International audienceThe design of a future multi-TeV muon collider needs new ideas to overcome the technological challenges related to muon production, cooling, accumulation and acceleration. The Low Emittance Muon Accelerator (LEMMA) concept *,** presents in this paper an upgraded layout of a positron driven muon source. The positron beam, stored in a ring with high energy acceptance and low emittance, is extracted and driven in a push-pull configuration to a multi-target system, to produce muon pairs at threshold on the target’s electrons. This solution alleviates the issues related to the power deposited and the integrated Peak Energy Density Deposition on the targets. Muons produced in the multi-target system will then be accumulated in many parallel rings before acceleration and injection in the collider. A special multi-target line lattice has been designed to cope with the focusing of both the positron and muon beams. Studies on the number, material and thickness of the targets have been carried out. A general layout of the overall scheme and a description is presented, as well as plans for future R&D

Antonelli, Mario

Bacci, Alberto

Bauce, Matteo

Biagini, Maria

Blanco-García, Oscar

Boscolo, Manuela

Cesarini, Gianmario

Chaikovska, Iryna

Chehab, Robert

Ciarma, Andrea

Collamati, Francesco

Giribono, Anna

Guiducci, Susanna

Liuzzo, Simone

Lucchesi, Donatella

Pastrone, Nadia

Raimondi, Pantaleo

Vaccarezza, Cristina

Variola, Alessandro

Hal-Diderot

Positron Driven Muon Source for a Muon Collider: Recent Developments

The design of a future multi-TeV μ collider needs new ideas to overcome the technological challenges related to μ production, cooling, accumulation and acceleration. In this paper an upgraded layout of a e+ driven μ source, known as the Low EMittance Muon Accelerator (LEMMA) concept developed at INFN-LNF, is presented. In this new scheme the e+ beam, stored in a ring with high energy acceptance and low emittance, is extracted and driven to a multi-target system, to produce μ pairs at threshold. Muons produced are accumulated in two rings before the fast acceleration and injection in a μ+ μ− collider

