Assmann R

Burkhardt H

Fartoukh S

Jeanneret J B

Jowett J M

Napoly O

Owen H L

Ruggiero F

Schulte D

Verdier A

Vos L

Wildner E

Zimmermann F

English

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DESIGN STATUS OF THE CLIC 3-TEV BEAM DELIVERY SYSTEM ANDDAMPING RINGSR. Assmann, H. Burkhardt, S. Fartoukh, J.B. Jeanneret, J.M. Jowett, F. Ruggiero, D. Schulte,A. Verdier, L. Vos, E. Wildner, F. Zimmermann, CERN, H. Owen, CLRC, O. Napoly, CEA/SaclayAbstractWe describe the present design status of beam delivery anddamping rings for CLIC at 3 TeV cm energy, and outlineour future plans. The beam delivery system comprises col-limation, final focus and post-IP exit line. Critical designaspects include halo collimation, machine protection, beamremoval, and thermal stability analysis. In order to attainthe design spot size at the collision point, the damping ringsmust provide beams of extremely small emittances. In thispaper, we focus on collimation and spent beam.1 BEAM DELIVERYThe beam delivery system encompasses collimation, fi-nal focus, interaction-point (IP) region, and exit line. A3.3-km long base-line final-focus optics for 3 TeV is de-scribed in a companion paper [1]. We are presently ex-ploring shorter alternatives either using dispersion acrossthe final doublet, as proposed by Raimondi [2], or usingmicrowave quadrupoles, proposed by Fartoukh and Jean-neret [3]. Ref. [4] compares stability issues for CLIC withthose for other linear colliders. Here, we report on collima-tion and spent beam.The collimation system should (1) remove the beam halowhich otherwise would cause unacceptable background inthe particle-physics detector, and (2) protect the down-stream systems against the impact of a mis-steered beam.Point (1) is achieved if the collimator shadows the finaldoublet apertures on the incoming side from the impactof beam particles or synchrotron radiation. To this end,the collimation aperture should be less than ±25 σx hor-izontally and ±80 σy vertically [5], assuming the finalquadrupole is a permanent magnet with 3.3 mm aperture, or400 σx and 1000 σy for a superconducting final quadrupolewith large bore. The apertures on the outgoing side neednot be shadowed by the collimation, since the incominghalo is tiny compared with the wide-angle debris and beam-strahlung coming from the collision point [6]. By the sameargument, background from synchrotron radiation emittedin the final quadrupoles is expected to be insignificant, aslong as it does not hit apertures on the incoming side.If a first beam halo collimation is performed prior to in-jection into the main linac, the halo at the entrance to thefinal focus, due to all known scattering sources, is esti-mated to be of the order of only 102 or 104 particles perbunch [7, 8]. Thus it appears that the collimators could beinstalled in the final focus itself, still retaining a tolerabledetector background. For example, one muon is generatedat the collimator for every few 104 scraped particles, whileit is likely that hundred muons passing through the detectorper bunch train are acceptable [9]. The above numbers im-ply a few orders of magnitude safety margin for the muonbackground, if the final-focus collimation is performed 1or 2 km upstream of the IP, and assuming that only a smallfraction of the muons produced there will reach the detec-tor. Collimation in the final focus does not only shorten theoverall system length, but, in addition, we can profit fromthe naturally large beta functions in this region. Also non-linear collimation systems a` la KEK [10] or TESLA [11]could be integrated into the final focus, e.g., utilizing thesextupoles in the two chromatic correction sections. Notethat up to a few percent of the beam had to be collimatedat the end of the SLC linac. We assume that a pre-linaccollimation will avoid a similar situation.In order that the collimators are not destroyed by surfacecracking during the impact of one bunch train, the lowerlimit on the beam size at the collimator is [12]:σxσy >αYσUTScpdEdxNbnb2π(1)where σUTS denotes the ultimate tensile strength, α the lin-ear thermal expansion coefficient, cp the heat capacity perunit density, Y the elastic modulus, dE/dx the ionisationenergy loss per unit length, nb the number of bunches pertrain, and Nb the bunch population. Table 1 lists exam-ple parameters for various materials and the minimum spotsize σmin =√σxσy according to Eq. (1). For copper, weTable 1: Physical properties of various materials and mini-mum beam size required for collimator survival.mat. α Y cp K[10−6/K] [GPa] [ MJm3K ] [ WKm ]Cu 16.5 120 3.4 390Ti 8.5 110 2.4 17W 4.4 411 2.5 200mat. σ dE/dx σUTS