Simulations for linear colliders are traditionally performed separately for the different sub-systems, like damping ring, bunch compressor, linac, and beam delivery. The beam properties are usually passed from one sub-system to the other via bunch charge, RMS transverse emittances, RMS bunch length, average energy and RMS energy spread. It is implicitly assumed that the detailed 6D correlations in the beam distribution are not relevant for the achievable luminosity. However, it has recently been shown that those correlations can have a strong effect on the beam-beam interaction. We present first results on CLIC simulations that integrate linac, beam delivery, and beam-beam interaction. These integrated simulations also allow a better simulation of time-dependent effects, like ground perturbations and interference between several beam-based feedbacks

Assmann, R

Blair, G

D'Amico, T

Guignard, G

Leros, N

Redaelli, S

Risselada, T

Schulte, D

Zimmermann, F

CERN Document Server

CLIC simulations from the start of the linac to the interaction point

Schulte, Daniel

Assmann, R W

Blair, G A

D'Amico, T E

Leros, Nicolas

Risselada, Thys

Zimmermann, Frank

  EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH  CERN - PS DIVISION       CERN/PS 2002-053 (AE) CLIC Note 529      CLIC SIMULATIONS FROM THE START OF THE LINAC TO THE INTERACTION POINT  D. Schulte, R. Assmann, G. Blair, T. D'Amico, G. Guignard, N. Leros, S. Redaelli, T. Risselada, F. Zimmermann           Abstract   Simulations for linear colliders are traditionally performed separately for the different sub-systems, like damping ring, bunch compressor, linac, and beam delivery. The beam properties are usually passed from one sub-system to the other via bunch charge, RMS transverse emittances, RMS bunch length, average energy and RMS energy spread. It is implicitly assumed that the detailed 6D correlations in the beam distribution are not relevant for the achievable luminosity. However, it has recently been shown that those correlations can have a strong effect on the beam-beam interaction.  We present first results on CLIC simulations that integrate linac, beam delivery, and beam-beam interaction. These integrated simulations also allow a better simulation of time-dependent effects, like ground perturbations and interference between several beam-based feedbacks.             Presented at the 8th European Particle Accelerator Conference, 3-7 June, 2002, Paris, France   Geneva, Switzerland 25 July 2002  CLIC SIMULATIONS FROM THE START OF THE LINAC TO THEINTERACTION POINTD. Schulte, R. Assmann, G. Blair, T. D’Amico, G. Guignard, N. Leros, S. Redaelli∗, T. Risselada,F. Zimmermann, CERN, Geneva, SwitzerlandAbstractSimulations for linear colliders are traditionally per-formed separately for the different sub-systems, like damp-ing ring, bunch compressor, linac, and beam delivery.The beam properties are usually passed from one sub-system to the other via bunch charge, RMS transverse emit-tances, RMS bunch length, average energy and RMS en-ergy spread. It is implicitly assumed that the detailed 6Dcorrelations in the beam distribution are not relevant forthe achievable luminosity. However, it has recently beenshown that those correlations can have a strong effect onthe beam-beam interaction. We present first results onCLIC [1] simulations that integrate linac, beam delivery,and beam-beam interaction. These integrated simulationsalso allow a better simulation of time-dependent effects,like ground perturbations and interference between severalbeam-based feedbacks.1 INTRODUCTIONTo design a linear collider and to investigate its potentialperformance, numerous simulations have to be performed.Four of the main areas of research are the main linac, thecollimation system, the final focus system and the beam-beam interaction. Each of these necessitates quite complexinvestigations which are usually performed by experts withspecialised simulation codes. The result of one simulationis fed into the next one in the form of simple values, e.g.bunch charge, RMS emittances and energy spread.This approach, to split the problem into sub-problems,allowed for very efficient development of solutions for eachpart. However, as the designs mature, it becomes nec-essary to improve the simulations by also taking corre-lation effects into account. An early attempt studied theimplications longitudinal effects in in the damping ring,bunch compressor and main linac had on the beam-beameffects [2]. Combined simulations of the TESLA linac andof the interaction of the colliding beams have been per-formed [3]. They showed that the luminosity reduction ascalculated from the emittance growth strongly underesti-mated the real effect.Simulations of the integrated systems are therefore im-portant. Two basic approaches are possible. One can tryto simulate all the systems with a single