Maria Biagini

Mario Antonelli

Alberto Bacci

Matteo Bauce

Oscar Blanco-García

Manuela Boscolo

Gianmario Cesarini

Iryna Chaikovska

Robert Chehab

Andrea Ciarma

Francesco Collamati

Anna Giribono

Susanna Guiducci

Simone Liuzzo

Donatella Lucchesi

Nadia Pastrone

Pantaleo Raimondi

Cristina Vaccarezza

Alessandro Variola

Archivio della ricerca- Università di Roma La Sapienza

POSITRON DRIVEN MUON SOURCE FOR A MUON COLLIDER:RECENT DEVELOPMENTS ∗M.E. Biagini†, M. Antonelli, M. Boscolo, O. R. Blanco-Garcia, A. Ciarma,A. Giribono, S. Guiducci, C. Vaccarezza, A. Variola, INFN-LNF, 00044 Frascati, ItalyM. Bauce, F. Collamati, G. Cesarini, R. Li Voti, INFN-Roma1, 00185 Roma, ItalyA. Bacci, INFN-MI, 20133 Milano, ItalyP. Raimondi, S. Liuzzo, ESRF, 38043 Grenoble, FranceI. Chaikovska, R. Chehab, IN2P3-LAL, 91440 Orsay, FranceN. Pastrone, INFN-TO, 10125 Torino, ItalyD. Lucchesi, Padova University, 35121 Padova, ItalyAbstractThe design of a future multi-TeV µ collider needs newideas to overcome the technological challenges related to µproduction, cooling, accumulation and acceleration. In thispaper an upgraded layout of a e+ driven µ source, known asthe Low EMittance Muon Accelerator (LEMMA) conceptdeveloped at INFN-LNF, is presented. In this new schemethe e+ beam, stored in a ring with high energy acceptanceand low emittance, is extracted and driven to a multi-targetsystem, to produce µ pairs at threshold. Muons producedare accumulated in two rings before the fast acceleration andinjection in a µ+µ− collider.INTRODUCTIONThe Low Emittance Muon Accelerator (LEMMA) con-cept [1–3] is based on µ production from a 45 GeV e+ beamannihilating with the electrons of a target close to thresholdfor µ+µ− pair creation, thus generating, without any cooling,µ beams with low enough transverse emittance for a highenergy collider. The initial design foresaw an e+ storagering with an internal target, in order to allow for multipleinteractions of the e+ with the electrons at rest in the tar-get. However, this layout has encountered several limitingdifficulties. An alternative design is presently under study,to identify the challenges within reach of the existing tech-nology, and those requiring further innovation. In the newscheme e+ bunches are extracted to impinge on multipletargets in a dedicated straight section. This scheme couldrelease the impact of the average power on the targets andalso reduce the number of e+ from the source.LAYOUT OF THE COMPLEXIn order to have a reliable µ production scheme, preciserequirements on the muon source chain timing have been set.The complete µ production cycle should be ∼410 µsec, ofthe order of the particle lifetime (467 µsec) at 22.5 GeV, thusreducing the intrinsic beam losses with respect to the accu-mulated µ intensity. After one production cycle, µ bunchesmust be immediately accelerated to increase their lifetime∗ Work supported by INFN, Frascati National Laboratories† e-mail address: marica.biagini@lnf.infn.itand reduce losses. Moreover, one complete cycle must ac-commodate enough time for the e+ production and damping,in the main Positron Ring (PR) or in a dedicated DampingRing (DR). Damping time must be compatible with a rea-sonable amount of synchrotron power emitted, ranging from10 msec in a 5 GeV DR to 80 msec in a 45 GeV PR. Thistime is needed even in case it is possible to recuperate the e+bunches “spent” in the µ production which, after interactingwith the targets, are strongly affected and have a degraded6D emittance. It is evident that the impact of the µ produc-tion on the e+ bunches should be minimized to allow forgenerating the maximum amount of µ for a single e+ bunchpassage, the study of different type of targets is in progress.Once an e+ bunch has been “spent”, it is mandatory to havea “fresh” e+ bunch for the µ accumulation cycle. Further-more, the different systems composing the µ source complexmust not show unrealistic performances, taking into accountthe state-of-the-art of the existing technology and the possi-bility to have a future R&D program to fulfill the requiredparameters. Three different accelerator complex layouts arecurrently being studied, in order to choose the one fulfillingall the requirements. In the following we will detail the ac-celerator complex Scheme III, which at the moment seemsthe most promising.Scheme III General LayoutScheme III main components are (see sketch in Fig.1):• e+ Source (PS) at 300 MeV plus 5 GeV Linac,• 5 GeV Damping Ring (DR),• SC Linac or Energy Recovery Linac (ERL) to acceler-ate to 45 GeV, and decelerate to 5 GeV after µ produc-tion,• 45 GeV PR to accumulate 1000 bunches needed for µproduction,• delay loops to synchronize e+ and µ bunches,• one or more Target Lines (TL) for the µ production,• 2 Accumulation Rings (AR) where µ are stored untilthe µ bunch has a suitable number of particles,• “embedded” e+ source, for the production of the e+needed to restore the design e+ beam current.The PS and the first Linac have to produce and inject1000 bunches of 5x1011 e+/bunch in the DR, which stores3.8 A e+ and has a short (∼10 msec) cooling time thanks10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW PublishingISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-MOZZPLS2MC1: Circular and Linear CollidersA09 Muon Accelerators and Neutrino FactoriesMOZZPLS249ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOIFigure 1: Layout of Scheme III for µ production via 45 GeV e+ beam.to damping wigglers. After the cooling, the e+ beam isextracted from the DR, accelerated in a SC Linac or ERLto 45 GeV and injected in the PR in 10 msec, with a pulsedcurrent