σmin[Ω−1m−1] [10−10 J/m] [MPa] [µm]Cu 6× 107 2.1 300 200Ti 2× 106 1.1 400 100W 2× 107 3.6 350 270find √σxσy > 200 µm or√βxβy ≥ 1000 km, and, fortitanium, √σxσy > 100 µm. In our present final-focusdesign, the maximum spot sizes for nominal beam param-Proceedings of EPAC 2000, Vienna, Austria522eters are √σxσy ≈ 45µm at the first three sextupoles, and√σxσy ≈ 76µm at the last one. These numbers are closeto the required minimum. Materials with a smaller productαE would be better suited for collimation. We are contem-plating the use of carbon composites [13]. Further workincludes the computation of time-dependent temperaturealong a batch in order to determine meltings limits, andthe study of active protection schemes with risk analysis.Also image currents can heat the collimator surface. Ta-ble 2 lists the maximum rise in surface temperature duringthe passage of a single bunch, ∆T1 or a full bunch train,∆TN , using the formulae derived in Ref. [14] and assum-ing a spot size (σxσy)1/2 ≈ 50 µm, andNb = 4×109. Fortitanium the temperature rise may exceed 1000 K. The tablefurther shows the thermal diffusion length over the bunchtrain, ldiﬀ =√Knbtb/cp, and the effective rf penetrationdepth [14], δe = Γ(3/4)√2σz/(cµσ), where K denotesthe thermal conductivity, nb the number of bunches (154for CLIC), tb the bunch spacing (0.67 ns), σ the conductiv-ity and µ the magnetic permeability.Table 2: Parameters for image current heating.material ∆T1 [K] ldiﬀ [µm] δe [µm] ∆TN [K]Cu 28 3.4 0.06 45Ti 40 0.85 0.3 1420W 14 2.9 0.1 131Fortunately, our above estimates are too pessimistic. Ifthe beam is deflected in the linac to an amplitude where itmight impinge on the final-doublet aperture, its emittanceat the end of the linac increases dramatically, by severalorders of magnitude, as is illustrated in Fig. 1. The emit-tance growth is explained by rapid filamentation due to thelarge energy spread. This suggests that typical values of√βxβy found throughout the final focus, i.e., significantlysmaller than 1000 km, might still ensure collimator survivalfor many conceivable failure modes.The exit line should cleanly remove the spent beamand debris of the beam-beam collision from the inter-action region, simultaneously ensuring acceptable back-ground in the detector and not exciting intolerable vibra-tions of the final-doublet quadrupoles. The outgoing beamhas a full energy spread of 100%, and there are copiousbeamstrahlung photons, carrying a third of the initial beampower, and almost as many coherent pairs as incoming par-ticles [16]. The radial power and energy distribution 2 mfrom the IP are shown in Fig. 2. The deposition of a smallfraction of the full beam power near the final quadrupolecould induce elastic waves with amplitudes well in excessof the 0.2 nm vertical jitter tolerance [17, 18]. If we con-sider a quadrupole a distance of 2 m away from the IP onthe outgoing side, and assume that its inner radius is R =10 mm, then, from the average power shown in Fig. 2 (top),the energy deposited in this quadrupole per bunch train isFigure 1: Relative increase in vertical emittance at the endof the linac for a mis-steered beam as a function of oscil-lation amplitude in units of rms beam size, simulated bythe code PLACET [15]. Shown are the results of a bunchdeflection after 10% and 90% of the full linac length.∆Etrain ≈ 1.6 J. It will be absorbed in a radial annulus∆R ≈ 0.14 m, e.g. of copper. The amplitude of radialelongation is about δRtrain ≈ α ∆Etrain ∆R/(CpV ) ≈ 1nm, where V denotes the volume in which the energy isabsorbed. The quadrupole motion induced on the incom-ing beam line will be much smaller and negligible.On the other hand, the absorption of the full beam power,11 MW pulsed at 100 Hz, requires some care. No solid ma-terial can stand this power and the thermomechanical vibra-tions due to beam impact can be orders of magnitude abovethe final-focus stability tolerances [18]. A dump made ofwater at 4◦C, where the density of water reaches a mini-mum, might suppress the vibrations to an acceptable level.Neutron generation in the vicinity of the detector is afurther concern. In addition, we anticipate the necessity ofnumerous spent-beam and beamstrahlung diagnostics, e.g.for luminosity optimisation, monitoring and stabilisation.A crossing angle is necessary at CLIC in order to (1) pro-vide sufficient space for the spent beam and collision de-bris, in particular for the opposite-charge pairs, and (2) sup-press the multi-bunch kink instability. The latter requiresa full crossing angle [19] φc ≥√Dxnb(3Dy)1/4φd ≈12 mrad, with nb = 154 the number of bunches; Dx =2Nbreσz/(γ(σx + σy)σx) = 0.12, and Dy = 5.2. Theformer favors