The High-Luminosity project (HL-LHC) of the
CERN Large Hadron Collider (LHC), requires low β* quadrupole
magnets in Nb$_3$Sn technology that will be installed on each side
of the ATLAS and CMS experiments. After a successful shortmodel magnet manufacture and test campaign, the project has
advanced with the production, assembly, and test of full-size 7.15-
m-long magnets. In the last two years, two CERN-built prototypes
(MQXFBP1 and MQXFBP2) have been tested and magnetically
measured at the CERN SM18 test facility. These are the longest
accelerator magnets based on Nb$_3$Sn technology built and tested
to date. In this paper, we present the test and analysis results
of these two magnets, with emphasis on quenches and training,
voltage-current measurements and the quench localization with
voltage taps and a new quench antenna

Bajko, M.

Bermudez, S. Izquierdo

Bottura, L.

Desbiolles, V.

Devred, A.

Fiscarelli, L.

Keijzer, R.

Lackner, F.

Mangiarotti, F.J.

Milanese, A.

Ninet, G.

Prin, H.

Ravaioli, E.

Russenschuck, S.

Takala, E.

Todesco, E.

Troitino, J. Ferradas

Willering, G.

English

The High-Luminosity project (HL-LHC) of the CERN Large Hadron Collider (LHC), requires low $\beta$∗ quadrupole magnets in Nb $_\text{3}$Sn technology that will be installed on each side of the ATLAS and CMS experiments. After a successful short-model magnet manufacture and test campaign, the project has advanced with the production, assembly, and test of full-size 7.15-m-long magnets. In the last two years, two CERN-built prototypes (MQXFBP1 and MQXFBP2) have been tested and magnetically measured at the CERN SM18 test facility. These are the longest accelerator magnets based on Nb $_\text{3}$Sn technology built and tested to date. In this paper, we present the test and analysis results of these two magnets, with emphasis on quenches and training, voltage-current measurements and the quench localization with voltage taps and a new quench antenna.</p

