The Large Hadron Collider extends the present state-of-the-art in stored beam energy by 2-3 orders of magnitude. A sophisticated system of collimators is implemented along the 27 km ring and mainly in two dedicated cleaning insertions, to intercept and absorb unavoidable beam losses which could induce quenches in the superconducting (sc) magnets. 88 collimators for the two beams are initially installed for the so called Phase 1. An optimized strategy for the commissioning of this considerable number of collimators has been defined. This optimized strategy maximizes cleaning efficiency and tolerances available for operation, while minimizing the required beam time for collimator setup and ensuring at all times the required passive machine protection. It is shown that operational tolerances from collimation can initially be significantly relaxed

Assmann, R

Bracco, C

Redaelli, S

Weiler, T

CERN Document Server

BEAM COMMISSIONING PLAN FOR LHC COLLIMATIONC. Bracco, R.W. Assmann, S. Redaelli, T. Weiler, CERN, Geneva, SwitzerlandAbstractThe Large Hadron Collider extends the present state-of-the-art in stored beam energy by 2-3 orders of magni-tude. A sophisticated system of collimators is implementedalong the 27 km ring and mainly in two dedicated cleaninginsertions, to intercept and absorb unavoidable beam losseswhich could induce quenches in the superconducting (sc)magnets. 88 collimators for the two beams are initially in-stalled for the so called Phase 1. An optimized strategyfor the commissioning of this considerable number of col-limators has been defined. This optimized strategy maxi-mizes cleaning efficiency and tolerances available for op-eration, while minimizing the required beam time for col-limator setup and ensuring at all times the required passivemachine protection. It is shown that operational tolerancesfrom collimation can initially be significantly relaxed.INTRODUCTIONThe LHC beam intensity and luminosity can be limitedby the efficiency of the collimation system in providingbeam cleaning and passive machine protection. This re-quires a specific setup of the full system for each opera-tional scenario.Phase 1 collimation system [1] includes 88 movable ringcollimators, for the two beams, which are set to differentopenings to implement a multi-stage cleaning system. Pri-mary (TCP) and secondary (TCSG) collimators plus ab-sorbers (TCLA) build up the two cleaning insertions of theLHC ring: momentum (IR3) and betatron (IR7) cleaning.Tertiary (TCT) collimators and special absorbers (TCLP)are placed closed to the interaction points (IP) to protectthe triplet magnets and catch the physics debris coming outfrom the collisions at the experiments. Additional collima-tors are used to intercept mis-kicked beams during injection(TDI, TCLI) and extraction (TCDQ, TCS).LHC collimators consist of two parallel, fully movablejaws which define a gap for the passage of the beam. Intotal one needs to control more than 340 independent de-grees of freedom [2] (two stepping motors per each colli-mator jaw).The setup of the collimation system is one of the most del-icate phases for the commissioning of the LHC with beam.In addition, performance optimization and machine pro-tection require to respect setting tolerances which becomemore demanding when increasing beam energy and inten-sity. An optimized commissioning strategy for the Phase 1collimation system is described in this paper. The requiredcomplements of collimators are defined for various stepsin beam commissioning. Moreover the tolerances for ma-chine and collimator setup are specified.GOALS AND ASSUMPTIONSThe LHC collimation system is the most elaborate sys-tem of this kind built to date. Setup and beam tolerancesare more demanding than in previous colliders. A learn-ing period for setting up the system and for optimum per-formance will be necessary and, ideally, it should be asshort as possible. It is therefore important to define an op-timized commissioning strategy which is matched to thebeam commissioning plan of the LHC and has the follow-ing overall goals:• Maintain at all times the required passive machineprotection for the given beam intensity.• Minimize the number of collimators required for eachstep in beam commissioning, such reducing the beamtime required for collimation.• Maximize the performance for each commissioningstep by defining the most efficient set of collimatorsto be used.