Prior C R

Rees G H

Schonauer H O

English

Prior, C R

Rees, G H

Schönauer, Horst Otto

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Synchrotron-Based Proton Drivers for a Neutrino Factory

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SYNCHROTRON-BASED PROTON DRIVERS FOR A NEUTRINOFACTORYC.R. Prior & G.H. Rees, CLRC, Rutherford Appleton Laboratory, Chilton, Didcot, U.K.H.O. Scho¨nauer, CERN, Geneva, SwitzerlandAbstractSynchrotron-based proton drivers for a neutrino factory areunder study at the Rutherford Appleton Laboratory (RAL),complementing a similar investigation using a high energylinac plus storage rings at CERN. Of the two sets of de-sign parameters being considered, one is based on foursynchrotron bunches per pulse at a final energy of 5 GeVand 50 Hz; the other is at 25 Hz with six bunches per pulseat 15 GeV. Both scenarios provide 4 MW of beam power.Bunch compression to 1 ns rms at the pion target is the mostchallenging aspect, and the driver parameters are dictatedby this requirement. The paper includes brief details of thetwo scenarios, covering the choice of low energy linac andthe rationale for the synchrotron lattices; injection, trap-ping and acceleration are described; and possible ways ofachieving the final bunch compression are discussed. Sim-ulation results are outlined, showing the feasibility of theproposals.1 INTRODUCTIONTwo rapid cycling synchrotron options are under study atRAL as a possible proton driver for a neutrino factory. Thefirst option (Driver 1, Figure 1) has an output energy of5 GeV at 50 Hz, and the other (Driver 2, Figure 2), 15 GeVat 25 Hz. Both must provide 4 MW of beam power withfinal output bunch durations of 1 ns rms. The 5 and 15 GeVoptions provide four and six bunches per cycle, respec-tively, and there is the potential for an upgrade to 6 MWfor the 15 GeV case.The energy of 5 GeV has been selected for study as thisis the lowest energy at which it appears practical to achievethe specified final synchrotron bunch durations. A lowerenergy of 2.2 GeV is under consideration at CERN, how-ever, in a scheme involving a full energy linac, a largecircumference accumulator ring and a separate compres-sor [1]. RAL collaborates with CERN on the design ofthese 2.2 GeV, 75 Hz rings.For the synchrotron options, a low linac injection en-ergy has been chosen to assist in achieving the bunch du-ration specifications. The scheme proposed for Driver 1uses a 180 MeV H  linac to feed two 50 Hz, 1.2 GeV syn-chrotrons, operating almost in phase; together these feedtwo 25 Hz, 5 GeV rings in alternate cycles; the combinedoutput, after bunch compression, is at 5 GeV and 50 Hz.For Driver 2, the basic scheme remains the same but the50 Hz, 1.2 GeV rings are replaced by a pair at 25 Hz and3 GeV, and the 25 Hz, 5 GeV rings by a pair at 12.5 Hz and15 GeV, with a combined output at 25 Hz. All rings use    180 MeV, 280 MHz H- LinacAchromatic arcfor collimationMomentum ramping 1.2 GeV, 50 HzBoosterSynchrotron5 GeV, 25 Hz RapidCycling Synchrotron.4 bunches of 2.5 1013protonsTwo rings, each,stacked verticallyFigure 1: Layout of the 5 GeV, 50 Hz, 4 MW proton driver.biased sine wave guide fields.The choice of the number of bunches per ring and the rfharmonic numbers is discussed in x2.2, together with theinjection and acceleration systems. Similar magnet latticesare chosen for the 1.2 GeV and 3 GeV booster synchrotronsto accommodate the H  injection scheme. The triplet lat-tice has been developed for the ESS [2] accumulator, and asimilar type is also considered for the CERN 2.2 GeV ac-cumulator. Simpler doublet lattices are proposed for the 5and 15 GeV rings, and for the CERN 2.2 GeV compressor,where a triplet is also a possibility. Final bunch compres-sion is addressed in x2.3.2 THE RAPID-CYCLINGSYNCHROTRON OPTIONS2.1 Injector LinacAn energy of 180.2 MeV has been chosen for the 