Brunner, O C

Herr, Werner

Keil, Eberhard

Lamont, M

Meddahi, Malika

Poole, John

Schmidt, R

Verdier, A

Von Holtey, G

Zhang, C

CERN Document Server

EXPERIMENTS WITH BUNCH TRAINS IN LEPO. Brunner, W. Herr, G. von Holtey, E. Keil, M. Lamont, M. Meddahi, J. Poole, R. Schmidt, A. Verdier,C. Zhang, CERN, Geneva, SwitzerlandAbstractDuring 1994 tests were made to evaluate the possibility of oper-ating LEP with four equidistant bunch trains in each beam dur-ing 1995. A train will consist of up to four bunches with a bunchspacing of some 75 m. The bunch trains collide head-on. Theyare separated at the parasitic collision points on either side of theinteraction points by vertical electrostatic separator bumps. Theaccumulation of bunch trains in a single beam was studied withmagnetically simulated bumps. Following the installation of ad-ditional separators late in 1994, the parasitic encounters of thebunch trains near the interaction points were studied for one trainof electron bunches colliding with one train of positron bunchesin two diametrically opposite interaction points. The higher-order mode losses in the superconducting RF cavities were mea-sured for several arrangements of beams, trains and bunches. Thesynchrotron radiation background, enhanced by the off-centredbeams in the quadrupoles inside the separator bumps, is reducedby super-imposing asymmetric magnetic bumps such that the off-set of the incoming beams is reduced.I. INTRODUCTIONBunch trains have been considered for the CESR storage ringat Cornell University since 1990 [1], and are now used in op-eration [2]. Thereafter, bunch trains with a horizontal crossingangle were proposed for LEP [3]. Tests in 1993 revealed in-surmountable difficulties with the synchrotron radiation back-ground, caused by the off-centred beams in the quadrupoles nextto the LEP detectors [4]. A proposal [5] which overcomes thisparticular difficulty is described and the results of tests in 1994are presented. The main changes are an increase of the bunchspacing and a vertical separation scheme which starts beyondthe quadrupoles next to the LEP detectors. The orbital effectsand background were studied with magnetic bumps, using verti-cal correctors. With up to four trains in a single beam, possiblebunch current limitations, due to collective effects and higher-order mode losses in superconducting RF cavities were investi-gated. During several technical stops, the extra vertical electro-static separators were installed [6]. Tests with colliding bunchtrains are described and the conclusions are given.A Bunch Train in LEP consists of up to four bunches. Thespacing between bunches, about s = 74 m, is much smaller thanthe spacing between trains. During tests in 1994, one train ofelectron bunches was collided with one train of positron bunchesin two diametrically opposite interaction points, IP4 and IP8. Thebunches were vertically separated at the three parasitic collisionpoints near by, on either side.II. CONCEPTWith the equipment installed for the tests, one bunch train ineach beam could be used. The number of bunches within thetrains and their spacing is determined by the requirement for suf-ficient separation at all parasitic encounters and the constraintfrom the LEP experiments that trains should not exceed a totallength of 750 ns [7]. This spacing must be a multiple of rfandthe final choice was a spacing of 87 rf.A. Separator bumpsThe separator bumps in the even insertions allow the beamsto be separated over the whole length of the bunch train duringinjection, ramping and squeezing as well as bringing them intocollision at the interaction point while keeping the beams sepa-rated at the parasitic encounters.In order to minimize the necessary changes, all existing verti-cal separators and 4 new ones were used [6]. At each experimen-tal interaction point six separators provide a symmetric electro-static bump over a length of about 240 m (Figure 1).ee +-Existing separatorsIPNew separators New separatorsFigure. 1. Separation bump in the even points at injection, fullline shows positron orbit and dashed line electrons. The two in-ner separator pairs are used to cancel the separation for physics.B. Side effectsThe amplitude of the electrostatic separator bumps shouldbe large enough to minimize the effects caused by the parasiticbeam-beam collisions, i.e. the beam-beam kicks, the beam-beamtune shifts, and their consequences. The bump amplitude shouldbe small to minimize the vertical dispersion Dyand the result-ing vertical beam emittance y. Practically all these effects alsodepend on the direction of the bumps in IP4 and IP8 – bumps ofopposite direction were used to minimize them. In Pits 4 and 8,jDyj is less than 1.4 mm, which at an rms momentum spread of1.210 3 results in an increase of the vertical beam size of theorder of 3%, which is negligible.Different bunches meet the bunches of the train(s) in the op-posite beam at the head-on collision points in IP4 and IP8, and atthree different parasitic collision points in their vicinity. Hence,different bunches receive