The collimation system of the Compact Linear Collider (CLIC) should simultaneously fulfill three different functions. It must (1) provide adequate halo collimation to render the detector background acceptable, (2) ensure collimator survival and machine protection against mis-steered beams, and (3) not significantly amplify incoming trajectory fluctuations via the collimator wake fields. We describe the present layout of CLIC post-linac collimation and characterize its potential performance

Assmann, R W

Burkhardt, H

Fartoukh, Stéphane David

Jeanneret, J B

Pancin, J

Redaelli, S

Risselada, Thys

Schreiber, H J

Zimmermann, Frank

CERN Document Server

Overview of the CLIC Collimation DesignR. Assmann, H. Burkhardt, S. Fartoukh, J.-B. Jeanneret, J. Pancin, S. Redaelli, T. Risselada,F. Zimmermann, CERN, Geneva, Switzerland; H.J. Schreiber, DESY Zeuthen, GermanyAbstractThe collimation system of the Compact Linear Collider(CLIC) should simultaneously fulfill three different func-tions. It must (1) provide adequate halo collimation to ren-der the detector background acceptable, (2) ensure colli-mator survival and machine protection against mis-steeredbeams, and (3) not significantly amplify incoming trajec-tory fluctuations via the collimator wake fields. We de-scribe the present layout of CLIC post-linac collimationand characterize its potential performance.1 INTRODUCTIONTwo different final-focus optics have been designed forCLIC at 3 TeV [1]. The shorter system provides a largerfree length from the IP, l∗, and generates no tails in thefinal-doublet region. Although, in principle, collimationcould be integrated into the final focus proper [2], the shortsystem does not easily allow for collimation of all types ofincoming beam tails or for machine protection, e.g., in caseof a mis-steered beam or large energy error. Therefore, adedicated collimation system upstream appears necessary,in particular for more frequent energy errors [3].As a preliminary baseline design we have scaled thecollimation optics adopted for NLC in Ref. [4] to the 3-TeV CLIC requirements. The optics consists of two parts,devoted to energy and betatron collimation, respectively.Compared to the original NLC design, the length of theenergy collimation section was increased by a factor of 8,which enlarges the spot size at the collimators to a valuewhere they can withstand the impact of a full bunch train ofnominal emittance, and we have reduced the dipole bend-ing angles by a factor 32, so as to keep the emittance growthfrom synchrotron radiation at a tolerable level. We havenot modified the optics of the betatron collimation sec-tion, since large betatron oscillations with small emittance-beams are not thought to occur frequently, and since thecollimators here are supposed to be replacable or renew-able as they are in the NLC scheme.The optical functions of a 3-TeV CLIC beam deliverysystem based on a short final focus are displayed in Fig. 1.The first 5.6 km accommodate the collimation system, thelast 550 m the final focus.The horizontal emittance growth from synchrotron ra-diation is given by the formula [6] ∆γx ≈ (4 ×10−18 m2GeV−6) E6∑i Li < Hi > /|ρi|3, where thesum is over all bending magnets, ρi the bending radius, Ethe beam energy, L the magnet length, andH the ‘curly H’function defined by Sands [7] (see also Ref. [8] for a moreprecise estimate of emittance growth due to synchrotron0.0 1000. 2000. 3000. 4000. 5000. 6000. 7000. 8000.s (m)L*= 4.3 m, Bx*= 0.8 cm, By*= 0.15 mmUnix version 8.51/05 16/05/01  18.32.360.020.40.60.80.100.120.140.160.180.200.β(m)[*10**(3)]-0.4-0.3-0.2-0.10.00.10.20.30.4D x(m)βxβyDxFigure 1: Optics of a 3-TeV beam delivery system (BDS)consisting of a 5.6-km long collimation section, scaledfrom the NLC design [4], and a 550-m final focus as pro-posed by Raimondi [5].radiation). Evaluating this expression for the baseline op-tics of Fig. 1 we obtain ∆γx ≈ 0.042 µm for the col-limation section, and ∆γx ≈ 0.163 µm for the final fo-cus, to be compared with a design normalized emittanceof γx ≈ 0.68 µm. The contribution from the collimationsection is reasonably smaller than the other two numbers.2 BACKGROUNDThe collimation system is designed to remove off-momentum particles and large amplitude halo particles.We studied the background in the detector, due to sec-ondary muons, produced in the collimation of high energybeam particles. Using the program MUBKG developed forTESLA [9], we simulated the production of muons on 1 ra-diation length carbon spoilers and the path of these muonsthrough the CLIC tunnel geometry. The results for thebaseline final-focus optics are illustrated in Fig.2, in termsof the ratio of beam particles removed by the collimation tothe