The LEP Superconducting RF system reached its maximum configuration of 288 four-cell cavities powered by 36 klystrons in 1999. In 2000, this system, together with 56 cavities of the original copper RF system, routinely provided more than 3630 MV, allowing the beam energy to be raised up to 104.5 GeV. This not only required operating the cavities more than 15% above their design gradient, but has also demanded a very high operational reliability from the entire system. This paper will describe the operation of the LEP RF system during 2000, including new features, operational procedures and limitations

Brown, P

Brunner, O C

Butterworth, A

Ciapala, Edmond

Frischholz, Hans

Geschonke, Günther

Peschardt, E

Sladen, Jonathan P H

CERN Document Server

ULTIMATE PERFORMANCE OF THE LEP RF SYSTEMP.Brown, O.Brunner, A.Butterworth, E.Ciapala, H.Frischholz, G.Geschonke, E.Peschardt, J.Sladen,CERN, Geneva, SwitzerlandAbstract:The LEP Superconducting RF system reached itsmaximum configuration of 288 four-cell cavities poweredby 36 klystrons in 1999.  In 2000, this system, togetherwith 56 cavities of the original copper RF system,routinely provided more than 3630 MV, allowing thebeam energy to be raised up to 104.5 GeV.  This not onlyrequired operating the cavities more than 15% above theirdesign gradient, but has also demanded a very highoperational reliability from the entire system.  This paperwill describe the operation of the LEP RF system during2000, including new features, operational procedures andlimitations.1. INTRODUCTIONIn preparation for the LEP energy upgrade, the firstprototype superconducting (SC) RF module with fourcavities was tested in LEP in September 1992.  Followingfurther development of this technology, installation of theseries production cavities started in 1995.  By October1995 in addition to the existing 128 copper cavities a totalof 64 SC cavities were operational, allowing a gradualincrease in beam energy from 45 GeV to 70 GeV.  In1996 with 144 SC cavities, Wr physics started at80.5 GeV per beam.  Subsequently more SC cavities wereadded, initially complementing and then, when allavailable space was filled, partially replacing, the originalcopper system.  The final configuration is given intable 1. The last two years of LEP physics centred on thesearch for the Higgs boson, consequently the highestpossible beam energy at good luminosity became themain priority.  The performance of the RF system waspushed far beyond its original specifications.  Theevolution of the nominal and the operationally availableRF voltage since 1995 as well as the beam energy isshown in figure 1.  Figure 3 shows the achievedaccelerating gradients with increasing beam energy.The circumferential voltage U0 required to replace thesynchrotron radiation losses per turn at a beam energy Eis given approximately by:]GeV[6.28]eV[ 40 EU  At 104.5 GeV the two counter rotating beams lose3.4 GeV per turn, corresponding to 3.2 % of their energy.In order to keep the beams stable, an over voltage ofabout 10-15 %, depending on the quantum lifetime, isrequired.  With a total accelerating voltage of about3.6 GV the LEP RF system was the biggest accelerationsystem built so far in a storage ring.The nominal circumferential voltage given in figure 1is calculated from the gradients specified at production ofthe cavities.  Throughout the upgrading programme therewas continuous pressure for the highest possible beamenergy.050010001500200025003000350040004/199510/19955/199612/19966/19971/19987/19982/19998/19993/200010/2000dateRF voltage [MV]405060708090100110LEP energy [GeV]Available RF voltageNominal RF voltageLEP energyFigure 1.: Beam energy, available and  nominal RFvoltage from 1995 until 2000.During the last two years the beam energy could besignificantly increased by several measures [1]:Increasing the bending radius by using the correctormagnets, changing the horizontal damping partitionnumber, implementing special operational proceduresand, principally by increasing the accelerating gradient ofthe SC cavities.This report describes the final performance of thissystem and the measures taken to achieve it.2. SYSTEM DESCRIPTIONThe Cu system is described in [2].  Details of the SCsystem can be found in [3] and [4] and consequently onlythe main features are given below.  The 352 MHz,standing wave, four-cell, SC cavities, were mounted ingroups of four in one common cryostat, called a"module".  They were cooled in a bath of liquid He at4.5 K, a total cooling capacity of 53 kW at 4.5 K wasavailable [4].  The first 16 superconducting cavities wereconstructed of solid Nb, all later ones were made bysputter-coating a Cu substrate with a thin layer of Nb.Each 1.3 MW klystron fed eight cavities in parallel,two klystrons shared one common high voltage DC powersupply in one "RF unit".  For reasons of compatibility thelayout of the RF plant for the SC system was essentiallythe same as for the Cu system.  The RF power wascontrolled by a "Voltage Loop", which maintained thesum of the eight individual cavity voltages at therequested level independent of beam loading.  