http://accelconf.web.cern.ch/AccelConf/IPAC2011/papers/tupz002.pdfInternational audienceWe describe a high β∗ optics developed for the ATLAS detector at the LHC interaction point IP1. Roman Pots have been installed 240 m left and right of IP1 to allow to measure the absolute luminosity and the total elastic cross section for ATLAS with ALFA (Absolute Luminosity for ATLAS). Ultimately, it is planned to perform these mea- surements at a very high β∗ of 2625 m. Here we describe a new, intermediate β∗ = 90 m optics, which has been opti- mized for compatibility with the present LHC running con- ditions. We describe the main features and the expected performance of this optics for ALFA

Burkhardt, Hans

Cavalier, S.

Peskov, A.

Puzo, P.

HAL-IN2P3

90 m β* OPTICS FOR ATLAS/ALFAS. Cavalier∗, P. Puzo, LAL,Universite´ Paris Sud 11H. Burkhardt, CERN, Geneva, SwitzerlandAbstractWe describe a high β∗ optics developed for the ATLASdetector at the LHC interaction point IP1. Roman Potshave been installed 240 m left and right of IP1 to allow tomeasure the absolute luminosity and the total elastic crosssection for ATLAS with ALFA (Absolute Luminosity forATLAS). Ultimately, it is planned to perform these mea-surements at a very high β∗ of 2625 m. Here we describea new, intermediate β∗ = 90 m optics, which has been opti-mized for compatibility with the present LHC running con-ditions. We describe the main features and the expectedperformance of this optics for ALFA.INTRODUCTIONALFA (Absolute Luminosity For ATLAS) aims to mea-sure absolute luminosity and total cross section for the AT-LAS experiment located at IP1. For this purpose, 8 RomanPots have been placed on both sides of the LHC ring, forbeam 1 on one side and for beam 2 on the other side, at 240m from the interaction point 1 (IP1) of the LHC. These Ro-man Pots (RPs) use scintillating fibers for particle detectionand should allow to detect elastic proton scattering down tovery low momentum transfer t of about−6.5×10−4GeV2.To reach this low value of t, the ATLAS/ALFA detectorsrequire special very high beta optics. For the design beamenergy of the LHC of 7 TeV, an optic with a β∗ of 2625m [1] was designed for this purpose. The measurementcan only be achieved with a good precision, if the beamdivergence σ′ =√/β∗ remains small compared to thescattering angle of, in this case, 3.5μrad. The roman potdetectors will have to operate very close to the beam, ata small number of σ’s, were σ stands for the r.m.s. beamsize.Table 1: 2625 m optics parameters for beam 1, at 7 TeVbeam energy and for an emittance of N = 1μm. D is thedispersion and σ′ the beam divergence.LHC version V6.503IP1 RPsn (μm) 1.0 βx (m) 95.2-97.9β∗x (m) 2625 βy (m) 123.9-117.1β∗y (m) 2625 σx (μm) 113-114α∗x 0.0 σy (μm) 129-125D∗y (m) 0.0 σ′x (μrad) 1.19-1.17D∗′y 0.0 σ′y (μrad) 1.04-1.07σ∗ (mm) 0.593 Δμx (2π) 0.534-0.541σ∗′(μrad) 0.226 Δμy (2π) 0.247-0.252∗ cavalies@lal.in2p3.frThe optics parameters are summarized in Table 1. The2625 m optics also require a normalized emittance close to1μm which is 2 times smaller than currently achieved andan inversion of the polarity of the Q4 magnet which is notcompatible with the present LHC operation.The intermediate β∗ = 90m optics described here wasdeveloped to allow to study how higher β∗ values can bereached in the LHC in conditions which are not too far fromstandard operation and at the same time to allow for firstphysics measurements in the very forward region.ATLAS/ALFA 90 M β∗ OPTICDESCRIPTIONThe 90 m β∗ optics has been described in [2] and [3].The main parameters useful for ATLAS/ALFA are the fol-lowing:Table 2: 90 m Optics Parameters for Beam 1. LHC opticsV6.503LHC version V6.503IP1 RPsn (μm) 3.75 βx (m) 193.5-124.2β∗x (m) 90 βy (m) 857.5-780.4β∗y (m) 90 σx (μm) 374-353α∗x 0.0 σy (μm) 926-883D∗y (m) 0.0 σ′x (μrad) 2.67-2.83D∗′y 0.0 σ′y (μrad) 1.08-1.13σ∗ (mm) 0.3 Δμx (2π) 0.515-0.519σ∗′(μrad) 3.33 Δμy (2π) 0.249-0.250As can be seen on Table 2, the main parameters used inthe 2625 m β∗ optics have been kept in the 90 m β∗ optic.In particular, the phase advance between the IP and the RPsis equal to 90 ˚ and the dispersion is equal