of the injector Linac of 8 mA. In the meantime the PScan continue to top-up the DR. Once 1000 bunches are storedin the PR, they are extracted to collide with the targets in theTL for µ production. This process can take 410 µsec. Afterµ production, degraded e+ bunches are sent back to the PRwith a reduced injection efficiency, estimated to 70% due tothe high energy spread generated in the targets interactions.With a slow extraction (∼20 msec) the “spent” e+ bunchesare slowly extracted from the PR, decelerated to 5 GeV inthe SC Linac, and sent back to the DR. At the same timethe produced µ bunches are extracted and accelerated tothe final collider energy in a separate accelerators chain. Inthe following 30 msec the DR provides for cooling of boththe e+ produced by the PS and those coming back fromthe PR. The cycle is then repeated at 20 Hz. Since the PRis not circulating any beam during the DR cooling phase,the synchrotron radiation emission duty cycle is reduced,decreasing also the total synchrotron power budget.POSITRON SOURCEThe e+ source has to provide trains of 1000 bunches with5x1011 e+/bunch to inject in the DR. With an e+ source likethe ILC [5] or CLIC one [6], which produce 1014 e+/sec, 5sec are needed to fill the DR. However, the source whichneeds to replace the e+ lost in the µ production process ischallenging, since the time available to produce, damp andaccelerate the e+ is very short. We assume that ∼70% ofthe “spent” e+ can be recovered, injected in the PR, slowlyextracted and decelerated and injected back in the DR. There-fore only 30% of the required e+ need to be produced in atime cycle tcycle = 50 msec, corresponding to the 20 Hzrepetition frequency. We assume to inject the bunches inthe DR during 20 msec and to store them for 30 msec todamp the emittance. The required e+ production rate is then3x1015 e+/sec. In order to achieve such a high rate of e+production we need to explore all the techniques developedfor the future linear colliders like hybrid targets (crystal tar-get + tungsten target) [6] and rotating targets [5] and we willdevelop an R&D program on new targets. The required en-ergy acceptance for the DR is demanding: it has to be of theorder of ±10%. An optimization of the e+ capture systemin order to take advantage of the large energy acceptancecould improve the e+ yield. Another possibility to reducethe requested e+ rate is to increase the energy acceptanceof the PR, so reducing the fraction of lost e+ to be replacedby the source. The option to use a Linac to compress the“spent” e+ beam, so to be able to re-inject at least 90% ofthe spent beam into the PR, is being also studied. Moreover,since the 45 GeV e+ passing through the µ targets produce alarge number of high energy photons, the feasibility of an“embedded” source [7] that uses these γ to produce new e+impinging on a 5 radiation length (5X0) tungsten target isunder study.POSITRON RINGSThe 45 GeV PR should have small beam emittance, mostlyround beams, and a large energy acceptance in order to beable to accommodate also the “spent” beam coming back af-ter the µ production. In order to store the 1000 bunches with5x1011 e+/bunch with less important synchrotron losses, a27 Km LHC-like ring was chosen.Table 1: 45 GeV PR Parameters for 3 Different EmittancesParameter 0.7 nm 6 nm 10 nmCircumference [Km] 27 27 27N. cells 64 32 32Ib [A] 0.89 0.89 0.89Npart /bunch 5x1011 5x1011 5x1011N. bunches 1000 1000 1000Nat. σz [mm] 1.9 3.6 3.8Energy spread 7x10−4 7x10−4 9x10−4τx,y [ms] 68 66 42Energy acceptance [%] ± 8 ± 6 ± 2SR power [MW] 106 109 170At present several lattices have been studied, with hori-zontal emittances ranging from 0.7 to 20 nm (see Table 1).Their energy acceptance ranges from ±2 to ±8 %, and work10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW PublishingISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-MOZZPLS2MOZZPLS250ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOIMC1: Circular and Linear CollidersA09 Muon Accelerators and Neutrino Factoriesis in progress to improve it. The lattices are all inspired tothe ESRF upgrade hybrid multi-bend achromat lattice [4],with different number of cells and different dipole lengthsin order to tune emittance and damping time.The 5 GeV DR should provide fast cooling of the e+produced by the source. A 6.3 Km long DR could providethe requested damping time (∼10 msec) with about 100damping wigglers, similar to those in the ILC TDR [5]. Apreliminary design was studied, based on the same lattice asthe PR, with 32 cells and an emittance of 70 pm. The latticeshows a good dynamic aperture and a ±10% momentumacceptance.MUON PRODUCTION LINESThe previous LEMMA scheme [2] showed both a largeinstantaneous and average energy deposition on the target(PEDD). The new scheme could solve these issues. In thefollowing a description of the studies performed for this op-tion is summarized. Two different configurations, describedin detail in [8], this conference, were studied.Multiple IPs, Ten Targets TLIn the first layout the e+ beam impinges on 10 targets fora total 0.3X0 of Be, split in 10 IPs separated by a transportline (TL) common to 3 beams (e+, µ+ and µ−), needed tofocus the beams at each IP to achieve the µ production withminimal emittance growth. The TL length should be as smallas possible in order to minimize µ decay issue. To focus3 beams at different energies imposes constraints on theminimum number of elements in the line. The chromaticitycannot be corrected with sextupoles because this would splitthe 3 beams, therefore other methods should be used. Thebest lattice design achieved so far has a total