There is a strong interest from the physics community for high-quality, high-intensity neutrino beams as produced by a neutrino factory. With the help of other European laboratories, CERN has started a study on some of the many technological challenges of such a facility. Present ideas concerning the proton driver, as well as the muon accelerator complex, are presented and plans for the future are described

Haseroth, H

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN - PS DIVISIONCERN/PS 2000-026 (PP)STATUS OF STUDIES FOR A NEUTRINO FACTORY AT CERNH. Haseroth (for the Neutriono Factory Working Group)AbstractThere is a strong interest from the physics community for high-quality, high-intensity neutrino beams as produced bya neutrino factory. With the help of other European laboratories, CERN has started a study on some of the manytechnological challenges of such a facility. Present ideas concerning the proton driver, as well as the muon acceleratorcomplex, are presented and plans for the future are described.7th European Particle Accelerator Conference, 26th-30th June 2000Vienna, AustriaGeneva, Switzerland11 July 2000STATUS OF STUDIES FOR A NEUTRINO FACTORY AT CERNH. Haseroth (for the Neutrino Factory Working Group)CERN, Geneva, SwitzerlandAbstractThere is a strong interest from the physics communityfor high-quality, high-intensity neutrino beams asproduced by a neutrino factory. With the help of otherEuropean laboratories, CERN has started a study onsome of the many technological challenges of such afacility. Present ideas concerning the proton driver, aswell as the muon accelerator complex, are presented andplans for the future are described.1  INTRODUCTIONThe production of neutrinos with accelerators (neutrinofactories) has gained considerable interest among itspossible future clients. The original interest started withthe study of muon colliders [1,2]. These colliders couldopen the way to lepton collisions at extremely highenergies. Circular electron colliders are limited in energydue to the high synchrotron radiation emitted by theelectrons. Although this radiation decreases with largerradius of the accelerator, it increases with the fourthpower of the relativistic J factor. For this reason it seemsunrealistic to build circular machines with higherenergies than LEP. The only possibility for higherenergies seemed to be linear colliders with all theirtechnical challenges. Another, more straightforwardsolution is the use of heavier leptons, as the synchrotronradiation at the same energy is much reduced because ofthe smaller J which is inversely proportional to the restmass. Muon beams seem to be possible candidates forthis purpose. Muons can be readily produced by thedecay of pions, which in turn can easily be produced bybombarding a target with high energy protons. The"only" remaining problem is the production of muonbeams with the required density in phase space,necessary for collider operation. In spite of someimpressive progress towards this goal, no technicallyfeasible solution has yet been found. A substantial R&Deffort will be required to make progress.With the apparent discovery of neutrino oscillationsand the increased interest in neutrino physics in general,the situation has changed drastically. What appeared tobe almost science fiction in the context of muoncolliders is now, due to the reduced requirements for aneutrino factory, much closer to reality. The R&D effortfor the muon colliders turned out to be very useful for aneutrino factory, and the increased interest from thephysics side has produced a high activity on theaccelerator side, so that considerable progress has beenmade towards a neutrino factory. It is interesting to notethat in turn a part of this progress is also beneficial forthe design of a muon collider.2  THE BASIC CONCEPT OF THENEUTRINO FACTORYThe first part of the basic concept has been outlinedabove: a proton driver which delivers a high-powerproton beam onto a target; a system for collection of theproduced pions and their decay products, the muons. Theresulting muon beam has to be accelerated and, toachieve an optimum intensity, the energy spread and thetransverse emittance have to be reduced. This is done by“phase rotation” and ionisation cooling. Afteracceleration with a linac and “RLAs” (RecirculatingLinear Accelerators) the muons are injected into astorage ring or rather decay ring, where they decay inlong straight sections in order to deliver the desiredneutrino beams. Different proposals have been made,e.g. [3], the latest being a study initiated by FNAL [4].This study has the advantage that all subsystems havebeen (at least almost) properly engineered and it hasdemonstrated the technical feasibility of such a machine.Our reference scenario (Fig. 1) does not pretend to bethe best solution or to describe necessarily the machinewhich will finally be built. It is rather intended as aworking scenario.Figure 1- Possible Layout of a neutrino factoryThe first part of a neutrino factory is the "protondriver" which, in order to provide an intensity of 1021neutrinos/year, should deliver a beam power of the orderof 4 MW. In the CERN scheme we have selected a linearaccelerator with a beam energy of 2.2 GeV [5,6]. Thereasons for this choice are the availability of LEPcavities and klystrons when this machine is shut down,and the desire to have a higher brilliance for the PSbeam intended for the LHC. This linac accelerates Hions and operates at 75 Hz and a pulse duration of 2.2 mswith 11 mA. The beam is injected into an accumulator(charge exchange injection) and subsequently into acompressor ring to produce the necessary short bunchlengths of the order of nanoseconds. 