ISBN 978-3-95450-115-1 - http://accelconf.web.cern.ch/AccelConf/IPAC2012/papers/moppc006.pdfInternational audienceA high β* = 90 m optics was commissioned and used for ﬁrst very forward physics operation in the LHC in 2011. The experience gained from working with this optics in 5 studies and operation periods in 2011 was very positive. The target β* = 90 m was reached by a de-squeeze from the standard 11 m injection and ramp optics on the ﬁrst attempt and collisions and ﬁrst physics results obtained in the second study. The optics was measured and corrected with good precision. The running conditions were very clean and allowed for measurements with roman pots very close to the beam

Burdhardt, H.

Cavalier, S.

Müller, G.J.

Redaelli, S.

Tomas, R.

Vanbavinckhove, G.

Wenniger, J.

White, S.

A high β* = 90m optics was commissioned and used for first very forward physics operation in the LHC in 2011. The experience gained from working with this optics in 5 studies and operation periods in 2011 was very positive. The target β* = 90m was reached by a de-squeeze from the standard 11 m injection and ramp optics on the first attempt and collisions and first physics results obtained in the second study. The optics was measured and corrected with good precision. The running conditions were rather clean and allowed for measurements with roman pots very close to the beam

Burkhardt, H

Müller, G

Redaelli, S

Tomas, R

Vanbavinckhove, G

Wenninger, J

Cavalier, S

White, S

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90M Optics Studies and Operation in the LHC

