The final focus systems for the future linear colliders need to focus beams to nm-range spot sizes at the collision point. The design spot size varies from several nm for 500 GeV to the one nm range for 3 TeV. In order to keep the beams in collision and to maintain the luminosity stringent stability optimization must be applied. We discuss different sources of beam perturbations and estimate the expected beamline stability based on previous experimental observations. Possible measures for beam stabilization are discussed and plans of further collaborative efforts are outlined

Adolphsen, C E

Assmann, R W

Brinkmann, R

Frisch, J

Jeanneret, J B

Montag, C

Phinney, N

Raubenheimer, T O

Reyzl, I

Seryi, Andrei

Tenenbaum, P G

Verdier, A

Vos, L

Walker, N

Wildner, E

Zimmermann, Frank

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN    SL DIVISIONGeneva, SwitzerlandJune 2000CERN - SL-2000 - 059    OPCLIC NOTE 448STABILITY CONSIDERATIONS FOR FINAL FOCUS SYSTEMS OF FUTURELINEAR COLLIDERS.R. Assmann, B. Jeanneret, A. Verdier, L. Vos, E. Wildner, F. ZimmermannR. Brinkmann, C. Montag, I. Reyzl, N. Walker - DESY, GermanyC. Adolphsen, J. Frisch, N. Phinney, T. Raubenheimer, A. Seryi, P. Tenenbaum - SLAC, USAAbstractThe final focus systems for the future linear colliders need to focus beams to nm-range spot sizes at thecollision point. The design spot size varies from several nm for 500 GeV to the one nm range for 3 TeV.In order to keep the beams in collision and to maintain the luminosity stringent stability optimizationmust be applied. We discuss different sources of beam perturbations and estimate the expected beamlinestability based on previous experimental observations. Possible measures for beam stabilization arediscussed and plans of further collaborative efforts are outlined.Paper presented at EPAC 200026 - 30 June 2000, Vienna, AutriaSTABILITY CONSIDERATIONS FOR FINAL FOCUS SYSTEMS OFFUTURE LINEAR COLLIDERSR. Assmann, B. Jeanneret, A. Verdier, L. Vos, E. Wildner, F. Zimmermann, CERN, SwitzerlandR. Brinkmann, C. Montag, I. Reyzl, N. Walker, DESY, GermanyC. Adolphsen, J. Frisch, N. Phinney, T. Raubenheimer, A. Seryi, P. Tenenbaum, SLAC, USAAbstractThe Final Focus systems of future linear colliders need to focusbeams to nm-range spot sizes at the collision point. The designspot size varies from several nm for 500 GeV to the one nm rangefor 3 TeV. In order to keep the beams in collision and to maintainthe luminosity stringent stability optimization must be applied.We discuss different sources of beam perturbations and estimatethe expected beamline stability based on previous experimentalobservations. Possible measures for beam stabilization are dis-cussed and plans of further collaborative efforts are outlined.1 INTRODUCTIONThe three major proposals for future linear colliders,NCL/JLC [1], TESLA [2] and CLIC [3], differ in theirdesign choices, especially for the main linac RF systems.This paper makes no attempt to review those choices norto explain their rationale. In spite of their differences, thedesigns all require similar values for the vertical beam sizeat the interaction point. The design vertical spot size is 5nm for TESLA [2] at 0.5 TeV, 2.7 nm for NLC/JLC at 1TeV and 1 nm for CLIC [4] at 3 TeV center of mass en-ergy. The nanometer size beams imply rigid stability re-quirements which are similar for all three proposals.The most severe stability tolerance is for the finalquadrupoles that focus the beams at the interaction point.A vertical displacement y of these quadrupoles causesroughly the same beam position offset at the interactionpoint. If the relative beam offsets at the IP are to be lessthan 10% of the vertical beam size, then the tolerance forvertical movements of the final magnets are roughly 0.5 nmfor TESLA, 0.27 nm for NLC and 0.1 nm for CLIC. Typ-ical tolerances for other elements in the final focus systemare about 10 times looser.Present experience with beam stabilization comes fromthe first prototype collider and the test facilities:1. SLAC Linear Collider (SLC): 45 GeV beams with 500nm vertical beam size were collided at 120 Hz for pro-duction of Z-bosons. The pulse-to-pulse jitter was es-timated to about 30% of the vertical beam size.2. Final Focus Test Beam (FFTB): For a 45 GeV beamat 30 Hz and an effective vertical spot size of 70 nma pulse-to-pulse jitter of 40 nm was measured (domi-nated by vibrations of the beam size monitor) [5].Many effects which contributed to this beam jitter havebeen understood and should be less important in future de-signs. These include poor design of quadrupole supports,large wakefields (which will be much smaller in all de-signs), absence of longitudinal beam collimation, etc. Nev-ertheless, the experience at the SLC and the FFTB showsthat the requirements for