University of Twente Research Information

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 32, NO. 6, SEPTEMBER 2022 4003305Power Test of the First Two HL-LHC InsertionQuadrupole Magnets Built at CERNF. J. Mangiarotti , G. Willering , L. Fiscarelli , M. Bajko , L. Bottura, V. Desbiolles,A. Devred , Senior Member, IEEE, J. Ferradas Troitino , S. Izquierdo Bermudez , R. Keijzer , F. Lackner ,A. Milanese , G. Ninet, H. Prin, E. Ravaioli , S. Russenschuck , E. Takala , and E. TodescoAbstract—The High-Luminosity project (HL-LHC) of theCERN Large Hadron Collider (LHC), requires low β* quadrupolemagnets in Nb 3Sn technology that will be installed on each sideof the ATLAS and CMS experiments. After a successful short-model magnet manufacture and test campaign, the project hasadvanced with the production, assembly, and test of full-size 7.15-m-long magnets. In the last two years, two CERN-built prototypes(MQXFBP1 and MQXFBP2) have been tested and magneticallymeasured at the CERN SM18 test facility. These are the longestaccelerator magnets based on Nb 3Sn technology built and testedto date. In this paper, we present the test and analysis resultsof these two magnets, with emphasis on quenches and training,voltage-current measurements and the quench localization withvoltage taps and a new quench antenna.Index Terms—Low beta quadrupole, Nb3Sn, quench,superconducting magnets.I. INTRODUCTIONA S PART of the HL-LHC project at CERN, the Nb-Ti innertriplet quadrupole magnets near the ATLAS and CMSinteraction points will be replaced with large aperture Nb 3Snquadrupole magnets, named MQXF [1], [2]. These magnets aredeveloped, manufactured, and tested in a collaboration betweenCERN and the US HL-LHC Accelerator Upgrade Project (AUP).The MQXF program includes the construction and test of severalshort-length model magnets, the 4.2-m-long magnets for Q1and Q3 (constructed by AUP [3]), and the 7.15-m-long magnetsfor Q2a and Q2b (MQXFB, constructed by CERN). The firsttwo MQXFB full-length prototype magnets (MQXFBP1 andMQXFBP2) were manufactured, assembled and cryostated atCERN [4]. MQXFBP1 was tested in summer–fall 2020, andMQXFBP2 was tested in winter–spring and fall 2021. KeyManuscript received November 26, 2021; revised February 13, 2022 andFebruary 21, 2022; accepted February 28, 2022. Date of publication March8, 2022; date of current version March 25, 2022. (Corresponding author: F. J.Mangiarotti.)F. J. Mangiarotti, G. Willering, L. Fiscarelli, M. Bajko, L. Bottura, V. Des-biolles, A. Devred, J. Ferradas Troitino, S. Izquierdo Bermudez, F. Lackner,A. Milanese, G. Ninet, H. Prin, E. Ravaioli, S. Russenschuck, E. Takala,and E. Todesco are with CERN, CH-1211 Geneva, Switzerland (e-mail:franco.julio.mangiarotti@cern.ch).R. Keijzer is with CERN, CH-1211 Geneva, Switzerland, and alsowith Universiteit Twente, 7522 Enschede, The Netherlands (e-mail:ruben.keijzer@cern.ch).Color versions of one or more figures in this article are available athttps://doi.org/10.1109/TASC.2022.3157574.Digital Object Identifier 10.1109/TASC.2022.3157574TABLE IMQXFBP1 AND MQXFBP2 MAIN PARAMETERSTABLE IICOILS USED IN MQXFBP1 AND MQXFBP2figures of these two prototype magnets are shown in Table I,and the coils used in each magnet in Table II.II. TEST PLAN AND FEATURESA. Magnet Powering, Quench Detection and ProtectionMQXFBP1 and MQXFBP2 were tested in the A1 horizontaltest bench at the SM18 magnet test facility at CERN [5]. Themagnets were powered with a 1-quadrant, 20 kA DC powerconverter. In addition, during the MQXFBP2 test campaign,an additional 1-quadrant, 2 kA DC power converter was usedfor trimmed powering tests: the current in one of the coils wasreduced for diagnostic purposes. The circuit diagram is shownin Fig. 1.The magnets were tested using the baseline quench detectionand protection systems for operation in the LHC: Quench detection with a universal quench detection system(uQDS, see [6]), which allows to set variable voltagethresholds and validation windows depending on the mag-net current. Magnet protection with outer-layer quench heaters [7],[8] and coupling-loss induced quench system (CLIQ [9]).Energy extraction by means of a dump resistor was notused for these magnets.This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/4003305 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 32, NO. 6, SEPTEMBER 2022Fig. 1. Circuit diagram for the test of MQXFB prototypes. PC indicatesthe DC power converters. Black shows the standard powering circuit. Bluecorresponds to the CLIQ system. Red shows the additional powering circuit forthe trimmed powering of pole P1. The blue and red circuits were not connectedsimultaneously.Fig. 2. Natural quench history at nominal test ramp rate of MQXFBP1 (top)and MQXFBP2 (bottom).B. Magnet InstrumentationThe