For mostRF units scalar sum detection was used, however in someunits a vector sum was implemented.  Table 1 gives themain parameters of the final configuration.0-7803-7191-7/01/$10.00 ©2001 IEEE. 1059Proceedings of the 2001 Particle Accelerator Conference, ChicagoThe cavities were installed in the straight sections oneither side of the four even numbered interaction points ofLEP.Table 1 The LEP RF system in 2000number of Cu cavities 56accelerating voltage per cavity 2.5 MVactive length of 4-cell cavity 1.7 mnumber of solid Nb SC cavities 16design gradient 5 MV/mnumber of sputtered Nb SC cavities 272design gradient 6 MV/mnumber of klystrons (1/1.3 MW each) 443. SC CAVITY PERFORMANCEThe solid Nb cavities were conditioned up to theirnominal design gradient of 5 MV/m and operated up to4.5 MV/m with beam.  The average gradient of thesputtered SC cavities was significantly increased abovethe design gradient of 6 MV/m, after a number ofreliability issues of ancillary equipment had beenaddressed and limitations had been removed [4].A few cavities degraded in performance duringoperation.  Since 1998 two modules were temporarilyremoved from the tunnel for repair of individual cavitiesduring the winter shut-downs.  Six cavities could berecovered by rinsing with ultra-pure water, in one case theNb film had to be re-sputtered.A hard limitation of cavity gradient was field emission,leading to local heating of the cavity walls and a sharprise in the pressure of the He bath, which triggered thesafety interlock.  In some cavities field emission loweredthe Q0-value and cavity dissipation exceeded the availablecryogenic cooling power, leading to a limitation of theaccelerating field.  The maximum cooling capacity percavity of approximately 200 W is given by the cryostatdesign.Apart from aggressive cavity conditioning, the increasein gradient was achieved by removing a number oflimitations, described below.3.1. Field EqualisationEight cavities were connected to a common klystron,with no control of RF power to individual cavities.The cavities were equipped with fixed power couplers.Due to mechanical tolerances and variations of fieldflatness of the four cells, the external Q-factor Qext of thepower couplers varied around the nominal value of 2*106by ±20 %.  The effect of extreme Qext of certain cavities,leading to particularly high fields, and therefore limitingthe whole group, was corrected by inserting O/4 plates ofappropriate thickness and distance from the powercouplers, into the waveguide.  This technique was alsoapplied to increase the field of some low-field cavities.Each cavity had its own frequency tuning circuit usingas reference signal the forward signal of a directionalcoupler upstream of the power couplers.  The reflectionfrom the cavities in the strongly beam loaded regime ofthe SC cavities varies between almost 100 % withoutbeam and 0 % at nominal beam current.  Insufficientdirectivity of the directional couplers can lead to beamcurrent dependent changes of cavity tuning.  Therefore,the directivity of all these couplers was adjusted to betterthan 30 dB.In order to achieve equal fields with maximum beamcurrent of about 6 mA, the electrical length of thewaveguides feeding a group of eight cavities had to beadjusted to a precision of about 3 degrees on all cavities.3.2. Cavity OscillationsPonderomotive oscillations had been a major limitationof cavity performance [6].  In the last year of operationthis problem was removed by implementing an activedamping system on all cavities, which made use of theexisting tuning system [4].3.3. Cavity ConditioningDuring the winter shut-downs the whole system wasbrought to room temperature, requiring reconditioningeach year.  The criteria for conditioning were localvacuum in the power couplers and x- and J- radiationfrom the cavities.Nb/Cu SC units - maximum field after conditioning (2000)0.001.002.003.004.005.006.007.008.009.00232-1233-1272-1431-2432-2433-2473-2472-2632-1633-1673-1672-1831-2832-2833-2873-2872-2average cavity field (MV/m)Figure 2:  Average accelerating gradient of RF units withsputtered SC cavities after conditioning in 2000.Both continuous (cw) and pulse conditioning withpulse length varying between 10 ms and 100 ms and dutycycle between 1 and 10 % were used.  A smooth switch-off of the RF pulse was implemented to avoid excessivepeak power reflections from the cavities causing arcs inthe waveguides.  The average gradients of all groups ofeight cavities after conditioning are shown in figure 2.The goal for conditioning in 2000 was 7.5 MV/maverage.  For stable operation with beam the total gradientwas about 5 % lower than the maximum achieved duringconditioning.  He processing had been applied in isolatedcases in the past with mixed results.  