future linear colliders are not eas-ily achieved. New technical solutions and methods of beamstabilization are required. In this paper we discuss funda-mental limits of beamline stability, given by ground mo-tion, consider the various techniques proposed to overcomethese limits, and outline plans for further collaboration.2 GROUND MOTIONThe natural stability of a beamline is limited by the motionof the supporting ground. Ground motion can be dividedinto “slow” (f < 0:1 Hz) and “fast” (f > 0:1 Hz). Fastground motion has been studied extensively and we sum-marize below its main qualitative features.The power spectrum of natural ground motion is a steepfunction of frequency which falls off as 1=f4. Above 1 Hz,the amplitude is affected by ”cultural” noise; the powerspectrum can be several orders of magnitude higher thanthe natural one. The correlation of the ground motionshows what part of the total motion is actually dangerousfor a collider. It depends on sound velocity in surroundingmedia and on distribution and nature of the noise sources.Sites on hard rock have high sound velocities and good cor-relation; sites on sediment tend to have poor correlation.A major part of the on-surface produced cultural noisedue to cities, traffic, etc. is localized within about onewavelength near the surface. Therefore shallow tunnels lo-cated in urbanized areas are usually noisy, like the HERAtunnel, regardless of the geology (rock/sediment). In con-trast, deep tunnels, like LEP, are much quieter. Table.1shows the typical rms amplitude of the ground motion inseveral frequency bands for a deep quiet tunnel with goodcorrelation (LEP) compared with a shallow noisy tunnelwith moderate correlation (HERA) [6, 15, 20]. The ”quiet”amplitude is better than required for any of the currentlyproposed linear colliders. A ”noisy” location would be un-acceptable unless feedbacks were able to damp the motion.Slow motion (f < 0.1 Hz) produces misalignments of thecollider components which would cause emittance growthif not reduced/removed by beam-based feedback. In con-trast with fast motion, it does not constitute an immediatelimitation for a linear collider. The diffusive ATL model ofslow ground motion [16] improved understanding and pre-Table 1: Typical rms amplitude of ground vibrations in quiet(LEP deep tunnel) and noisy conditions (HERA shallow tunnel).Single point motion (“total”) and the uncorrelated part for L =20 m. Note that relative motion isp2 the uncorrelated part.f rms y ”Quiet”, nm rms y ”Noisy”, nmHz total uncorrelated total uncorrelated3–1 0.2 0.02 50 2010–1 0.05 0.015 5 530–1 0.01 0.007 3 3dicts that the misalignment of two points is proportional totheir distance L and elapsed time T : hy2i = ATL. A is asite/condition/geology specific parameter whose observedrange is 10−9 – 10−4 m2/s/m. There is still controversyover the region of validity. The T dependence has beenconfirmed in the minute to month time scale. For years,the T dependence may change to T 2 due to systematic mo-tion [17]. Careful investigation of the spatial dependenceof slow motion is subject of further studies.The noise generated in the tunnel is of particular con-cern. Accelerator equipment which is not carefully de-signed may produce unacceptably large noise. For exam-ple, the ventilation system located near point 5 of LEP in-creases the amplitude for f > 3 Hz by more than a factor often [6]. Key factors to reduce the in-tunnel generated noiseare careful design of accelerator equipment, avoidance ofresonances with modes which propagate along the tunnel,use of passive/active vibration damping.Special care must be taken with beam-induced vibrationin the interaction region. Analytical studies of the beam-induced thermal shock waves suggest that significant vi-bration may be induced in the beam dump [8]. Proper mea-sures must be taken to minimize those vibrations and todecouple them from the magnets. The noise in the experi-mental areas is another concern. Detector equipment mayproduce additional noise. Also, tunnel discontinuities (likelarge experimental halls) may alter/destroy the correlationof motion. The relative motion measured 17 meters leftand right across a HERA IP was found to be about 100 nmabove 1 Hz [7]. Further investigations of the contributingfactors should be done in HERA and other colliders.Three key approaches can be relied upon to ensure suf-ficient stability of the final focus: a naturally quiet site,proper design of accelerator equipment, development anduse of correction methods that overcome vibration prob-lems. For the three different designs of a linear collider,the balance between these three approaches is different, asdescribed in the following sections.3 PASSIVE AND ACTIVE SUPPORTThe magnets are carried by support structures that unavoid-ably transport