The present parameters of the CLIC study require the collision of small emittance beams with a vertical spot size of 1 nm. The tolerances on vertical quadrupole vi-bration (above a few Hz) are as small as a few nm in the linac and most of the Final Focus. The final focusing quadrupole has a stability requirement of 4 nm in the horizontal and 0.2 nm in the vertical direction. Those tol-erances can only be achieved with the use of damped support structures for CLIC. A study has been set-up at CERN to explore the application of stabilization devices from specialized industry and to predict the time-dependent luminosity performance for CLIC. The results will guide the specification of required technological im-provements and will help to verify the feasibility of the present CLIC parameters

Aleksa, Martin

Assmann, R W

Coosemans, Williame

Guignard, Gilbert

Leros, Nicolas

Mayoud, Michel

Redaelli, S

Ruggiero, F

Russenschuck, Stephan

Schulte, Daniel

Wilson, Ian H

Zimmermann, Frank

CERN Document Server

THE CLIC STUDY OF MAGNET STABILITY AND  TIME-DEPENDENT LUMINOSITY PERFORMANCE  M. Aleksa, R. Assmann, W. Coosemans, G. Guignard, N. Leros, M. Mayoud, S. Redaelli, F. Ruggiero, S. Russenschuck, D. Schulte, I. Wilson, F. Zimmermann, CERN, Geneva, Switzerland  Abstract The present parameters of the CLIC study require the collision of small emittance beams with a vertical spot size of 1 nm. The tolerances on vertical quadrupole vibra-tion (above a few Hz) are as small as a few nm in the li-nac and most of the Final Focus. The final focusing quad-rupole has a stability requirement of 4 nm in the horizon-tal and 0.2 nm in the vertical direction. Those tolerances can only be achieved with the use of damped support structures for CLIC. A study has been set-up at CERN to explore the application of stabilization devices from spe-cialized industry and to predict the time-dependent lumi-nosity performance for CLIC. The results will guide the specification of required technological improvements and will help to verify the feasibility of the present CLIC pa-rameters.  1 INTRODUCTION The Compact Linear Collider (CLIC) study at CERN proposes an e+e- collider that combines a 3 TeV centre-of-mass energy, high-gradient acceleration of 150 MV/m, and a high luminosity of 1035 cm-2 s-1 [1]. Some important CLIC parameters are listed in Table 1. The required spot sizes in the interaction point (IP) are σx* = 43 nm in the horizontal and σy* = 1 nm in the vertical plane. This is below the smallest single beam size of 70 nm so far ob-served [2]. The SLC beams were reliably collided with a vertical spot size of 500 nm [3].  The optical design of a CLIC beam delivery section has been described for a standard [4] and short [5] version. The small IP spot sizes for CLIC and the required small beam emittances greatly enhance the sensitivity of the luminosity to imperfections [6]. Imperfections occur along the whole linac and beam delivery sections (align-ment offsets, mechanical vibrations, ) and reduce the achievable luminosity in two ways:  1. Imperfections generate emittance growth ∆γε(t). The required vertical IP emittance (γεIP = 0.02 mm-mrad) is small and ∆γε(t) must be kept small as well (~0.005 mm-mrad). This imposes tight tolerances on the stability of transverse quadrupole positions. 2. Imperfections (mainly transverse quadrupole offsets) generate trajectory errors that are different for the two colliding beams, resulting in a time dependent beam-beam offset ∆yIP. The design luminosity L0 is then reduced to a value L, which we approximate as:    22 */ 20yIPyL L e       . (1) \ Table 1: Some basic CLIC parameters [1]. Parameter Symbol Unit Design value Beam energy E GeV 1500 Luminosity L cm-2 s-1 1035 IP spot size σx* nm 43  σy* nm 1 Repetition rate frep Hz 100 Bunches/train Nb  154 Bunch charge Ne  4 109 Bunch distance ∆Tb ns 0.67  The beam-beam offset must be a fraction of the IP spot size. This imposes stringent tolerances on the quadrupoles, with the tightest requirement occurring for the final quadrupoles.  The values for ∆γε(t) and ∆yIP and therefore the lumi-nosity depend on the performance of all systems after the damping rings and cannot be studied in test facilities. They must be predicted based on the measured properties of accelerator components and detailed simulations. Most important is the time-stability of the quadrupole magnetic centres. The magnetic centre of a quadrupole in the nanometre regime is affected by the mechanical position of the overall magnet, internal mechanical deformations, and changes of the quadrupole field with fixed geometry (for example heat induced in a permanent magnet). The CLIC stability study aims at estimating the time-stability of the magnetic quadrupole centre and at predict-ing the achievable luminosity, after application of correc-tion schemes (e.g. feedbacks). For example, extending the formalism from [7], we can write the time average of the beam-beam offset ∆yIP as an integration over frequency ω and wave number k:   20 0( , ) ( ) ( ) ( )2 2IP tdk dy P k T G k F           . (2)  Here, P(ω,k) describes the spectrum of noise like ground motion, cooling water, or heat-induced errors. T(ω) is the transfer function to the magnet (e.g. from magnet sup-ports), G(k) is the lattice response, and F(ω) is the feed-back transfer function. The CLIC stability study should estimate all those functions, investigate the best available technologies (e.g. determine T(ω) for actively damped magnet supports), and predict the achievable CLIC lumi-nosity. The final goal is to establish the feasibility of the CLIC design parameters in a realistic accelerator envi-ronment. 