Since 1995 LEP is operated with the new bunch train scheme. This scheme allows head-on collisions of four trains of up to four bunches within a train. The first experience with this new scheme and the problems encountered during the commissioning and the operation are reviewed and discussed. The performance of LEP and the results from dedicated experiments are shown and compared with expectations. The modifications and improvements to allow a successful operation at LEP2 energies are discussed and the performance at energies above 80 GeV is presented

Goddard, B

Herr, Werner

Keil, Eberhard

Lamont, M

Meddahi, Malika

Peschardt, E

CERN Document Server

Experience with Bunch Trains in LEPW. Herr, B. Goddard, E. Keil, M. Lamont, M. Meddahi and E. PeschardtCERN, Geneva, Switzerland1 ABSTRACTSince 1995 LEP is operated with the new bunch trainscheme. This scheme allows head-on collisions of fourtrains of up to four bunches within a train. The first expe-rience with this new scheme and the problems encounteredduring the commissioning and the operation are reviewedand discussed. The performance of LEP and the resultsfrom dedicated experiments are shown and compared withexpectations. The modifications and improvements to al-low a successful operation at LEP2 energies are discussedand the performance at energies above 80 GeV is presented.2 INTRODUCTIONThe bunch train scheme for LEP was developed in 1994[1, 2, 3], and then commissioned operationally in 1995 [4,5, 6]. The two motivating factors for the scheme were: To increase the luminosity at Z energies by increas-ing the number of bunches To raise the maximum current per bunch in 8 bunchesper beam for operation at W energies.The 8 bunch pretzel scheme [7, 8] was limited, at injec-tion energy, to bunch currents significantly less than theexpected 1mA. For operation at 45 GeV this limitation wasnot a problem since the beam-beam effect limits the bunchcurrent to around 350 A, which is easily attainable at in-jection energy with pretzel. However at higher energies, itis desirable to collide much larger intensities and this lim-itation becomes a problem. It follows that a new schemeshould be flexible enough to permit an optimization of thenumber of bunches and bunch intensity, depending on theconstraints and requirements.The plan was to operate LEP in 1995 with four equidis-tant trains of bunches in each beam. The number ofbunches per train is determined by the maximum lengthof the train, limited by the separation scheme and experi-mental constraints [5]. Simultaneously, the scheme was op-timized and modified for running at LEP2 energies whereeight bunches per beam and higher intensities per bunchwere envisaged.3 CONFIGURATION IN 1995The 1995 bunch train scheme in LEP used electrostatic sep-arators to provide extended local separation bumps aroundthe interaction points (IP) such that: a) trains of up to fourbunches separated by 87 RFcould 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) bunches in the counter-rotating e+and e  beams were separated at all parasitic encountersat physics energy; d) collisions could take place betweene+ and e  bunches at the four experimental IPs; e) a ver-tical ‘vernier’ bump could be superimposed at these pointsto allow adjustment of the collision for luminosity optimi-sation. In each such pit six separators are necessary, sincethe beams need to be brought into collision at the IP whileremaining separated at the parasitic encounters, essentiallycreating a closed electrostatic 3 ”corrector” bump on eachside of the IP. The small closed vernier bump was superim-posed using two of the separator pairs.Each of the odd (non-experimental) pits was equippedwith four electrostatic separators, to create an extendedbunch train bump which separated all e+ and e  bunches[2, 6].The direction of the separation bumps can be chosenfreely but since some of the side effects of the bumps, (e.g.dispersion and orbit effects, see later section), can accu-mulate or cancel depending on the relative direction of theorbit distortion, the directions of the separation bumps werechosen to minimize these effects by a partial compensation.4 SIDE EFFECTS AND BEAM DYNAMICS4.1 Vertical dispersionThe vertical separation bumps induce a residual verticaldispersion proportional to the bump amplitude which mustbe kept as small as possible to avoid an increase of the ver-tical emittance or the excitation of synchro-betatron reso-nances in the RF cavities. An insufficient separation how-ever, would lead to other effects, i.e. large beam-beam tuneshifts and beam-beam induced orbit effects. This wouldresult in low life-times and reduced luminosity. A compro-mise has to be found to meet all requirements simultane-ously.4.2 Effects from parasitic beam-beam interactionsFurther insight into the side effects of the parasitic en-counters can be gained by a first-order calculation, startingwith the vertical orbits caused by the electrostatic separa-tor bumps. The vertical orbit kick, y0, the horizontal andvertical beam-beam tune shifts, xand y, at