We describe various parameter scenarios for a ring-linac ep collider based on LHC and an independent electron linac. Luminosities between $10^{31}$ and $10^{33} cm^{-2}s^{-1}$ can be achieved with a s.c. linac, operated either in pulsed or in cw mode, with optional recirculation, at a total electric wallplug power of order 20 MW. Higher luminosities, of several $10^{33} cm^{-2}s^{-1}$ can be reached by investing more electric power or by energy recovery. Finally, merits of a linac-ring ep collider are discussed

Aksahal, H

Bordry, Frederick

Braun, H H

Brüning, Oliver Sim

Chattopadhyay, S

Dainton, J B

de Roeck, A

Eide, A

Garoby, R

Holzer, B J

Klein, M

Linnecar, T

Mess, K H

Osborne, J

Rinolfi, L

Schulte, D

Sultansoy, S

Tomás, R

Tückmantel, Joachim

Vivoli, A

Willeke, F J

Zimmermann, F

Çiftçi, A K

CERN Document Server

LHC-PROJECT-REPORT-113704Sep2008F. Zimmermann, H. Aksahal1), F. Bordry, H.H. Braun, O.S. Brüning, A.K. Ciftci2),S. Chattopadhyay3), J. Dainton4), A. de Roeck, A. Eide5), R. Garoby, B.J. Holzer6), M. Klein4),T. Linnecar, K.H. Mess, J. Osborne, L. Rinolfi, D. Schulte, S. Sultansoy7), R. Tomas,J. Tuckmantel, A. Vivoli8), F.J. Willeke9)EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsLINAC-LHC EP COLLIDER OPTIONSWe describe various parameter scenarios for a ring-linac ep collider  based on LHC and an independentelectron linac. Luminosities between  1031 and 1033 cm-2s-1 can be achieved  with a s.c. linac, operated eitherin pulsed or in cw mode, with optional recirculation, at a total electric wallplug power of order 20 MW.Higher luminosities, of several 1033 cm-2s-1 can be reached by investing more electric power or by  energyrecovery. Finally, merits of a linac-ring ep collider are discussed.Large Hadron Collider ProjectAbstractLHC Project Report 1137CERN,CH-1211 Geneva 23Switzerland1) Nigde Univ. Turkey, 2) Ankara Univ. Turkey, 3) Cockcroft Institute UK, 4) Liverpool Univ. UK ,5) EPFL, Switzerland, 6) DESY, Germany , 7) TOBB ETU. Turkey, 8) LAL, France, 9) BNL, USACERN, Geneva, SwitzerlandPresented atEPAC'08, 11th European Particle Accelerator Conference, Genoa, Italy - June 23-27, 2008LINAC-LHC ep COLLIDER        OPTIONSF. Zimmermann, F. Bordry, H.-H. Braun, O.S. Bru¨ning, H. Burkhardt, A. de Roeck, R. Garoby,T. Linnecar, K.-H. Mess, J. Osborne, L. Rinolfi, D. Schulte, R. Tomas, J. Tuckmantel, CERN,Switzerland; A. Eide, EPFL, Switzerland; F.J. Willeke, BNL, U.S.A.; S. Chattopadhyay, Cockcroft I.,UK; B.J. Holzer, DESY, Germany; J. Dainton, M. Klein, Liverpool U., UK; A. Vivoli, LAL, France;S. Sultansoy, TOBB ETU, Turkey; A.K. Ciftci, Ankara U., Turkey; H. Aksakal, Nigde U., TurkeyAbstractWe describe various parameter scenarios for a linac-ringep collider based on LHC and an independent electronlinac. Luminosities between 1031 and 1033 cm−2s−1 canbe achieved with a s.c. linac, operated either pulsed or incw mode with optional recirculation, at a total electric wall-plug power of order 20 MW. Higher luminosities of several1033 cm−2s−1 can be reached by investing more electricpower or by energy recovery. Finally, merits of a linac-ringep collider are discussed.SCENARIOSColliding the LHC 7-TeV protons with a 25–140 GeVe± beam would both extend the discovery reach of theLHC, and enable precision physics with LHC data [1, 2].e+p collisions are desirable in addition to e−p. One wayto realize such ep collider is by installing a new leptonring in the LHC tunnel, implying a new lepton injector too[3, 4]. The THERA study [5] inspired looks at an alter-native LHC-based “QCD Explorer” colliding LHC protonswith electrons delivered by a linac [6].Possible linac-ring scenarios include a n.c. linac, apulsed s.c. linac, and a cw s.c. linac with or without energyrecovery in various configurations, as shown in Fig. 1.LHCsc or ncpulsed linacLHCsc cw linacLHC2 pulsed sc linacswith energy recoveryLHCenergyrecoverys.c. linacLHChigher -energyenergyrecoverys.c. linacLHC1 pulsed sc