ISBN 978-3-95450-115-1 - http://accelconf.web.cern.ch/AccelConf/IPAC2012/papers/weppr076.pdfThe full physics program of a future Large Hadron electron Collider (LHeC) requires both pe+ and pe- collisions. For a pulsed 140-GeV or an ERL-based 60-GeV Linac-Ring LHeC this implies a challenging rate of, respectively, about 1.8 *10 15 or 4.4 *10 16 e+/s at the collision point, which is about 300 or 7000 times the past SLC rate. We consider providing this e+ rate through a combination of measures: (1) Reducing the required production rate from the e+ target through colliding e+ (and the LHC protons) several times before deceleration, by reusing the e+ over several acceleration/ deceleration cycles, and by cooling them, e.g., with a compact tri-ring scheme or a conventional damping ring in the SPS tunnel. (2) Using an advanced target, e.g., W-granules, rotating wheel, sliced-rod converter, or liquid metal jet, for converting gamma rays to e+. (3) Selecting the most powerful of several proposed gamma sources, namely Compton ERL, Compton storage ring, co- herent pair production in a strong laser, or high-ﬁeld undulator radiation from the high-energy lepton beam. We describe the various concepts, present example parameters, estimate the electrical power required, and mention open questions

Braun, H.-H.

Brüning, O.S.

Bulyak, E.V.

Klein, M.

Papaphilippou, Y.

Rinolfi, L.

Schulte, D.

Sievers, P.

Variola, A.

Yakimenko, V.

Zimmermann, F.

Zomer, F.

The full physics program of a future Large Hadron electron Collider (LHeC) [1] requires both pe+ and pe− collisions. For a pulsed 140-GeV or an ERL-based 60-GeV Linac-Ring LHeC this implies a challenging rate of, respectively, about 1.8 × 1015 or 4.4 × 1016 e+/s at the collision point, which is about 300 or 7000 times the rate previously obtained, at the SLAC Linear Collider (SLC). We consider providing this e+ rate through a combination of measures: (1) Reducing the required production rate from the e+ target through colliding e+ (and the LHC protons) several times before deceleration, by reusing the e+ over several acceleration/deceleration cycles, and by cooling them, e.g., with a compact tri-ring scheme or a conventional damping ring in the SPS tunnel. (2) Using an advanced target, e.g., W-granules, rotating wheel, slicedrod converter, or liquid metal jet, for converting gamma rays to e+. (3) Selecting the most powerful of several proposed gamma sources, namely Compton ERL, Compton storage ring, coherent pair production in a strong laser, or high-field undulator radiation from the high-energy lepton beam. We sketch some of these concepts, present example parameters; estimate the electrical power required, andmention open questions

Zimmermann, F

Brüning, O S

Papaphilippou, Y

Schulte, D

Sievers, P

Rinolfi, L

Variola, A

Zomer, F

Braun, H H

Yakimenko, V

Bulyak, E V

Klein, M

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Positron Options for the Linac-Ring LHeC