57 mA,H  injector linac, allowing small longitudinal bunch areasin the rings at reasonable transverse space charge levels.The injector has an overall length of 129 m and consistsof a 2.493 MeV, 280 MHz RFQ, a chopper section and aneight tank, 280 MHz Alvarez linac. Each of the eight tanksrequires a 3 MW peak power rf (radio frequency) genera-tor, including 25% additional power for field control. Thechopping section provides a 70% beam duty cycle at a sub-963Proceedings of EPAC 2000, Vienna, Austria    180 MeV, 280 MHz H- LinacTwo rings each,stacked vertically15 GeV, 12.5 Hz Rapid CyclingSynchrotron6 bunches of 1.11 1013 protons3 GeV, 25 HzRapid CyclingSynchrotronAchromatic arcfor collimationMomentumrampingFigure 2: Layout of the 15 GeV, 25 Hz, 4 MW protondriver.harmonic of 280 MHz which is close to the rf frequency ofthe booster rings (RCS1) at injection.Between the linac and rings is a beam line that providessome debunching of the linac microbunches, subsequentmomentum reduction, collimation and ramping, and ver-tical beam separation for each RCS1. Separate 280 MHzcavities are required for each stage. The four period,41.6 m, achromatic collimator bends the H  beam through180 and includes some reverse bends and two bunchercavities. A range of fractional momentum ramping up to4 10 3 is provided, with normalised transverse rms emit-tances of approximately 0:26 ()rad.m.2.2 Injection, Trapping and AccelerationThe use of booster and main rings allows the differentrequirements of injection and bunch compression to betreated separately. A triplet lattice has been adopted forthe boosters, providing regions of high dispersion for injec-tion and momentum collimation, and long dispersion-freestraights for the rf systems, betatron collimation and fastextraction. Typical lattice functions are shown in Figure 3.Driver 1 has three superperiods and mean radius 32.5 m,and Driver 2 has four superperiods and radius 50 m. In-jection is via an Al2O3stripping foil in a low field dipole( 0:05T) where the normalised dispersion Dh=phis inthe range 1.6-1.8. 160 turns are injected into Driver 1 over200s and 70 turns into Driver 2 over 134s. The inter-vals are timed symmetrically about the minimum of the ac-celerating wave-form, injecting into a decelerating bucketat the start of injection and an accelerating bucket at theend. Momentum painting of longitudinal phase space im-proves the accumulated beam distributionand enhances thebunching factor. Vertical orbit bump magnets and disper-sion coupling into the transverse horizontal plane controlthe transverse distribution and the parameters are chosen to0 20 40 60  0.00  5.00 10.00 15.00 20.00 25.00 30.00DISTANCE (m)HV0 20 40 60-0.75 0.75 2.25 3.75 5.25 6.75DISTANCE (m)DISPERSION (m) DHFigure 3: Lattice functions for the booster synchrotrons.reduce subsequent traversals of the stripping foil by recir-culating protons. Excessive foil temperatures are therebyavoided.Low loss levels are required to permit hands-on main-tenance in the rings. H  and proton interactions with thefoil account for about 0.01%; otherwise the main source ofloss stems from the mechanism for trapping the beam in thefirst stages of acceleration. The latter can be eliminated us-ing a suitably programmed system of rf voltages, with an rffrequency range allowing limited radial steering after injec-tion. A single harmonic h = 2 system with maximum peakvoltage ^V = 0:28MV is used in Driver 1, while Driver 2has h = 3 and ^V = 0:4MV. Two bunches each of 2:5 1013protons and three of 1:11 1013 protons are accumulated inthe drivers respectively. At ejection, the respective bunchdurations are 100 ns and 50 ns.2.3 Final Bunch CompressionFor each driver, the bunches in the two vertically-stackedbooster rings are extracted and transferred together to ei-ther the upper or the lower of the two main synchrotrons inalternate