The efficiency of the magnet protection by quench heaters was studied using a novel method which derives the temperature profile in a superconducting magnet during a quench from measured voltage signals. In several Large Hadron Collider single aperture dipole models, temperature profiles and temperature gradients in the magnet coil have been evaluated in the case of protection by different sets of quench heaters and different powering and protection parameters. The influence of the insulation thickness between the quench heaters and the coil has also been considered. The results show clear correlation between the positions of quench heaters, magnet protection parameters and temperature profiles. This study allowed a better understanding of the quench process mechanisms and the efficiency assessment of the different protection schemes

Maroussov, V

Sanfilippo, S

Siemko, A

CERN Document Server

[LEE 1 W S A C T I O N S  ON APPLIED SUPEKCONDUCTIVI'I'Y, VOL. 10, NO. I, MARCH 2000 661 Temperature Profiles During Quenches in LHC Superconducting Dipole Magnets Protected by Quench Heaters. V. Mnroussov, S. Sanfilippo, A, Siemko CBRN, LI-IC Division, 121 I Gcnevrr 23, Swit7.crland Abstract-Tlic efficiency of the magnct protection by quench heatcrs was studicd using a novel method which h i v e s  tlic temperntiire profile in a superconducting magnct during a qucnch from niensiacd voltage signals. I n  sevcril Largc Iladmn Collidcr single aperture dipole iuodcls. tcmpcraturc profiles and tcmperatiirc gradients in  thc magnct coil have been cvaluated in thc case of protection by diffcrcnl sets of quench heaters nnd dirkrent powering and prutcction parameters. The influence of tlic insulation thickness bciween thc qucncli heatcrs and the coil has also bccii considered. The rcsults show clear corrclition bctween thc positions of quench licntcrs, magnet protcciion pnranictcrs and tempcrahm profilcs. This study allowed a bctter mndcrstanding of thc quench pruccss mechanisms and the efficiency nssessmcnt of the diffcrcnt protection sdieines. Tlic Large Hadron Collidcr (LIIC) main superconducting dipole magnets will store a substantial amount of electromagnetic encrgy [l] .  In 15 m long dipoles 7 MJ is storcd at 8.3 T. For this magnetic held the quench propagation velocity is equal to 20-30 mls and is not sulficienl to makc the magnet self protccting. A fast and rcliahle magnct protection system is thciefore essential to avoid an over-vnltrige and/or ai l  overheating of the mngnct component i n  casc of quench [ 2 ] .  Qucnch heaters are thercrore included to sprertd rapidly the normal zone acrnss the coil. Iu order to bctter understand the efficiency of the magnet protcclion, sevcral one-meter long diprdc models have been built and tested with variuus design parameters. Concerning thc quench heaters the main qualitntivc variants were thc fnllowing: a) the positions of quench heaters were in the outer layer of the coil eithcr at the outcr radius refcrrcd to as outer radius quench hcitcr (ORQH) or between thc inner and the outer layer referred to as inter radius quench hcaters (IRQH). b) Two insulation foils 75 and 200 pm thick placed between thc hcater strip iind the magnet coil were tested. During each tcst, voltage signals were measured to dcrive tcmperature profiles and tu localise: the critical points in the sliperconducting magnet during the qucnch 131. In particular this method determined the nveragc temperature occurring In the hottest turn and the tem~ieraturc gradient across the hottest block. These parametcrs were of crucial importancc as it was observed that high hot spot temperatures and large temperature gradients could cause II significant performance degradation [4]. Ttic magnet protection by sevcrnl sets of quench heaters is comparcd here for selcctcd short dipole magnets with 6-blocks cui1 dcsign. '['lie magnet electrical circuit during a quench can be represented BS an equivalcnt scrial connection of notilincar inductnncc LE (I) nnd a timc rlependenl rcsistancc R(t). At tlw beginning of thc quench, a pure