number of muons reaching the detector. For the last pairof spoilers (SPX4, SPY4), at about 500 m distance from theIP, this ratio reaches about 10−4. Lowering the c.m. energyto 500 GeV increases the number of beam particles thatcan be collimated for a given muon flux in the detector byabout an order of magnitude. The dependence on the dis-tance is weak, and we expect to obtain similar numbers forthe shorter final focus and the dedicated collimation sys-tem. We also studied the reduction of the muon flux by asystem of three massive, oppositely magnetized (2 Tesla)0-7803-7191-7/01/$10.00 ©2001 IEEE. 4071Proceedings of the 2001 Particle Accelerator Conference, Chicago10 310 410 510 610 710 80 500 1000 1500 2000 2500 3000 3500Source location, in meterEl. Beam Lost / Muon in DetectorE=1.5 TeV, 1 rad.len. CBethe Heitlerx(IP) = 0.0 cmStandard case3 Tunnel Fillers added  (a 10m)3 Tunnel Fillers added  (a 30m)SPX1SPY1SPX2SPY2SPY3SPX3SPY4SPX4→ → → → → → → →CLIC BDS base line opticsFigure 2: Number of lost electrons per muon passingthrough a detector with 7.5-m radius as a function of po-sition along the baseline final focus. Potential collimatorlocations are indicated. The IP is at 3282 m.“Iron Tunnel Fillers” [10] placed downstream of the lastpair of spoilers. The results for 10 m and 30 m tunnel fillersare also shown in Fig.2. The reduction is more pronouncedfor muons produced in the last spoilers.3 COLLIMATOR SURVIVALPassive survival of the collimators is a concern. In Fig. 3,the design beam sizes at spoiler locations in the energyand betatron collimation sections, the compact final focusand the alternative baseline final focus are superimposedon spoiler survival curves for various materials, which takeinto account the energy deposition by both ionization andby image current ohmic heating [11]. In the energy colli-mation section, the rms radial beam size σr =√σxσy is147 µm and 1.862 mm at the spoilers and the absorbers,respectively, assuming nominal emittances. Figure 3 illus-trates that this should be sufficient to guarantee survival ofthe energy-collimation spoilers for beam impact, providedthat the spoilers are made from beryllium, carbon, or pos-sibly titanium [11]. We also note that beam sizes in thebaseline final focus are slightly more forgiving than thosein the compact final focus, and that the spoilers in the beta-tron collimation section are ‘sacrificial’ and will certainlybe destroyed by beam impact.4 WAKE-FIELD EFFECTSThe deflection of the beam centroid by a tapered circularcollimator is [12, 13]∆y′ =2Nbreγσz[(4λσz)1/4g3/2+LF (λσz)1/22√πg3]y (1)where y is the offset from the center of the chamber,λ[m] = ρ[Ωm] /(120π), LF the length of the collimator1e-061e-050.00010.0010.011e-06 1e-05 0.0001 0.001 0.01sigma y[m]sigma x [m]CuTiBeCC_no-icBaseline FFCompact FFDedicated momentumSacrificial betatronFigure 3: Vertical vs. horizontal beam sizes required forspoiler survival [11] and typical values at prospectivespoiler locations in the CLIC beam delivery system, ascomputed from the design emittance and energy spread.The lowest curve refers to carbon without image current(diamond).flat part, and g the half gap. The taper angle is assumed tobe optimally chosen as θopt ≈ 1.1(λσz/g2)1/4 [12]. Equa-tion (1) is correct for √σzλ  g √σzλσz /λ [12],which, e.g., for ρ ≈ 4 × 10−8 Ωm gives 60 nm  g  16mm.If the trajectory of the incoming beam changes, thischange may be amplified by the collimator wake fields,possibly resulting in an enhanced displacement at the in-teraction point (IP). We have computed the combined ef-fect of 4 vertical spoilers and 4 absorbers located in thebetatron collimation section of Fig. 1. We assume that thespoilers are made from beryllium and extend over 0.5 radi-ation lengths (r.l.), or 177 mm, and that the copper-coatedtitanium absorbers are 712 mm long (20 r.l.). Note thatsolid copper would not be an adequate absorber material,since, even with protective spoilers upstream, copper couldnot withstand the stress induced by the impact of a bunchtrain [14].The collimation depth is set to 80σy, leaving a few σmargin [1] to ensure that all beamline elements downstreamare shadowed by the collimators. If needed, octupole mag-nets upstream of the final doublet, which ‘fold in’ the beamtails [15], could be employed to further increase the re-quired collimation depth.Figure 4 shows the IP displacements at the IP for sine-like and cosine-like trajectories with a 1σ initial amplitude.For the nominal charge, the IP displacements never exceed1σ, i.e., ‘enhancement’ of the initial 1σ position changeessentially is absent. The IP displacement significantly in-creases only for bunch populations larger than 2 × 10 10,or 5 times the nominal. It is