We present a design for the CLIC #nal focus system which authorizes the stable multi#bunch operation with a 5 mrad crossing#angle. The loss of luminosity is limited to less than 20# in suchaway that crab#crossing cavities may not be required. The main feature of the design is that the vertically focusing #nal quadrupole is split into a 48 mm large bore superconducting quadrupole and a 13.7 mm bore normal conducting one. In that way# the o##axis outgoing beam is de#ected by the #rst quadrupole into the #eld#free region of the second one.  CEA#DAPNIA#SEA#97#17  1 Introduction The preceding #nal focus systems for CLIC had been designed for the single bunch op# eration at zero crossing#angle. With the current CLIC parameters# partially recalled in Table 1# 60 bunches separated by 20 cm #0.67 ns# form a bunch train repeated at 511 Hz rate. Toavoid unwanted collisions at multiples of 10 cm distances away on both sides from the IP# it is necessary to include a non#zero crossing#angle. This c..

Olivier Napoly

We present a design for the CLIC final focus system which authorizes the stable multi-bunch operation with a 5 mrad crossing-angle. The loss of luminosity is limited to less than 20% in such a way that crab-crossing cavities may not be required. The main feature of the design is that the vertically focusing final quadrupole is split into a 48 mm large bore superconducting quadrupole and a 13.7 mm bore normal conducting one. In that way, the off-axis outgoing beam is deflected by the first quadrupole into the field-free region of the second one

