The 1997 LEP collider run was the first year of the LEP2 program devoted to stable high luminosity running. The good performance of the LEP superconducting RF system allowed to run at beam energies of 91.5 GeV. In addition to the to usual optimisation procedures to minimise the vertical beam emittances, the horizontal beam sizes were reduced by increasing the horizontal damping partition number and reducing the betatron function at the interaction points. Vertical beam-beam tune shifts in excess of 0.05 were achieved with total beam currents of 5 mA. The total integrated luminosity of 73 pb-1 is the highest ever recorded in a single year of LEP operation

Arduini, Gianluigi

Bailey, R

Burkhardt, H

Collier, Paul

Cornelis, Karel

Faugier, A

Jonker, M

Lamont, M

Morpurgo, G

Roy, G

Uythoven, J

Wenninger, J

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN    SL DIVISIONGeneva, SwitzerlandJune, 1998CERN SL-98-047  OPLEP performance at 91.5 GeVG. Arduini, R. Bailey, H. Burkhardt, P. Collier, K. Cornelis,A. Faugier, M. Jonker, M. Lamont, G. Morpurgo, G. Roy,J. Uythoven, J. WenningerAbstractThe 1997 LEP collider run was the first year of the LEP2 program devoted to stablehigh luminosity running. The good performance of the LEP superconducting RF systemallowed to run at beam energies of 91.5 GeV. In addition to the to usual optimisationprocedures to minimise the vertical beam emittances, the horizontal beam sizes werereduced by increasing the horizontal damping partition number and reducing the betatronfunction at the interaction points. Vertical beam-beam tune shifts in excess of 0.05 wereachieved with total beam currents of 5 mA. The total integrated luminosity of 73 pb-1 isthe highest ever recorded in a single year of LEP operation.Presented at EPAC-98, 6th European Particle Accelerator ConferenceStockholm22-26 June 1998LEP PERFORMANCE AT 91.5 GEVG. Arduini, R. Bailey, H. Burkhardt, P. Collier, K. Cornelis, A. Faugier, M. Jonker,M. Lamont, G. Morpurgo, G. Roy, J. Uythoven, J. Wenninger, CERNAbstractThe 1997 LEP collider run was the first year of the LEP2program devoted to stable high luminosity running. Thegood performance of the LEP superconducting RF systemallowed to run at beam energies of 91.5 GeV. In additionto the usual optimisation procedures to minimise the ver-tical beam emittances, the horizontal beam sizes were re-duced by increasing the horizontal damping partition num-ber and reducing the betatron function at the interactionpoints. Vertical beam-beam tune shifts in excess of 0.05were achieved with total beam currents of 5 mA. The to-tal integrated luminosity of 73 pb 1 is the highest everrecorded in a single year of LEP operation.1 BEAM ENERGY AND RFDuring 1997 LEP was operated routinely at 91.5 GeV. Thechoice of the energy was dictated by the available total RFvoltage. The design gradient of 6 MV/m for the 224 super-conducting Nb/Cu cavities was generally met. Taking intoaccount those design fields and the gradients provided bythe copper RF system, the total voltage that could be deliv-ered was 2650 MV. In practice about 95% of the voltage,2540 MV, was available for operation. This would allowfor a trip of a high voltage power converter, which supplies16 cavities, without losing the beams at 91.5 GeV. Such se-vere trips accounted for about 7% of all RF trips. The meantime between trips increased from about 50 to 70 minutesin the course of the run. 10% of the LEP fills were lost dueto RF trips.2 LEP PERFORMANCE IN 1997To maximise the performance of LEP the bunch currentsare first increased until the beam-beam limit is exceededwith 4 bunches per beam. Then the number of bunches isincreased to 6 or 8 per beam as long as they are no limita-tions to the total beam currents.In 1997 LEP ran with 4 bunches per beam and a totalcurrent around 5.5 mA, because the available cryogenicspower prevented running with much higher total intensities.Significantly higher beam currents would have been re-quired to reach equivalent performance with 6 or 8 bunchesper beam, making life difficult for the RF and the vacuumsystems.Total beam currents of 5.4 mA, limited by synchro-betatron resonances, were typically accumulated at injec-DayIntegrated Luminosity (pb-1)199619970204060800 50 100 150Figure 1: Evolution of the integrated luminosities duringthe 1996 and 1997 LEP runs. When the best periods of thetwo runs are compared, the slope for 1997 is a factor 2.3steeper than for 1996.tion into 4 bunches per beam. In collisions the total currentwas usually 5.0 mA at the start of fill, the ramp transmis-sion efficiency being 90%. Those currents were about 20%larger than in 1996 thanks to a reduction of the transverseimpedance due to the replacement of copper RF cavities bysuperconducting cavities.During 1997 LEP delivered a total integrated luminosityof 73.3 pb 1 (Figure 1). 