, NO. 2, JUNE 2006Fig. 4. Interface turrets provide connection to services. Their installation tookmost of the time prior to start of vacuum pumps. Pressure and leak test werealways performed on four independent cooling circuits of each BT coil. Thejoints were welded in both turrets.vacuum pumps, Magnet Control (MCS) and Magnet SafetySystems (MSS) [7]. The cryogenic system includes the 1.2 kWrefrigerator cold box, compressors, the pump cryostat con-taining two immersed centrifugal pumps, distribution valvebox, liquid nitrogen pre-cooling unit and two transfer linesfeeding the test benches. The power system comprises a6 V/24 kA switched-mode power converter and a diode/re-sistor dump unit with an electro-mechanical circuit breaker.MCS provides regulation and monitoring of all the functionalparameters of the magnet. It consists of sensors & actuators,the supervisor system and fast and slow Data Acquisition. Foreach coil a certain sequence of operations is required to preparethe coil for the acceptance test and to connect it to the externalservice systems.A. Installation in Test Bench and Interface TurretsThe BT coils are assembled in the final toroid at different an-gles but they were installed and tested horizontally (worst caseposition) close the magnetic mirror at the test facility. Eightmain support units (MSU) link the room temperature heads ofthe tie-rods to the main frame of the mirror. Each MSU with-stands about 25 MN of magnetic force acting between the coiland the mirror. In the overall schedule, movement of the coil tothe test slot and its alignment with respect to the magnetic mirrorwere done in one day and never cause a problem. Mounting ofthe MSU was also made in the shadow of other activities. Twointerface turrets are used for the connections to the external ser-vices. The current leads turret (CLT) at the top of the BT testset up contains the electrical and cryogenic connections andjoints to the bus bars via current leads. The cryogenic supplyturret (CST) at the bottom houses the interconnections of mainand redundant cooling circuits to the cryogenic transfer linesand the electrical junction between the two double pancakesFig. 5. The typical pump down curve where each change of slope correspondsto a start of one type of pump: at the beginning rotary piston, then roots anddiffusion at the end. Cryopumping is not seen here but move the vacuum furtherdown to a level of 10 mbar.(see Fig. 4). The assembly of these turrets was the most timeconsuming operation because of many various tests and checkswhich had to be done before vacuum closing. They includewelding of conductors and the connection to the current leads,connections of the cold mass and the thermal shield coolinglines and their leak and pressure testing, installation of diag-nostics and vacuum sealing of the turrets. All these steps werewell organized and were normally completed within one and ahalf week. The preparation works were always planned in a wayto be finished before weekends to save working time on roughpumping.B. Vacuum and LeaksThe vacuum system has two identical branches, one for eachtest station. Each branch has three pumps, six valves and fourpressure gauges. All devices are connected and operated via aremote control system. The pump configuration is designed tocover the necessary range of pressure regimes. The rotary pistonpump pumps from atmospheric pressure down to some tens ofmbar, when the roots pump is started. The roots pumps bringthe pressure to below where the diffusion pumpis started. The diffusion pump goes down to aboutwhen the cooling down starts. Fig. 5 shows the vessel pressureduring the first pumping down of Barrel Toroid coil 7 (it is atypical pumping down curve). The pumping down of the 60vacuum vessel filled with air at atmospheric pressure to the pres-sure needed for cool down took about 30 hours.During cooling down the pressure in the vessels is reduced fur-ther by two or three orders of magnitude, reaching an ultimatepressure level about (values varying fromcoil to coil). During a fast dump the cold mass temperature risesquickly, and as consequence gas molecules are freed from thesurfaces, which causes an increase of the vessel pressure. Aftera fast dump at high current the vessel pressure was found to in-crease about two decades during the first hour after a fast dump.With the diffusion pump turned on, the pressure went downagain to the ultimate pressure level after two hours. Only onefailure happened in the test period: during the rough pumpingof the first coil, a part of superinsulation was stuck at the intakeDUDAREV et al.: ON-SURFACE TEST OF THE ATLAS BARREL TOROID COILS: OVERVIEW 511to the vacuum system. This was due to the fact that the roughingline was evacuated before and a big corner valve that connectsthe line to the vacuum vessel was opened. The pressure in theline was a few mbar while it was still one atmosphere in thevacuum vessel, a strong air flow was created inside the vesseland it has sucked in the port one blanket of superinsulation. Thesuperinsulation was repaired and the necessary precautions havebeen taken by installing small bypasses in