Autin B

Cappi R

Chanel M

Gareyte J

Garoby R

Giovannozzi M

Haseroth H

Hofmann I

Koscielniak S

Martini M

Metral E

Mohl D

Prior C R

Rees G H

Schindl K

Schonauer H

English

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DESIGN OF A 2.2 GEV ACCUMULATOR AND COMPRESSOR FOR ANEUTRINO FACTORYR. Cappi, B. Autin, M. Chanel, J. Gareyte, R. Garoby, M. Giovannozzi, H. Haseroth, M. Martini,E. Me´tral, D. Mo¨hl, K. Schindl, H. Scho¨nauer, CERN, Geneva, SwitzerlandI. Hofmann, GSI, Darmstadt, GermanyC. Prior, G. Rees, RAL, Chilton, Didcot, U. K.S. Koscielniak, TRIUMF, Vancouver, CanadaAbstractThe proton driver for a neutrino factory must providemegawatts of beam power at a few GeV, with nanosecondlong bunches each containing more than 1 × 1012 protons.Such beam powers are within reach of a high-energy linac,but the required time structure cannot be provided with-out accumulation and compression. The option of a linac-based 2.2 GeV proton driver has been studied at CERN,taking into account the space charge and stability prob-lems which make beam accumulation and bunch compres-sion difficult at such a low-energy. A solution featuringtwo rings of approximately 1 km circumference has beenworked out and is described in this paper. The subjects de-serving further investigation are outlined.1 THE H− BEAM DELIVERED TO THEPROTON DRIVERSIn the scenario presented in this paper, the beam deliveredto the accumulator and compressor (see Table 1) is gen-erated by the proposed H− Superconducting Proton Linac(SPL) [1] based on the use of the 352 MHz LEP cavi-ties. The driver beam parameters are mainly dictated bythe properties of the systems used to generate the pions(produced by the proton beam), and to collect and cool themuons (produced by the pion decay) to be used in neutrinogeneration.For this, a new and promising scenario [2] is under studyat CERN. It is proposed to use 40 and 80 MHz RF sys-tems to capture, cool and phase-rotate pions and muons,downstream of the target. In contrast to induction linacscenarios [3], the number of proton bunches per pulse is notlimited. This has deep implications on the design of the ac-cumulator and compressor, as it opens up the possibility ofincreasing the number of bunches in the machine, thus re-ducing the intensity per bunch. There is a beneficial impacton many parameters in this scenario: the linac duty cycle,ring lattice, instability rise-times and bunch compression.2 PROTON DRIVER RINGS2.1 General LayoutThe geometry of the proton driver rings is determined bythe preference that has been given to install the two ma-chines in the former ISR tunnel. This tunnel has 150 mmean radius, with a width of about 15 m and height of 4 m.Table 1: Summary of linac output beam characteristics.Parameter Unit ValueMean beam power MW 4Kinetic energy GeV 2.2Rep. rate Hz 75Pulse duration ms 2.2# of micro-bunch trains × turns 140× 660Pulse intensity 1.5× 1014Train spacing ns 22.7# of micro-bunches/train 5Micro-bunch spacing ns 2.84Micro-bunch intensity 3.3× 108Micro-bunch length ns 0.5Energy spread (2σ) MeV 0.2Energy jitter within pulse MeV ±0.2Energy jitter between pulses MeV ±2∆p/p (2σ) 0.08× 10−3l eVs 0.2× 10−3∗h/v = β γ σ2h,v/βh,v µm 0.6The choice of performing the bunch compression bymeans of a non-adiabatic process, imposes the use of twoseparate rings: one for accumulation of the linac micro-bunches and one for compressing the resulting macro-bunches. The RF cavities needed to perform the bunch ro-tation would be a potential source of problems if installedin the accumulator ring. Their high impedance and the un-avoidably long filling time could drive beam instabilitiesduring the accumulation period. The beam characteristicscommon to the two rings are given in Table 2.The accumulator lattice has been designed at RAL. Allthe optics computations for the compressor have been car-ried out at CERN by using different codes [4, 5] includingspace charge effects.2.2 Accumulator RingThe accumulator ring is fed directly by the linac througha transfer line where beam collimation is also performed:140 buckets out of 146 available are filled using an H− in-jection scheme over 660 turns. Five linac micro-bunches,chopped from eight, are injected into each bucket. The ringis equipped with a 44 MHz RF system of the type devel-921Proceedings of EPAC 2000, Vienna, Austriaoped for LHC in the