63.8 pb 1 were produced at beamenergies ranging from 90.5 to 92 GeV, with 92% of the lu-minosity delivered at 91.5 GeV. The best performance over24 hours resulted in 1.9 pb 1. Peak luminosities reached5  1031cm 2s 1 at the beginning of fills and beam-beamtune shifts ybeyond 0.05 where recorded. 2.3 pb 1 weredelivered at 45 GeV for the LEP detector calibrations. Adedicated run at 65 and 68 GeV to check the existence ofa signal observed in 1995 by one of the LEP experimentsproduced 7.3 pb 1. LEP was colliding beams for physicsduring 44% of the allocated time. 7% of the scheduled timewas also used for beam energy calibration. The down timewas dominated by vacuum problems caused by the highlevels of synchrotron radiation, by the RF and by the cryo-genics systems. About 20% of the fills were lost due toequipment failures. The average turn-around time from abeam dump to the next collisions was 108 minutes.For most of the year LEP was running with an opticshaving an arc phase advance per cell of 90 in the horizon-tal and 60 in the vertical plane (90/60). For the last 10 daysa 102/90 optics yielding smaller horizontal emittances wastested in operation. The performance reached levels similarto the 90/60 optics and the integrated luminosity deliveredwith the 102/90 optics was 2.5 pb 1. This optics will beused for operation in 1998.3 BEAM SIZE OPTIMISATIONIn e+e  storage rings the dependence of the horizontalbeam emittance "xon the beam energy E and on the hori-zontal damping partition number is"xE2Jx(1)for a given optics and phase advance in the arcs. The nat-ural increase of "xwith beam energy leads to higher back-grounds in the experiments and lower luminosities. To re-duce the impact of the emittance increase, Jxwas shiftedfrom the usual value Jx= 1 to Jx= 1:6. This change wasobtained with a +120 Hz change of the RF frequency. Sincethe change of damping partition number also increases thebeam energy spread, a higher RF voltage is required for thesame beam energy to guarantee a sufficient quantum life-time. At LEP the top beam energy being limited by theavailable RF voltage, the beam energy must be reduced byabout 0.4 GeV with respect to Jx= 1 to guarantee a suffi-ciently large quantum lifetime of the beams.A further reduction of the horizontal beam size was ob-tained by squeezing the horizontal betatron function at theinteraction points xfrom 2.0 to 1.5 m. Figure 2 shows thehorizontal beam sizes at the interaction points for xof 2.0and 1.5 m and Jx= 1:6. Backgrounds in the experimentsdid not increase significantly.The Jxand xchanges resulted in an increase of the lu-minosity of about 1.4 with respect to 1996. The overall im-provement of roughly a factor 2.3 in luminosity productionrate compared to 1996 seen in Figure 1 is also due to 20%higher average bunch currents (factor 1.5) and to a fasterturnaround time.To ensure small vertical beam sizes in collision the cou-pling between the horizontal and vertical planes is carefullyadjusted with skew quadrupoles. The amount of couplingis measured at LEP by the closest tune approach [1]. It iscorrected during the LEP startup period down to the mea-surement resolution.The performance is steadily improved during the runwith the help of “Golden Orbits” [1]. Such orbits give re-producible high performance with optimum vertical emit-tance "yand are obtained by trial and error using variousorbit correction algorithms. In 1997 the smallest emit-tances of "y= 0.25 nm corresponded to vertical beam sizesof 3.5 m at the interaction points. When the coupling be-tween the two planes is well corrected, "yseems to be lim-ited by the residual vertical dispersion and beam-beam ef-fects. This minimum of "yvaries from one running periodσx (µm)Entriesβ*  = 1.5 mβ*  = 2.0 m010203040160 180 200 220 240 260Figure 2: Horizontal beam sizes xmeasured with beam-beam deflections scans for x= 1.5 and 2.0 m (Jx= 1:6).The mean ratio is 1.15 as expected from the xratio.I (µA)ξ yε y = 0.25 nmε y = 0.28 nm00.010.020.030.040.050.060 200 400 600Figure 3: Vertical beam-beam tune shift yas a functionof the bunch current (x= 1.5 m). There