program or onecan define interfaces which allow to use different programsfor the different sub-systems. The first approach may sim-∗PhD student of the University of Lausanne, Institut de Physique desHautes Energies (IPHE), Switzerland.plify the simulation of the whole machine by a single per-son. The second approach seems simpler if the subsystemsare simulated by different persons with different preferredcodes. It also allows one to easily include specialised sim-ulation codes which are relevant for only one sub-system.We followed both approaches. The first, by extend-ing PLACET [4] to also cover the beam delivery system;a similar extension has been made at SLAC by combin-ing [5] LIAR [6] and DIMAD [7] and at DESY by extend-ing MERLIN [8] to include the simulation of acceleratingstructures. The second approach led to defining a simplefile format which can be used to transfer beams from onecode to another one. This interface has been implementedfor the beam dynamics codes MAD [9], DIMAD [7], MER-LIN and PLACET, for the collimation system simulationBDSIM [10] and for the beam-beam simulation programGUINEA-PIG [11].Comparison between the results of different codes forthe tracking through the final focus system has been per-formed, and the results are reported elsewhere [12]. Cur-rently, a comparison of the results of tracking through thewhole machine including the main linac using MERLIN,LIAR/DIMAD and PLACET is ongoing [13].2 THE PERFECT MACHINEWe first consider a machine without any imperfections.The main linac lattice is taken from [14] and the beam-delivery system (BDS) from [15]. The tracking in the linacand BDS is performed with PLACET; the beam-beam in-teraction is simulated with GUINEA-PIG. The initial nor-malised emittances used are x = 680 nm and y = 5nm,0501001502002503002.7 2.75 2.8 2.85 2.9 2.95 3dL/dE cm [1034cm-2 s-1 TeV-1 ]Ecm [TeV]Figure 1: The luminosity spectrum for the perfect machine.−600 −400 −200 0 200 400 600−20−15−10−505101520X [nm]Y [nm]Figure 2: Particle distribution in the x-y plane at the IP.0.60.811.21.41.61.822.22.40.05 0.1 0.15 0.2 0.25 0.3ratio actual to geometric ∆L 1/L1geometric ∆L/LFigure 3: The loss of luminosity in the peak, L1, due tothe static imperfection in the main linac of CLIC. The ef-fect of the BDS and the beam-beam forces have also beenincluded, but for them no imperfections were taken into ac-count.and the beam is accelerated from 9GeV to 1.5TeV. Thebeta-functions at the interaction point (IP) are βx = 6mmand βy = 70µm. In the main linac the emittance is per-fectly preserved, while it grows in the beam delivery sys-tem. At the IP we find RMS beam sizes of σx ≈ 100 nmand σy ≈ 2.2 nm and emittances x ≈ 3µm and y ≈33 nm. The luminosity is L ≈ 12 × 1034 cm−2s−1; theluminosity spectrum is shown in Fig. 1.More important than the total luminosity is the frac-tion that is useful for the physics experiments. This partvaries, depending on the actual investigation; we use L1 asa figure of merit, which is the fraction of the luminositywith Ecm > 0.99Ecm,0. For the perfect machine we findL1 ≈ 4.9× 1034 cm−2s−1.The simulation was also performed using MAD for thebeam delivery system. The x-y distribution of the particlesat the IP is shown in Fig. 2.-10-8-6-4-202468100 10 20 30 40 50 60 70 80 90 100y [nm]t [s]beam 1beam 2Figure 4: The relative offsets of the two beams at the IP dueto ground motion in the main linacs and the beam deliverysystems of CLIC. No feedbacks are included.3 MAIN LINAC ALIGNMENTThe beam-based correction of static imperfections hasbeen simulated [14, 16]. In [16] the beam-beam interactionhas also been included. But in these simulations the effectof the beam delivery system has been ignored, the wholesystem being replaced by a simple transfer matrix. As indi-cated above, the BDS strongly changes the beam distribu-tion. We have therefore repeated the simulations includingfull tracking through the BDS. For the linac and the BDS,PLACET is used; for the beam-beam, GUINEA-PIG. InFigure 3 the relative luminosity reduction is shown for 50different cases, compared to a perfect machine, if beam-beam forces are neglected. The ratio of actual reductionwith beam-beam forces to the geometric is also shown. Ob-viously the beam-beam effect increases the loss. In thesesimulations the centres-of-charge of the beams were col-liding. Introducing small angles in the bunches may poten-tially lead to an improved luminosity [3], but this needs tobe studied.4 GROUND MOTIONTo simulate the effect of ground motion, a model devel-oped by A. Seryi [17] has been implemented in PLACET.The model takes into account correlations of the groundmotion in space and time. It has been used to fit data ob-tained by measurements at CERN [18]. Figure 4 showsthe motion of the two beams at the interaction point in theabsence of feedbacks.5 PLACET/BDSIM INTEGRATIONThe definition of a common