linacwith energy recoveryvia turnaround loopFigure 1: Scenarios for the linac-ring ep collider.A s.c. linac can accelerate long trains of bunches with25 or 50 ns spacing, matching the LHC fill pattern. There-fore, its luminosity in collisions with the LHC can be muchhigher than for a short-pulse low-charge and less efficientn.c. linac. The s.c. linac may have an rf frequency around1.3 GHz, similar to ILC, or near 700 MHz, as for the CERNSPL. The bunch spacing is roughly 1/10 of the ILC’s, at re-duced charge per bunch. PLACET [7] simulations suggestthat even with the ILC bunch charge of 2 × 1010 e− thebeam can be stabilized by a 0.1% detuning of the linac cav-ities. Energy recovery may be realized via a second linacpointing at the first one, in which the beam from the lat-Table 1: Proton beam scenariosNb,p Tsep pγp β∗pLHC 1.7× 1011 25 ns 3.75 μm 0.25 mLHC∗ 5× 1011 50 ns 3.75 μm 0.10 mter is decelerated [8]. More conventional energy recoveryis possible by means of a recirculating linac, e.g. similarto ELFE [9], or with a turnaround loop as proposed for afuture X-ray FEL [10]. Possible layouts are sketched inFig. 2. For highest beam energy, the ERL with its recircu-Figure 2: Example linac layouts on LHC site.lating arcs could be installed in the LHC tunnel itself, blur-ring the distinction between ring-ring and linac-ring LHeCoptions [11].Two proton scenarios are listed in Table 1: (1) the nom-inal LHC beam combined with reduced proton interaction-point (IP) beta functions β∗p of 0.25 m as foreseen for 2013,and (2) a higher brightness beam [corresponding to sce-nario “LPA” of [12]] available from an upgraded LHC in-jector chain, including a 5 GeV s.c. proton linac (SPL) anda 50-GeV synchrotron (PS2), by 2017. This second sce-nario also assumes β∗p = 0.1 m, which may be possible by(1) focusing only one of the two proton beams, thereby re-laxing aperture constraints; (2) dedicated ep runs, allowingfor a chromatic correction twice as strong as for two low-βIPs; (3) reducing the free length l∗ between the last protonquadrupole and the ep collision point; or (4) the hypotheti-cal availability of (stronger) Nb3Sn magnets.ELECTRIC POWERThe cryogenics electric power for a s.c. linac is [13]Pcr = AE/g + BDEg , (1)where the first and second terms describe the static and dy-namic heat loads, respectively, making explicit their scalingwith the rf duty factor D, the accelerating gradient in unitsof eV/m g, and the energy gain over the length of the linacE. Inserting the parameters of the ILC and XFEL designs,we infer the values of the two coefficients A and B at 1.3GHz [13]: A ≈ 350 W/m and B ≈ 10−10 Wm/(eV)2.A lower frequency of 700 MHz is likely to require lesscryopower (up to 2 times less from BCS theory); the truedifference depends on the residual resistance of the cavityNiobium, and other details. Figure 3 shows the cryogenicelectric power for a 70 GeV beam as a function of accel-erating gradient at D = 1 (cw mode) with and without a4-pass recirculation, and at D = 0.0075 (pulsed mode).17010 20 30 4010520330P  [MW]crg [MeV/m]pulsed, D=0.75%cw, 4-pass       recirculatorcw, single pass 1.3 GHz700 MHz1.3 GHz700 MHz1.3 GHz700 MHzFigure 3: Cryogenics el. power vs. accelerating gradient.The active length of the XFEL and ILC linacs is about60% of the total. With a cavity gradient of 30 MV/m, thefull length of a 60-GeV linac, without recirculation, wouldthen be 3.3 km, and with 10 MV/m gradient it would be10 km. The β = 1 section of the CERN SPL will be upto about 400 (or 256) m long and accelerate a beam by amaximum of 4.4 (or 2.9) GeV. With a 4-pass recirculationthrough this part of the SPL, an electron energy close to 20GeV might be reached.The rf electric power for a given beam power is P rf =Pbeam/(ηrf→beamηwp→rf)(1 − ηER). From the XFEL andILC designs, we infer a wall-plug power to rf efficiencyηwp→rf of 50% for s.c. linacs. In cw modes of opera-tion the rf to beam power conversion efficiency η rf→beamis