The first version of the LHC prototype half-cell has been in operation since February 1995. It consists of one quadrupole and three 10-m twin aperture dipole magnets which operate at 1.8 K. This experimental set-up has been used to observe and study phenomena which appear when the systems are assembled in one unit and influence one another. The 18-month long experimental program has validated the cryogenic system and yielded a number of results on cryogenic instrumentation, magnet protection and vacuum in particular under non-standard operating conditions. The program was recently complemented by the cycling experiment: it consisted in powering the magnets following the ramp rates which will be experienced by the magnets during an LHC acceleration. In order to simulate 10 years of routine operation of LHC, more than 2000 1-hour cycles were performed interleaved with provoked quenches. The objective of this experiment was to reveal eventual flaws in the design of components. The prototype half-cell performed to expectations showing no sign of failure of fatigue of components for more than 2000 cycles until one of the dipoles started exhibiting an erratic quench behavior

Casas-Cubillos, J

Collier, Paul

Cruikshank, P

Dahlerup-Petersen, K

Desforges, D

Hilbert, B

Krainz, G

Momal, F

Rijllart, A

Rodríguez-Mateos, F

Saban, R I

Schmidt, R

Serio, L

Desforges, B

CERN Document Server

LHC Division, *SL DivisionEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsEXPERIMENTS AND CYCLING AT THE LHC PROTOTYPE HALF-CELLJ. Casas-Cubillos, P. Collier*, P. Cruikshank, K. Dahlerup-Petersen, B. Desforges*,B. Hilbert, G. Krainz, F. Momal, A. Rijllart, F. Rodriguez-Mateos, R.  Saban, R.Schmidt, L. SerioThe first version of the LHC prototype half-cell has been in operation since February 1995. It consists of onequadrupole and three 10-m twin aperture dipole magnets which operate at 1.8 K. This experimental set-up has beenused to observe and study phenomena which appear when the systems are assembled in one unit and influence oneanother. The 18-month long experimental program has validated the cryogenic system and yielded a number ofresults on cryogenic instrumentation, magnet protection and vacuum in particular under non-standard operatingconditions. The program was recently complemented by the cycling experiment: it consisted in powering themagnets following the ramp rates which will be experienced by the magnets during an LHC acceleration. In order tosimulate 10 years of routine operation of LHC, more than 2000 1-hour cycles were performed interleaved withprovoked quenches. The objective of this experiment was to reveal eventual flaws in the design of components. Theprototype half-cell performed to expectations showing no sign of failure of fatigue of components for more than2000 cycles until one of the dipoles started exhibiting an erratic quench behavior.Presented at 1997 Particle Accelerator Conference, Vancouver, 12-16 May 1997.Geneva, Large Hadron Collider ProjectCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 110Abstract03/06/97EXPERIMENTS AND CYCLINGAT THE LHC PROTOTYPE HALF-CELLR.  Saban, J. Casas-Cubillos, P. Collier, P. Cruikshank, K. Dahlerup-Petersen, B. Desforges,B. Hilbert, G. Krainz, F. Momal, A. Rijllart, F. Rodriguez-Mateos, R. Schmidt, L. Serio,CERN, Geneva, Switzerland1 INTRODUCTIONThe first version of the LHC Prototype Half-Cell [1,2]has been in operation since February 1995. It consists ofone quadrupole and three 10-m twin aperture dipolemagnets which operate at 1.9 K. One electrical circuitpowers all the magnets in series. This experimental set-uphas been used to observe and study phenomena whichappear when the systems are assembled in one unit andtherefore influence one another.2 EXPERIMENTSTwo experimental runs have been performed in the twoyears since commissioning began in December 1994 [2].RUN2 started with the addition of a third dipole and astructural modification to the cryogenic system [3]. TheLHC half-cell systems, the experiments and their resultshave been described and reported in detail in specializedconferences [4,5,6]: what follows is a concise report of theexperiments carried-out with the facility.The 1.9 