cycles. Each transfer line contains a pulsed dipolemagnet to provide vertical beam splitting.The design of the main synchrotrons is dictated by therequirements of the final 1 ns rms bunch compression. Ex-tensive simulation studies have suggested that this can beachieved by working close to transition energy. The latticerequirement that t> at top energy has been adoptedProceedings of EPAC 2000, Vienna, Austria964to meet this condition, avoiding crossing transition andany subsequent instabilities. Simple doublet lattices areproposed with small transverse space charge tune shifts,and the dispersion functions are designed to resist depres-sion of tas the bunches compress (Figure 4). Inductiveimpedances Zk=n 5-10 are envisaged for the metallicand ceramic vacuum chambers to reduce longitudinal spacecharge effects.0 40 80 120  0.00  5.00 10.00 15.00 20.00 25.00DISTANCE (m)HV0 40 80 120-0.40 0.40 1.20 2.00 2.80 3.60 4.40DISTANCE (m)DISPERSION (m) DHFigure 4: Lattice functions for the main synchrotrons.Acceleration is achieved by cavities with programmed rffields. For Driver 1, where the rings contain four bunches,these are at h = 8 with maximum ^V = 0:575MV. Addi-tional h = 24 cavities are brought into play over the last1 ms of the cycle, with peak total voltage rising linearlyto 0.5 MV. Simulations show a final rms bunch duration of1 ns, with peak momentum spread p=p 1.6% and lon-gitudinal emittance L1.0 eV.s. In the case of Driver 2,h = 36 cavities with ^V rising to 1.7 MV in mid-cycle, thenfalling to 0.6 MV, give similar results with the final condi-tions achieved from adiabatic bunch compression.A consequence of working close to transition is that sec-ond order momentum effects need to be taken into ac-count. With a particle’s increased path length defined byL = L0(1 + 0 + 12+ O(3)), where  = p=p, thechange in revolution period istt=0 12+1 02+2+ 2222+O(3):In the CERN 2.2 GeV compressor ring, the first term dom-inates. However, for the two synchrotron-based protondrivers, both terms are comparable. An arrangement ofsextupole magnets has been devised to correct the chro-maticities and simultaneously over-compensate 1, therebyeliminating both second order path-length and kinematiceffects.3 SUMMARYA summary of the main parameters of the two drivers isgiven in Table 1.In a proton driver of fixed power, the product of the out-put energy and the repetition rate offers a crude figure ofmerit for the pion target. This suggests some bias towardsthe 15 GeV option. A further advantage of Driver 2 is thatthe main ring would fit into the CERN ISR tunnel.Table 1: Main parameters of the two synchrotron drivers.Parameter Driver I Driver IIBooster Synchrotrons:Kinetic energy (MeV) 180! 1200 180! 3000Number of bunches 2 3Bunch intensity 2:5 1013 1:1 1013No. of injected turns 160 70Injection period (s) 200 134Mean radius (m) 32.5 50.0-transition 4.73 5.97Normalised dispersionDh=phat foil (m1=2) 1.6 1.8Qh, Qv4.24, 4.35 5.70, 4.32Harmonic number 2 3Peak rf voltages (MV) 0.28 0.40Main Synchrotrons:Kinetic energy (GeV) 1.2!5.0 3.0!15.0Number of bunches 4 6Mean radius (m) 65.0 150.0-transition 6.50 17.14 2.28!6.33 4.20!16.99Qh, Qv7.78, 5.80 20.41, 15.24Qh, Qv-0.07, -0.13 -0.02, -0.02Chromaticities -1.15, -1.21 -1.15, -1.31Harmonic numbers 8 and 24 36Peak rf voltages (MV) 0.58/0.5 1.7REFERENCES[1] R. Cappi et al: “Design of a 2.2 GeV Accumulator and Com-pressor for a Neutrino Factory”. Proceedings of this confer-ence.[2] G. Bauer, T. Broome, D. Filges, H. Jones, H. Lengeler,A. Letchford, G. Rees, H. Stechemesser, G. Thomas (eds):“ESS: A Next Generation Neutron Source for Europe. VolIII: The ESS Technical Study”. November 1996.965Proceedings of EPAC 2000, Vienna, Austria

Synchrotron-based proton drivers for a neutrino factory

Synchrotron-based proton drivers for a neutrino factory

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