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POSITRON OPTIONS FOR THE LINAC-RING LHECF. Zimmermann, O.S. Bru¨ning, Y. Papaphilippou, D. Schulte, P. Sievers, CERN, Geneva,Switzerland; V. Yakimenko, BNL, Upton, NY, USA; L. Rinolfi, JUAS, Archamps, France;A. Variola, F. Zomer, LAL, Orsay, France; E.V. Bulyak, NSC/KIPT, Kharkov, Ukraine;H.-H. Braun, PSI, Villigen, Switzerland; M. Klein, U. Liverpool, UKAbstractThe full physics program of a future Large Hadron elec-tron Collider (LHeC) [1] requires both pe+ and pe− col-lisions. For a pulsed 140-GeV or an ERL-based 60-GeVLinac-Ring LHeC this implies a challenging rate of, re-spectively, about 1.8 × 1015 or 4.4 × 1016 e+/s at thecollision point, which is about 300 or 7000 times the ratepreviously obtained, at the SLAC Linear Collider (SLC).We consider providing this e+ rate through a combinationof measures: (1) Reducing the required production ratefrom the e+ target through colliding e+ (and the LHC pro-tons) several times before deceleration, by reusing the e+over several acceleration/deceleration cycles, and by cool-ing them, e.g., with a compact tri-ring scheme or a con-ventional damping ring in the SPS tunnel. (2) Using anadvanced target, e.g., W-granules, rotating wheel, sliced-rod converter, or liquid metal jet, for converting gammarays to e+. (3) Selecting the most powerful of several pro-posed gamma sources, namely Compton ERL, Comptonstorage ring, coherent pair production in a strong laser, orhigh-field undulator radiation from the high-energy leptonbeam. We sketch some of these concepts, present exam-ple parameters, estimate the electrical power required, andmention open questions.MOTIVATIONIt is the physics beyond the standard model and thesearches for it which pose the highest demands on the e+pluminosity. Important example processes which requirehigh-statistics positron (and electron) data include the de-termination of the fermion number of leptoquarks, disen-tangling the nature of new contact interactions, resolvingdifferences in the strange (and top) quark and anti-quarkdistributions, accessing valence quarks at low x and gener-alized parton distributions at low Q2, and a precision mea-surement of the longitudinal structure function FL.POSITRON REQUIREMENTSTable 1 compares the e+ beam flux foreseen for theLHeC with those obtained at the SLC, and targeted forCLIC and the ILC. The requested LHeC flux for pulsed op-eration at 140 GeV (a factor 300 compared to SLC) couldbe obtained, in a first approximation, with 10 e+ target sta-tions working in parallel. Several more advanced solutionsare proposed to meet the requested LHeC flux for the CWoption (a factor 7300 compared to SLC).Table 1: Comparison of e+ rates in various colliders. ForSLC, CLIC and ILC the energy quoted refers to the exitof the damping ring. The CLIC parameters are for 3 TeVc.m. energy.SLC CLIC ILC LHeC LHeCpulsed ERLE [GeV] 1.19 2.86 4 140 60γx [μm] 30 0.66 10 100 50γy [μm] 2 0.02 0.04 100 50e+[1014s−1] 0.06 1.1 3.9 18 440MITIGATION SCHEMESTwo main approaches can lessen the demands on the rateof positrons to be produced at the source, namely• Recycling the positrons after the collision, with con-siderations on e+ emittance after collision, emittancegrowth in the 60-GeV return arc due to synchrotronradiation, and possible cooling schemes, e.g. introduc-ing a tri-ring system with fast laser cooling in the cen-tral ring (see below), or using a large damping ring. If90% of the positrons are recycled the requirement forthe source drops by an order of magnitude.• Repeated collisions on multiple turns, e.g. usinga (pulsed) phase-shift chicane in order to recover 60GeV when reaching the collision point again on thefollowing turn.COOLING OF POSITRONSOne of the most challenging problems associated withthe continuous production of positrons is cooling (damp-ing) of the positron beam emerging from a source or be-ing recycled after the collision. Possible cooling scenariosinclude pushing the performance of a large conventionaldamping ring with the size of the SPS, and a novel com-pact tri-ring scheme.Damping RingThe 6.9-km SPS tunnel can accommodate a train of 9221bunches with 2.5 ns bunch spacing. A tentative parameterlist for low (10 Hz) and high repetition rate (100 Hz) isshown in Table 2, considering 234 bending magnets of 0.5-m long dipoles with 1.8-T bending field. The wiggler fieldfor the high-repetition option of 1.9 T along with a wigglerperiod of 5 cm is within the reach of modern hybrid wigglerWEPPR076 Proceedings of IPAC2012, New Orleans, Louisiana, USAISBN 978-3-95450-115-13108Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)03 Particle Sources and Alternative Acceleration TechniquesA16 Advanced Conceptstechnology. A big challenge is the high energy loss per turnfor this case, which requires around 300 MV of total RFvoltage and implies an average synchrotron-radiation (SR)power of 25 MW. In the low repetition case, the RF voltageand SR power are an order of