K superfluid helium cooling scheme wasexperimentally validated in steady state conditions andduring transients [7]. The thermohydraulic effects of aresistive transition of a string of magnets have beeninvestigated. A model was developped and wasextrapolated to a full-cell of LHC [8]; this allowed thereduction the number of valves and definition of theirtechnical characteristics. Heat loads on the cryogenicsystem have been measured in steady state and withdegraded insulation vacuum [9]. They confirmed thecalculated heat loads and in some cases highlightedpossible improvements.The quench detection and magnet protection system[10,11] was experimentally validated. Quench propagationbetween magnets has been studied by provoking a quenchin the quadrupole and observing its propagation to theadjacent dipole [12].Experiments to determine the upper limit on pressurein the insulation vacuum enclosure prior to cool-downhave confirmed that positive displacement pumpsachieving a pressure of 1 Pa are sufficient [13]. Thelimited propagation speed of a helium leak along acryogenically cooled tube has been measured on the 1.9 Kcold bore beam tube [14] showing agreement within 20%of the theoretical model. Accidental loss of insulationvacuum has been studied by venting the cold LHCPrototype Half-Cell to atmospheric pressure.The displacements of the magnet cold masses relativeto their cryostats, for long term stability measurements,during quenches and during electrical cycles weremonitored [15].3 THE CYCLING EXPERIMENTThe cycling experiment was performed in order toreveal eventual flaws of components of the half-cell whichcould only be seen after some time of operation of thecollider.After 18 months of routine current ramping to 12.5and 13.1 kA, the half-cell was electrically cycled from 800A to 12.5 kA repeatedly in a cycle that lastedapproximately 43 minutes. The ramp rates (10 A/s) weresimilar to those which will be experienced by the magnetsduring the acceleration phase. Assuming 200 days ofoperation per year and one injection per day, theexperiment simulated 10 years of operation of the collider.05102:52 4:04 5:16 6:28 7:401.861.871.88CTemFigure 1: Current and temperature stability3.1 Conditions of the experimentThe experiment was conducted from the accelerator controlroom under the supervision of the operators who areinvolved in the running of SPS and LEP. Therefore, theprocedures for cycling the LHC Prototype Half-Cell had tobe simple, secure and efficient to be executed by non-specialised personnel.The area was closed with fences and video cameraswere installed for remote visual inspection of the site.Two X-terminals were used to monitor the cryogenics, thevacuum and the power converter. Only limited control ofthe cryogenics system was available to prepare for cyclingor put the cryogenic system in stand-by mode. Theprogram controlling the power converter allowed theoperator to start, stop or interrupt the continuous cycling.A completely linear downwards ramp at -10 A/s wasachieved despite the inverse voltage limitation set by thefree-wheeling diodes of the power converter. Authorizationfor powering was granted only by the magnet protectionteam at the beginning of a cycling period or after theinterlocks had been broken.All the systems (vacuum, cryogenic, magnetprotection, power converter, general mains) were hardwareinterlocked. Depending on the gravity of a failure of anyof the systems, the string was either slowly discharged orthe magnets were quenched by firing the protectionheaters.The experiment, which lasted eleven weeks, wasconducted 24 hours a day and seven days a week; it wasonly interrupted by access requirements to the restrictedarea of the LHC Prototype Half-Cell or the unavailabilityof peripheral systems (e.g. central refrigeration plant). Theaccess procedure was remotely controlled by the acceleratorcontrol room operators: it mainly involved the loweringof the current to 50 A. The power converter was rarelyturned-off because the operation would break the interlockchain.3.2 IncidentsAlthough a number of minor incidents occurred during thecycling experiment, all were adequately handled either bythe emergency systems which had been engineered