the ground motion to the magnet. The driv-ing ground motion is usually somewhat enhanced, leadingto larger magnet motion. Measurements at LEP show anamplification of between 10 and 30 in the range of 0-100Hz with peaks at the acoustical resonance frequency of thequadrupole stand (33Hz) and the acoustical resonance fre-quency of the LEP tunnel (diameter 3.76 m). It has beenshown at the FFTB that a well designed stand can signifi-cantly reduce this amplification. In the vertical plane, thedifference between the motion of a magnet and the tunnelfloor was measured to be 2 nm for f > 3 Hz while theaccelerator was operating [9]. In contrast, the horizontalplane had a much greater difference between magnet andfloor, likely because the magnet stands are rather narrowand high  2 m. Alternately, passive damping can be usedto damp the high frequency motion. For example, passivedamping pads were placed between the supports and thefloor at the Advanced Photon Source to reduce the highfrequency motion which drove girder resonances [19]. An-other example, is the multi-layered approach taken at LIGOto stabilize the interferometer mirrors [18].Active magnet supports can also be used to damp themagnet motion. Here, the magnet motion is continuallymonitored with accelerometers. Piezo-actuators are incor-porated into the design of the support and are used within afeedback circuit to compensate the measured motion. Suchan active support system has been built and tested at DESY[10]. Damping was achieved in the frequency range from5 Hz to 25 Hz. The magnet motion was measured to be10 nm above 4 Hz, about a factor of three smaller than themotion of the supporting ground. It is believed that activestabilization schemes can stabilize magnets at the nm- orsub-nm level. Special schemes have been proposed for thestabilization of the final doublet that has the tightest toler-ances. A so called optical anchor could measure the stabil-ity in the interaction region using laser interferometry [11];experimental studies are ongoing.4 BEAM-BASED FEEDBACKAll linear collider designs have tolerances on the align-ment of beamline components which require continuousbeam-based feedback to counteract performance deterio-ration. Misalignments can change the trajectories so thatthe beams no longer collide optimally. They can also causeemittance growth and introduce aberrations which increasethe beam size. NLC plans to use a multi-layered approachon different time scales. A slow ”feedback” will movequadrupoles and structures onto the beam trajectory every˜30 minutes to compensate for slow ground motion. Inter-pulse feedback in a few locations will correct the accumu-lated trajectory error between beam-based alignments. Fastkickers may also be used to straighten out the bunch train.Finally, a very fast feedback capable of acting within thebunch train will be used to keep the beams in collision.Similar systems are proposed for the other collider designs.An important topic for further study is to simulate and un-derstand the interaction of these different systems whichmust work together.Inter-pulse feedback will be used to measure and cor-rect the average properties of a bunch train throughout alinear collider. Such systems were used extensively atthe SLC but their performance was poorer than predicted.These problems are now understood from simulations andan improved algorithm has been developed [12]. The maxi-mum rate of beam measurements used for inter-pulse feed-back is set by the repetition frequency of the machine. TheSLC feedback system was optimized to damp motion atfrequencies below about 120 of the sampling rate; a moreaggressive algorithm might have a cross-over point as highas 16frep. The response time of this type of feedback mustbe carefully optimized with respect to the position monitorresolution. This has been studied in detail for TESLA.Intra-pulse feedback operates at high frequency andacts within a bunch train. The operational scenario of lu-minosity stabilization has been studied in detail for TESLA[13]. In total, three intra-bunch train feedback systems areenvisioned: one at the end of each main linac (optional),one in each chromatic correction section (CCS) and one atthe IP. At the end of the linac the feedback removes verticalposition and angle jitter expected to be 0:5. In the CCS,it will correct the angles at the IP. At the IP, it removes therelative orbit jitter between the two beams by measuringthe beam-beam deflection and steering the beams back intocollision. The large bunch spacing of 337 ns in TESLAmakes it possible to damp frequencies of up to 170 KHz,covering most sources of noise. In TESLA, a train-to-trainjitter up to 35y in beam separation and 5y′ in crossingangle is predicted (with 70 nm uncorrelated quadrupole vi-bration in linac