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90M OPTICS STUDIES AND OPERATION IN THE LHCH. Burkhardt, G. Mu¨ller, S. Redaelli, R. Tomas, G. Vanbavinckhove, J. Wenninger, CERN, Geneva,Switzerland, S. Cavalier, LAL,Universite´ Paris Sud 11, S. White, BNL, Upton, New YorkAbstractA high β∗ = 90m optics was commissioned and usedfor ﬁrst very forward physics operation in the LHC in 2011.The experience gained from working with this optics in 5studies and operation periods in 2011 was very positive.The target β∗ = 90m was reached by a de-squeeze fromthe standard 11 m injection and ramp optics on the ﬁrstattempt and collisions and ﬁrst physics results obtained inthe second study. The optics was measured and correctedwith good precision. The running conditions were ratherclean and allowed for measurements with roman pots veryclose to the beam.INTRODUCTIONHigh β∗ optics have been requested by the ATLAS-ALFA and TOTEM experiment to be used in special physicruns for low angle precision measurements of the proton-proton scattering in the LHC using roman pot detectors in-stalled in the very forward region around the LHC interac-tion points IP1 and IP5 [1, 2].The experience with the 90m optics, has been very pos-itive in 2011. We had 5 short special running periods dedi-cated to the operation at 90m adding up to a total 4.2 daysof running time [3]. We succeeded to reach 90m usinga de-squeeze from the standard 11m injection optics andramp in a single ﬁll [4] using the main quadrupoles of allLHC arcs for the external tune compensation and to com-mission the new 90m optics with parallel separation bumpsin the second special run. This allowed to bring beams intocollisions at the end of the second special run dedicated to90m, see Fig. 1 [5], and to leave beams into collisions forabout an hour which was used by the TOTEM experimentto obtain a ﬁrst measurement of the total proton-protoncross section at LHC energies to a precision of 3% [6].19:00 20:00 21:00 22:00 23:00 00:00 01:00 02:00 03:00 04:00051015202530pilotsbeam 1beam 2Intensity,  in 109 protons450 GeV3.5 TeV11m30m90m020406080100?*?* in mseparation offstart optics measurementsoptics correctionFigure 1: Intensities, energy and β∗ as a function of timeduring the 2nd study dedicated to 90m.The following three dedicated 90m sessions were usedto re-measure the optics, adjust collimators and roman potsclosely around the beam and for data taking for the TOTEMand ALFA experiments.The beam intensity in the special 90m runs is much re-duced (nearly 1000 times) compared to standard high lu-minosity operation. This allows for much tighter collima-tor and roman pot settings at 90m than in standard physics.The crossing angle is turned off for high-β operation. Theminimal bunch spacing to avoid parasitic crossings withoutcrossing angle is 524 ns, which limits the maximum num-ber of bunches to 156, see also Table 1. For the special90m runs described here, we used typically 3 bunches, twowith an intensity of 7 × 1010 protons and a third very lowintensity probe bunch.Table 1: Comparison of the main intensity related beamparameters for the 90m runs, compared to standard 2011high luminosity operation and LHC design parameters.90m standard LHC design2011bunch spacing ≥ 524 ns 50 ns 25 ns#bunches ∼ 3, ≤ 156 1380 2808total intensity 2.5× 1011 2× 1014 3.2× 1014In this paper, we summarize the experience with the 90moptics commissioning and operation in 2011. Future plansand prospects are described in [7]. Precise local measure-ments of optics parameters performed with the TOTEM de-tector are described in [8].GETTING TO 90MThe 90m optics have been designed to use as much aspossible the standard LHC injection and energy ramp. Theaim is to minimize the extra set-up time required in spe-cial running conditions. The injection optics is the stan-dard optics, with β∗ = 11, 10, 11, 10m in the LHC inter-action points IP 1, 2, 5, 8 which uses crossing angles in allinteraction points. The procedure for the ramping up to thephysics energy in 2011, Eb = 3.5GeV, is the same as forstandard operation with one exception : the parallel separa-tion is kept always at a constant±2mm in IPs 1 and 5. Thecrossing angles in IP1 and IP5 are turned off at top energybefore the de-squeeze to 90m.The target value of β∗ = 90 m is reached by a de-squeeze from the injection β∗ = 11 m using 17 interme-diate steps. A major challenge in the simultaneous de-squeeze of IP1 and IP5 from β∗ = 11m to β∗ = 90mis a signiﬁcant tune reduction of up to 0.45. It is com-pensated by an increase of the strength of the main LHCquadrupoles (QD, QF) during the de-squeeze, which mod-MOPPC006 Proceedings of IPAC2012, New Orleans, Louisiana, USAISBN 978-3-95450-115-1130Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)01 Circular and Linear CollidersA01 Hadron Collidersiﬁes the optics all around the LHC and induces a few percent of β-beating [9]. Orbit and tune feedbacks were usedduring the de-squeeze, and the required corrections incor-porated as feed-forward for the following de-squeeze [4],which allowed to reduce the number of stops during thede-squeeze in the subsequent 90m runs.OPTICS MEASUREMENTS-0.4-0.2 0 0.2 0.4 0  5000  10000  15000  20000  25000??/?yLongitudinal location [m]?* = (90,10,90,10)mBefore correctionAfter correction-0.4-0.2 0 0.2 0.4??/?xIR3 IR4 IR5 IR6 IR7IR2 IR8 IR1LHCB1 3.5 TeV-0.4-0.2 0 0.2 0.4 0  5000  10000  15000  20000  25000??/?yLongitudinal location [m]?* = (90,10,90,10)mBefore correctionAfter correction-0.4-0.2 0 0.2 0.4??/?xIR1 IR2 IR3 IR4 IR5IR8 IR6 IR7LHCB2 3.5 TeVFigure 2: Measured β-beat at, 90m, for beam 1 x,y (top)and beam2 x,y (bottom).Global optics measurements of β, coupling and disper-sion were performed using the LHC AC-dipoles, whichinduce transverse oscillations [10]. In addition, the β∗and waist were measured locally at IP1 and IP5 using K-modulation [11]. The initial β-beating (which is the nor-malized difference between measured and model β) in theﬁrst 90m study was found to be between 20 and 30% [4].Based on these measurements, corrections were calculatedand applied for the subsequent 90m runs. The global cor-rections were calculated using a response matrix approach.No common correctors were used for these corrections.Fig. 2 shows the measured β-beat and Fig. 3 the dispersionmeasured at 90m before and after correction. The β-beatafter the corrections was reduced to 10− 15%.The experiments require an accurate knowledge of theeffective lengthLy =√β∗βrp sinμ , (1)-0.04-0.02 0 0.02 0.04 0  5000  10000  15000  20000  25000?D/?x1/2 [m1/2 ]Longitudinal location [m]IR3 IR4 IR5 IR6 IR7IR2 IR8 IR1LHCB1 3.5 TeV?