140 bunches at afrequency of 40 MHz will be sent onto a target toproduce short pion bunches decaying into muons whichare captured, and reduced in energy spread and intransverse emittance. The muons are then accelerated to50 GeV and injected into a storage ring (decay ring)where they are kept for the duration of their lifetime(1.2 ms at this energy). The muons decaying in the longstraight sections of this ring produce the requiredneutrino beams. 3  THE MAIN SUBSYSTEMS 3.1  The LinacThe SPL (superconducting proton linac), actually anH linac, as proposed [5,6] is based on the idea of re-using the LEP cavities and ancillary equipment after itsdecommissioning. It would use the E=1 cavities of LEPin the energy range from around 1 to 2.2 GeV, andE=0.52, 0.7 and 0.8 cavities from 120 MeV to 1 GeV.The development of the E=0.7 and 0.8 cavities, beingvery much based on the original LEP cavities, a goodpart of the hardware can be recuperated. The E=0.52cavities, however, need some more development. Below120 MeV a drift tube linac will be used, being fed froman RFQ at 7 MeV. The RFQ will be split at 2 MeV forthe installation of a chopper, necessary for minimisingbeam losses at the high energy side. First engineeringlayouts on the CERN site have been made. 3.2  Accumulator and Compressor RingsThe CERN-specific 2.2 GeV/75 Hz scenario, using theSPL, features an Accumulator and a Compressor ring inthe ISR tunnel. Both rings have high-Jt lattices ensuringfast debunching of the linac microbunches, as well asvery fast bunch rotation (~7 turns) in the compressor.The feasibility of H injection and of the final bunchrotation has been shown. The accumulator lattice and thelattice of the intersecting compressor have been designed.More refined simulations, including the effect of spacecharge on momentum compaction and of the microwaveinstability, have given very satisfactory results [7]. Thelength of the ejected pulses is given by the circumferenceof the ring: 3.3 Ps. It was possible to increase the numberof bunches, originally 12 to 140 because of the use of rfcavities instead of an induction linac as in the FNALstudy. This helps with space charge problems.3.3  Alternative Proton DriversAs it may turn out that 2.2 GeV is not an ideal energyfor a neutrino factory, alternative scenarios to 4 MWproton drivers have been studied.In close collaboration with RAL, a scenario for a site-independent synchrotron was established. A 5 GeV/50 Hz and recently, also a 15 GeV/25 Hz scenario wasinvestigated, lattices were designed and H injection andfinal bunch compression was studied and shown to befeasible [8]. In case that low repetition rates would beneeded, we have explored a 30 GeV/8 Hz configuration,using the ISR tunnel for the driver [9]. The high-Jt-lattice of the latter provides naturally short buncheswithout compression.3.4  TargetThis device, which has to survive a 4 MW protonbeam, is an extremely critical item, although only asmall fraction of the beam power is lost in the target. Forthe moment we have kept only the specification of a4 MW beam power, as established at the Nufact99workshop in Lyon. No design has yet been made, butmodel tests are under way [10] for a pulsed liquid metal(Hg) target. So far the model has only operated withwater (similar viscosity), but tests are foreseen in thenear future with Hg. The next step will be to inject theHg jet into a high magnetic field (which maybenecessary for the collection of the pions) and/or byexposing it to the ISOLDE beam at CERN. This beam,which has only 1.4 GeV, can however be adjusted todeliver the same power density as in our favouredscenario. The repetition rate of course will be lower bytwo orders of magnitude. It might be worthwhile to pointout a few characteristics implied by our scenario(2.2 GeV, 75 Hz operation):x the energy dumped in the target will be higherthan in the case of a proton beam with higherenergy (typically by a factor 2).x the energy per unit volume is also likely to behigher.x however, the energy per pulse is reduced becausein our scenario the repetition frequency is higher.In case a low-Z material can be used (e.g. Li), a sub-stantial gain is expected for the production of positivepions. To profit from this fact, however, it is necessaryto switch from high Z to low Z between different runs.The HARP experiment [11] is intended to measure theproduction of pions and their characteristics for differenttarget materials. The results of this experiment may bedecisive for the future orientation of our study.3.5  Decay channel, phase rotation, cooling,capture and first accelerationThe pions produced in the target have to be collectedso