for thispurpose, or by the control room operators.3.2.1 Interruption of the Cryogenic SupplyOn several occasions, the liquid helium level in the maindewar supplying the LHC Prototype Half-Cell amongother users, fell below a threshold. The operators wereinformed by an alarm signal and were advised to stopcycling in order to avoid quenching the magnets due toloss of nominal cryogenic conditions.3.2.2 Quench Heater Power Supply FailureCapacitor banks which are discharged in the protectionheaters when a quench occurrs are continuously monitored.Two different internal power supplies failed on twooccasions when the magnets were at 12.5 kA and 1.8 kA.As a result, the interlock chain was broken and themagnets slowly discharged.3.2.3 Mains FailureA mains failure on the electricity grid occurred during thefalling phase of the cycle when the magnets were excitedat 9.8 kA. The protection heaters were fired and themagnets were quenched. The emergency procedure duringthe cycling experiment foresees this in order to simplifythe recovery.3.3 ObservationsBefore the experiment started, the highest current levelattained without quenching any of the magnets on twooccasions was 13.5 kA. A number of cycles were executedand then the current raised to the same value: if a magnetquenched before 13.5 kA, a degradation of the quenchingmagnet was assumed.After a relatively low number of cycles (184), the 13.5kA limit could not be attained; the third dipole quenchedjust below 13.5 kA. Instead of trying to confirm thisquench level, it was decided to double the number ofcycles (300) before the second dipole was quenched at 13.2kA. The number of cycles were increased first to 500 thento 1000 before the test quench. They all confirmed adegradation in the quench level of the second dipole. Thecycling experiment was ended after 2152 cycles.11.512.513.50100020003000Feb-96 May-96 Aug-96 Dec-96CyclesQuench CurrentCurCyFigure 2 : Quench Current and Number of Cycles versustimeThe degradation of the second dipole was investigatedby repeatedly pushing the LHC Prototype Half-Cell to itshighest natural quench. All the quenches which wereobserved during this campaign occurred in the first pole ofthe first aperture of the second dipole. The most salientpattern is the erratic quench behaviour characterized byquenches approaching the highest limit (13.2 kA)followed by quenches at much lower level (11.7 kA). Insome cases, during the raising phase, intermediate highercurrent levels were attained; this wrongly suggested aretraining of the dipole (see Figure 3). The highestdifference in current for two consecutive quenches was 1.6kA. In total 10 natural quenches were caused during thiscampaign.In the course of the campaign it was suggested that thequenches might be caused by a non-uniform currentdistribution in the superconducting cable [16]; this wouldstart a persistent current loop which could saturate one ofthe strands and cause the quench. A number of attempts tohighlight this effect were made, but all failed.11.512.012.513.013.51 2 3 4 5 6 7 8 9 10 11 12CuQuench NumberFigure 3 : Quench pattern after the cycling experimentIt was decided to remove the second dipole from theLHC Prototype Half-Cell during the shutdown followingthe cycling experiment. The dipole (MBL1A2) wasexchanged with a spare (MBL1N1) which had beenrecently re-assembled and measured on the LHC MagnetTest Bench.MBL1A2 has been since then measured on the LHCMagnet Test Bench [17]. Preliminary data, which has notyet been thoroughly analyzed, confirm what has beenobserved after the cycling experiment: the quenches arealmost always localized to the same region of pole 1. Anyattempt to identify the effects of cycling on MBL1A2remains, so far, speculative.Other than the degradation of the second dipole, noother effect of the cycling experiment has been identified.In the coming run, cycling will be resumed both tocontinue the artificial ageing of the components and, toeventually reveal other effects which might be hidingbehind the degradation of the second dipole.It is worthwhile mentioning that 85 natural orprovoked quenches were performed during the twoexperimental runs: 45 of these were at nominal current orabove. Another 15 quenches