different beam-beam kicks, travel ondifferent closed orbits, and all their orbital parameters are dif-ferent. In particular, the simulated vertical orbit position y andthe vertical orbit slope y0 at IP4 and IP8 are different for all fourbunches in a train. The orbit difference between the highest andthe lowest bunch is y  2:3 m whilst the nominal verticalbeam radius is y 8 m. The difference between the high-est and lowest slope is y0  60 rad which is still small com-pared to the crossing angle ' above which synchro-betatron res-onances [8] are excited: ' = y=s 0:5 mrad, where s15 mm is the bunch length. The expected tune and chromaticitysplits between the bunches for 0.3 mA/bunch are Qx 0:003,Qy 0:005,Q0x 0:01, Q0y 2:0. BothQxand Qyshould be small compared to the synchrotron tuneQs 0:065 inorder to avoid synchro-betatron resonances. We attribute occa-sionally observed losses on individual bunches to the chromatic-ity splits.III. SINGLE BEAM EXPERIMENTSIn order to obtain information on how the separator bumps forthe bunch trains affect the beam behaviour, magnetic correctorswere first used to excite bumps which follow as closely as possi-ble the proposed electrostatic bumps. The required aperture andoptical effects were studied. Bump amplitudes up to 2.7 times thenominal value did not affect the injectionor the beam lifetime [9].The reduced aperture at the beam dump in point 5 was also stud-ied and with bunch train bumps of 1.8 times the nominal valueno effects on the lifetime, the intensity and the background wereobserved [9]. Long bumps extending into the RF cavitites weresuccessfully tested and no effects were observed on the beam.If the relative bump polarity is well chosen, the residual verticaldispersion can be minimized,During one experiment, the verti-cal rms dispersion was reduced from 48 cm to 20 cm by usingalternating polarity.The injection and accumulation in a single beam were testedby injecting one bunch of each train at a time. The maximum to-tal intensity was obtained when bunches are filled in order, start-ing at the head of the train. The first injection of four trains offour bunches spaced by 87rfreached a total DC current of 5mA. After the installation of additional separators, further singlebeam studies were performed. With four trains of four bunches,320A per bunch were accumulated, limited by transverse modecoupling instabilities (c.f single bunch limit for one beam of630 A). The trailing bunches suffered more and saturated atlower levels. The losses were negligible when the beam wasramped to 45.6 GeV and squeezed; the emittances were measuredto be x= 30 nm and y= 3 nm, and there were no notice-able variations amongst the bunches in a train. Longitudinal os-cillations, larger for the trailing bunch, and a variation of the syn-chrotron tune along the trains were observed. These effects werepredicted from the beam loading and RF modulation.IV. HIGHER-ORDER MODE LOSSESThe superconducting (SC) RF cavities are equipped withhigher order mode couplers which can extract a few hundredwatts of RF power which is sufficient for four or eight equidis-tant bunches in each beam. The damping due to HOM cou-01234233-1233-2233-3233-4273-1273-2273-3273-4673-1673-2673-3673-4673-9673-10673-11673-12633-5633-6633-7633-8HOM Power Relative to Incoherent Power(Power/Incoherent Power)1T of 4 in 1 beam2T of 4, each in 1 beam4T of 4 in 1 beamPincohCavity NumberLevel corresponding tocoherent addition of HOM'sLevel corresponding to incoherent addition of HOM'sFigure. 2. HOM power measured on 20 cavities with differentsingle beam bunch train configurations.plers is, however, too small to damp the HOM losses betweenbunches in a train. Therefore the knowledge of the HOM lossesand frequencies in each cavity becomes crucial since one cannot exclude, by design, that any arbitrary bunch spacing is anexact multiple of the wavelength of a higher order mode. Whenthis happens, the HOM fields of the k bunches of a train mustbe added coherently, instead of simply adding their power (in-coherent addition). The same argument holds for the counterrotating beams since the bunch trains in the e+ and e  beamspass through the cavities too close in time for damping betweenpassages.The results of measurements of the HOM power on 20 SC RFcavities are shown in Figure 2; there is no indication of full posi-tive coherent addition of fields. The HOM power is always aboutor even below the incoherent value, except for two cavities wherethe power corresponds to about twice that for simple addition.With the configuration installed in 1994, the effect of the twocounter-rotating beams could not be checked because the trainscrossed several kilometers away from the RF stations.V. COLLIDING BUNCH TRAINSWith standard injection conditions the bunch current for onetrain of four bunches per beam was limited to around 300 A.Following a low intensity trial during the setup period, the firstattempt to establish colliding bunch trains successfully brought279 - 247 - 221 - 230 A of positrons and 302 - 303 - 316 -281A of electrons into collision. After appropriate orbit correc-tions the collimators were moved into position allowing ALEPHand DELPHI to take data. The lifetimes were good and the beamsizes comparable to normal 4 bunch operation.Physics conditions were established three more times in thefollowing three weeks [9] during which time the luminosity andbackground were optimised. Luminosity scans using the data foreach collision within a train were performed. Figure 3 shows theluminosity for the crossings of bunches a, b, c and d during such ascan. The results confirmed the expected variation in vertical off-sets caused by the beam-beam kicks from the parasitic encountersand both experiments gave consistent values for the luminositiesof the order of 5 - 61030cm 2s 1.Figure. 