CERN-PS [6]. The RF voltage is in-creased during beam accumulation to compensate continu-ously the longitudinal space charge. Injection simulationspredict four subsequent proton foil traversals, on average,after the initial H− beam stripping, resulting in acceptablefoil temperatures for the carbon stripping foil. The simula-tions also show that the fast synchrotron motion due to thelarge |η| helps obtaining smooth macro-bunches at the endof the process.Table 2: Summary of output beam characteristics commonto accumulator and compressor rings.Parameter Unit ValuePulse duration µs 3.3RF freq. fRF MHz 44.02# of bunches/h 140/146Bunch intensity protons 1.1× 1012l eVs 0.1∗h/v = β γ σ2h,v/βh,v µm 50The lattice of this machine, based on triplet cells, hasbeen designed to optimise the H− injection. In particu-lar, the value of the dispersion function at the stripping foilhas been adjusted to allow ease of painting. The values ofthe optical parameters for half a super-period are shown inFig. 1, while the main lattice parameters are summarised inTable 3.10 20 30 40 50 60s m510152025, D mFigure 1: Optical parameters for the accumulator ring (βhsolid, βv dashed, Dh dotted line).Table 3: Summary of geometrical and optical parametersof the accumulator and compressor ring.Parameter Accumulator CompressorRadius (m) 151 151Main dip. field (T) 0.69 0.49γtr 14.837 15.089η −0.0848 −0.0849βhmax/βvmax (m) 26.4/26.2 24.5/25.5Dminh /Dmaxh (m) 0.0/4.6 0.0/1.8Qh/Qv 11.23/13.30 17.18/16.40Super-symmetry 8 8Vac. pipe half width (m) 0.09 0.09Vac. pipe half height (m) 0.09 0.09# of dip.-quad./super-per. 3− 22 5− 17Length of s.s./super-per. (m) 73.4 86.22.3 Compressor RingOnce the accumulation process is over, the 140 macro-bunches are transferred to the compressor ring where theirlength is reduced by bunch rotation in less than 30 µs (seeFig. 2). For that purpose the compressor ring is equippedwith four cavities at 44 MHz [6], providing a total volt-age of 2 MV, and with one cavity at 88 MHz [7] deliver-ing 350 kV. Prior to beam transfer, RF power is applied tothese cavities at a slightly offset frequency, so that the fullvoltages are present when the beam enters the machine. Assoon as beam circulates, a peak beam current of about 15 Aflows in the cavities. Since the bunch compression is muchfaster than the cavity filling time (100 µs), the phase shiftdue to the transient beam loading increases almost linearlyFigure 2: Bunch rotation in compressor ring. The phaseportraits are shown in the upper part (initial left, final right).The projections along the direction of the azimuthal angle θare shown in the lower part (total horizontal scale ±1.23◦,1◦ = 9.21 ns).with time and can be compensated by an appropriate fre-quency difference between generator and beam frequen-cies (≈ 20 kHz at 44 MHz). The compression processbrings the bunches from the estimated initial r.m.s. lengthof 3.5 ns (∆p/p is 1.5×10−3 at 2σ) to the final one of 1 ns(∆p/p is 5.0× 10−3 at 2σ).The lattice of the compressor machine, based on doubletcells, has been designed to maximise the free space in thestraight sections to allow the installation of the RF cavities.To this aim, the dispersion function is zero in the straightsections and special care has been taken to minimise thevalue of Dmaxh to reduce the transverse beam size of the ro-tated bunch. The values of the optical parameters for halfa super-period are shown in Fig. 3, while the main latticeparameters are summarised in Table 3.2.4 Space Charge and Other Collective EffectsCollective effects have been studied for both linac micro-bunches and macro-bunches at the end of the accumulationand compression processes. The transverse incoherent tuneshift (for a particle at the centre of a transverse Gaussian,Proceedings of EPAC 2000, Vienna, Austria92210 20 30 40 50 60s m510152025, D mFigure 3: Optical parameters for the compressor ring (βhsolid, βv dashed, Dh dotted line).and longitudinally parabolic, bunch) of the macro-bunch atthe end of the compression process is about −0.2, whichis within the classical synchrotron limit. The RF voltageneeded to match the macro-bunch at the end of the ac-cumulation, including longitudinal space charge, is about250 kV.At the end of the accumulation, the Sacherer for-mula [9] for the macro-bunch interacting with the trans-verse resistive-wall impedance (using a resistivity