crossing angles larger than 20 mrad.The disadvantage of the crossing angle is the loss ingeometric luminosity, e.g., 14% loss for φc = 20 mrad.The luminosity is recovered by crab crossing with dipole-mode rf cavities producing a z-dependent displacement atthe IP of ∂x∗/∂z = φc/2 or φc/2 = R12eVmaxkrf/E.With R12 ≈ 23 m, an rf voltage of Vmax = 1 MV isneeded at 30 GHz. If the relative phase of crab cavitiesjitters, the collision will not be head-on. The phase toler-ance for a 2% luminosity loss reads [20] ∆z ≤ 4σx/(5φc)or ∆φrf ≤ krf4σx/(5φc) ≈ 0.06◦ at 30 GHz. The phasetolerance is proportional to the rf frequency.523Proceedings of EPAC 2000, Vienna, AustriaFigure 2: Radial power and energy distribution of de-bris particles 2 meters away from the IP, as simulated byGUINEA-PIG. Top: integrated power outside radius r vs.r; bottom: average energy vs. r.Table 3: Beam parameters required from the damping ringsNormalised emittance γεx/γεy 430/3 nmParticles/bunch Nb 4× 109Bunches/train kbt 154Bunch spacing lb 0.2 mRep. rate fr 100 Hz2 DAMPING RINGSThe CLIC damping rings must provide e+ and e− beamswith the demanding parameters given in Table 3.The emittances of the beam on injection into the ring de-termine the storage time in the ring. The positrons will havethe larger injected emittances, taken to be γεx = 63 µm,γεy = 1.5 µm, as they come from a pre-damping ring.Although a detailed study has been done for the require-ments of a CLIC operating at centre-of-mass energy of1 TeV [21], the design of the damping rings for 3 TeV isnot complete. However a similar parameter optimisationindicates qualitatively similar basic choices for the designof the optics.The optimum beam energy is around 1.98 GeV and thedamping time required in a ring of 380 m circumference isaround 30 ms. As in [21], the ring will have a racetrackshape, with a TME (Theoretical Minimum Emittance [22])lattice for its arcs and strong wigglers to provide additionaldamping in the straight sections. The TME optics will beadjusted to balance the need for a small emittance againstthat for a high momentum compaction to maximise thethreshold impedance for turbulent bunch-lengthening.The energy lost by synchrotron radiation is predomi-nantly in the wiggler sections and these are likely to presentthe major difficulties for the final design. In order to avoidunwanted emittance generation due to the build-up of thedispersion functions in each wiggler period, the periodmust be reduced to a few cm. This raises issues related totolerances on alignment and field quality in the wigglers.Given the small beams in the ring also intra-beam scat-tering will contribute to the final emittance. In addition, fastbeam-ion instability, single-bunch beam break up drivenby the electron-cloud, coherent synchrotron radiation, andspace charge effects need to be addressed.Finally, there may exist a diffractive quantum limit onthe IP spot size [23], given by σy > cf/(Eδ0) withf ≈ 5 m the final focal length and E the energy. Viaδ0 ≡√/(eB) this is related to the wiggler field Bin the damping rings. For 3-TeV CLIC parameters andB ≈ 1.5 T, the above limit is σy ≈ 0.03 nm, a factor of30 below our design goal.3 SUMMARYConservative design approaches for beam delivery anddamping rings have been described, which are based onmoderate extensions of proven strategies, adapted to thespecific requirements of CLIC at 3 TeV c.m. energy. REFERENCES[1] F. Zimmermann, et al., these proceedings.[2] P. Raimondi, private communication (2000).[3] S. Fartoukh and J.B. Jeanneret, these proceedings.[4] R. Assmann, et al., these proceedings.[5] O. Napoly, unpublished.[6] R. Brinkmann, private communication (1999).[7] H. Burkhardt, CERN-SL-057-AP and CLIC Note 416 (1999).[8] R. Brinkmann, talk at LC99, Frascati (1999).[9] R. Settles, private communication (2000).[10] N. Akasaka et al., “JLC Design Study”, KEK (1997).[11] R. Brinkmann and N. Walker, LC99, Frascati (1999).[12] R. Brinkmann, et al., TESLA 95-25 (1995).[13] F. Caspers, private communication (1999).[14] X.E. Lin and D.H. Whittum, SLAC-PUB-8393 (2000).[15] D. Schulte, these proceedings.[16] D. Schulte, PAC99, New York (1999).[17] O. Napoly, CLIC note 414 (1999).[18] J.B. Jeanneret and E. Wildner, CERN-SL-99-072 (AP) andCLIC Note 42 (1999).[19] K. Yokoya, P. Chen, USPAS Hilton Head Island (1990).[20] NLC ZDR, SLAC Report 474 (1996).[21] J.-P. Potier, T. Risselada, these proceedings.[22] L. Teng, FNAL TM-1269 (1984).[23] C. Hill, FNAL preprint, subm. to Phys. Rev. D (2000).Proceedings of EPAC 2000, Vienna, Austria524

Design status of the CLIC 3-TeV beam delivery system and damping rings

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