0-7803-7191-7/01/$10.00 ©2001 IEEE. 3774Proceedings of the 2001 Particle Accelerator Conference, Chicago2 CLIC REQUIREMENTS In the first phase of the study the requirements were re-viewed for the CLIC linac and the beam delivery:  (a) Linac quadrupoles:  Number: 1300 for each of two linacs Field:  200 T/m Transverse size: 0.15 × 0.11 m  (width × height) Length:  0.46  2.08 m Weight:  69  312 kg   Goal: 1.3 nm (vertical) rms  uncorrelated motion above 4 Hz  (b) Final Focus quadrupoles (short or standard solution):  Number: 2 Field:  388  or  450 T/m Transverse size: 4.3  or  2.0 cm   (outer radius) Length:  3.50  or  4.75 m Weight:  250  or  50 kg Distance to IP: 4.3 or 2.0 m Goal:  4.0 nm (horizontal)   0.2 nm (vertical)  rms  uncorrelated motion above 15 Hz For the linac, the estimated requirements are based on an extrapolation of the existing CLIC Test Facility quad-rupoles. For the final focusing quadrupole the vibration tolerances were calculated with the program FFADA, as discussed in [4]. A permanent ring magnet was found to be the most adequate solution, in view of the very limited space, the necessary high field gradient and the stringent requirements on the mechanical stability. Figure 1 shows how the 16 premagnetized sectors will be assembled into a ring of permanent magnet material (zero clearance de-sign). Because of its thermal stability and radiation hard-ness, Sm2Co17 was found to be the best suited material. Field computations with ROXIE [8] showed that the re-quired field gradient of 450 T/m is feasible. To avoid de-magnetization effects, the outside field (e.g. detector so-lenoid field) has to stay below 2T. The frequency requirements, as listed for the tolerable motion of linac and final focusing quadrupoles, are based on the feedback performance, that was achieved at the SLC [9]. We note that the linac quadrupoles require less stringent, but quite challenging vertical position stability, down to lower frequencies. A large number of quadru-poles must be controlled to the level of 1.3 nm.  The final quadrupoles in the beam delivery have a strict vertical stability tolerance of 0.2 nm, but above a higher frequency (faster feedback on the IP offset). Only two quadrupoles need to be controlled, but they are located inside the particle physics detector with severe mechani-cal constraints, mainly arising from the crossing angle [10] and the properties of the spent beam (see Figure 2). We note, that the natural ground motion in quiet por-tions of the LEP tunnel was measured to be smaller than 0.2 nm above 4 Hz (accelerator off) [11]. However, levels approached 20 nm with the accelerator equipment  Figure 1: Cross section of the final focusing quadrupole. G=468 T/m for the permanent magnet material VACO-MAX 225HR Sm2Co17 (coercitivity of 820 kA/m). 18 mDetectorSolenoidfieldExit beam (e-)0.6 mQuad QuadExit beam (e+)18 mDetectorSolenoidfieldExit beam (e-)0.6 mQuad QuadExit beam (e+) Figure 2: Top view of the CLIC IP region with the detec-tor, the colliding beams, and the final quadrupoles for the standard (left) and short (right) beam delivery design. Crab cavities, fast kickers, beam monitors are required as well, but are not included. Scales are indicated; the trans-verse radius of the detector is about 17 m. switched on. The IP environment is known to be espe-cially noisy, with measured motion in large particle phys-ics detectors being in the order of a few 100 nm [12]. A proper design can improve those levels, but technical en-hancement of natural ground motion should be expected and prepared for. 3 VIBRATION TEST STAND A vibration test stand has been set-up at CERN. The lo-cation was required to have decent ground motion (not too noisy, not too quiet), electricity and cooling water for magnets, and sufficient space for a test set-up. The base-ment of a building on the CERN Meyrin site, also used for CLIC alignment studies, was selected. The room was equipped with a granite table and measurement equip-ment, among those two geophones (GSV-310 from Geo-Sig). The geophones have a frequency range from 1-315 Hz with a nominal resolution of 15.3 nm/s. Having established the stability requirements for CLIC, the next step was to demonstrate vibration measurements with sub-nm resolution and to characterize the ground motion in our test stand. The measurements provide ve-3775Proceedings of the 2001 Particle Accelerator Conference, Chicago0 50 100 150 200 