four coils of both prototypes were instrumented with 8voltage taps in each of their two layers, for a total of 64 voltagetaps per magnet. Of these, only 8 (plus redundancy) are requiredfor LHC operation, one voltage tap at each extremity of eachcoil. The remaining 56 voltage taps were installed to be used forquench localization and performance analysis.In addition, the magnets were instrumented with strain gaugeson the rods and with optical fibers with Bragg gratting (FBG)sensors in the coil poles [10]; MQXFBP1 was also equippedwith strain gauges on the magnet shell. The mechanical mea-surements will be reported in another publication.On the test bench, the magnets were equipped with an anti-cryostat, that allows the bore to be accessible at room tempera-ture during magnet operation. This allowed us to use a dedicatedarray of induction coils that can be used for quench localization,usually referred to as quench antenna, and a stretched wire or arotating-coil magnetometer for magnetic field measurements.III. TEST AND MEASUREMENT RESULTSA. Quench PerformanceThe natural quench history at nominal test ramp rate (20 A/s)of the two magnets is shown in Fig. 2. Training was done at1.9 K. MQXFBP1 reached a quench limit during the first ramp toquench, at 15.17 kA (93.4% of the nominal current). MQXFBP2Fig. 3. Quench current vs. ramp rate at 1.9 and 4.5 K of MQXFBP1 andMQXFBP2. Quenches in MQXFBP2 were all quench location 2.A; the locationof quenches in MQXFBP1 is noted next to each point.Fig. 4. Longitudinal location (top) and in the cross section (bottom left) of therecurring quenches in the MQXFB prototypes, based on the voltage taps andquench antenna measurements.reached its limit at the second ramp to quench, at 15.95 kA(98.2% of nominal current). In each magnet, the quenches atthe quench limit had the same signature, in terms of resistivevoltage growth rate, coil segment that quenched, and quenchantenna signals.Further investigation of the quench performance was done atdifferent temperature levels (1.9, 2.15 and 4.5 K) and ramp rates(1, 20, 50, 100, 150 and 200 A/s). The quenches at 100 A/s andbelow were in the inner-layer pole turn straight section, close tothe magnet mechanical center. In MQXFBP1, these quencheswere in pole P3, on the left of the pole at low ramp rate (location1.A) and on the right of the pole at higher ramp rate (1.B). InMQXFBP2, the quenches were on pole P1, always on the leftof the pole (location 2.A). Quenches above 100 A/s were in theinner-layer multi-turn blocks, near the magnet heads, and areattributed to heating due to high AC losses. A summary of thequench current at 100 A/s and below is shown in Fig. 3. Thequench locations are shown in Fig. 4.The resistive voltage development after quench was comparedto previous experience in MQXF short-model magnets. For bothmagnets, the resistive voltage growth was similar or slowerthan in the short models at similar current, see an example forMQXFBP2 in Fig. 5. The quench propagation velocity at 1.9 Kin both magnets is around 60–70 m/s.Longitudinal quench localization was done by shifting thequench antenna along the magnet axis: coarse localization wasdone with nine 600-mm-sectors that cover most of the magnet.MANGIAROTTI et al.: POWER TEST OF THE FIRST TWO HL-LHC INSERTION QUADRUPOLE MAGNETS BUILT AT CERN 4003305Fig. 5. Resistive voltage development in the recurring quench location at20 A/s, 1.9 K in MQXFBP2 (black) compared to quenches in the short modelsat similar current. In all cases the quench location is similar (inner-layer poleturn or adjacent turns).Fig. 6. Example of quench antenna signals after a quench in MQXFBP2.Signals CF1 through CF4 correspond to the 50 mm coils; CF5 corresponds tothe 100 mm coil. In this case, quench antenna segments CF3 and CF4 react tothe quench first, around time −10 ms, indicating that the quench starts in thatlocation.Once a recurring quench location was identified, the localiza-tion was narrowed-down by shifting the quench antenna 200–300 mm along the axis. A last step of localization was done byplacing the quench antenna end (consisting of four 50 mm, one100 mm and one 300 mm induction coils) around the quenchlocation. This last step allows to identify the quench locationwithin ±50 mm. One example of quench antenna signal for thelast localization step is shown in Fig. 6.B. Magnetic MeasurementsMagnetic field measurements were performed in both mag-nets at low current and room temperature with rotating-coilmagnetometers, and at the highest stable current at 1.9 K withthe stretched wire. In addition, for MQXFBP2 the magnetic fieldwas measured with rotating-coil magnetometers at 1.9 K. Thetransfer function measured on MQXFBP2 is +60 units in 10000with respect to MQXFBP1. This difference is not visible onmeasurements at ambient temperature. About +20 units can beexplained by the presence of magnetic shims. A summary oftransfer function measurements at 1.9 K is given in Fig. 7.The field quality in both magnets is within specifications.A systematic b6 component of about −5 units is present onMQXFBP1. It has been later corrected by removing 0.125 mmfrom the midplane before impregnation. The effectiveness ofthe correction is visible on MQXFBP2. In addition, based onFig. 