length of < 5 m,with quadrupole gradients of 200 T/m, 1 cm of apertureradius, separated by drift spaces of ∼20 cm. Two tripletsare used to focus the 45 GeV e+ and 18 GeV µ+ µ− onboth transverse planes. These triplets are asymmetric inorder to partially cancel chromaticity at 45 GeV as in the“apochromatic” design [9]. While the chromaticity is wellcorrected for the e+ beam, for the µ beam it is not. As aconsequence, the µ beam shows a rapid emittance growthas shown in Fig. 2 (magenta line). The e+ beam spot onthe target here is σe+ = 150 µm. Between the first andsecond IP, the emittance grows because of the combinationof non corrected chromaticity and large energy spread ofthe µ beam, since they are produced between 18.5 GeVand 26 GeV, a ±18% energy spread. The final achievedemittance after 10 targets is just below 200 nm, a valuelarger than a factor of two of the initial one. Therefore, anew layout with a single IP, with several targets very closeto each other, has been studied.Single IP, Ten Targets TLThe single IP line consists in one region where e+ collidewith one target sliced in 10, each slice separated by a verysmall drift in order to still allow for power dissipation. Fora σe+ = 150 µm beam spot at the first target, the total µemittance growth is 100 nm because of the µ beam multiplescattering within targets, see Fig. 2 (green line). Of coursethe smaller the σe+ at the targets, the smaller will be thefinal µ emittance. At present, different e+ beam spots arebeing studied, since this parameter is crucial both for the µemittance and for the amount of PEDD and temperature riseof the target.Figure 2: Comparison of µ emittance growth in the Multiple(magenta) and Single (green) IP schemes, as a function ofthe target number (0 to 9). σe+=150 µm in both cases.The design of two µ+ and µ− AR to store the µ producedover several passages of the e+ beam, was also performed [8].These rings must be short (∼120 m) in order to complete1000 turns before µ decay. In the current scheme, µ are notinjected but generated directly inside the ring. Therefore,the interaction region is common to a transport line for e+and two accumulator rings, one for µ+ and one for µ−, thisposes some design issues currently under study.FUTURE R&DA solid R&D program to increase the µ beams quality,and consequently the final luminosity, is foreseen. The useof H2 targets, with respect to Be and C, could improve theintegrated thickness, reducing the number of passages andincreasing the rate of “fresh” bunches/passage. With a lineardependence on the µ/bunch number, a quadratic increase ofthe final luminosity can be expected, a simple scaling with Zgives a factor 15 increase of the luminosity. An increase inthe efficiency of the PS, such as the rotating target conceivedfor ILC and the possibility to develop immersed e+ capturesystems with very high peak B field in the AMD (20 T as inMAP [10]) and in the capture solenoid, could increase therepetition rate of a factor 5-10, with a linear dependence onthe luminosity. To further reduce the production emittance,since the LEMMA higher production energy results in alonger µ lifetime, also a moderate cooling mechanism, suchas stochastic, optical stochastic, crystal, and electron coolingcan be envisaged. A full evaluation of these mechanismscould reduce the µ emittance by 1-2 order of magnitude,with a linear impact on the final luminosity.10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW PublishingISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-MOZZPLS2MC1: Circular and Linear CollidersA09 Muon Accelerators and Neutrino FactoriesMOZZPLS251ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOIREFERENCES[1] M. Antonelli and P. Raimondi, “Snowmass Report: Ideasfor µ production from positron beam interaction on a plasmatarget”, INFN-13-22/LNF, 2013.[2] M. Boscolo et al., “Low EMittance Muon Accelerator Studieswith Production from e+ on Target”, Phys. Rev. Accel. andBeams, vol. 21, p. 061005, 2018.[3] M. Antonelli, M. Boscolo, R. Di Nardo and P. Raimondi,“Novel proposal for a low emittance muon beam using positronbeam on target”, Nucl. Instr. Meth., A807 101-107, 2016.[4] P. Raimondi, “The ESRF low emittance upgrade”, Proc.IPAC’16, Busan, Korea, May 2016, paper WEXA01, pp.2023-2027.[5] C. Adolphsen et al., “The International Linear Collider,Thechnical Design Report”, Vol.3.II: Accelerator BaselineDesign, ISBN 978-3-935702-77-5, 2013.[6] M. Aicheler et al. , “CLIC Conceptual Design Report: AMulti-TeV Linear Collider Based on CLIC Technology”,CERN-2012-007, 2012.[7] F. Collamati et al., “Positron regeneration in LEMMA”, talkat POSIPOL Workshop, CERN, 2018, https://indico.cern.ch/event/727621/contributions/3109138.[8] O.R. Blanco-García et al., “Multi-Target lattice for µ produc-tion from positron beam annihilation on target”, presentedat IPAC19, Melbourne, Australia, May 2019, paper MO-PRB003, this conference.[9] C. A. Lindstrøm and E. Adli, “Design of general apochro-matic drift-quadrupole beam lines”, Phys. Rev. Accel. Beams,vol. 19, 2016.[10] MAP: dedicated JINST volume on “Muon Acceleratorsfor Particle Physics”, https://iopscience.iop.org/journal/1748-0221/page/extraproc46.10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW PublishingISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-MOZZPLS2MOZZPLS252ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOIMC1: Circular and Linear CollidersA09 Muon Accelerators and Neutrino Factories

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Positron Driven Muon Source for a Muon Collider: Recent Developments

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