and final focus). Simulations have shownthat the beam-beam interaction can be successfully stabi-lized within 0:1y and 0:10y except for the first 3% ofeach bunch train. An intra-pulse IP feedback option hasbeen studied for NLC [14] and CLIC but the shorter bunchspacing makes its implementation more challenging.5 BEAM SIMULATIONSSimulation of beam stability in linear colliders is a de-manding enterprise. The simulation must include realis-tic time-dependent models of initial beam conditions (posi-tion, angle, charge, longitudinal and transverse beam distri-butions), alignment changes (two dimensional ground mo-tion model with correlations), magnetic field variations,and correction systems (orbit correction, slow and fastfeedbacks). The proper design of the collimation systemrequires an accurate tracking of beam tails from the damp-ing rings, through the bunch compressors, the linac, thecollimation, into the Final Focus.Some requirements have been implemented into trackingprograms. LIAR [21] has been developed for linac simula-tions, verified during SLC operation, and includes detailedground motion and feedback models. It can be run on par-allel computing facilities, opening new possibilities for thestudy of beam tails and halos. It is planned to incorporatebunch compression and the Final Focus into this program.MERLIN [22] has been developed for the TESLA Final Fo-cus system. It includes imperfections and feedback models.It is planned to incorporate a collimation model.6 CONCLUSION AND FURTHER PLANSThe stability requirements for the Final Focus systems offuture linear colliders have been reviewed. In most cases,the required stability is close to the limit due the naturalground motion, without added human-made noise. Thecultural and technical noise levels found in existing accel-erators are larger than tolerable, but well constructed tech-nical equipment with good passive damping is expectedto improve these levels significantly. Supports with activedamping can further reduce vibrations although these tech-niques need further verification from planned experiments.If the vibration of magnetic elements cannot be com-pletely avoided, beam-based feedback can compensate forthe effect on the beam. The TESLA design with a longbunch spacing supports a very efficient intra-bunch trainfeedback, which would make it possible to tolerate eventhe large magnet motion observed at DESY. The shorterbunch spacing for NLC and CLIC make it more difficulttechnically to implement intra-bunch feedback, especiallyfor CLIC. An intra-bunch train feedback will, however, stillrelax the tolerances for NLC and to some extent for CLIC.Inter-pulse feedback is included in all designs to compen-sate for the deteriorating effects of slow alignment drifts.Further development of simulation programs is needed inorder to verify the feasibility of the design choices.Additional topics that require further studies and werenot discussed in this paper include temporal stability ofmagnetic field centers in the nm-regime and the effects oftime-varying stray fields.7 REFERENCES[1] T. Raubenheimer (ed.). SLAC Report 474.[2] R. Brinkmann. Proc. EPAC 1998.[3] J.P. Delahaye et al. Act.Phys.Polon.B30(2029-2039).[4] F. Zimmermann et al. These proceedings.[5] M. Woods et al. FFTB98-03.[6] V.M. Juravlev et al. CERN-SL-93-53.[7] C. Montag. These proceedings.[8] B. Jeanneret, E. Wildner. CERN-SL-99-072-AP.[9] R.J. Fenn, T. Slaton, M. Woods. LCC-0036 (2000).[10] C. Montag, NIM A378(1996)369-375.[11] T. Markiewicz, T. Maruyama. Proc. 1999 LCWS.[12] L. Hendrickson et al. SLAC-PUB-8055 (PAC99).[13] I. Reyzl. These proceedings.[14] D. Schulte. CERN-OPEN-99-243.[15] C.Montag, V. Shiltsev, in Proc. PAC95.[16] B.Baklakov, et al. Tech. Phys. 38, 894 (1993).[17] R.Pitthan, SLAC-PUB-7043,8286, 1995,1999.[18] J. Giaime et al., Rev. Sci. Instrum. 67, 208-214 (1996).[19] G. Decker et al. In Proc. PAC97.[20] A. Seryi. These proceedings.[21] R. Assmann et al. SLAC/AP-103.[22] N. Walker. http://www.desy.de/~njwalker/merlin.htm

Stability Considerations for Final Focus Systems of Future Linear Colliders

The Final Focus systems of future linear colliders need to focus beams to nm-range spot sizes at the collision point. The design spot size varies from several nm for 500 GeV to the one nm range for 3 TeV. In order to keep the beams in collision and to maintain the luminosity stringent stability optimization must be applied. We discuss different sources of beam perturbations and estimate the expected beamline stability based on previous experimental observations. Possible measures for beam stabilization are discussed and plans of further collaborative efforts are outlined

Stability Considerations for Final Focus Systems of Future Linear Colliders

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