A technique for splitting bunches into three has recently been proposed as an alternative to debunching-rebunching in a synchrotron[1]. The preservation of longitudinal emittance and the possibility of maintaining a gap in the bunch train are key features expected from this new process. A proof of principle has been established experimentally at the CERN PS in 1999. This showed that the nominal longitudinal characteristics of the proton beam for the LHC could be achieved at the exit of the PS with the help of triple splitting. The method is described and experimental results presented. Other possible applications are sketched

Garoby, R

Hancock, S

Vallet, J L

Vallet, JL

CERN Document Server

Demonstration of bunch triple splitting in the CERN PS

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN - PS DIVISIONCERN/PS 2000-038 (RF)DEMONSTRATION OF BUNCH TRIPLE SPLITTING IN THE CERN PSR. Garoby, S. Hancock, J.L. ValletAbstractA technique for splitting bunches into three has recently been proposed as an alternative todebunching-rebunching in a synchrotron[1]. The preservation of longitudinal emittance and thepossibility of maintaining a gap in the bunch train are key features expected from this new process.A proof of principle has been established experimentally at the CERN PS in 1999. This showed thatthe nominal longitudinal characteristics of the proton beam for the LHC could be achieved at theexit of the PS with the help of triple splitting. The method is described and experimental resultspresented. Other possible applications are sketched.7th European Accelerator Conference, 26 - 30 June 2000, Vienna, AustriaGeneva, Switzerland2 August 2000DEMONSTRATION OF BUNCH TRIPLE SPLITTING IN THE CERN PSR. Garoby, S. Hancock, J.L. ValletCERN, Geneva, SwitzerlandAbstractA technique for splitting bunches into three hasrecently been proposed as an alternative to debunching-rebunching in a synchrotron[1]. The preservation oflongitudinal emittance and the possibility of maintaininga gap in the bunch train are key features expected fromthis new process. A proof of principle has beenestablished experimentally at the CERN PS in 1999. Thisshowed that the nominal longitudinal characteristics ofthe proton beam for the LHC could be achieved at theexit of the PS with the help of triple splitting. The methodis described and experimental results presented. Otherpossible applications are sketched.1  INTRODUCTIONIn order to meet the stringent requirements of the LHCconcerning transverse and longitudinal beam emittance,important modifications have been made and new rfhardware has been installed in the PSB and PS[2,3,4]. Inthe transverse phase planes, the specified beamcharacteristics have been obtained at the exit of the PS[5].In the longitudinal phase plane, the emittance budget wasknown to be particularly tight because of debunching-rebunching[6] and, in practice, instabilities in thedebunched beam lead to a final emittance which is 40%above the specifications[3,4,5]. Consequently, a newscheme has been proposed which avoids debunching andachieves the required bunch spacing using a series ofsplitting processes. One such step is a new method fortriple bunch splitting.2  PRINCIPLE2.1  Multiple splitting schemeThe complete process is sketched in Fig. 1. Sixbunches delivered in two batches by the PSB are capturedon harmonic h=7 in the PS. Triple splitting is started assoon as the second batch is received, which provides18 consecutive bunches on h=21. The beam is thenaccelerated on this harmonic up to the 25 GeV flat-top,where each bunch is twice split in two to give72 consecutive bunches on h=84. Finally, these bunchesare compressed by stepping up, in 25 Ps, rf voltages at40 MHz (h=84) from 100 to 300 kV and at 80 MHz(h=168) from 0 to 600 kV. This leaves a 320 ns gap in thebunch train for the rise-time of the ejection kicker.Simulations show that the emittance blow-up isnegligible.Uncertainty in the performance of this scheme islimited to the triple splitting, since double splitting hasbeen amply demonstrated with excellent operationalexperience[3,4]. 2.2  Triple bunch splittingTriple splitting requires three simultaneous rfharmonics (h=7, 14 and 21). The voltages of these threecomponents and the corresponding evolution of thedistribution of particles in the longitudinal phase planeare represented in Fig. 2. A stable phase on h=21 and anunstable phase on h=14 coincide with the stable phase onh=7. Starting with h=7 alone, the effect of increasing thevoltages on h=14 and 21 is to flatten the bunch (t=7 ms inFig. 2). In phase space, two new stable points emergeclose to the initial one, encircled by 3 buckets. If the rateof change of the voltages is sufficiently slow, the particlesof the initial bunch are gradually captured in these newbuckets, whose area grows as the voltage decreases onh=7 and increases on h=21 (t=14 ms in Fig. 2). Usingnumerically determined laws of variation, the three areasare kept equal throughout the process, so that layers ofincreasing emittance in the initial bunch are progressivelypeeled off and accumulated evenly into the three newbuckets. Three equal bunches are finally obtained, eachTriple splittingat 1.4 GeVQuadruple splitting at 25 GeVPS injection:6 (4+2) buncheson h=7 in 2 batchesEmptybucketAcceleration of18 buncheson h=21PS ejection:72 buncheson h=84 in 1 turn320 ns beam gapFigure 1: Multiple splitting scheme