magnitude more relaxed.Tri-Ring SchemeAnother possible solution to cool down a continuouspositron beam, both the recycled beam and/or a new beamfrom a source, is the tri-ring scheme illustrated in Fig. 1.Figure 1: Tri-ring scheme.In this scheme, the basic cycle lasts N turns, duringwhich the following processes happen simultaneously: N -turn injection from the ERL into the accumulating ring(bottom); N -turn cooling in the cooling ring (middle) pos-sibly with fast laser cooling [3]; and N -turn slow extrac-tion from the extracting ring (top) back into the ERL. Atthe start of the cycle there is a one-turn transfer from thecooling ring into the extracting ring, and a one-turn trans-fer from the accumulating ring into the cooling ring. Theaverage current in the cooling ring is N times the averageERL current.Table 2: Parameter List for a Damping Ring in the SPS Tunnelwith High and Low Repetition-Rate Optionsrepetition rate [Hz] 100 10energy [GeV] 10 7bunch population [109] 1.6 1.6bunch spacing [ns] 2.5 2.5number of bunches/train 9221 9221repetition rate [Hz] 100 10damping times trans./long. [ms] 2/1 20/10energy loss/turn [MeV] 230 16horizontal norm. emittance [μm] 20 100total wiggler length [m] 800 -longitudinal norm. emittance [keV-m] 10 10RF voltage [MV] 300 35average SR power [MW] 23.6 3.6PRODUCTION SCHEMESPositrons can be produced by pair creation when high-energy electrons or photons hit a target. Conventionalsources, as used at the SLC, send a high-energy electronbeam on a conversion target. Alternatively, a high-energyelectron beam can be used with a hybrid-target configura-tion where the first thin target is used to create high-energyphotons, through a channeling process, which are then sentonto a thick target. The prior conversion into photons re-duces the heat load of the target for a given output inten-sity and it may also improve the emittance of the gener-ated positrons. There exist a number of other schemes thatcan accomplish the conversion of electrons into photons.Several of them employ Compton scattering off a high-power laser pulse stacked in an optical cavity. According tothe electron-beam accelerator employed, one distinguishesCompton rings, Compton linacs, and Compton ERLs. Analternative scheme uses the photons emitted by an electronbeam of very high energy (of order 100 GeV) when pass-ing through a short-period undulator. Finally, there evenexists a simpler scheme where a high-power laser pulse it-self serves as the target for (coherent) pair creation.TargetsFor the positron flux considered for the LHeC the heat-ing and possible destruction of the target are important con-cerns. Different target schemes and types can address thesechallenges: (1) multiple, e.g. 10, target stations operatingin parallel; (2) He-cooled granular W-sphere targets; (3)rotating-wheel targets; (4) sliced-rod W tungsten conver-sion targets; (5) liquid mercury targets; and (6) runningtape with annealing process.The LHeC ERL option requires a positron current of6 mA or 4 × 1016 e+/s, with normalized emittance of≤50 μm and longitudinal emittance ≤5 MeV-mm. Fora conventional conversion target with optimized lengththe power of the primary beam is converted as fol-lows Pprimary(100%) = Pthermal(30%) + Pγ(50%) +Pe−(12%) + Pe+(8%). The average kinetic energy of thenewly generated positrons is < Te+ >≈ 5 MeV, whichallows estimating the total power incident on the targetas Ptarget = 5 MV ×6 mA / 0.08 = 375 kW. Assum-ing an electron linac efficiency of ηacc ≈ 20% we findPwall = Ptarget/0.2 = 1.9 MW. This wall-plug powerlevel looks feasible and affordable. However, also consid-ering a capture efficiency (for the ‘useful’ e+) of about 5%,Pwall becomes 38 MW.Presently with conventional targets, the transverse nor-malized rms beam emittances, in both planes, are in therange of 6000 to 10 000 μm. A strong reduction of emit-tances is mandatory for the requested LHeC performance.For Compton sources the conversion of gammas topositrons is a bottleneck, which requires a study andoptimization of effective convertor targets such as thesliced-rod converter. A typical tungsten convertor op-timized for Compton gammas with a maximal energyof 20 MeV can deliver 0.02 positrons per incident scat-Proceedings of IPAC2012, New Orleans, Louisiana, USA WEPPR07603 Particle Sources and Alternative Acceleration TechniquesA16 Advanced ConceptsISBN 978-3-95450-115-13109 Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)Table 3: IP e+ current and the implied minimum e− beamcurrent in a Compton Ring. Electron-beam currents below5 A are considered achievable.LHeC pulsed LHeC ERLIe+ at IP [μA] 290 7050typical Ie− [A] 4.3 105.7Ie− with 5 J [A] 0.46 11.2Ie− with 5 J+1 m rod [A] 0.065 1.6tered gamma. A sliced-rod convertor target may produce0.07/0.13 positrons per gamma for a 1 m or 3 m long rod,respectively [2].Compton SourcesIn Compton sources (polarized) positrons are generatedby scattering of an electron beam off a higher-power laserpulse, and