7. Transfer function measurements in MQXFBP1 and MQXFBP2, per-formed during a stairstep current cycle. The transfer function measured onMQXFBP2 is +60 unit compared to MQXFBP1.TABLE IIIFIELD QUALITY IN MQXFBP1 AND MQXFBP2, IN TERMS OF INTEGRALFIELD, AT 50 MM REFERENCE RADIUS. VALUES IN UNITS OF 10−4. MAGNETICSHIMS WERE INSTALLED IN MQXFBP2 TO CORRECT THE B3 COMPONENTFOLLOWING THE MEASUREMENTS AT 300 Kthe room temperature measurements, magnetic shims were in-stalled in MQXFBP2 to correct b3. The correction resulted tobe effective at nominal level (b3 < 2 units). A summary of themagnetic field quality at both temperatures in Table IIIC. V-I MeasurementsV-I measurements (see [3], [12]) have been done in bothMQXFB prototype magnets. During uniform stair-step currentcycles in MQXFBP1 decaying voltages were observed in thequenching locations and the segment between them (corre-sponding to the inner layer pole turn head) upon reaching acurrent plateau near the quench current. The voltage decay isnegative in quench location 1.A, and positive in the other twosegments. At that current level, the voltage decays to a levelabove the previous current plateau, indicating the start of thesuperconducting normal transition. This is shown in Fig. 8.In MQXFBP2, during a similar uniform stair-step currenycycle, the decaying voltage in the quench location is muchsmaller, and mainly a non-decaying voltage increase is observed.Further investigations were done with a current cycle with ahigher density current step near the quench current, and thestart of the superconducting-normal transition could be bettermeasured. In Fig. 9 the results in the quench location segment for4003305 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 32, NO. 6, SEPTEMBER 2022Fig. 8. V-I measurements in MQXFBP1, shown as current and voltage versustime, during an uniform stair-step current cycle. During the last current plateaua voltage offset and decay can be observed.Fig. 9. V-I measurements the quench location segment in MQXFBP2, at 1.9(blue) and 4.5 K (red), during a current cycle with high density of current plateausnear the quench current. The current has been normalized to the short samplelimit at each temperature. Note that the magnetic field in the measured segmentincreases with the current. The quench happens at a lower fraction of shortsample limit at 1.9 K. Error bars in the vertical axis are around 0.1 μV (notshown).such tests is shown, at 1.9 and 4.5 K, with the current normalizedto the short sample limit indicated in Table I. The inner poleturn segments of the other coils also show an early start of thetransition, at up to 1/5 of the voltage level of the quench-locationsegment.D. Trimmed Powering of MQXFBP2In preparation for the trimmed powering of MQXFBP2, thepowering setup, quench behavior and agreement with simula-tions were tested on the short magnet MQXFS7b [13], [14], andthe protection scheme without CLIQ was verified at increasingcurrents in MQXFBP2. The magnet was then ramped to quenchat 1.9 and 4.5 K, using a trim current of 0, 2.5 and 5% of the maincurrent. The results of trimmed powering is shown in Fig. 10.With a 5% trim, pole P3 quenches in location 2.C at 20 A/s (1.9and 4.5 K), pole P2 quenches in location 2.B at 1 A/s, 4.5 K,and no quench is recorded up to 17 kA at 1 A/s, 1.9 K. Allquenches were in the inner layer pole turn straight segment ofthe respective coils.IV. DISCUSSIONBoth magnets have similar behavior: they have a quench cur-rent limit in the inner pole turn of one coil, near the mechanicalFig. 10. Trimmed powering quench summary of MQXFBP2. Quenches in P2are in quench location 2.B, and in P3 in quench location 2.C.center. Their ramp rate and temperature dependency is mostly“normal”. Both magnets show early superconducting-normaltransition observed in the quench location segments, with de-caying voltages. The quench propagation velocity is similar toother MQXF short magnets at similar quench current. All theseindicators point towards local damage of the superconductor, asopposed to distributed (over several twist pitches) degradationas was seen in early short models of the CERN 11 T Nb 3Sndipole [15], [16].The decaying voltages in the MQXFBP1 V-I measurementsduring current flat-tops indicate current redistribution betweenstrands, likely caused by a