The inherent risks ofprocessing in situ using klystrons as RF power sourceswith strongly overcoupled power couplers wereconsidered too high for systematic application of thistechnique and it was no longer applied in the LEP tunnel.1060Proceedings of the 2001 Particle Accelerator Conference, Chicago05101520253044.34.64.95.25.55.86.16.46.7 77.37.67.98.28.58.89.19.4Accelerating field [MV/m]Number of cavities96 GeV100 GeV104 GeVFigure 3.:  Histogram of cavity gradient distribution forthree different maximum beam energies in 1999 and2000.During the year the maximum accelerating field foreach RF unit was set individually, such that stablemaximum voltage operation with beam was possible.This level was regularly adjusted.  Some units had to betemporarily reduced, but overall most were slowlyincreased.  By permanently running at or near the limit,the maximum total accelerating voltage was increased toa record of 3666 MV.  This in spite of the loss of fourcavities permanently detuned because of damage andanother three cavities which had to be partly detuned toachieve stable operation.4. OPERATIONAL PROCEDURES ANDSYSTEM PERFORMANCEThe exploitation of the highest cavity gradients wasonly possible by a new LEP running scenario andmodified RF operational procedures.In previous years, the number of different beamenergies was limited by the LEP physics experiments tothree.  However, during 2000, the accumulation ofluminosity at the highest possible energy  resulted in analmost continuous distribution of energies during physicscoasts.Trips of the RF interlock system occurred mainly dueto field emission, on a statistical basis.  An RF trip lostthe voltage from eight cavities, about 100 MV,corresponding to 0.8 GeV beam energy. A trip of a HVsupply (two klystrons) lost 200 MV and 1.6 GeV.The beam energy was determined by the available RFvoltage on a fill by fill basis.  At the beginning of aphysics coast the beam energy was chosen such that thebeam survived the loss of 16 cavities, i.e. between 102.0and 102.8 GeV.  During the coast, the beam energy was"mini-ramped" by 0.8 GeV, reducing the margin to a tripof one klystron.  Another miniramp of 0.8 GeV at the endof the coast reduced the quantum lifetime such, that aklystron trip would cause the loss of the beam.The average time between RF trips was 14 min andrecovery normally took about 2 min, except in the case ofequipment failure.  The average length of a physics coastwas 1.5 hours.A GPS timing based diagnostics system wasintroduced, which allowed precise determination of thesequence of all unit trips, distributed over 26.7 km.  Thesystem could distinguish losses of the circulating beamsdue to RF trips or due to other reasons.  This systemproved very important for efficient fault finding.At injection energy the beam induced electric field inthe cavities was much higher than the required cavityfield, therefore particular care was taken to correct tuningof the cavities, since small errors could lead to a run-awayof the cavity tuning.  In particular, during the power rampthe tuning state of cavities was checked and the maximumfield of RF units was adjusted dynamically.Permanent operation of the system at - and sometimesbeyond - safe limits caused damage to components of theRF plant.  In particular arcing in waveguides led todestruction of several components and to damage of threecirculators.5. CONCLUSIONThe performance of the RF system contributedprincipally to the increase in LEP centre of mass energyof 7 GeV since 1999, and allowed the Higgs bosondiscovery limit (5 V) to be pushed to 113 GeV/c2 [7].With the shut-down at the end of 2000 LEP operationcame to an end.  The RF system is now dismantled.ACKNOWLEDGEMENTSThe successful operation of the LEP RF system and itsultimate performance was only possible due to thededication of all members of the SL LRF group whoparticipated in the daily performance meetings and in thevery demanding stand-by service.  We are pleased to alsoacknowledge the important contributions from all othergroups involved in RF system operation, in particular thecryogenics and the operations teams.6. REFERENCES[1] R.Assmann, "LEP Operation and Performance withElectron - Positron collisions at 209 GeV", Proc. XIChamonix workshop 2001, CERN-SL-2001-003 DI[2] G.Geschonke, S.Hansen, "Status and Performance ofthe LEP RF System", Conf. on High Energy Accelerators,Tsukuba 1989, Part Acc. 28(1990), p. 779[3] D.Boussard, "Performance of the LEP2 SRF System",PAC, Vancouver 1997, p. 2879[4] P.Brown, O.Brunner, A.Butterworth, E.Ciapala,H.Frischholz, G.Geschonke, E.Peschardt, J.Sladen,"Performance of the LEP200 Superconducting RFSystem", 9. Workshop on RF Superconductivity, 1999Santa Fe, LA-13782-C, p. 1[5] M.Sanmarti, Ph.Gayet, S.Claudet, "Cryogenic Issuesfor LEP in 2000", LEP –SPS Performance ws. ChamonixX, CERN-SL-2000-007 DI, p.226[6] D.Boussard, P.Brown, J.Tückmantel, "ElectroacousticOscillations in the LEP SC Cavities", EPAC Sitges 1996,p.187[7] P.Janot,CERN/EP, private communication1061Proceedings of the 2001 Particle Accelerator Conference, Chicago

Ultimate Performance of the LEP RF System

Ultimate Performance of the LEP RF System

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