by converting the resulting gammas in a target.Compton Ring Table 3 illustrates that a Compton-ringsource equipped with an array of optical resonators yield-ing a total (single-IP ‘equivalent’) laser-pulse energy of5 Joule, together with a sliced-rod conversion target, mayproduce the desired flux of polarized positrons even for theLHeC ERL option. The emission of 30-MeV gammas atthe required rate can induce significant beam energy spreadin the Compton ring, which requires further studies and op-timization.Compton Linac An optimistic power analysis for asingle-pass Compton linac using a CO2 laser shows that thewall plug power for generating the Compton-linac electronbeam alone exceeds the limit of 100 MW set for the entireLHeC project.Compton ERL A high current ERL appears to be apossible approach, e.g. a 3-GeV 1.3-A ERL with 2-micronwavelength optical enhancement cavities would providethe desired e+ rate, with “only” 50 MW of wall plugpower, and with upper-bound estimates on the transverseand longitudinal emittances for the captured positron beamof γ⊥ ≤ 1.5 m, and ||,N ≈ 450 μm.The desired emittances are not reached from any Comp-ton scheme source, even if the target is immersed in astrong magnetic field. Therefore, cooling or scrapingwould be required.Undulator SourceAn undulator process for e+ production could be basedon the main high-energy e− (or e+) beam. The LHeC un-dulator scheme can benefit from the pertinent developmentwork done for the ILC. The beam energy at LHeC would belower, e.g. 60 GeV, which might possibly be compensatedby more ambitious undulator magnets, e.g. ones made fromNb3Sn or HTS. However, the requested photon flux callsfor a careful investigation. The undulator scheme couldmost easily be applied for the 140-GeV pulsed LHeC.Coherent Pair CreationThe normalized transverse emittance of all positronsfrom a target is of order N ≈ 1− 10 mm, to be comparedwith a requested emittance of N = 0.05 mm. Therefore,a factor 100 emittance reduction is required. Possible solu-tions are cutting the phase space or damping. A third solu-tion would be to produce positrons in a smaller phase spacevolume. Indeed the inherent transverse emittance from pairproduction is small. The large phase space volume onlycomes from multiple scattering in the production target.Pair production from relativistic electrons in a stronglaser field would not need any solid target, since the laseritself serves as the target, and it would not suffer from mul-tiple scattering. This process has been studied in the 1960’sand 1990’s [4, 5, 6]. It should be reconsidered with state-of-the-art TiSa lasers and X-ray FELs.CONCLUSIONSThe challenging requirements for the LHeC Linac-Ringpositron source may be relaxed, to a certain extent, by e+recycling, e+ re-colliding, and e+ cooling. The compacttri-ring scheme is an attractive proposal for recooling thespent and recycled positrons, with a pushed conventionaldamping ring in the SPS tunnel as an alternative solution.Assuming some of the aforementioned measures aretaken to lessen the required positron intensity to be pro-duced at the source, by at least an order of magnitude, andalso assuming that an advanced target is available, severalof the proposed concepts could provide the intensity andthe beam quality required by the LHeC ERL.For example, the Compton ring and the Compton ERLare viable candidates for the Linac-Ring LHeC positronsource. Coherent pair production and an advanced undu-lator represent other possible schemes, still to be exploredfor LHeC in greater detail. The coherent pair productionwould have the appealing feature of generating positronswith an inherently small emittance.In conclusion, it may be possible to meet the very de-manding requirements for the LHeC positron source. A se-rious and concerted R&D effort will be required to developand evaluate a baseline design for the linac-ring positronconfiguration. Among the priorities are a detailed optics& beam-dynamics study of multiple collisions and of thetri-ring scheme, a theoretical exploration of coherent pairproduction, and participation in experiments on Comptonsources, e.g. at the KEK ATF.REFERENCES[1] LHeC Study Group, C. Adolphsen et al, “A Large HadronElectron Collider at CERN,” LHeC-Note-2011-003 GEN,CERN, 2011, to be published.[2] E. Bulyak, “Optimal γ → e+ conversion target for Comptonsources,” ALCPG11, Eugene, U.S.A. (2011).[3] E. Bulyak, J. Urakawa and F. Zimmermann, “Asymmetriclaser radiant cooling in storage rings,” PAC2011.[4] T. Erber, Reviews of Modern Physics, vol. 38, no. 4 (1966).[5] P. Chen and R.B. Palmer, SLAC-PUB-5966 (1992).[6] C. Bamber et al, Phys.Rev.D60:092004 (1999).WEPPR076 Proceedings of IPAC2012, New Orleans, Louisiana, USAISBN 978-3-95450-115-13110Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)03 Particle Sources and Alternative Acceleration TechniquesA16 Advanced Concepts

Positron Options for the Linac-Ring LHeC

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