at nominal current or abovewere experienced by the magnets of the LHC Magnet TestBench. This is far more than any dipole is expected toexperience in the lifetime of LHC.4 RUN3ARUN3A will start in early June 1997. Following theshutdown which lasted five months, the LHC PrototypeHalf-Cell is now equipped with additional cryogenicinstrumentation: wetting indicators in the longitudinalheat exchanger, level meter in the phase separator, flowmeters in the discharge line and additional thermometers.The first beam screen [1] has been installed in the thirddipole. The installation of diode directly on the bus-barsentering the third dipole will permit investigation ofquench propagation between dipoles.5 THE SECOND GENERATION TESTFACILITYAfter RUN3, the present LHC half-cell will bedismantled. The construction of a second generation testfacility will commence beginning 1999. It will includethe separate cryogenic supply line and new generation 15-m dipoles with all the corrector magnets. For the year2000 it is planned to start experimenting with a full106-m LHC cell including three 12.5 kA circuits: one forthe main dipoles, one for the focussing quadrupoles andone for the defocussing quadrupoles. Twelve additional600 A circuits will power the multipole corrector andlattice magnets attached to the main magnets of theregular cell of an LHC arc.6 ACKNOWLEDGMENTSThe authors wish to thank the operators of the CryogenicPlant in SM18 and the operators of the Prévessin ControlRoom for their active participation: their autonomy duringthe experiment has given an insight on the reliability ofthe systems of the LHC Prototype Half-Cell.REFERENCES[1] The Large Hadron Collider, Conceptual Design, CERN 95-05,October 1995[2] P.Faugeras for the String Team, Assembly and Commissioningof the LHC Test String, 1995 PAC ’95 , Dallas, Texas, May1995[3] A.Bézaguet et al. , Cryogenic Operation and Testing of theExtended Prototype Magnet String, paper presented at theICEC16, Kitakyushu, Japan, May 1996.[4] R.Saban et al., The Control and Data Acquisition of the LHCTest String, ICALEPCS, Chicago, Illinois, November 1995.[5] Specification for a Data Acquisition System for the LHC String,CERN-AT Group Note 94-02 (IC)[6] R.Saban et al.,  The LHC Test String: First OperationalExperience, EPAC ‘96, Sitges, Spain, June 1996.[7] A.Bézaguet et al. , The Superfluid Helium Cryogenics System forthe LHC Test String: Design, Construction and First Operation,Cryogenics Engineering Conference, Columbus, Ohio, July1995.[8] M.Chorowski et al., Thermohydraulics of resistive transitionson the LHC prototype magnet string: theoretical modelling andexperimental results - to be presented at the CEC/ICMC 97 -Portland USA[9] G.Riddone, Theoretical Modelling and ExperimentalInvestigation of the Thermal Performance of the PrototypeLattice Cryostats, Doctoral Thesis, Politecnico di Torino,January 1997.[10 ] F.Rodriguez-Mateos  et al. , Electrical Performance of a String ofMagnets Representing a Half-Cell of the LHC Machine, MT14,Tampere, Finland, June 1995.[11 ] G.Krainz, Quench Protection and Powering in a String ofSuperconducting Magnets for the Large Hadron Collider,Doctoral Thesis, Technical University of Graz, February 1997.[12 ] L.Coull, D.Hagedorn, G.Krainz, F.Rodriguez-Mateos,R.Schmidt, Quench Propagation Tests on the LHCSuperconducting Magnet String, EPAC ‘96, Sitges, Spain, June1996.[13 ] P.Cruikshank et al. , Investigation of Thermal and VacuumTransients on the LHC Prototype Magnet String, paperpresented at the ICEC16, Kitakyushu, Japan, May 1996.[14] E.Wallen, Experimental Test of the Propagation of a HePressure Front in a long, Cryogenically Cooled Tube, LHCDivision Vacuum Group Internal Note 96-12[15] D.Missiaen, Metrology of Superconducting Magnets, FourthInternational Workshop on Accelerator Alignment, Tsukuba,Japan, November 1995.[16] A. Verweij, Current Redistribution in the Cables of the LHCMagnets, LHC Project Note 90, May 1997[17] A.Siemko et al., paper in preparation for Magnet TechnologyMT15, Beijing, China, October 1997

Experiments and cycling at the LHC prototype helf-cell

Experiments and cycling at the LHC prototype helf-cell

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