for the LHCwith the same distribution of particle density as the initialone (t=25 ms in Fig. 2).2.3  BenefitsThe following beneficial consequences are expected:x instabilities should be less of a problem as the beam isnever debunched and its momentum spread is greater,x performance should be very reproducible thanks to thecontinuous action of a beam phase loop. 3  EXPERIMENT3.1  ImplementationThe 10 ferrite-loaded cavities of the PS are split into4 groups. During triple splitting, one group of 4 cavitiesis stopped and three pairs of cavities are active on h=7, 14and 21. At the end of the process, all groups are tuned onh=21 and the full rf voltage is available for acceleration.The low-level rf is based on the infrastructure developedfor the AD production beam[7]. Each cavity is driven bya dedicated Direct Digital Synthesizer (DDS) whoseharmonic is computer- controlled by a function generator.All DDS are driven by a common clock on h=128 andsynchronized with a common train at the revolutionfrequency whenever they are at an integer harmonic.A supplementary DDS is used as a phase reference forthe measurement of the beam phase. The beam phaseloop controls the h=128 synthesizer. It is kept lockedduring all the gymnastics, switching the reference DDSfrom h=7 to 21 when both spectral components of thebeam current are of similar amplitude (t=20 ms in Fig. 3).3.2  Results of triple splittingThe process has been adjusted at 1.4 GeV to simplifythe cycle and make better use of the dynamic range of thecavity voltage servo-system. After precise adjustment ofthe relative phases of the different harmonics, goodresults were obtained (Fig. 4). Emittance is indeed preserved and performance isreproducible with up to 4 bunches from the PSB. Thevariation in bunch population is only +/– 10% (Fig. 5). Figure 3: Beam spectral amplitudes onh=7 and 21 during triple splitting.Figure 2: Simulation of triple splitting.Figure 4: Triple splitting at 1.4 GeV in the PS.Figure 5: Splitting of 4 bunches into 12 at 1.4 GeV.In order to stabilize the beam and minimize bunchshape oscillations upon crossing transition, somecontrolled blow-up is applied at the injection energy.Four bunches of nominal intensity and 0.6 eVs emittanceare then accelerated to the high-energy flat-top withoutlosses or any measurable blow-up (Fig. 6).3.3  Status of the remaining gymnasticsThe synchronization at 25 GeV, which starts at thebeginning of the flat-top, is disturbed by phase shifts inthe amplifier chains during the voltage reductions prior tothe first double splitting from h=21 to h=42. Bunch shapeoscillations are triggered which degrade the final result.Nevertheless, both splitting steps have been demonstratedand 48 distinct bunches have been obtained (Fig. 7).Improvements to the low-level hardware are beingprepared. 4  FUTURE PLANSThe demonstration that small emittances can bemaintained up to 25 GeV makes other bunch trainsconceivable. For example, 120 ns gaps every 8 bunchesare feasible (Fig. 8), as is 50 ns between bunches. Suchpossibilities are especially important to help the SPS, andpossibly the LHC, study and fight against electron cloudinstabilities[8], although this has to be balanced againstsingle bunch instabilities in these machines.5  CONCLUSIONSTriple splitting has been successfully demonstrated inthe PS with one and four PSB bunches of the nominalintensity. The experimental results show that:x the nominal LHC bunches can be obtained thanks tothe small emittances obtained at 25 GeV,x exotic gymnastics are also feasible which will helpstudy and possibly fight against electron cloud effects,first in the SPS and later in the LHC.However, hardware improvements are still needed tobring the double splittings at 25 GeV to the proper levelof performance. Developments are in progress and testswith beam will resume in July 2000.REFERENCES[1] R. Garoby, “Bunch merging and splitting techniquesin the injectors for high-energy hadron colliders”,CERN/PS 98-048 (RF), publ. at HEACC98 (1998).[2] LHC Study Group, “The Large Hadron Collider –conceptual design”, CERN/AC/95-05 (LHC) (1995).[3] M. Benedikt (ed.), “The PS Complex as proton pre-injector for the LHC – design and implementationreport”, CERN 2000-003 (2000).[4] A. Blas, R. Cappi, R. Garoby, S. Hancock,K. Schindl, J.L. Vallet, “Beams in the CERN PSComplex after the rf upgrades for LHC”, EPAC98,Stockholm, (1998).[5] M. Benedikt et al., “The PS Complex produces thenominal LHC beam”, this Conference.[6] R. Garoby, “Longitudinal limitations in the PSComplex for the generation of the LHC proton beam,Particle Accelerators, 58 1-4, p.121 (1997).[7] R. Garoby, S. Hancock, A. Ozturk, J.C. Perrier,J.L. Vallet, “Preparation of the nominal productionbeam for the AD”, PS/RF/Note 99-01 (1999).[8] R. Cappi, R. Garoby, “Generation of gaps in thebunch train of the PS beam for LHC”, Chamonix XWorkshop, CERN-SL-2000-007 DI, p.155 (2000).Figure 7: Bunches at the end of the 25 GeVflat-top.Figure 6: Capture, splitting and accelerationup to 25 GeV of 4 PSB bunches.Figure 8: "Exotic" splitting giving 120 ns beam gapsDouble splittingat 1.4 GeVQuadruple splitting at 25 GeVPS injection: 7(4+3) bunches onh=7 in 2 batchesAcceleration of14 buncheson h=21PS ejection:56 buncheson h=84 in 1 turn7 x 4 empty buckets7 empty buckets120 ns beam gap

Demonstration of Bunch Triple Splitting in the CERN PS

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