inductive voltage is mensuicd by most of thc voltage taps. Tlic reason is that thc mqjor part of the coil rcmains superconducting becnusc thc quencil occurs in a limited cablc length of the magnct. The first step of thc method aimed to calculnte tlic inductive voltage distribution of the coil parts from the quench recording. For the cahlc lcngth betwccn two voltage taps. the partial inductance L,, defined in ( I )  was cnlculated using tieatcr provoked qiicnches (for dctails see [3]). By choosing For each signal bctween two voltage taps thc maximum value or each I-,. thc minimum Joule licat released during the quenclics could be ubtaincrl using (2). t Q(t)= [ V i  t ' )  I( t')dt' J 1 L1 In (2) V represents the total voltiige across thc magnet and to a time before thc quench wherc the magnet was still in rhc supercnnducting state. Knowing the masses of thc copper and of thc superconductor and knowing the dependcncics of the spccific heat vcrsus temperaturc, the average tcmpcrntiire of the part of the cihle between thc two voltiigc taps could lie obtained. The advantage of this method is tlint onc does not nccd to calculate the rcsistance of thc turn which depends on thc local ratio between tiic electron mean h e  pnth with respect tn the interfilamcnt and thc inter bunch spacing. The peak temperature obtained by this method i s  by definition lower thnn the hot spot temperature extracted from the commonly uscd MIITS inethod because temperatures are nvcraged itlong the cable length bounded hy two voltage taps. Fig. I prcscnts examples of thc tcmperaturc profiles in  the coil for a naturel Manuscript rcccived Seplcinhcr 27, 1999. 662 Fig 1. Temperature profile for twining qucnch witlinui cncrgy cxlracliun at 13967A (left) and for a spot heatcr provokcd qticncli a1 12850A (right). qucnch and a quench provoked by the firing of a spot heater. Both quenches were performed without cncrgy cxtriiction. As onc can see in Fig. 1 for quenches without cncrgy extraction the highest temperature was rcaclied in the pole turn of the outer lnyer independent of the qixcnch origin [5]. This particular turn of the outcr layer (in the pole where the quench started), and the whole block were developing the highest temperatures. This was causcd by the higher current density of thc mtcr  layer :is coinpared to the inner layer, coupled with the existence of a higher magnetic field in this region with rcspect to the rest of the outer layer. For spot heater induced qiicnchcs the siinilar temperature profiles wcrc rccorded. In the following the variation of the temperature of this turn 'rh[ i.e. the peak temperature and the temperature difference between thc polc turn and the average temperature of the related block (block 2) ATLC will be considered. where ATllb is a measurc of thc tcmpernture gradient across the sixteen turns of the block 2. A. E$twt of the Quench Healer Posiriori Fig 2. Layout Ibr a moddel equippcd with OKQH and IRQH W ~ c t o f  the quench heater pallton on Thl. " .,......, " xx ~ 25 I,+,& , I -*m -clRcH a v...I " Zm m Em 1cm3 11w 1m C I " l [ A ]  Fig. 3. Hottcst tcmperatiire (11) ontl tempcraturc gmdicnt (b) vs ciirrent whcn magnet protection was pcifortned by IRQH or ORQl I. For high crfiiciency the heater strip milst bc in close thermal contact with the coil. Studies showcd that the optimum place was bctwcen the coil layers [6]-[7]-[SI. A dipole model was equipped with twti heating circuits betwecn thc iniicr and the outer layers in addition to the "classical" hearers cavcring the outer radius of thc outer layer. For redundancy thcrc wcrc two circuits for each typc and coil quadrant. Wcatcrs which covered the high ficld and the low fkId regions wcre respectively called ITF and 1.J.  