Napoly, O

CERN Document Server

Design of a nal focus system for CLIC in themultibunch regimeOlivier NAPOLYCEA DSMDAPNIASEACESaclay F GifsurYvette Cedex FranceDecember  AbstractWe present a design for the CLIC nal focus system which authorizes the stablemultibunch operation with a  mrad crossingangle The loss of luminosity is limitedto less than  in such a way that crabcrossing cavities may not be required Themain feature of the design is that the vertically focusing nal quadrupole is splitinto a 	 mm large bore superconducting quadrupole and a  mm bore normalconducting one In that way the oaxis outgoing beam is deected by the rstquadrupole into the eldfree region of the second oneCEADAPNIASEA IntroductionThe preceding nal focus systems for CLIC had been designed for the single bunch operation at zero crossingangle With the current CLIC parameters partially recalled inTable   bunches separated by 	 cm  ns form a bunch train repeated at  Hzrate To avoid unwanted collisions at multiples of  cm distances away on both sides fromthe IP it is necessary to include a nonzero crossingangle This crossing angle must belarge enough to stabilize the bunch train from the multibunch kink instability generatedby the parasitic crossings If the crossing angle is larger than the diagonal angle of thebeam    xz the loss of luminosity is signicant It can be completely recovered withthe crabcrossing scheme using transversely deecting RF cavities to rotate the bunchesaround their vertical axis This scheme adopted in the NLC  and JLC 	 designs cancertainly be adapted to the CLIC parameters as wellIn this study we would like to investigate an alternative scheme where crabcrossingis not required but left as an optional improvement This is possible because the diagonalangle   is 	 mrad for the CLIC parameters that is small enough to consider a  mradcrossing angle with a loss of luminosity limited to 	 and even less with some dispersioninduced crabcrossing  much simpler to implement We therefore attempt to design a nalfocus system less complex than the NLC design that is with fewer and simpler quadrupolesand no crabcrossing cavities In this design the beam extraction angle starting from theinitial crossingangle of  mrad is increased to  mrad by the deection provided by therst quadrupole with a large aperture to the oaxis outgoing beam trajectory Thistrajectory then crosses the eldfree region of the other smaller aperture quadrupoles ofthe last doubletBeam sizes  x  ynm 	  Emittances  x  ym   Bunch length zm Bunch population Ne Number of bunch per train nbBunch spacing tbns Divergences  x  yrad   Beta 	 x 	 ymm   Table  CLIC beam parameters at the IP forps   GeV CrossingAngle and Multibunch Kink InstabilityIn this section we discuss the choice of the crossing angle and its implication on the luminosity	0.80.820.840.860.880.90.920.940.960.9810 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 0.005L/LoCrossing-angle  [rad]Luminosity Lossrigid beamsGUINEA_PIG0501001502000 0.02 0.04 0.06 0.08 0.1 0.12Average Angle <y’>  [mu-rad]Vertical Offset  [mu-m]VERTICAL BEAM-BEAM DEFLECTIONrigid beamsGUINEA_PIGa bFigure  Luminosity loss vs crossing angle a and beam deection vs oset b  Single bunch luminosityFor rigid beams the luminosity as a function of the crossing angle   is given by thefollowing expressionL  Lq  tan 	  where the diagonal angle is    xz 	 mrad for CLIC It is therefore expected thatthe luminosity loss is small for crossing angles smaller than    The actual luminosity lossincluding the beambeam eect simulated with GUINEA PIG  is shown in Figa incomparison with the rigid beam expression The expected loss is 	 for a  mrad crossingangle However because of the energy correlation along the bunch the luminosity at nitecrossingangle can be better optimized for the nonzero horizontal dispersion that achievesa partial crabrotation  of the bunches The expected gain in luminosity has not beencalculated   Multibunch luminosityTo estimate the loss of luminosity from the multibunch instability induced by the parasiticcrossings one must rst calculate the coherent beambeam kick at the IP which is the mainsource of the instability as a function of the beam oset This is done in Figb wherethe result of a beambeam deection scan simulation with GUINEA PIG  is comparedto the rigid beam expression The focal distance of the coherent beambeam kick derivedfrom the slope of the deection curves at the origin is fcoh  m for the simulation andfcoh  m for rigid beamsThe rst value is used to calculate the loss of luminosity averaged over the bunch trainas a function of the incoming bunchtobunch vertical jitter The results are shown inFig	 for three dierent values of the crossingangle For simplicity in this calculationFigure 	 Multi bunch luminosity loss from bunch to bunch vertical jitterthe  m long colliding bunch trains have been assumed to travel along a straight orbitaround the IP and to be separated in two dierent beam pipes at 	 m from the IP Asshown in Fig in the proposed doublet design the rst 	 m space actually includes adefocusing quadrupole