that they are useful for the final production of a high-quality muon beam. The FNAL study and most otherproposals use a 20 T solenoidal field for this purpose.Hence we foresee tests of our Hg target with a highmagnetic field. Another possibility we explore is thepossibility of horn focusing, where CERN hasconsiderable experience. The critical issue is the highoperating frequency of 75 Hz. More calculations, andtests, are required to establish the feasibility. Theadvantage of a horn could be its lower price and perhapsbetter collection efficiency.The decay of the pions would take place in any case ina solenoidal channel where debunching occurs, i.e. thereis a correlation between position and energy of theresulting muons. This energy spread will be reducedwith rf cavities working with modest gradients in therange of 2-4 MV/m at 40-80 MHz. This rf scenario[12,13] is well adapted to our high repetition rate(75 Hz) and the large number of proton bunchesimpinging on the target (12 originally, now 140).Another advantage is that it works immediately with abunched beam that is adapted to further acceleration inthe subsequent linac. The characteristics of the rfcavities are similar to the American designs (closed withBe foils or grids), but have a more modest powerconsumption. The cooling follows basically the“standard” scheme of ionisation cooling, i.e. reduction ofthe total momentum of the muons (by ionisation lossesof the beam passing through liquid H2, LiH or Li) andreconstitution of the longitudinal momentum componentwith rf cavities. The following muon linac will havesolenoid focussing up to around 1 GeV beforeacceleration to 2 or 3 GeV, operating at a multiple of thefrequency used in the cooling channel. 3.6  Further Muon acceleration and Decay RingTwo RLAs take the muons from the first linac to afinal energy of 50 GeV, an energy which seems to befavoured by most of our possible future clients.Acceleration has to be fast, because of the limitedlifetime of the muons, and to accelerate with a normallinac would be very costly.To allow the muons to decay in a “useful” way, i.e. byproducing neutrino beams in the desired direction andwith a good quality, a muon storage ring operating at50 GeV with a triangular shape, extending also in thevertical dimension, has been designed [14]. It sendsneutrino beams from two long straight sections, in whichthe muon beam divergence is small, to two distantdetectors at 1000 and 3000 km distance. It is calculatedfor a normalised emittance of about 1.67 mm and an rmsmomentum spread of 0.5%. The verification of theoptical design included the tracking of many thousandsof muons during their lifetime. The total height of thering is less than 250 m and hence smaller than thethickness of the molasse near the CERN site. 4  CONCLUSIONS A preliminary scheme for a CERN neutrino factoryhas been proposed. It is intended to continue this studyand future work may well show that some elements ofthis scenario need substantial modification, or evenreplacement by other components. The results of theHARP [11] experiment expected for next year may alsoprovoke some modifications. 5  ACKNOWLEDGEMENTS The present work is the result of the effort of theNeutrino Factory Working Group at CERN. The help ofother laboratories, in particular RAL, but also GSI, FZJand CEA, and of members of the American NeutrinoFactory and Muon Collider Collaboration is gratefullyacknowledged.REFERENCES[1] C.M. Ankenbrandt et al., Status of Muon ColliderResearch and Development and Future Plans, Phys.Rev. ST Accel. Beams 2, 08001 (August 1992).[2] B. Autin, A. Blondel, J. Ellis (Eds.), ProspectiveStudy of Muon Storage Rings at CERN, CERN 99-02, ECFA 99-197 (April 1999).[3] R. Palmer, C.D. Johnson, E. Keil, A Cost-EffectiveDesign for a Neutrino Factory, CERN SL-99-070,(1999).[4] N. Holtkamp, D. Finley (Eds.), A Feasibility Studyof a Neutrino Source Based on a Muon StorageRing, FERMILAB pub-00/08-E, (2000).[5] M. Vretenar (Editor), Report of the Study Group ona Superconducting Proton Linac as a PS Injector,CERN/PS 98-063 (RF-HP), (1998).[6] R Garoby, M. Vretenar, Status of the Proposal for aSuperconducting Proton Linac at CERN, CERN/PS/99-064 (RF), (1999).[7] R. Cappi et al., Design of a 2 GeV Accumulator-Compressor for a Neutrino Factory, this conference.[8] C.R. Prior et al, Synchrotron Based Proton Driversfor a Neutrino Factory, this conference.[9] H. Schonauer et al, A Slow-Cycling Proton Driverfor a Neutrino Factory, this conference.[10]C.D. Johnson, Study of the Behaviour of a PulsedLiquid Jet Target, CERN NUFACT-Note 27, (2000).[11]Spokesperson: F. Dydak, Proposal to Study HadronProduction for the Neutrino Factory and for theAtmospheric Neutrino Flux, HARP PS-214, CERN-SPSC/99-35, SPSC/P315, (1999).[12]A. Lombardi, A 40-80 MHz System for PhaseRotation and Cooling, CERN NUFACT-Note 20,(2000).[13]A. Lombardi et al., Phase Rotation, Cooling andAcceleration of Muon Beams: a Comparison ofDifferent Approaches. To be published in theproceedings of LINAC2000.[14]E. Keil, A 50 GeV Muon Storage Ring Design,CERN SL/2000-013 AP, CERN NUFACT-Note 26,(2000).

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