25010-510-410-310-2Frequency, HzRms motion with f>f 0, µm0 50 100 150 200 25010-1210-1010-810-6Frequency, HzPower Spectral Density, µm2 /HzDifference Difference  Figure 3: Power spectral density (left) and integrated rms motion above a given frequency (right), measured in the vibration test stand with two equal sensors. Shown are the absolute measurements and the difference between the two sensors for the vertical direction. locities v(n), observed at times n∆t, where n is an integer and ∆t is the sampling time. The power spectral density with frequencies fk=k/(N ∆t) for N measurements is then:   2322 21( ) ( )2Nikn Nd knN tP f v n ek   (3) The integrated rms motion I(fk0) above a given fre-quency fk0 is:     00/2''1 Nk d kk kI f P fN t  (4) An example of power spectral density and integrated motion is shown in Figure 3. More detailed measurements are described in [13]. The resolution of the available equipment was determined to be about 30 nm/s above 6 Hz. The motion of the concrete floor in the test stand was measured to have the following rms motion: • Above 4 Hz: 5.4 nm ± 0.88 nm • Above 15 Hz: 3.7 nm ± 0.54 nm • Above 100 Hz: 0.1 nm ± 0.04 nm The error is an upper estimate, based on the difference between the measurements from two identical sensors located side-by-side. We note that the concrete floor ex-hibits a good stability (taking account the location and industrial-like environment), which is however 5-10 times above the CLIC goal. 4 STABILIZATION TECHNOLOGY The vibration test stand will be used to install state-of-the-art stabilization equipment from specialized industry. Mounting quadrupoles on top of the actively and pas-sively damped tables, we will measure the magnet mo-tion. The effect of cooling water and other accelerator supports will be quantified. At the present time, the equipment is not yet acquired. However, as a first exam-ple we studied the effect of putting rubber feet on one sensor, while connecting the other sensor directly to the supporting table. The result is shown in Figure 4, illustrat-ing the benefit of passive damping for higher frequencies, while lower frequency perturbations are enhanced. 0 50 100 15010-410-310-210-1Frequency, HzRms motion with f>f 0, µmWithout rubber feetWith rubber feet Figure 4: Integrated rms motion above a given frequency with and without rubber feet under the sensors. 5 SUMMARY AND OUTLOOK As part of the CLIC study at CERN, we initiated a study on magnet and luminosity stability. The goal of this study is to establish the feasibility of the present CLIC design parameters, especially the collision of beams with a vertical spot size of 1 nm.  The stability study was approved in January 2001. By now, we did some preliminary magnet and IP design work, we estimated the stability requirements for the pre-sent CLIC parameters, we set-up a vibration test stand at CERN, we established vibration measurements with sub-nm resolution, and we characterized ground motion in the test stand.  To achieve a rms vertical magnet vibration of 1.3 nm above 4 Hz (linac) and 0.2 nm above 15 Hz (IP), we fore-see to use modern stabilization technology. State-of-the-art equipment will be acquired and tested for usage in the environment of particle accelerators. The measured data will be put into simulations to predict the luminosity, achievable in a realistic accelerator environment. 6 REFERENCES [1] G. Guignard (ed) et al, A 3 TeV e+e - Linear Collider Based on CLIC Technology, CERN 2000-008. [2] V. Balakin et al, Focusing of submicron beams for TeV Scale e+ e- linear colliders, Phys. Rev. Lett. 74 (1995) 2479-2482. [3] P. Raimondi et al, Recent Luminosity Improvements at the SLC, HEACC98. SLAC-PUB-7955. [4] F. Zimmermann et al, Final-Focus System for CLIC at 3 TeV, EPAC2000, CERN-SL-2000-057-AP. [5] F. Zimmermann et al, Final-Focus Schemes for CLIC at 3 TeV, HEACC2001, CERN-SL-2001-010-AP. [6] R. Assmann et al, Stability Considerations for Final Focus Systems of Future Linear Colliders, CERN-SL-2000-059-OP. [7] C. Adolphsen et al, Zeroth-Order Design Report for the Next Linear Collider, SLAC Report 474. [8] S. Russenschuck et al, Integrated Design of Superconducting Mag-nets with the CERN Field Computation Program ROXIE, ICAP2000, CERN-LHC-Project-Report 446. [9] L. Hendrickson et al, Beam Based Feedback Testing and Simula-tions for the SLC linac, Linac2000, SLAC-PUB-8585. [10] D. Schulte et al, The crossing angle in CLIC, these proceedings. [11] V.M. Juravlev et al, Seismic vibration studies for future linear colliders, EPAC94, CERN SL 94-41 RF. [12] A. Seryi, A Shaky Road to Subnanometer Beams: NLC Ground Motion, Vibration and Stabilization Studies, HEACC2001, SLAC-PUB-8825.  [13] R. Assmann et al, Measuring Vibrations with Sub-Nanometer Accuracy, to be published. 3776Proceedings of the 2001 Particle Accelerator Conference, Chicago

The CLIC Study of Magnet Stability and Time-dependent Luminosity Performance

The CLIC Study of Magnet Stability and Time-dependent Luminosity Performance

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