from  to 	 m which provides an additional horizontal separationof the bunch trains One can therefore expect the luminosity to be slightly larger thancalculated Moreover although perfectly adequate to predict the onset of the instabilitythe linear approximation used for the beambeam kicks overestimates this eectFig	 reveals an instable bunch train collision for  and  mrad and a rather stableregime at  mrad For instance at this value the multibunch beambeam eect onlyreduces the luminosity by  in addition to the expected  reduction for a jitter ofhalfy Final Doublet and Interaction Region DesignThe layout of the interaction region with a  mrad crossing incoming orbits is shown inFig Its main characteristics are summarized in Table 	 Each nal doublet includesa focusing quadrupole QF and a split defocusing quadrupole QDQD	 Splitting thedefocusing quadrupole is necessary for the spent beam extraction as discussed in Sect~~QD is a large aperture superconducting quadrupole  it is identical to the nal doubletQuadrupoles Length m Aperture diameter mm Gradient TmQD 	  	QD	   	QF   	last drift between quads Table 	 Doublet parametersIPQD1QD2 QFQD1QD2QFe+e−Figure  Magnet layout with a  mrad crossingangle The eand eorbits solid linescorrespond to the nominal energy of 	 GeVquadrupole designed for TESLA  from the LHC twoinone lattice quadrupoles  Thequadrupoles QD	 and QF are normal conducting  they have the same aperture and crosssection as the JLC nal doublet quadrupoles 	 and we have assumed a poletip eld of TTaking an outer radius of  mm for the QD cryostat like for TESLA and a cylindrical mask thickness of  mm around it the dead cone in the forward region of the detectorwould be limited to an angle of  mrad smaller than the JLC design Furthermorethe  m distance between QD and the warm magnet QD	 is enough for the endcapof the cryostat If the quadrupoles QD	 and QF because of their iron yoke need to beshielded from the detector solenoid eld a superconducting compensating solenoid couldbe included in the same cryostat Synchrotron Radiation from the Final DoubletWhile the incoming beam stayclear conditions are xand y the clearing of thesynchrotron radiation from the last doublet quadrupoles requires about xand 	ytransverse collimation of the beams as shown in Figs This is rather tight verticallywhen compared to the yrequired for the JLC design with a 	 times smaller emittanceThe prole of the synchrotron radiation from the upstream doublet for 	xand ycollimation requirements at that phase is shown by Fig not to be a problemAs shown in Fig the Oide eect induced by the synchrotron radiation in the lastdoublet has a negligible impact on the spot sizes at the IP for the design emittances Spent Beam and Beamstrahlung Photon ExtractionThe main characteristics of the spent beams and beamstrahlung photon beam are givenin Table  Their angular and energy distributions are also represented in FigsNotice the asymmetry in the horizontal angular distribution of the photons due to thenite crossingangle These distributions are calculated from a GUINEA PIG  beambeam simulation using   macroparticlesSpent BeamsEmittances x ypm   Divergences x yrad   Beta 	x 	ymm   Beamstrahlung PhotonsAverage angle hxi  hyi rad   Divergences x yrad   Table  Spent beam and beamstrahlung photon beam parametersWith their very small angular spread the extraction of the beamstrahlung photonbeams is not a problem On the contrary extracting the low energy particles of the spentbeams puts a constraint on the length of the rst quadrupole QD If too low in energy orat a too large angle at the IP these particles hit the exit face of the quadrupole Given its mmaperture diameter the acceptance of this quadrupole in the energy E and horizontalangle xphasespace of the particles originating from the IP is shown in Fig for threedierent quadrupole lengths Using a  m long quadrupole would simplify the doubletdesign by allowing to remove the second defocusing quadrupole QD	 Unfortunately Figclearly shows that such a quadrupole would be hit by a large number of outgoing particlesUsing the 	  macroparticle sample of the simulation one can roughly estimatethat the 	 m QD quadrupole should be hit by a fraction of the spent beam smaller than representing about 	 W of beam power This is certainly too large to be sustainedby the cryostat A ner estimate of the power deposited at the edge of the cryostat shouldinclude more particles in the beambeam simulation as well as taking the contribution fromthe bremmstrahlung particles into account If this power is too large a back up solutionshould be found with a  m QD quadrupole or with a larger bore quadrupole like thelow	 LHC ones These quadrupoles with a larger outer diameter would however increasethe dead cone in the detector	 Optics of the Chromaticity CorrectionThe doublet design described above can be combined into a nal telescope with 	demagnication Including a chromatic correction section with 	 pairs of sextupoles thenal focus system shown in Fig is  m long Its bandwidth is plotted in Fig  itis about  wide If this is too small