to zero.The general scheme aims at focussing the particles withthe same angle at the same position in the detectors. They-position in the roman pots is directly related to the scat-tering angle θ∗y at the IP byy =√βyβ∗y θ∗y , (1)where βy is the vertical β-function at the roman pot. Usingthe relation, the scattering angle at the IP is measured asvertical position at the Roman pot detector.The 90 m β∗ optics described here and shown in Figs. 1and 2 was designed to be similar to the 90 m optics de-veloped for the TOTEM experiment installed at IP5 to al-low for an efficient commissioning and operation of bothTUPZ002 Proceedings of IPAC2011, San Sebastián, Spain1798Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)01 Circular CollidersA01 Hadron Collidersoptics in parallel. As TOTEM Roman Pots are located at220 m instead of 240 m for ATLAS/ALFA, a rematch toobtain the π/2 phase advance was required. The initial IP5quadrupole strengths have been used as start values for therematch for IP1.0.0 200. 400. 600. 800. 1000. 1200.IP1_90m_b1 MAD-X 4.01.47  24/02/11 14.51.200.0200.400.600.800.1000.1200.1400.1600.1800.2000.-1.0-0.50.00.51.01.52.02.53.0β x β y Dx Dys (km)Dx,y (m)  β x , β y (m)RP240 mQ4Q5Q6 Q4 Q6Q13Figure 1: 90 m ATLAS/ALFA optics for beam 1.0.0 200. 400. 600. 800. 1000. 1200.IP1_90m_b2 MAD-X 4.01.47  24/02/11 14.51.360.0200.400.600.800.1000.1200.1400.1600.1800.2000.-1.0-0.50.00.51.01.52.02.53.0β x β y Dx Dys (km)Dx,y (m)  β x , β y (m)RP240 mQ4Q5Q6 Q4 Q6Q13Figure 2: 90 m ATLAS/ALFA optics for beam 2.This optics is reached by an un-squeeze fromβ∗ = 11 m. used at injection and throughout the energyramp, in 17 steps by strength interpolation between 17intermediate optics files. All the un-squeeze files weregenerated for IP5 files and the quadrupole strengths areidentical until the last intermediate steps at β∗ = 75m.The quadrupole strengths ratio r beam 1 over beam 2,0.5 < r < 2, has been kept in the allowed range. Onlythe final step at β∗ = 90 m differs between IPs 1 and 5 tofulfill the different requirements for the ATLAS/ALFA andTOTEM experiments (The phase advance reaches its finalvalue of 90 ˚ only for 90 m).Throughout the un-squeeze, the beams are kept sepa-rated using a parallel constant separation at the IPs. Inthe LHC the standard injection and ramp uses a separa-tion which is horizontal in IP1 and a vertical crossing an-gle as illustrated in Fig. 3, whereas at IP5 the separationis vertical with a horizontal crossing angle. This requires acompletely different set of parallel separation bumps for allintermediate steps from 11 m to 90 m to conserve the hor-izontal separation in IP1 and the vertical separation in IP5up to a β∗ = 90m.0.0 200. 400. 600. 800. 1000. 1200.s (m)+/-7 sigma sep in x for EmitNorm=3.75mum MCBXvalue=-13.98e-06 MAD-X 4.01.47  24/02/11 18.00.13-0.005-0.004-0.003-0.002-0.0010.00.0010.0020.0030.0040.005x (m)xbeam2beam1IPFigure 3: Horizontal separation bump at IP1. The sepa-ration is ±2mm at the IP which corresponds to ±6.65σat 3.5 TeV at β∗ = 90m for a normalized emittance ofN = 3.75μm.As can be seen in Fig. 4 and Fig. 5, the calculated aper-ture at 90 m is not critical and well above the specifica-tion. The calculation was done for the standard normalizedemittance of 3.75μm at 3.5 TeV. The specification is thatthe aperture shown in terms of a figure of merit value ”n1”remains well above the specified value of 7 including sep-aration over the full interaction region.0.0 200. 400. 600. 800. 1000. 1200.                               s (m)IR1 b1 3.5TeV n1 aperture0.05.10.15.20.25.30.35.40.aperture n1specFigure 4: 90 m ATLAS/ALFA optics. Left for beam 1The un-squeeze from β∗ = 11 m to 90 m results in a sig-nificant loss in phase advance which has to be compensatedin the rest of the LHC to keep the overall tunes constant.The numerical values are given in Table 3. A machinestudy performed earlier this year in the LHC has demon-strated, that it is possible to provide the external tune com-pensation by ramping up all main LHC arc quadrupoles(QF, QD) during the un-squeeze, such that the tunes arekept constant and the beam lifetime is not reduced [4].Proceedings of IPAC2011, San Sebastián, Spain TUPZ00201 Circular CollidersA01 Hadron Colliders 1799 Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)0.0 200. 