parallel to big valves.The leak test of the vacuum vessel started immediately afterreaching a reasonable level of the background leak rate. As arule leak test is performed in two steps. The first step aimsto detect big leaks and to fix them quickly in order to startcooling down. The second and more careful search is normallydone during cool down when the vacuum is improved by cry-opumping. In the case of internal leaks of helium lines the leaktests was done just after the power test when the helium wasevacuated from the lines. The leakage rates of each internal lineat 20 bars of He pressure is measured at the same time. No sig-nificant leaks were found. Details on the leak rates are givenin [6].C. Cool Down and Warm UpUnder normal operating conditions the coil is cooled withforced flow liquid helium flowing through pipes attached to thecold mass. The cryogenic system allows the ATLAS coils to becooled down from ambient temperature to about 80 K, by meansof a 10 kW pre-cooler and capable to maintain a differentialtemperature of 40–50 K between the inlet and outlet. The furthercooling of the cold mass, from 80 K to 4.5 K, is achieved using a1.2 kW at 4.5 K refrigerator. The refrigerator cools at the sametime the thermal shields of the coil cryostat. After 12 days ofcooling the operating temperature is reached. The heat loadsof the cold masses at 4.5 K were obtained by measuring thehelium vapor mass flow returning to the refrigerator from thephase separator. The measured load is about or less than 80 Wfor all coils and it is almost twice lower than the acceptablevalue. In details the results of cryogenic tests are presented in[8]. It is important to mention the nonfailure performance of thecryogenic plant during whole test period. The warming up wasdone by applying some current through the coil and in additionby circulating the He gas through the coil and the heat exchangerof the pre-cooler. The warm up period varies from 5 to 8 daysdepending on the applied current and the He mass flow.D. Powering, Losses and Mechanical BehaviorThe test program with a current took normally one week andit included powering to 5 kA and testing of heaters, ramp up to16 kA and slow discharge, powering to 22 kA in steps and fastdump, the steady state test at 20.5 kA for 10 h with ramp up at theend to 22 kA and slow discharge. The nominal current 20.5 kAof the Barrel Toroid was reached without any training in all coils.Only once a training quench was observed in the first doublepancake. The mechanical behavior was always monitored andno problems were found. The dynamic losses were measured aswell for each coil. The results of power test of the coils are welldescribed in [6], [9] and [10].IV. CONCLUSIONThe acceptance test of the Barrel Toroid coils was success-fully completed in June 2005. Initially being planed to be per-formed in 15 month the period of the acceptance test was re-duced to 12 months including two additional cool down cam-paigns of the third tested BT coil. Most of the coils have passedthe acceptance criteria without any problems. A reduced groundinsulation was found in one coil but it could finally be accepted.Only one spontaneous quench has occurred almost at the max-imum test current on the first manufactured double pancake. Allcoils are finally accepted and they are already installed in the un-derground area of the ATLAS experiment at CERN.ACKNOWLEDGMENTThe authors would like to acknowledge all members of thesupport teams of CERN and CEA-Saclay for their contin-uous support of all operations during the BT coils testing andcommissioning.REFERENCES[1] P. N. Vedrine et al., “Completion of the Manufacturing of the ATLASBarrel Toroid Magnet at CERN,” this conference.[2] H. H. J. ten Kate, “ATLAS superconducting toroids and solenoid,”IEEE Trans. Appl. Supercond., vol. 15, no. 2, pp. 1267–1270, 2005.[3] J.-M. Rey et al., “Cold Mass Integration of the ATLAS Barrel ToroidMagnets at CERN,” this conference.[4] N. Dolgetta et al., “Review of the ATLAS B0 model coil test program,”IEEE Trans. Appl. Supercond., vol. 14, no. 2, pp. 495–504, 2004.[5] A. Dudarev et al., “First full-size ATLAS barrel toroid coil successfullytested up to 22 kA at 4 T,” IEEE Trans. Appl. Supercond., vol. 15, no.2, pp. 1271–1275, 2005.[6] C. Berriaud et al., “On-Surface Test of the ATLAS Barrel Toroid Coils:Acceptance Criteria and Results,” this conference.[7] P. Miele et al., “The ATLAS magnet test facility at CERN,” IEEETrans. Appl. Supercond., vol. 11, pp. 1713–1716, 2000.[8] K. Barth et al., “Results of the cryogenic tests of the superconductingmagnets forming the barrel toroid of the ATLAS experiment,” pre-sented at the CEC-ICMC, Keystone, Colorado, USA, 2005, 29/8-2/9,unpublished.[9] C. Mayri et al., “Suspension System of the Barrel Toroid Cold Mass,”this conference.[10] J. J. Rabbers et al., “Theoretical and Experimental Investigation of theRamp Losses in the Conductor and Coil Casing of the ATLAS BarrelToroid Coils,” this conference.

On-Surface Test of the ATLAS Barrel Toroid Coils: Overview

On-Surface Test of the ATLAS Barrel Toroid Coils: Overview

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