for the single bunch energy spread more sextupolescan be added in order increase the bandwidth  ConclusionWe have proposed a nal focus system design for CLIC based on a small crossing angle of mrad and a scheme where the beams are focused and extracted by an hybrid doubletThe main advantage of the system over a crabcrossing scheme with a larger crossing angleas chosen for the NLC design lies in its simplicity  fewer and simpler quadrupoles easierdesign of the vacum chamber around the IP no crabcrossing cavitiesHowever the viability of the proposed design must be studied in more details Inparticular the following questions should be investigated Validate the choice of a  mrad crossingangle by detailed simulations of the multibunch kink instability For instance the additional separation induced by the rstdefocusing SC quadrupole will reduce the sensitivity of the luminosity to the incomingjitter	 Introduce crabcrossing with dispersion at the IP This will reduce the 	 expectedloss of luminosity due to the crossing angle Characterize the spent beam energy spectrum in particular the crossover at lowenergy with the irreducible bremsstrahlung spectrum Check the dependance of the beambeam eect on the shape of the longitudinaldistribution~~ Rene the beam loss estimate from the extracted beam Increase the FFS bandwidth by adding sextupoles in the CCS Study the transverse coupling eect induced by the detector solenoid and the nonzero crossing angleAcknowledgementsI thank Daniel Schulte for many discussions and Ian Wilson for inviting me several timesto do this studyReferences Zerothorder Design Report for the Next Linear Collider LLBNLPUB	 SLACReport  UCRLID 	 	 JLC Design Study Report DRAFT April   I thank T Tauchi for providingme with the chapter  of this report R Brinkmann Crabcrossing with dispersion  unpublished D Schulte Study of Electromagnetic and Hadronic Background in the InteractionRegion of TESLA PhD thesis University of Hamburg  and DESY reportTESLA  April  Conceptual Design of a  GeV eeLinear Collider with Integrated Xray LaserFacility R Brinkmann G Materlick J Rossbach and A Wagner Editors DESY and ECFA 	  JM Riet et al Cryogenic and Mechanical Measurements of the rst two LHClattice Quadrupole Prototypes EPAC Conf London Great Britain  R Brinkmann Optimization of a Final Focus System for Large Momentum Bandwidth DESYM a bFigure  Longitudinal proles of the synchrotron radiation from the last doubleta horizontal plane b vertical planeQD1 CROSS-SECTION QD2 and QF CROSS-SECTIONa bFigure  Transverse proles of the synchrotron radiation at the entrance small rectangleand exit large rectangle faces of  a QD b QD	 right and QF left in the extractionlineCLIC97Figure  Longitudinal proles of the synchrotron radiation from the last but one doubletCLIC_97Figure  Oide eect in last doublet0500010000150002000025000-0.4 -0.2 0 0.2 0.4dNe/Ne/dx’   [1/rd]x’  [mrad]HORIZONTAL ANGULAR DISTRIBUTIONinitialBeam 1Beam 2RMS Gaussian020004000600080001000012000-0.4 -0.2 0 0.2 0.4dNe/Ne/dy’   [1/rd]y’  [mrad]VERTICAL ANGULAR DISTRIBUTIONinitialBeam 1Beam 2RMS Gaussiana bFigure  Spent beam angular distribution a horizontal plane b vertical plane-0.001-0.000500.00050.0010.00150.0020 50 100 150 200 250"beam1.dat""beam2.dat"lq = 1.5 mlq = 1.2 mlq = 1.0 mFigure  Acceptance of the large aperture quadrupole QD of  m 	 m or  m length inthe E GeVxrad plane These limits are compared to the distribution of the 	  spent beam particles as predicted by GUINEA PIG	0.00010.0010.010.1110140 160 180 200 220 240 260dNg/Ng/dE   [1/GeV]E  [GeV]BEAM ENERGY DISTRIBUTION : clic97Beam 1Beam 20.00010.0010.010.11100 20 40 60 80 100dNg/Ng/dE   [1/GeV]E  [GeV]PHOTON ENERGY DISTRIBUTION : clic97Photons 1Photons 2a bFigure  Energy distribution a spent beam b beamstrahlung photon010002000300040005000600070008000-0.4 -0.2 0 0.2 0.4dNg/Ng/dx’   [1/rd]x’  [mrad]PHOTON HORIZONTAL ANGULAR DISTRIBUTIONPhotons 1Photons 2x-RMS Gaussian01000200030004000500060007000-0.4 -0.2 0 0.2 0.4dNg/Ng/dy’   [1/rd]y’  [mrad]PHOTON VERTICAL ANGULAR DISTRIBUTIONPhotons 1Photons 2x-RMS Gaussiana bFigure  Beamstrahlung photon angular distribution a horizontal plane b verticalplane0.0 50. 100. 150. 200. 250. 300. 350. 400. 450. 500. 550. 600.s (m)δ E/ p 0c = 0 .Table name = TWISS clic97          ( 12.0 x 30.0 ) FFSHP/UX version 8.21/11 21/11/97  15.31.440.010.20.30.40.50.60.70.80.β(m)[*10**(3)]-.05-.04-.03-.02-.010.0.01.02.03.04.05D(m)β x β y DxDyFigure 	 Optics of the chromatic correction section-.010 -.006 -.002 .002 .006 .010δ E/ p 0cs (m) =  558.515Table name = TWISS clic97          ( 12.0 x 30.0 ) FFSHP/UX version 8.21/11 21/11/97  15.31.440.0.00331.00663.00994.01325.01656.01988.02319.02650.02982.03313βx(m)0.0.4280E-01.8560E-01.1284.1712.2140.2568.2996.3424.3852.4280βy(m)[*10**(-3)]β x β yFigure  Bandwidth of the chromatic correction section

Design of a Final Focus System for CLIC in the Multi-bunch Regime

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Design of a final focus system for CLIC in the multibunch regime

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