Following an intensive MD program in 1994, a bunch train scheme was adopted as the operational mode for LEP. The configuration was used throughout 1995 and produced record luminosities. The year culminated in a high energy bunch train run which produced encouraging results for LEP2. In spite of this, the bunch train scheme met with varying degrees of success and the overall performance was not as good as expected. The performance of the machine is presented, together with the problems encountered and the various optimisation techniques used. The performance of related hardware and instrumentation is discussed

Collier, Paul

Goddard, B

Lamont, M

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EUROPEAN LABORATORY FOR NUCLEAR RESEARCHCERN - SL DIVISIONCERN-SL-96-22 OPOPERATION OF LEP WITH BUNCH TRAINSP. Collier, B. Goddard, M. LamontAbstractFollowing an intensive MD program in 1994, a bunch train scheme wasadopted as the operational mode for LEP. The configuration was usedthroughout 1995 and  produced record luminosities. The year culminated in ahigh energy bunch train run which produced encouraging results for LEP2.In spite of this, the bunch train scheme met with varying degrees of successand the overall performance was not as good as expected. The performance ofthe machine is presented, together with the problems encountered and thevarious optimisation techniques used. The performance of related hardwareand instrumentation is discussed.Paper presented at the 5th European Particle Accelerator Conference (EPAC’96),10-14 June 1996, Sitges (Barcellona), Spain.Geneva, Switzerland26 June, 1996OPERATION OF LEP WITH BUNCH TRAINSPaul Collier, Brennan Goddard and Mike Lamont, CERN, Geneva, SwitzerlandABSTRACTFollowing an intensive MD program in 1994, a bunchtrain scheme was adopted as the operational mode forLEP. The configuration was used throughout 1995 andproduced record luminosities. The year culminated in ahigh energy bunch train run which produced encouragingresults for LEP2.In spite of this, the bunch train scheme met with varyingdegrees of success and the overall performance was not asgood as expected. The performance of the machine ispresented, together with the problems encountered and thevarious optimisation techniques used. The performance ofrelated hardware and instrumentation is discussed.1. INTRODUCTIONThe bunch train scheme for LEP was partially developedin 1994 [1], and then commissioned operationally in 1995[2]. The two prime motivating factors in developing thescheme were:1. To increase the luminosity at Z0 energies byincreasing the number of bunches;2. To raise the maximum bunch current in 8 bunchesper beam for operation at W pair energies [3].The 8 bunch Pretzel scheme [4] was limited at injectionenergy to bunch currents significantly less than the 1mAhoped for. For 45 GeV operation this limitation was nota significant problem, since the beam-beam effect limitedthe bunch current to around 350 m A, well below themaximum attainable at injection energy with pretzel.The plan was to operate LEP in 1995 with fourequidistant trains of bunches in each beam. Each train wasabout 220 m long and consisted of up to four bunches.The results from MD in 1994 had led to high expectationsof possible deliverable luminosity. The results were notas good as expected.  2. CONFIGURATIONThe bunch train scheme used electrostatic separators toprovide extended local separation bumps around theinteraction point (IP) such that: a) trains of up to fourbunches, separated by 87 l RF could be accommodated; b)all bunches in the counter-rotating e+ and e- beams wereseparated at all encounters at injection energy and duringthe energy ramp; c) separation was provided for allparasitic encounters during physics; d) collisions couldtake place at the four experimental IPs e) a vertical‘vernier’ bump could be superimposed at these points toallow fine adjustment of the collisions. The bunch trainbump in an experimental IP is shown in figure 1.- 8-1212- 4 ZL7 ZL4 ZL7ZL2 ZL4+--++-+-++840IPZL2e-e+Orbit excursion  mmFigure 1 : The bunch train separation scheme for oneexperimental interaction point, with collisions at the IP.3. OPERATION WITH BUNCHTRAINS  IN 19953.1 CommissioningDuring the commissioning of the bunch train schemeparticular attention was given to the control of radiationto the experiments at injection, bunch train bump closureat 20 and 45 GeV and the background seen by theexperiments.The first physics runs with two bunches per train wereperformed with a total beam current of 3 mA. During thisperiod problems with separator sparking caused many fillsto be lost. Many studies were also undertaken to try andunderstand a significant non-closure of the bumps, both atinjection and during physics. Injection efficiency wasrather low and the radiation doses received by theexperiments seemed to be sensitive to the bumpamplitude in the non-experimental interaction regions.Later, an aperture restriction was found in one of theseregions, caused by a mis-aligned vacuum chamber.The logbook during this period reads like a litany ofvernier optimisations, background spikes, poor lifetimesand coherent oscillations as operations struggled to masterbunch train running.  