* = (90,10,90,10)mBefore correctionAfter correctionFigure 3: The normalized horizontal dispersion for beam1, before corrections (red) and after corrections (blue). 0 1 2 3 4 5 6 240  250  260  270  280  290  300  310?2L y  [m]IP1?RPAIP1?RPBFigure 4: χ2 versus measured Ly , using different opticsmodels generated by Monte-Carlo simulation.where β∗ and βrp are the β-function at the IP and romanpot, respectively; μ is the phase advance between the ro-man pot and the IP.Ly translates an angular kick at the IP toan offset at the roman pot (equivalent to the transfer matrixelementR34). Two different techniques were used to deter-mine the effective lengthLy from the optics measurements.The ﬁrst is a segment-by-segment technique, referred to asSBS [12], in which the measured optical functions at thenearest available BPMs are used to obtain the values of theright hand side of Eq. 1. This approach assumes an idealmodel between the BPMs, the IP and the roman pots. Ta-ble 2 shows the most precise values obtained by SBS forthe TOTEM roman pots. The accuracy is 3% on Ly .The second method is based on Monte-Carlo simulationand does not require to assume an ideal optics. Instead,100 machine models are generated using different randomsets of quadrupole errors and compared to the AC-dipolemeasurements. Fig. 4 shows the χ2 versus the correspond-ing Ly for the different models. Table 3 lists the Ly valuesobtained by both techniques which agree within the errorbars. The Ly values obtained at the minimum χ2 fromProceedings of IPAC2012, New Orleans, Louisiana, USA MOPPC00601 Circular and Linear CollidersA01 Hadron CollidersISBN 978-3-95450-115-1131 Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)Monte-Carlo modeling are expected to be less affected bysystematic errors and are slightly higher than the values ob-tained from SBS.Table 2: Measured vertical β functions at the TOTEM ro-man pots and the IP, phase advance and effective length.RP βrp β∗ μ/2π Ly[m] [m] [m]A 871± 3 89.7± 5.6 0.26± 0.01 277± 9B 793± 7 89.7± 5.6 0.27± 0.01 264± 8Table 3: Ly values obtained using the Segment-By-Segment technique (SBS) and Monte-Carlo (MC) model-ing for the TOTEM roman pots.Roman pot Approach Ly[m]A SBS 277± 9A MC 283± 8B SBS 264± 8B MC 270± 8ROMAN POTS AND COLLIMATIONFigure 5: Closing the primary collimators to 5σ. Loss ratesrecorded as a function of time for three 10μm (or 0.036σ )steps, in 90m operation at the 20 October 2011 at 10:02.Collimators and roman pots are set from the LHC con-trol room using the same motorization and procedures.They are centered around the beams based on the obser-vation of the local beam loss rates using a semi-automaticbeam-based alignment procedure [13]. This is illustratedin Fig. 5. The loss rate can be seen to increase sharply ateach step and to decay within seconds to the previous level,showing that halo particles were removed. Using this pro-cedure, the primary vertical collimators were closed downto 4.6σ, in the last 90m special run at a total intensity of2 × 1011 protons / beam, mostly on two bunches of each7 × 1010 protons (with σ calculated for a nominal 3.5μmnormalized emittance). Towards the end of the ﬁnal study,the vertical roman pots were closed to a minimum of 4.8σ,remaining just within the shadow of the primary collima-tors, but receiving already increased background rates byintercepting some of the secondary halo. This showed, thatit is in principle possible to bring roman pots very close tothe beam for small beam intensities. It also showed, thatit would be desirable to further reduce the emittances (herewe had on average 2.2μm) and to leave more margin be-tween primary collimators, secondary collimators and ro-man pots.SUMMARYThe initial experience with operation at higher β∗ in theLHC has been very positive and allowed to achieve all ourgoals for this special running mode in 2011 in just fourdays of dedicated operation. The simultaneous de-squeezeto 90m in IP1 and IP5 worked on the ﬁrst attempt in-spiteof the major tune changes. Collisions were reached in thesecond dedicated 90m session and allowed for a ﬁrst totalproton-proton cross section measurement at the LHC. Theoptics was well measured and corrected. Data was recordedby the ALFA and TOTEM collaborations over ∼ 10 hoursin stable conditions at 90m with roman pots very close tothe beam, down to 4.6σ for primary collimators and 4.8σfor the vertical roman pots in the ﬁnal 90m session in 2011.ACKNOWLEDGMENTThanks to the LHC operation and TOTEM and ATLAS-ALFA teams for the excellent and stimulating collabora-tion.REFERENCES[1] TOTEM Collaboration, G. Anelli et al., “The TOTEM ex-periment at the CERN LHC”, JINST 3 (2008) S08007[2] ATLAS Collaboration, ATLAS-TDR-018 ; CERN-LHCC-2008-004, 2008[3] A. Macpherson, “LHC AVAILABILITY AND PERFOR-MANCE IN 2011”, Proc. Chamonix 2012.[4] H. Burkhardt et al., “Un-squeeze to 90 m”, CERN-ATS-Note-2011-032 MD[5] S. Cavalier, P. Puzo, and H. Burkhardt, “90 m Optics Com-missioning”, Proc. IPAC 2011[6] TOTEM Collaboration, G. Antchev et al., EPL 96 (2011),no. 2, 21002, CERN-PH-EP-2011-158[7] H.Burkhardt et al., “Plans for high beta optics in the LHC”,this conf. MOPPC007.[8] H. Niewiadomski et al., “LHC Optics Determination withProton Tracks Measured in the Roman Pots Detectors of theTOTEM Experiment”, this conf. MOPPC008.[9] H. Burkhardt and S. White, LHC Project Note 431[10] R. Tomas et al., PRSTAB 13 (2010) 121004[11] R. Calaga et al., “β∗ Measurement in the LHC Based onK-modulation”, Proc. IPAC 2011[12] M. Aiba et al., PRSTAB 12 (2009) 081002[13] G. Valentino et al., “Semi-automatic beam-based alignmentalgorithm for the LHC collimation system”, Proc. IPAC2011MOPPC006 Proceedings of IPAC2012, New Orleans, Louisiana, USAISBN 978-3-95450-115-1132Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)01 Circular and Linear CollidersA01 Hadron Colliders

90m Optics Studies and Operation in the LHC

90m Optics Studies and Operation in the LHC

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