taken to be 100%. In pulsed operation mode, addi-tional rf energy is needed to pre-fill the accelerating struc-tures, and the efficiency can be estimated from a refer-ence design (subindex ’ref’; here the ILC) via η rf→beam ≈Tb/(Tb + (Trf,ref − Tb,ref)Iref/I) with Tb the beam pulselength, Trf the rf pulse length, and I the beam current in thepulse. When we consider energy recovery options, the en-ergy recovery efficiency factor is taken to be ηER ≈ 0.95.In all other cases ηER is set to zero.LUMINOSITYThe electron beam size is assumed to be matched tothe size of the protons, σ∗p = σ∗e , as a smaller electronbeam could have adverse effects on the proton beam life-time. Round-beam collisions are considered, since thelinac beam generated by an e− gun is naturally round asare the LHC protons, and round beams maximize the lumi-nosity for constant beam-beam tune shift. The luminosityformula then becomesL =14πeNb,pp1β∗pIe Hhg(β∗eσz,p,ep), (2)where e denotes the electron charge, and the subindices por e refer to protons or electrons. The luminosity (2) de-pends only on the p beam brightness (Nb,p/p) with Nb,pthe number of protons per bunch and p the geometric emit-tance, on β∗p , on the electron beam current Ie, and on thehourglass factor Hhg. The term (Nb,p/p) is limited byspace charge in the proton injector complex and by thepp beam-beam tune shift. The electron current Ie is con-strained by the linac technology, by the available electricalpower, and by linac beam dynamics. The proton IP betafunction β∗p is confined, on the proton side, by the IR lay-out, as well as by the chromatic correction scheme, and onthe electron side by the reduction factor due to the hour-glass effect, Hhg, given byHhg(x, r) = 2√πxre4x2r21+r2 erfc(2xr√1 + r2). (3)Figure 4 shows the minimum β∗p that can be achieved forreasonable values of Hhg as a function of the normalizedemittance at 60 GeV, and also Hhg vs. β∗ for typical emit-tance values. To set this into perspective, the normalizedemittance γee of the DESY XFEL design is 1.4 μm, whilethe smallest normalized emittance in LEP at 60 GeV wasabout 2 mm, i.e. ∼ 1000× larger. Figure 4 underlines thepotential of a low emittance linac beam to take advantageof small proton beta functions.0 500 1000 1500 20000.00.20.40.60.81.01.2γε  [μm]eβ  [m]∗pH  =0.9hgH  =0.5hg0.0 0.2 0.4 0.6 0.8 1.00.00.20.40.60.81.0γε  =2000 μmeβ  [m]∗pγε  =200 μmeγε  =20 μmeH  hgFigure 4: Proton IP beta function β ∗p vs. normalized elec-tron emittance for two constant values of Hhg (left) andHhg vs. β∗p for three values of γee (right), assuming 60GeV electron energy and σz,p = 0.075 m.Figure 5 illustrates the luminosity reach vs. energy forvarious linac options considering a fixed wall plug powerof 20 MW. One case with 100 MW was added to reveal thedependence on available power. The solid lines refer to thenominal LHC proton beam, the higher dashed curves to the‘LHC∗’ parameters of Table 1.pulsed100 MW0 20 40 60 80 100 120 140 1029 1030  1031  1032 5x1032E  [GeV]eL [cm  s  ]-2  -1cw 1-pass20 MWcw 4-pass20 MWpulsed20 MWcw 4-passERL, 20 MWFigure 5: Luminosity vs. e− energy for cw & pulsed linac.SOURCESThe e− beam can be produced from a polarized dc gun.Depending on the bunch charge, a normalized rms emit-Table 2: Electron-beam parameters for various (s.c.) linac-ring LHeC scenarios. The β ∗ values are calculated for anormalized e- emittance of 20 μm. Parameters marked by asterisks refer to ‘LHC ∗’ of Table 1.energy [GeV] 20 20 60 60 60 120option cw 4-pass cw 4-p. ERL cw 4-pass cw 4-p. ERL pulsed pulsedbunch population Nb,e [109] 0.06, 0.12∗ 1.3, 2.6∗ 0.1, 0.2∗ 0.3, 0.6∗ 17, 34∗ 7, 14∗average current [μA] 400 8650 74 2050 820 340beam power at IP [MW] 8.0 172 4.5 120 49 48IP beta function [m] 0.25, 0.098∗ 0.25, 0.098∗ 0.74, 0.30∗ 0.74, 0.30∗ 0.74, 0.30∗ 1.72, 0.69∗luminosity [1031 cm−2s−1] 2.7, 20∗ 58, 430∗ 0.5, 3.7∗ 14, 100∗ 5.5, 41∗ 2.3, 17∗total electrical power [MW] 20 20 20 20 100 100tance between 10 and 100 