However by the end of this periodthe typical total currents were around 4.5 mA and thebeam-beam tune shift ( x y) was above 0.03 [5].Although the performance was improving, it was still notoptimal with two bunches per train.  It was clear that thepush had to be towards trains of three and four bunches.As proof of principle a small number of fills wereperformed with three bunches per train. These attemptswere reasonably successful with around 6 mA total beamcurrent, peak luminosities of 1.2 1031 cm-2s-1 and a x y ofaround 0.02. Thus encouraged, trains of four bunches wereattempted. During the last week of running before thetechnical stop, extra damping wigglers werecommissioned and used to accumulate 9 mA.  This couldbe ramped to physics energies, but many problems wereencountered going into collision. Low lifetimes wereobserved, especially for the outermost electron bunches.Six physics fills were performed during this week: all buttwo had missing bunches. Typical physics conditionswith the full compliment of 32 bunches saw currents ofaround 250 m A /bunch (~6mA total), peak luminosities of1.5 1031 cm-2s-1 and a x y of around 0.02.In a little over a month and a half, operation with twobunches per train had been successfully proven manyproblems resolved. Principle among these were: sparking,mis-alignments, radiation at injection and vernierscanning. Operation with three bunches per train had beendemonstrated, and the first dogged attempts with trains offour had been made.3.2  Energy ScanFor the energy scan of LEP in 1995 it was decided to usethree bunches per train, after several more failed attemptsto use the full four bunch per train configuration. Thebunch configuration and other parameters of the machinehad to be fixed in order to provide a stable platform for thepainstaking process of scanning.Concentrating on a single configuration allowed a slowbut steady improvement in the performance, and by themiddle of the period over 8 mA were regularly beingbrought into collision. Typical initial luminosities werearound  1.5 1031 cm-2s-1, corresponding to a x y of about0.03. The bunch train scheme was at least equalling thebest performance of pretzel [6], and peak luminositiescontinued steadily to evolve, as shown in figure 2.05101520251-May-95 31-May-95 30-Jun-95 30-Jul-95 29-Aug-95 28-Sep-95 28-Oct-95P k30 cm-2 s-1Figure 2 : Evolution of peak luminosity with bunchtrains through the commissioning and energy scanperiods.The main bunch train related problem during this periodwas the continued experimental background storms,apparently related to beam instabilities. Towards the endof the period things had settled down so much that theprinciple question had become: “why is the x y so low?”(Recall that x y = 0.045 had been achieved with pretzel, andsteady running above x y = 0.04 was common.)Steady running continued until the end of dedicated Zophysics with special attention paid to regular vernier scans[7] and, by varying the RF frequency between scans,measurement of residual vertical dispersion at eachexperimental IP [8].The bunch train scheme at this point was a qualifiedsuccess. A lot of effort had gone into operating LEP in acompletely new way, resulting in a reasonable perform-ance. However, the effects of parasitic encounters [9,10]had lead to the abandonment of four bunches per train, andeven with three bunches per train had resulted in lowerbeam-beam tune shifts than expected.3.3 The High Energy RunThe final running period of LEP was planned atintermediate energies, between 65 and 70 GeV per beam.The commissioning of the machine for higher energieswas very quick and within four days the first 65 GeVphysics run took place. Because of a cautionary totalbeam current limit imposed by the RF group operationwas with one bunch per train.In the final physics fill of 1995 a current, of 5.6 mA wasput into two bunches per train and collided at 68 GeV.This was a big success, producing peak luminosities of3.4 1031 cm-2s-1. In another experiment using two bunchesper train at 45 GeV, values of x y in excess of 0.04 wererecorded, due to the ability to collide both bunches headon in this configuration [11]. 