a parasitic en-counter are given by:y0=  2Nredx=Nrex2d2y=  Nrey2d2(1)The separation between the beams at the parasitic en-counter is d. It is assumed that the vertical r.m.s. beam ra-dius is much smaller than the separation at the parasitic en-counter, y d; N is the intensity of the opposite bunch,reis the classical electron radius, and  is the usual rela-tivistic factor. Any vertical orbit kick y0 causes a verticalorbit distortion y and a vertical orbit slope y0 at any obser-vation point around LEP which are given by the standardequations for the closed orbit and its slope. The closed or-bit position and slope of a bunch are obtained by adding thecontributions of all parasitic encounters with the bunches ofthe counter-rotating beam.In general, different bunches in different trains meet thebunches of the opposite beam at different parasitic encoun-ters. Therefore, different bunches travel on different ver-tical orbits and have different vertical slopes around LEP.Hence, different vertical collision offsets y and differentslopes y0 exist between any two bunches colliding at thehead-on interaction points. From symmetry arguments itis evident that, for an ideal machine without imperfectionsand equal bunch populations, the first bunch of a train hasan orbit offset of the same magnitude and opposite signas the last bunch of the equivalent counter-rotating bunchtrain. Similar arguments hold for each bunch of a train, re-sulting in an asymmetric orbit for the bunches along a train.It is easy to remove the average vertical offset by vernieradjustments, but it is impossible to remove the spread in thevertical offset between the bunches. For trains of only twobunches the above mentioned symmetry allows a vernieradjustment to collide both bunches of a train head on, al-though not on the same orbit.Not only at the interaction point the orbit of a bunch ischanged, the separation at a parasitic encounter is also af-fected and such a change of separation is not taken into ac-count in the perturbative approach. A self consistent treat-ment of the problem becomes necessary.4.3 Self consistent calculationThe first-order calculation mentioned does not include theconsequences of the beam-beam interaction at the parasiticencounters. These effects are included in a self consis-tent computation which is embedded in a computer pro-gram train[10]. It finds the individual closed orbits ofall bunches, as well as their vertical dispersion, tunes andchromaticities. The understanding and evaluation of theside effects via the self consistent calculation was impor-tant in understanding some of the limitations of the scheme.The comparison is made for bunch trains of three bunchesper train since most of the time LEP was operated with suchtrains and experimental data is available.4.3.1 Self consistent orbitsTab. 1 shows the results of calculations of the separation syat the collision point in m for the three bunches, labelleda, b and c in a typical bunch train in the even-numberedpits. The vertical separation syis symmetrical between theleading and trailing bunches in a train as expected. Themeasurement of the vertical separation between bunches inTable 1: Self consistent results for the separation syin mfor the three bunches in a train. The bunch current is I =0:25 mA, the beam energy is E = 45:6 GeV.Bunch IP2 IP4 IP6 IP8a 1.42 -1.59 1.82 .15b .32 .07 .61 2.09c 1.42 -1.59 1.82 .15a train is a by-product of the luminosity optimisation byvernier scans. Fig. 1 shows the results of a typical scan.The difference between the optimal position for familiesa, b and c gives a measure of the shape of the trains andthe width of the scan a measure of the vertical beam sizewhich is significantly larger than the separation betweenthe bunches. A rather good agreement between the calcu-lated and measured results was found [6].c024681012-7.9 -2.5667 2.7667 8.1Vernier setting ( µm)Luminosity (1030 cm-2  s-1 )IP2      IP2   Scan Families012345670 10Vernier setting ( µm)Luminosity (1030 cm-2  s-1 )IP4      IP4   Scan 012345678-6.1 -0.7667 4.5667 9.9Vernier setting ( µm)Luminosity (1030 cm-2  s-1 )IP6      IP6   Scan 01234567810 20Vernier setting ( µm)Luminosity (1030 cm-2  s-1 )IP8      IP8   Scan a bFigure 1: Vertical scans to optimize luminosity and to mea-sure individual orbits4.3.2 Tune and chromaticity splitsSimilar differences between the bunches within a train ex-ist for the tunes, the chromaticities and the dispersion. Typ-ically one finds in the calculation splits up to 0.02 for thefractional part of the tune for bunch currents of 0.5 mA. Thechromaticity spread can reach values up to Q0  1.0.These calculations have been confirmed by measurements[6].5 PERFORMANCE5.1 Life timeIn the original design it was foreseen to operate LEP withfour bunches per train, i.e. each individual bunch havingthree parasitic encounters at each interaction region. Thetypical separation d at the three encounters in an even inser-tion are shown in Tab.2. Also shown are the calculated cor-responding horizontal and vertical beam-beam tune shiftsTable 