is no strong evi-dence for a beam-beam limit. The lines show the expecteddependences for two vertical emittances "y.to the next at energies above 80 GeV. The smallest value of"y= 0.15 nm was measured in 1996.Figure 3 shows the correlation between the verticalbeam-beam tune shift yand the bunch currents for all LEPfills with x= 1.5 m. Although there is a slight levelling ofat high currents, there is no strong evidence that the beam-beam limit has already been reached.Single beam lifetimes at high energy are, for good vac-uum conditions, limited to  40 hours by Compton scat-Time (h)Lifetime (h)CollisionsRF Trip10203040500 2 4 6Figure 4: Evolution of the beam lifetime during a physicsfill at 91.5 GeV. The initial lifetime is  40 hours. Thesudden drop below 10 hours occurs when the beams arebrought into collision. The lifetime increases with time asthe bunch currents and the beam-beam tune shifts go down.tering on thermal photons [4]. In collisions the lifetimesare dominated by beam-beam bremsstrahlung and are in-versely proportional to y. Figure 4 shows the evolution ofthe beam lifetimes at 91.5 GeV.4 OPTIMISATION TOOLSVertical collision offsets between the beams are correctedwith beam-beam deflection scans [2]. The beam-beam kickbetween the 2 beams in collision is reconstructed from theclosed orbit readings and measured for each IP as a func-tion of the vertical separator settings which are used to cre-ate a closed local bump around the IP. An example of ascan, which takes about 4 minutes, is shown in Figure 5.The stability of the offsets is 1-2 m over a few weeks, tobe compared with beam sizes of 4 m.Once the collision offsets are corrected, the beam-beamtune shifts yare optimised using either the luminositymeasurements or beam size signals from various LEP emit-tance measurement devices. Even when the absolute emit-tance values are not very accurate, the relative signals givevaluable information for machine tuning.In the later part of the 1997 run first attempts were madeto measure continuously the various beam-beam modes andin particular the  and  modes using a small but controlledexcitation of the beams [3]. This instrument will be furtherdeveloped in the future and promises to become a good toolfor performance optimisation.5 ENERGY CALIBRATIONA large amount of time was devoted to improve the knowl-edge of the beam energy at high energy. For LEP200 thebeam energy is obtained by interpolation up to 91.5 GeV ofprecise resonant depolarization calibrations in the range of40 to 55 GeV using Nuclear Magnetic Resonances probesand flux measurement loops. The accuracy of this proce-dure depends on the highest energy at which resonant de-polarization is possible as well as on the lever arm of cali-brated energies. Between 1996 and 1997 the lever arm wasincreased from 5 to 14 GeV and the highest calibrated en-ergy was increased from 50 to 55 GeV [5].6 CONCLUSIONHigher bunch currents and smaller horizontal beam sizesmade it possible to boost the LEP performance by morethan a factor two between 1996 and 1997. Record beam-beam tune shifts over 0.05 were observed. A further re-duction of the transverse impedance in 1998 and the use ofan optics with smaller horizontal emittance could furtherimprove the performance in the coming years.7 REFERENCES[1] R. Bailey et al., “Operational Procedures to Obtain HighBeam-beam Tune Shifts in LEP Pretzel Operation”, Proc.of the 1995 Particle Accel. Conf., Dallas, USA, May 1995.[2] C. Bovet et al., “Luminosity Optimisation Using Beam-beam Deflections at LEP”, Proc. of the 5th EPAC, Sitges,Spain, June 1996.[3] O.Berrig et al., “Real-time monitoring of beam-beam modesat LEP”, these proceedings.[4] H. Burkhardt, “Energy Dependence of Beam-beam Interac-tions at LEP”, Proc. of the 1997 Particle Accel. Conf., Van-couver, Canada, May 1997.[5] M. Placidi et al., “Recent Results on Energy Calibration atLEP”, these proceedings.-50-2502550-10 0 10 20 30 40Separation Change ∆y (µm)Beam-beam Defl. θ bb (µrad)Correction Optimum :  14.9 +-  0.2 µmσx = 194 +- 3 µmσy =  3.7 +- 0.2 µmIb e+/e- =   383/  446 µA ξx/ξy =  0.023 / 0.040Figure 5: Example of a beam-beam deflection scan used tooptimise the setting of the vertical separators. The beamsizes at the collision point xand yas well as the beam-beam tune shifts xand yare extracted from the fit to thedata (line).

LEP Performance at 91.5 GeV

LEP Performance at 91.5 GeV

Abstract

Similar works

Full text

Available Versions

CERN Document Server