of10−6Ωm), yields rise-times of about 400 ms for the head-tail single-bunch instability, and 10 ms for the coupled-bunch instability. The growth-times are long compared tothe 2.2 ms accumulation time.The transverse mode-coupling instability [10] as well asthe beam-breakup instability [11, 12] have also been in-vestigated and are not harmful. The longitudinal coupled-bunch instability has been studied using the known HigherOrder Modes (HOMs) of the CERN-PS 40 MHz RF cav-ity [8]. The computed rise-times [13] of the damped HOMsare much longer than the accumulation time. Finally, thelongitudinal microwave instability has been studied usingthe coasting-beam formalism with the bunch peak current.In this computation, a broad-band impedance has been as-sumed at the cut-off frequency of the vacuum chamber(≈ 530 MHz) with a unity quality factor. The plot of therise-time vs. the broad-band impedance Zbb/n is shownin Fig. 4. The accumulator ring should be carefully de-signed to have a broad-band impedance Zbb/n ≤ 1 Ω. It2 4 6 8 10Zbbn ohms0.20.40.60.81Risetime msFigure 4: Microwave-instability rise-time (neglecting Lan-dau damping) for the macro-bunch at the end of the accu-mulation process vs. Zbb/n.can be seen from Fig. 4 that in this case, the rise-time isabout 0.6 ms, which is to be compared with the accumu-lation time (2.2 ms). Although the microwave instabilitycould be a source of concern at first sight, preliminary sim-ulation results show that it should not be a problem (tails ofthe distribution provide Landau damping): the value of 1 Ωcan be considered to be a conservative estimate.3 CONCLUSIONS AND OUTLOOKThe study of a 2.2 GeV accumulator-compressor for a neu-trino factory has shown that such an option is very promis-ing, and no major obstacles have been found. The variousparameters are not frozen, as their values depend on keycomponents downstream of the proton drivers, such as tar-getry and cooling system.Future activity will be devoted to refining the parame-ter list and to determining the optimal relative location ofthe two rings (whether they should lie on the same planeor should be separated vertically) with special emphasis oninjection/extraction sections from the accumulator ring tothe compressor ring.More detailed investigations will be performed, bymeans of both analytical and numerical tools, to understandbetter the effects of the microwave instability and to inves-tigate collective effects induced by electron clouds.Finally, a campaign of numerical simulations will be car-ried out to study the issue of halo formation and beamlosses in both rings.4 REFERENCES[1] R. Garoby, M. Vretenar, “Status of the Proposal for a Super-conducting Proton Linac at CERN”, CERN PS (RF) 99-064(1999).[2] A. Lombardi, “A 40-80 MHz System for Phase Rotation andCooling”, CERN NuFact Note 20 (2000).[3] R. Scrivens, “Example Beam Dynamic Designs for a Neu-trino Capture and Phase Rotation Line Using 50 m, 100 m,200 m Long Induction Linacs”, CERN NuFact Note 14(2000).[4] B. Autin, C. Carli, T. D’Amico, O. Gro¨bner, M. Martini,E. Wildner, CERN 98-06 (1998).[5] P. Bryant, WinAgile, private communication.[6] R. Garoby, D. Grier, E. Jensen, “The PS 40 MHz bunchingcavity”, CERN PS (RF) 97-39 (1997).[7] D. Grier, E. Jensen, R. Losito, K. Mitra, “The PS 80 MHzcavities”, CERN PS (RF) 98-021 (1998).[8] E. Jensen, R. Hohbach, A. K. Mitra, R. L. Poirier, inEPAC’96, edited by J. Poole et al. (IOP, London, 1996)p. 2068-70.[9] F. J. Sacherer, in Int. Conf. on High Energy Accelerators,edited by US Atomic Energy Commission (CONF 740522,Washington D.C., 1974) p. 347-51.[10] R. D. Kohaupt, “Head Tail Turbulence and the TransversePETRA Instability”, DESY 80-22 (1980).[11] K. Yokoya, “Cumulative Beam Breakup in Large-ScaleLinacs”, DESY 86-084 (1986).[12] D. Brandt, J. Gareyte, in EPAC’88, edited by S. Tazzari(World Sci., Singapore, 1989) p. 690-92.[13] F. J. Sacherer, in PAC’77, edited by IEEE (Trans. Nucl. Sc.,24, 1977) p. 1393-95.923Proceedings of EPAC 2000, Vienna, Austria

A 40-80 MHz System for Phase Rotation and Cooling”,

Cumulative Beam Breakup in Large-Scale Linacs”,

Example Beam Dynamic Designs for a Neutrino Capture and Phase Rotation Line Using 50 m, 100 m, 200 m Long Induction Linacs”,

Design of a 2.2-GeV accumulator and compressor for a neutrino factory

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