interface describing thebunch structure enabled the interfacing of PLACET to aGEANT4-based program BDSIM. Geant simulation [19]has traditionally been used in the modelling of particledetectors and, for this purpose, it includes a wide rangeof physics processes describing the interaction of elemen-tary particles with materials. BDSIM combines this Geantfunctionality with accelerator-style particle tracking and isthereby well suited to simulating in detail the interactionsof beam particles with materials in the beam delivery sys-tem, and subsequent tracking of secondaries.A prime example is the interaction of off-momentumparticles with spoilers and collimators, where the detailsof edge-scattering will affect the collimation efficiency, asindeed will the tails of phase-space distributions of bunchesexiting the linac. For this reason, interfacing the codes sothat the detailed bunch structure is mapped to full simu-lation of downstream beam element geometries, may beimportant to the final design of the beam delivery system.These effects may be particularly important for the detailedtreatment of halo collimation, where the shapes of tails inthe distributions will be the dominant factors determiningthe global collimation requirements. In this case, smallregions of phase-space may match holes in the collima-tion efficiency, and predicting the occupancy of these holescould be the key to designing an effective collimation sys-tem. The need for exploring integrated simulations in thiscase is thus apparent.As a first step in this direction, this idea was applied tooff-momentum particles in the CLIC baseline beam deliv-ery system [20] where a bunch leaving the linac with an en-ergy 2% lower than nominal was generated with PLACETand interfaced to BDSIM. The results are presented inFig. 5 and show the energy deposition in the beamline ele-ments after the spoiler, which for this simulation was takenas one radiation length of graphite. The absorber, takenas 1 m of iron, clearly absorbs a large fraction of the totalenergy. However, studies of this nature allow subsequentoptimisation of relative positions of spoilers and absorbersin the collimation system. The energy deposition beforethe spoiler is due to synchrotron radiation.6 CONCLUSIONFor the design of future linear colliders it is very im-portant to be able to simulate the whole machine in a con-sistent way. This requires that full advantage be taken ofthe information obtained from the simulation of the differ-ent sub-systems. Therefore integrated simulations are vital.Within the CLIC study, we started this integration processby defining and implementing interfaces and by extendingthe capabilities of programs. First results demonstrate theusefulness of this approach.We would like to thank N. Walker for his support in im-plementing MERLIN, and also Andy Wolsky, M. Hayesand R. Tomas.7 REFERENCES[1] G.Guignard (ed.). CERN-2000-8.[2] F. Zimmermann, K.A. Thompson and K.L.F. Bane, SLAC-CN-417 (1997).z (m)Energy Loss (GeV/m)Energy SpoilerEnergy AbsorberFigure 5: Energy loss in the BDS determined using BDSIMas a function of distance from the linac for a CLIC bunchgenerated in PLACET with energy 2% lower than nominal.The energy loss before the spoiler is due to synchrotronradiation. 1000 1.5 TeV electrons plus secondaries havebeen tracked for this plot. Only the first part of the colli-mation system is shown, since little energy is lost furtherdownstream. Simulation with better statistics needs to beperformed to obtain meaningful results for this region.[3] R. Brinkman, O. Napoly, D. Schulte. CLIC-Note 505 andProc. PAC 2001, Chicago, Illinois, USA.[4] E. D’Amico, G. Guignard, N. Leros, D. Schulte. CERN/PS2001-028 (AE) and PAC 2001, Chicago, Illinois, USA.[5] P. Tenebaum. Private communication.[6] R. Assmann et al. SLAC-AP 103 (1997).[7] R. Servranckx, K. Brown, L. Schachinger, D. Douglas,SLAC-0285 (1990)[8] N. Walker. See ttp://www.desy.de/ merlin[9] H. Grote and F. C. Iselin, CERN-SL-90-13-AP-REV.2.[10] G.A.Blair. CLIC Note 509 (2002).[11] D. Schulte. CERN/PS 99-014 (LP) and Proc. ICAP 1998,Monterey, CA, USA.[12] S. Redaelli, R. Assmann, G. Blair, D. Schulte, F. Zimmer-mann. This conference.[13] D. Schulte, P. Tenenbaum, N. Walker, A. Wolsky, M. Wood-ley. To be published (CLIC-Note 513).[14] D. Schulte. CERN/PS/98-018 (LP) and Proc. EPAC 1998,Stockholm, Sweden.[15] F. Zimmermann et al. This conference.[16] N. Leros and D. Schulte. CERN/PS 2001-029 (AE) and PAC2001, Chicago, Illinois, USA.[17] A. Seryi, T. Raubenheimer. SLAC-PUB-8595 and LINAC2000, Monterey, CA, USA.[18] A. Seryi. PAC 2001, Chicago, Illinois, USA.[19] G.Abril et al.. N.I.M. to be published 2002.[20] F. Zimmermann et al. PAC2001, Chicago, CERN-SL-2001-036 AP, and CLIC Note 493.

CLIC simulations from the start of the linac to the interaction point

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