4. HARDWARE PERFORMANCE4.1Electrostatic SeparatorsWith bunch train all 40 separators are operated at highvoltage during physics. This increased the probability ofseparator sparking and related beam loss, and  systematicsparking was observed in several separators, cured eitherby operating with positive high voltage only, or byreducing the bunch train bump. The high energy run wasan important test of the sensitivity of the separators tohigher energy photons ( e c = 230 keV at a beam energy of68 GeV), with only one separator spark recorded despitehigh radiation doses of over 107 rad measured at someseparator locations [12].4.1 Radio FrequencyBunch train operation allowed high currents to beaccumulated which facilitated the debugging of the newsuperconducting RF systems. Higher order modemeasurements showed that full coherent addition of cavityfields did not occur, although in some cases the higherorder mode power exceeded that expected from addition ofcavity powers. The longitudinal feedback system ran witha fixed bunch spacing of 87 or 174 l RF the optimum forfour bunches per train. For 1996 this will be changed to118 l RF to allow operation with two bunches per train.The modified transverse feedback system performed verywell, making accumulation easier and preventing beamlosses at the start of the ramp. The system was also usedin physics to prevent background bursts in the LEPexperiments.4.3  Beam InstrumentationThe bunch current transformers used fast-samplingoscilloscopes to allow measurement of single bunchcurrents, and the new bunch current equaliser wasindispensable for filling the machine. It was demonstratedthat a rapid optimisation of luminosity could be made byscanning the beam separation whilst measuring the orbitdistortion due to the beam-beam effect [13]; this will beimplemented in 1996.The synchrotron radiation telescopes were able to measurebeam sizes of individual bunches in a train, although theabsolute calibration of the instrument was only as good(or as bad) as the knowledge of the dispersion and betafunctions at the instrument location. More realisticinformation should be available in 1996. The polarimeterworked well, although synchrotron radiation causeddamage to a dielectric mirror and out-gassing from thesupport structure, and these components have beenupgraded for 1996.5. CONCLUSIONSInitial tests in 1994 with two trains of four bunchescolliding in two interaction points had raised expectationsof the potential of bunch trains to almost hystericallevels. The reality, necessarily, fell short.First, a totally new way of running the machine had to bemastered by the operators. New methods of optimisationhad to be developed. These concerned not just luminosity,but backgrounds and radiation. At the end of the day,however, it was felt within operations that the bunch trainscheme was in fact easier to control than pretzel. Secondlykey equipment, such as separators, were pushed to newlimits, forcing the understanding of their performance up asteep learning curve. Thirdly, theoretical and practicalunderstanding of the many side effects that arise fromcolliding bunch trains, in particular the effects of theparasitic encounters, had to be mastered. This inevitablytook some time.However, a radically new way of running LEP wascommissioned successfully. It was challenge to theequipment groups, beam instrumentation, operations,accelerator physicists and management, and as suchstimulated frank open discussion, improvements andunderstanding. Not withstanding the lower than expectedluminosity yield, the final results of the year bodeextremely well for LEP2 [14].6. ACKNOWLEDGMENTSThe work of all members of the 1995 bunch train studygroup is gratefully acknowledged, as are the considerableefforts made by the equipment and operations teams.REFERENCES[1] Final report of the 1994 bunch train study group,C.Bovet et al., CERN SL/94-95 (1994).[2] Report of the 1995 bunch train study group, C.Bovetet al., CERN SL/96-12 (1996).[3] Bunch trains for LEP2, E.Keil. Proceedings of thefifth workshop on LEP performance, Chamonix1995, CERN SL/95-08 (1995).[4] The 8-bunch pretzel scheme, J.Jowett. Proceedings ofthe first workshop on LEP performance, Chamonix1991, CERN SL/91-23 (1991).[5] Performance in physics, H.Burkhardt. Proceedings ofthe sixth LEP performance workshop, CERN-SL-96-05(DI) (1996).[6] Review of bunch train running (1995) and pretzelrunning (1994), R.Bailey. Proceedings of the sixthLEP performance workshop, CERN-SL-96-05(DI)(1996).[7] Systematic analysis of the vernier scans during theLEP 1995 run, P.Puzo. To be published.[8] Measurement of collision offsets and difference invertical dispersion at the LEP interaction points,M.Böge et al. These proceedings.[9] Side effects of bunch trains with actual separatorbump amplitudes, E.Keil. CERN SL Note 95-88(AP) (1995).[10] Truly self-consistent treatment of the side effects ofbunch trains, E.Keil. CERN SL/95-75 (1995).[11] Removal of the beam-beam tune shift limit for bunchtrains, K.Cornelis et al. CERN SL-MD Note 190(1995).[12] Separator performance with bunch trains and pretzel,B. Goddard. Proceedings of the sixth LEPperformance workshop, CERN-SL-96-05(DI) (1996).[13] Measurement of beam-beam deflections at theinteraction points of LEP, J.Wenninger, CERNSL/96-01 (1996).[14] Luminosity expectations at 90 GeV, J.Gareyte.Proceedings of the fifth workshop on LEPperformance, Chamonix 1995, CERN SL/95-08(1995).

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