μm is expected after bunchingand acceleration.If the spent high-power e− beam is not passed througha turnaround loop to recover the e− energy [10], it can beused for e+ production with subsequent interleaving of e−and e+ bunches, or bunch trains, in the linac. This is pos-sible since the colliding beams are round and the e− beamis only moderately disrupted by the collision (Fig. 6). Thedisruption angle θ0 = Dσ∗/σz,p = Nb,pre/(γeσ∗) [14]represents an upper bound on the divergence, and emit-tance, of the spent e− beam (right picture). The conceptis sketched in Fig. 7, which also presents results of prelim-inary target and capture simulations, indicating a yield ofup to 19 e+ per e− for e+ emittances γee of a few mm.0.2 0.4 0.6 0.8 1.0110.550220330β  [m]∗pDx,yLHCLHC*0 200 400 600 800 1000110550220LHCLHC*γ ε  [μm]eΔθ    /σ  rms     ee'∗Figure 6: e− beam disruption parameter vs. β∗p , and relativerms divergence increase in collision vs. initial γee.LHCLinacspent e-e+ Pre-injectorIPe+p+Targete- 60 GeV 101520050 9yield [e+/e-]γ  ε         [mm] e+;x,yε      [MeV-cm]e+,z1 2 3 4 5 6 7 8d   /X0We+Figure 7: Schematic linac-ring collider with integratedpositron production (left) and simulated e+ yield for amor-phous W target of varying thickness hit by a 60-GeV e−beam [γee = 20 μm, σx,y,e = 20 μm, β = 10 m] (right).MERITS(1) Thanks to the small e− emittance γee the luminos-ity can be increased for reduced β ∗p , and in addition thee− quadrupoles be positioned far from the collision point.The first elements are warm separation dipoles with 0.5–2 T field. A schematic IR layout is shown in Fig. 8. (2)The e&p beams can be collided head-on and no crab cavi-ties are required, since the electrons are dumped and, withthe assumed IR layout, residual parasitic collisions uncrit-ical. (3) A staged construction is possible. A first stagecould use the SPL as recirculator for alternating e−/p oper-ation, bringing the electrons to some 20 GeV, which wouldDump ore+ sourceProton Triplet Proton Tripletbendse trip- letL=40mL*=10-20mProton (beam 2)Figure 8: Schematic of linac-ring interaction region.probe physics beyond HERA. Such SPL-based recircula-tor could later serve as injector for an electron ring or foran extended linac. The beam energy could be raised insteps by adding further linac segments. (4) As the linacis not affected by synchrotron radiation, there is no funda-mental limit on the electron beam energy. (5) Implementa-tion of energy recovery could ultimately gain another orderof magnitude in luminosity. (6) Except for the collisionpoint, the linac tunnel would be fully separate from theLHC, minimizing construction downtime. (7) The linac-ring collider would benefit from the planned proton infras-tructure upgrades, i.e. linac4, SPL and PS2. (8) The e−beam can be 80% polarized. (9) Numerous important syn-ergies with CLIC and ILC (beam dynamics, e+ production,tunnel, etc.) may well prepare the ground for a future linearcollider. (10) Electron-ion collisions, as well as, via laserCompton backscattering, γ-proton and γ-nucleus collisionswould also be possible.REFERENCES[1] S. Sultansoy, Eur. Phys. J. C33: S1064 (2004);[2] J.B. Dainton et al, this conference.[3] J.B. Dainton et al, JINST 1: P10001 (2006).[4] H. Burkhardt et al, this conference.[5] TESLA Design Report, DESY-01-011, Pt. 6, Ch. 2 (2001).[6] D. Schulte, F. Zimmermann, Proc. EPAC’04 Lucerne, p. 632(2004); H. Aksakal et al, Proc. PAC’05, p. 1207 (2005).[7] D. Schulte, Proc. EPAC’00 Vienna, p. 1402 (2000).[8] M. Tigner, Nuovo Cim. 37: 1228 (1965).[9] K. Aulenbacher et al, ELFE at CERN, CERN-99-10 (1999).[10] J. Sekutowicz et al, PRST-AB 8:010701 (2005).[11] V. Litvinenko, CERN AB Forum 11 March 2008.[12] F. Zimmermann, Proc. PAC’07, p. 714 (2007).[13] A. Eide, Project Report, EPFL Lausanne (2008).[14] P. Chen, K. Yokoya, PRD 38, 3, 987 (1988).

Linac-LHC EP Collider Options

Linac-LHC EP Collider Options

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