400. 600. 800. 1000. 1200.                               s (m)IR1 b2 3.5TeV n1 aperture0.05.10.15.20.25.30.35.40.specaperture n1Figure 5: 90 m ATLAS/ALFA optics. Right for beam 2.Table 3: External Tune Compensation Required for the 90 mbeam 1 beam 2ΔQx ΔQy ΔQx ΔQyIP1, 90 m 0.2251 0.0569 0.2237 0.0550IP5, 90 m 0.2219 0.0546 0.2203 0.0528total 0.4470 0.1115 0.4440 0.1078ATLAS/ALFA SIMULATIONS WITH THE90 M β∗ OPTICThe Roman Pots are movable in the vertical position andthe closer they can go to the beam axis the better it is. Forthe measurement, a smaller emittance than N = 3.75μmwould also be useful. A smaller emittance allows mea-surements down to smaller scattering angle and momentumtransfer. All the simulations in this study have been madefor 3.75 μm but as it has been seen during the commission-ing of the optic, the emittance value is close to 2.0 μm. Thissmaller emittance will be very useful.An important parameter for the measurement is the ac-ceptance which is the ratio between the elastic protons col-lected after the tracking at the RPs and the generated elas-tic protons at the IP. This acceptance can only be knownby the simulation. Indeed, when elastic protons are col-lected at the RPs, the way to know the initial t-spectrumgoes through the acceptance value.The simulation for ATLAS/ALFA is made of three dif-ferent steps [5]:• the generation of elastically scattered protons usingthe Monte Carlo generator PYTHIA [6], defining alsovertex smearing, angular divergence and energy dis-persion.• the tracking along the LHC beam line with MadX-PTC [7]• the collection of the datas at the RPs and acceptancecalculationThe acceptance is shown on Fig. 6. The plot shows twodifferent parts : one above log10(|t|) = −1.2 linked tothe losses due to the transport and the other one below thesame value, linked to the detector geometry. What is rele-vant looking at the acceptance plot is the values above 50%.We can see that the acceptance is much bigger with a peakof 0.8 for 8σ when the distance of the pots is closer to thebeam as this can be seen for three different values corre-sponding to 8.1 mm, 6.8 mm and 5.4 mm ( 12σ , 10σ and8σ).Accept_log)t(10=logτ-5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0Acceptance00.10.20.30.40.50.60.70.8Optical and detector acceptance8 sigmas10 sigmas12 sigmasFigure 6: Acceptance as a function of log(|t|) for differentσ values : 8, 10 and 12This illustrates the great interest to put ATLAS/ALFAdetectors close to the beam.CONCLUSIONThe intermediate 90 m β∗ optics described here presentsa major milestone in the use of very high-β∗ optics in theLHC. First studies with beam have already demonstratedthat the major external tune compensation required for highβ∗ optics can be well controlled using the main LHC ringquadrupoles. The 90 m optics will now be used for preci-sion measurements and adjustments of the optical parame-ters and initial physics studies.REFERENCES[1] ATLAS collaboration, “ATLAS Detectors for Measurementof Elastic Scattering and Luminosity”, ATLAS-TDR-018,2008, CERN-LHCC-2008-004.[2] H. Burkhardt, S.M. White, “High β∗ optics for the LHC”,CERN, June 2010, LHC Project Note 431.[3] H. Burkhardt, S. Cavalier, “Intermediate 90 m Optics forATLAS-ALFA”, CERN, Avril 2011, CERN-ATS-Note-2011-027 PERF.[4] H. Burkhardt, M. Fitterer, S. Redaelli, R. Tomas, G. Muller,S. Cavalier, “90 m optics commissioning”, IPAC’11, San Se-bastian, September 2011, these proceedings.[5] S. Cavalier, M. Heller, P. Puzo, H. Burkhardt, S.M. White,“Tracking and Tolerances study for the ATLAS High-beta op-tics”, PAC’09, Vancouver, May 2009, WE6PFP036, p. 2573,http://www.JACoW.org.[6] http://home.thep.lu.se/ torbjorn/Pythia.html[7] http://wwwslap.cern.ch/mad/Optics in IP1 and IP5TUPZ002 Proceedings of IPAC2011, San Sebastián, Spain1800Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)01 Circular CollidersA01 Hadron Colliders

90 m β * optics for ATLAS/ALFA

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