clear that the IP displace-4072Proceedings of the 2001 Particle Accelerator Conference, Chicagoment depends on the betatron phase of the incoming oscil-lation. Selecting always the betatron phase with maximumdisplacement, Figs. 5 and 6 display the dependence on thebunch population and the collimator beta function (main-taining a constant collimation depth). From these figureswe conclude that the jitter enhancement due to collimatorwake fields is not a severe limitation in CLIC, and that thebeta function is close to optimum. It is interesting that forcertain values of beam current the centroid-beam jitter atthe IP is strongly reduced. A collimator wake acts on thecoherent beam motion like an additional quadrupole, andfor certain wake-field strengths the ‘coherent’ beta functionat the IP becomes smaller. In these cases, the displacementof the bunch tail at the IP is opposite to that of the head.0 0.5 1 1.5 2 2.5 3−3−2−10123Bunch population, 1010Y* IP/σ* IPY0 = (σ0,0)Y0 = (0,σ‘0)Figure 4: IP orbit displacement for a 1σ change in incom-ing beam position or slope (the two curves) as a function ofbunch population. The nominal value is N b = 4× 109.0 0.5 1 1.5 2 2.5 300.511.522.53Bunch population, 1010Maximum Y* IP/σ* IPFigure 5: Maximum IP orbit displacement for a 1σ changein incoming beam trajectory as a function of bunch popu-lation Nb. The nominal value is Nb = 4× 109.5 CONCLUSIONSWe have presented the first design of a CLIC collima-tion system for 3 TeV. The system guarantees spoiler sur-vival in the energy collimation section and it exhibits sur-prisingly benign wake field effects. Suppression of muonbackground at 1.5 TeV is more difficult than at lower en-ergies. A drawback of the proposed system is its substan-tial length of about 6 km. It could be shortened by about30%, if we relax the requirements on emittance growth due400 450 500 55000.511.522.53βy, mMaximum Y* IP/σ* IPFigure 6: Maximum IP orbit displacement for a 1σ changein incoming beam trajectory as a function of beta functionat the collimator. The nominal beta function is 483 m.to synchrotron radiation and omit the betatron collimationsection, which does not appear to be essential [3].6 REFERENCES[1] F. Zimmermann, et al., “Final-Focus Schemes for CLICat 3 TeV,” HEACC’01 Tsukuba, CERN-SL-2001-010-AP,CLIC-Note-476 (2001).[2] R. Assmann et al., “Design status of the CLIC 3-TeV beamdelivery system and damping rings,” EPAC 2000, Vienna,CERN-SL-2000-058-AP (2000).[3] D. Schulte and F. Zimmermann, “Failure Modes at CLIC”,these proceedings.[4] P. Tenenbaum, et al., “Studies of beam optics and scatteringin the Next Linear Collider Postlinac Collimation System,”LINAC 2000 Monterey (2000).[5] P. Raimondi and A. Seryi, “A Novel Final Focus Design forHigh Energy Colliders”, EPAC 2000, Vienna, p. 492.[6] R. Helm, M. Lee, P. Morton, M.Sands, “Evaluation of Syn-chrotron Radiation Integrals,” SLAC-PUB-1193 (1973).[7] M. Sands, “The Physics of Electron Storage Rings,” SLAC-121 (1970).[8] F. Ruggiero and B. Zotter, “Emittance Growth induced byRadiation Fluctuations in the Final Focus of a Linear Col-lider,” EPAC 88, Rome (1988).[9] H. J. Schreiber, “Muon Background.” In 7th Workshop ofthe 2nd ECFA/DESY Study of Physics and Detectors fora Linear Electron-Positron Collider, Hamburg, Germany,2000., http://www.desy.de/∼ecfadesy/transparencies/-Det HJSchreiber1.pdf.[10] L. P. Keller, “Muon background in a 1-TeV linear collider,”SLAC-PUB-6385, LC 93, Stanford (1993).[11] S. Fartoukh, J.B. Jeanneret, J. Pancin, “Heat deposition bytransient beam passage in spoilers,” submitted to PRST-AB,CERN-SL-2001-012-AP, CLIC-Note-477 (2001).[12] J. Irwin in Chapter 9 of the “Zeroth-Order Design Report forthe Next Linear Collider,” The NLC Design Group, SLAC-Report-474 (1996).[13] K. Yokoya, “Impedance Of Slowly Tapered Structures,”CERN-SL/90-88 (AP) (1990).[14] J.B. Jeanneret and E. Wildner, “Thermal and acoustic effectsin CLIC beam absorbers,” CERN-SL-99-072-AP.[15] P. Emma, et al., AIP Conf. Proc. 468, 143, and SLAC-PUB-7958 (1998).4073Proceedings of the 2001 Particle Accelerator Conference, Chicago

Overview of the CLIC Collimation design

The collimation system of the Compact Linear Collider (CLIC) should simultaneously fulfill three different functions. It must (1) provide adequate halo collimation to render the detector background acceptable, (2) ensure collimator survival and machine protection against mis-steered beams, and (3) not significantly amplify incoming trajectory fluctuations via the collimator wake fields. We describe the present layout of CLIC post-linac collimationand characterize its potential performance

Overview of the CLIC Collimation design

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