non-uniform degradation component.Also, negative decaying voltages cannot be explained by dam-age in the segment measured. This topic is discussed in detailin [12]. For such type of degradation, the expected degradationlocation would be the quench location at high ramp rate andhigh temperature, in this case location 1.B. In MQXFBP2, theexpected degradation location is the quench location.V. CONCLUSIONThe first two MQXFB prototypes magnets have been powertested at the CERN SM18 test facility. MQXFBP1 reached15.17 kA (93.4% of nominal current, corresponding to an op-eration energy of 6.5 TeV) and MQXFBP2 reached 15.95 kA(98.2% of nominal current, corresponding to 6.9 TeV).The field quality of the two tested magnets resulted to bewithin specifications. A systematic b6, present on MQXFBP1and already seen on the short models, has been successfullycorrected by shimming the coil midplane. On MQXFBP2, theb3 component has been corrected by applying magnetic shimsin the bladder slots.The performance limitation in both MQXFB prototypes isexpected to be caused by local damage of the superconductor,in the quench location at high ramp rate. This location wasidentified within±50 mm, to allow further investigation at roomtemperature.Trimmed powering of the limiting coil in MQXFBP2 allowedthe other three coils to be powered above the performance limit.With a 5% trim, other two coils quenched at around 500–600 Ahigher than the limiting coil at the same temperature and ramprate conditions.MANGIAROTTI et al.: POWER TEST OF THE FIRST TWO HL-LHC INSERTION QUADRUPOLE MAGNETS BUILT AT CERN 4003305REFERENCES[1] E. Todesco et al., “A first baseline for the magnets in the high luminosityLHC insertion regions,” IEEE Trans. Appl. Supercond., vol. 24, no. 3,Jun. 2014, Art. no. 4003305.[2] E. Todesco et al., “The high luminosity LHC interaction region mag-nets towards series production,” Supercond. Sci. Technol., vol. 34, 2021,Art. no. 053001.[3] G. Ambrosio et al., “Lessons Learned From the Prototypes of the MQXFALow-Beta Quadrupoles for HL-LHC and Status of Production in the US,”IEEE Trans. Appl. Supercond., vol. 31, no. 5, Aug. 2021, Art. no. 4001105.[4] S. I. Bermudez et al., “Progress in the development of the Nb3Sn MQXFBquadrupole for the HiLumi upgrade of the LHC,” IEEE Trans. Appl.Supercond., vol. 31, no. 5, Aug. 2021, Art. no. 4002007.[5] M. Bajko, “SM18 the CERN magnet test facility,” Accessed: Feb. 11, 2022.[Online]. Available: https://indico.cern.ch/event/704235/contributions/2931988/[6] R. Denz et al., “Quench detection and diagnostic systems for the super-conducting circuits for the HL-LHC,” in Proc. 10th Int. Part. Accel. Conf.,Melbourne, Australia, May 2019, Art. no. THPTS036.[7] S. I. Bermudez et al., “Overview of the quench heater performance forMQXF, the Nb3Sn Low-β quadrupole for the high luminosity LHC,” IEEETrans. Appl. Supercond., vol. 28, no. 4, Jun. 2018, Art. no. 4008406.[8] E. Ravaioli et al., “Quench protection performance measurements in thefirst MQXF magnet models,” IEEE Trans. Appl. Supercond., vol. 28, no. 3,Apr. 2018, Art. no. 4701606.[9] E. Ravaioli, “CLIQ,” Ph.D. dissertation, Univ. Twente, 2015.[10] L. Bianchi, “Strain measurements on MQXFBP1 superconducting mag-net,” CERN, Tech. Rep. EDMS 1851752, 2021.[11] F. J. Mangiarotti et al., “Powering Performance and Endurance BeyondDesign Limits of HL-LHC Low-Beta Quadrupole Model Magnets,” IEEETrans. Appl. Supercond., vol. 31, no. 5, Aug. 2021, Art. no. 4000805.[12] R. Keijzer et al., “Modelling V-I measurements of Nb3Sn acceleratormagnets with conductor degradation,” IEEE Trans. Appl. Supercond., tobe published, doi: 10.1109/TASC.2022.3153247.[13] F. Mangiarotti, “MQXFS7b trim test results,” CERN, Tech. Rep. EDMS2621655, 2021.[14] E. Ravaioli, “Analysis of the electrical and thermal behavior of coil 211of the MQXFS7b magnet,” CERN, Tech. Rep. EDMS 2645040, 2021.[15] G. Willering et al., “Comparison of cold powering performance of 2-m-Long Nb3Sn 11 T. model magnets,” IEEE Trans. Appl. Supercond., vol. 28,no. 3, Apr. 2018, Art. no. 4007205 .[16] F. J. Mangiarotti et al., “Quench propagation in Nb3Sn cos-theta 11 T.dipole model magnets in high stress areas,” IEEE Trans. Appl. Supercond.,vol. 28, no. 4, Jun. 2018, Art. no. 4008204.

Power Test of the First Two HL-LHC Insertion Quadrupole Magnets Built at CERN

Mangiarotti, F J

Willering, G

Fiscarelli, L

Bajko, M

Bottura, L

Desbiolles, V

Devred, A

Ferradas Troitino, J

Izquierdo Bermudez, S

Keijzer, R

Lackner, F

Milanese, A

Ninet, G

Prin, H

Ravaioli, E

Russenschuck, S

Takala, E

Todesco, E

CERN Document Server

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Power Test of the First Two HL-LHC Insertion Quadrupole Magnets Built at CERN

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