2: Separation and normalized beam-beam tune shiftfor parasitic encountersEncounter Separation x/y[10 3]1 10-12 mm 2.7/1.02 15-20 mm 0.3/1.53 5-7 mm 1.1/7.5at 45.6 GeV and for 0.5 mA bunch current. It can be ob-served that the encounter farthest from the interaction pointexperiences a much larger vertical shift, caused by the sig-nificantly smaller separation [6]. During the running periodwhen LEP was operated with four bunches per train, fre-quent life time problems were experienced for the bunchescorresponding to this close encounter. As a consequence,it was decided to abandon the fourth bunch and continuethe operation with three bunches per train, thus avoidingthe encounter with the smallest separation. Furthermore,since the contribution of the 4th bunch to the spreads ofthe tunes and chromaticities is large, the overall spreadswere also reduced. The machine was much easier to oper-ate with these shorter trains and the life-time of all remain-ing bunches was acceptable [6].5.2 Luminosity and beam-beam tune shiftWhen LEP was operated with three bunches per train, theluminosity was not fully up to the expectations and partic-ularly the beam-beam tune shift achieved was lower thanwas hoped for. Values between 0.025 and 0.030 were thebest found during the year. This should be compared withtune shifts of 0.03 to 0.04 regularly obtained with fourbunches and the Pretzel scheme with 8 bunches, and withbest values of around y 0.045.It was already demonstrated (Fig.1 and Tab.1) that with 3bunches per train the bunches do not collide head on. It wasbelieved that the lower beam-beam tune shift was causedby the offset collison. In a dedicated a run with only twobunches per train where all bunches can be collided headon, the beam-beam tune shift quickly reached values above0.040 with a maximum at 0.045.6 EXPERIENCE AT LEP26.1 Configuration in 1996Unlike LEP running at 45.6 GeV, LEP2 is not beam-beamlimited and it is advantageous to concentrate the availableintensity into fewer bunches. While the aim at LEP1 was toincrease the number of bunches the main issue at higher en-ergies is to remove the intensity limits at injection and runwith a smaller number of bunches. The original schemewas designed that it could operate with any number ofbunches between one and the maximum of four per trainand the bunch spacing was kept flexible to optimize it forthe number of bunches, however fulfilling the constraintsdictated by the hardware. For the first runs of LEP2 in1996, no hardware modifications were necessary on theseparation scheme but the bunch spacing was increased tominimize the residual beam-beam effects from unwantedparasitic beam-beam encounters [9]. The chosen spacing of118 RFis compatible with the existing longitudinal feed-back system and has a minimum impact on the performanceof the orbit measurement system.6.2 Luminosity and intensityIn 1996 LEP was run at two energies: 80.5 and 86.0 GeV.During most of the year, the total current was limited torather low values due to RF considerations and thereforethe machine was operated with single bunch trains, i.e. fouron four bunches. Furthermore, several low emittance lat-tices were tried [11] with varying success. However a fewruns were made with trains of two bunches and the resultswere very promising. The beam-beam tune shift achievedwas the same as for single bunches at equivalent bunch in-tensities and the resulting luminosity was as expected. Thetotal current was always limited due to the commissioningof the large LEP2 RF system.In a dedicated experiment [12] the maximum intensity atinjection was studied for different RF configurations and nobunch train related problems were found up to intensitiesabove 0.550 mA per bunch, where the intensity could notbe further increased due to RF limitations. This is a verypromising result for a good luminosity in future runs.7 REFERENCES[1] W. Herr; Bunch trains without a crossing angle; Proc.4th workshop on LEP performance, (Chamonix 1994)CERN SL/94-06 (DI) (1994) 323.[2] C. Bovet et al., Final report of the 1994 bunch train studygroup; CERN SL/94-95 (AP) (1994).[3] O.C. Brunner et al., Proc. 1995 Part. Accel. Conf. (Dallas1995) 514.[4] P. Collier, B. Goddard and M. Lamont, CERN-SL/96-22 (OP), Proc. 5th Eur. Part. Accel. Conf. (Sitges 1996)[5] C. Bovet et al., Report of the 1995 bunch train study group;CERN SL/96-12 (AP) (1996).[6] B. Goddard et al.; Bunch trains for LEP; Submitted to Part.Accelerators, CERN SL/97-02 (AP) (1997).[7] C. Rubbia, Proc. 1st Eur. Part. Accel. Conf. (Rome 1988)290.[8] J.M. Jowett, W. Kalbreier and S. Myers, Proc. 2nd Eur. Part.Accel. Conf. (Nice 1990) 403.[9] W. Herr; Bunch trains at high energy; Proc. 6th work-shop on LEP performance, (Chamonix 1996) CERN SL/96-05 (DI) (1996) 125.[10] C. Iselin, private communication.[11] Y. Alexahin et al., these proceedings.[12] D. Boussard et al., More tests on Injection In-tensities with Various RF Configurations; CERN SL-MD Note 233 (1997).

Experience with Bunch Train in LEP

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