3. Luminosity variation during a vertical separator scanin ALEPH.The experiments had minimal problems and were able to fullyexplore their ability to take data with colliding bunch trains. Pre-cise luminosity measurements were achieved for each bunch andmeasured online. Online tagging of the bunch number can also beobtained with high efficiency. The loss efficiency for the ALEPHhadron calorimeter and muon detector, a potential source of con-cern, appeared to be small and acceptable.The combination of the vertical separation bump and poweringthe skew quadrupole magnets caused extremely large beam sizes("x100 nm) when the beams were brought into collisionduringphysics operation with 4-on-4 bunches. The skew quadrupolesare inside the separation bumps and are used to compensate thesolenoidal field of the experimental detectors. This effect was notobserved with collisions only in points 4 and 8 during the tests,only when the bunch train bumps were on, the skew quadrupolesin 4 and 8 were powered and there were collisions in all 4 exper-iments. It was found that removing any one of these three con-ditions reduced the beam size to normal values.VI. BACKGROUNDMeasurements with two colliding bunch trains [9] have shownno increase of the off-momentum background in the experimen-tal detectors. However, a steep increase of the synchrotron radi-ation (SR) photon background has been observed in ALEPH andDELPHI for vertical bump amplitudes larger than 10 mm (Fig-ure 4). Monte Carlo (MC) simulations confirm that this is dueto photons, radiated from the peak of the bump. These photonsstrike the jaws of downstream vertical collimators and are backscattered into the detectors. For bump amplitudes below 10 mmmost photons from the peak of the separator bump pass the in-teraction point without hitting downstream collimators and onlya small increase of the photon background with amplitude is ob-served. The SR photon background with large separation bumpscan be substantially reduced by superimposing asymmetric mag-netic bumps in such a way that the amplitude of the incomingbeam is reduced below the critical value of 10 mm, while keepingthe separation constant. The increased radiation of photons alongthe larger amplitude of the outgoing beam does not increase the 0 5 10 1510 0 10 1 10 2 10 3 10 4 bump amplitude at QS4  (mm)photons/ BX*mA DELPHI   ALEPH  MC Figure. 4. Measured and simulated (MC) SR photon back-ground in ALEPH and DELPHI as function of the separation.SR rate, as these photons only strike the vacuum system a longway from the IP. The effect of the asymmetric correction bumphas been experimentally verified [9].VII. CONCLUSIONSThe results of the tests in 1994 were judged encouragingenough for LEP to be adapted during the 1994/95 shutdown, foroperation with four trains in each beam. Extra electrostatic sep-arators were installed near IP2 and IP6, the separators near theodd pits were moved [6] and the skew quadrupole magnets weremoved outside the separation bump. The application softwareand databases were modified and most of the other equipmentgroups also had to make changes to accommodate bunch trainoperation [9].References[1] R.E. Meller, ‘Proposal for CESR Mini-B’, CESR Opera-tions Note CON 90-17 (1990).[2] D. Rubin for CESR Operations Group, ‘CESR Status andPlans’, this conference.[3] E. Keil, ‘Lattice and Interaction Region Design for Z Fac-tories’, Proc. of the Joint US-CERN School, Benalmadena,Springer, 1994 - (Lecture Notes in Physics No. 425) 106-126.[4] R. Bailey et al., ‘Studies of Bunch Trains in LEP’, Proceed-ings of the 4th EPAC (1994) 445[5] W. Herr, ‘Bunch Trains without a Crossing Angle’, inProc. 4th Workshop on LEP Performance, ed. J. Poole,CERN SL/94-06 (DI) (1994) 323-332.[6] B. Goddard et al.,‘Modification of the LEP ElectrostaticSeparator Systems for Operation with Bunch Trains’, thisconference.[7] T. Camporesi et al., ‘Report from the Bunch Train Work-ing Group’, CERN/LEPC 94-13 (1994), CERN/LEPC 94-16 (1994).[8] A. Piwinski, ‘Satellite Resonances due to Beam-beam In-teraction’, IEEE Trans. Nucl. Sci. NS-24 (1977) 1408.[9] C. Bovet et al., ‘Final Report of the 1994 Bunch TrainStudy Group’, CERN/SL 94-95 (AP) (1994).

Experiments with bunch trains in LEP

Experiments with bunch trains in LEP

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