The insulntion lhickncss between, the coil and thc qucncli heater (inter and outer radius) was 75 pm for this test. Fig. 2 shows the position o f  the OKQH and the IRQII io thc cross section of this dipole. The variation of Tht and AThb vs magnet current is shown in figs. 3a and 3b when the magnet was protected by the ORQH nnd the IRQH. Quenchcs wcre prvvoked at different currents by firing n spot heater locntetl in  the outer laycr. At low currcnts (I < 10 kA) the protection by the two types of hcaters wiis in  practice equivalent. The influence of the heater position occurred at higher currents. For an ultimate current I = 12850 A, a rcdiiction of 35 K and 20 K was measured for Tht and Ar l ,b  respectively when the magnet was protected by thc IRQH. This means a lowering of about 13% in terms of maximum tcmperature and of 20% for thc temperature gradiciit evaluated i n  the holtest block. Thc improved efficiency of thc lRQH tind nt least two origins. First, due to a keystone angle of the cable lRQH covers four . .  . . . . . . . . . . . . . . . . . . . . . . . .  lurns more than ORQII. Second, the IRQH wcrc locatcri in high magnctic ricld regions and therefore thc cifccctivc tcmpcratiire margin was lower and ICSH cntlialpy was needed to raise thc temperntute of the coil tn thc critical one. The protection id thc magnc( using the IRQH wiis inore cfricicnl because the dclay nccdcrl to triggcr D qiiench was shortened. During the training, quenches without cncrgy cxtraction wcre protected by firing successively the four  mter  radius and thc four iolcr radius quench heaters. As one can scc iii ltig, 4, tlic magnetic field level at quench druppcd altcr seveIal quenchcs witlimt cncrgy extraction IO IIYD diffe.rent levels (Djl,,t,, 13,,,,,,~) depending 011 tlic typc of [he protection. Protection by the  lRQH limitcd thc drop of the magnetic field to an  avcrnge value of 8.7 T and to 8.4 T (no,,,,,) in case or protection by the ORQH. This de-training cff'cci illready observccl [or scvcral six-block coil models had ii thermu-mcchanical origin induced by the coexistence of a mechanical wc;tk rcgioii in the coil and a temperatiire rise occurring when all thc storcd ciici.gy is ilissipnterl in the magnet. In order to evalunte the importance c)f thc thermal effect in the insfability of the trnining. the TI,( and ATbb wcrc calculated foi. quciiches without energy e x t r a d o t i .  As unc cnn scc in Figs. 5~ a n d  5 b  ii clew corrclirtion cxistcd between Tht arid AT,, dctcrmincd by the position of the quench hentcrs and thc qiicncli Icvel. The drop of the magnetic field to l),,,,,, (or tlic incrcasc to Di,),cr ) was preceded by n qtiencb during which thc magiict wiis prritcctcd by the ORQII (or by the IRQH) i.e. by the quench numbcr 33 (thc qucnch number 38). The drop to n lower magnetic field at qucnch appeared after an increase of TI,, and (increase of 35 K and 26 K respectively) cnlculated for the case when thc magnct w a s  protected by the outer radius quench licaters. I, 4.8 8.1, x '1 8.1 x A.3 Protection by Different Sets of IRQH 'I'he efficicncy cif diffcrcnt sets high field (HI:) and  low field (LF) quench tienters was separately tested in the m e  uf intcr radiiis qucnch hcatcrs (SCC Fig. 2 fiw the positinn of thcsc stups). This study aiincd at optimisatioii of the protection scheme by assessing its reilundmcy. For quenchcs A III~+'I.Y m IIF e 1,P Q om III? I ................................. ...... . . . . . . . . . . . . . . . . . . . . .  ................. . . . . . . . . . . . . . . . . . . . . . . . .  . . .  i . i .  . .i ...... : ................. ............ i. . <  , .  50no 7000 9000 limo Fig. 6. Effccr of protcctiaii by scvcral scts of lliQ11 oil a) ' 1 ' 1 ~ ~  mid b) AThh. 664 l~rovoked by firing a spot heater, the magnet wns successively protected by all the four IRQH (HP+LF), by only the two high field strips IRQI-I (I-IF), by only one high f ieIrl strip IRQH (onc rrr;? and finally by only the two low field strips 1RQI-I (LF). For each case the teinpcri\tiire profilc of the magnet WiiS determined. Figs. 6a and 6b present the cnlculated values of Tht and AThb for four currents i.e. for I = 6000 A, I = 7500 A, I = 9000 A, I = 10500 A. It wils fciuntl that protection by all the heaters or only by HF is equivalent. This suggests that thc dclays till the beginning of the quenches were higher when the magnet was protected only by LF. This assumption was confirmed by the high peak tempersttires and the high gradients obtained in the case of protcction by only this set of heaters (Fig. 6). In the case of protcction only by half high field IRQH produced temperatures which stayed well within the design specification. Thus for adequate redundancy protection by one S C ~  of HF IRQH is fully sufficient. n. Efect of the Irisdration Tlrickness Two different Kapton@ insidation layers between the coils and the outer radius strips, 75 pm thick and 200 pn thick, were tested. This test aimed at investigating how the thcrinal conductivity between the tieaters and the coil affected Thl and AThb. Quenches were provoked by firing the spot heater and the magnet was protected by all thc ORQH. Figs. 7a and 7b show the variation of Thl nnd of  AThh for currents tip to 13 kA. Obviously Thr and AThh increased with the insulation thickncss. At high currents the increase was important and Fig. 7. EKcct 01' the insulation rliickncss on a) Ti,, and b) AThb cqiial to about 80 K and 54 K rcspcctively. Hent transfer wns significantly slowed down by the addition of one 125 pm thick shcct of Kapton* and led for the nominal ctirretit (1 1850 A) to. temperatures close to 300 K. The meanired heater delays for 200 ptn thick ORQH increased as expected by a factor of two. Thc ct'ficicncy of thc protection of a selected LHC short dipolc magncts by scvcral SCIS of  hcatcrs has been compared. The highest temperature reached in the coil and the highest temperature gradient were considered. We find that an insulation thickness of 200 pm between thc heater and the coils rcsults in pcnk tcmpcratures cxccediiig design maxiinum temperature. Protection by heaters located on the inter layer rndius is much more efficient than by those located on the otiter rndius of the coil. In the first case protection by only high field heaters is sufficient. Evaluation OF thc temperature profiles during quenches was found to be very helpful in the inicrpretation of the quench bchaviour of magncts. Slroiig correlation between temperature profile and the de-training effcct has becn dcmonstratcd. This method i s  an efficient and independent crosscheck of the temperature valucs ohtaincd from the MIITS method and provides results for validation of simulatiun models of the quench process. ACKNOWLEDGEMENT The authors would like to thank ttic MMS and the MTA teams for building and testing the LIIC short models, F. Rodrigticz-Mateos, R. Schmidi, F. Sonncmnnn for i'ruitliil discussions. In particular, we wish to thank N. Siege1 and l'ommasini for their contribution. REFWENCES Thc LHC Study Group, "The large hadron collitler conceptual design. L. Coull, U. Hagciidorii, V .  Rcmondino and P. Rodriguez-Mntcos, "LIIC qwnch protection system." /LEE 'li.n/is. Mfl.r/wf, 30, pp. 1525- 1529, 1994. V. Mnroussov and A. Sicmko,"R iiicthod to cuduntc tlic tcinpcrnhirc profile in a superconducting magnci during a qiicnch," I E I X  Tram. A ~ p l i e d S ~ p e r r ~ u n d u r t i v i f ~  9, pp. 1153-1  156, 1998. A. Sicmko CI aI,, "Powcr ICSI results on thc long models and full scnlc Iirototypc nf thc sccmd generation I.HC arc dipoles," in  iCECI6/lCMC P I C K ,  Kirakiushu, Japan. May 1996, T~ Hariiynma. T~ Mitwi, K. Ynmnfuji, Eds, Uscvirr'  Srierrre, pp. R37-842, 1'197. P. Rutlriguez-Mriteos, R. Schmidt, F. Sonncman nnd A. Sicmko, "Quench proccss and protcction of the LtlC dipolc inngncts," CERN- L1.K Project Nutc 184, 1999. G. Ganetis and A, Stevens, "Resiilt uf qtianch protection experiment on DM I-03 1 ," SSC Tech Nole 12, ONL, 1984. I.., Bolturn el al., "PerIormance 01' the I-meter male1 o l  the 70 mni aperture quadrupole Tor the I.HC low-bcta inserlions." CERN-LHC Project Kcport-24O. 1 g9X. P. Clinpuis,"Modt~isation dn comportcmcnt dlcctrorhermiquc dcs nirnants supraconducteurs en cns de transition rbistive," DEA rcporl LHC, 1998. CEKN," CERNIAC:I95-05 (LHC:), 1995, YCIIOW Book. 

Temperature Profiles During Quenches in LHC Superconducting Dipole Magnets Protected by Quench Heaters

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