• Maximize the tolerance budget available for collima-tor setup errors or machine imperfections for each stepof commissioning.The collimation cleaning efficiency and induced lossesin sc magnets depend strongly on imperfections [3]. Locallosses are expected to be up to a factor of 11 higher whenrealistic imperfections are assumed, this safety margin hasbeen considered when determining the achievable perfor-mance of the system during commissioning. Besides, con-servative assumptions are made (i.e. 0.1 h beam lifetime τduring the energy ramp) to take into account further safetymargins and uncertainties (e.g. on the quench limit). Allthe estimates presented here are related to these assump-tions. Should beam lifetime and quench limit vary, thanthe maximum achievable beam intensity would have to bereevaluated.TOLERANCE BUDGETThe tolerance budget Tb for collimator positioning is de-fined as:Tb = n2 − n1 (1)where n1 is the half-gap of the primary collimators and n2is the half-gap of the second closest collimator to the beam(nominally secondary collimators). Tb is attributed to thefollowing contributions:• Collimator setup and imperfections (40% of Tb)Proceedings of PAC09, Vancouver, BC, Canada WE6PFP013Circular CollidersA01 - Hadron Colliders 2507• Transient orbit at collimator (30% of Tb)• Transient beta-beat at collimator (30% of Tb)The nominal retraction between TCP and TCSG is equal to1σβ , where σβ =√εβ gives the normalized beam size. Thebeam is adiabatically damped during the acceleration andσβ (at the location of primary collimators) is reduced from1 mm at 450 GeV to 250 μm at 7 TeV. The optimum clean-ing efficiency is obtained when all collimators are closedfollowing the beam size reduction (scaled with energy set-ting). Gaps and tolerances decrease when increasing thebeam energy (see Fig. 1) and collimator operation becomestherefore more demanding. During the first stages of the0.45 1 2 3 4 5 6 700.20.40.60.811.2Energy [TeV]T b [mm]  Constant & Tolerance optimizedScaled with energyFigure 1: Tolerance budget Tb as a function of beam energyfor different collimator settings.commissioning, the reduced beam intensity allows to usemore relaxed collimator settings and tolerances. Two alter-native setups are proposed:• Constant setting: collimators are not moved during theramp• Tolerance optimized setting: TCP follow the accelera-tion beam damping. Remaining collimators are closedby keeping the retraction (in mm) with respect to theTCP unchanged.These options make operation easier (see Fig. 1) with theprice of a lower cleaning efficiency [4].The proposed commissioning strategy is base on themost favorable trade off between machine performance andoperation.COMMISSIONING STRATEGYThe commissioning plan has been defined based on ex-perimental results and a large number of simulations fordifferent optics configurations. Optimum Collimator set-tings have been worked out for four reference beam inten-sities [5] as presented in the following.Pilot BunchA pilot bunch (up to 5 · 109 protons) will be used for ini-tial commissioning of the LHC. Such a low intensity beamcan be handled with a minimal set of collimators that im-plements a one-stage cleaning system. Only 15 collima-tors per beam are needed: primary collimators in the clean-ing insertions and protection collimators (installed for 2008run [4]).Experimental tests showed that, on average, 20 minuteshave to be taken into account for the manual alignment ofeach collimator with respect to the beam. A minimum timeof 5 h is therefore expected for aligning the one-stage sys-tem.The maximum achievable beam intensity Imax is de-fined as [6]:Imax =τ · Rqηc(2)where Rq is the quench limit of the sc magnets [4] andηc is the local cleaning inefficiency of the collimation sys-tem (number of particles locally lost with respect to parti-cles absorbed by the collimators [3]). Imax is presented inFig. 2 as a function of beam energy for the settings definedabove. Lines referring to the beam intensities as foreseen0.45 1 2 3 4 5 6 7101010111012101310141015Energy [TeV]I max [protons]  Constant settingTolerance opt. settingScaled with energyPilot bunch43 bunches156 bunchesNominalFigure 2: Maximum beam intensity reach for a minimalone-stage cleaning system. A 0.1 h beam lifetime is con-sidered.for commissioning and nominal operation are also shown.The one-stage system can withstand the pilot beam up to7 TeV with constant gap (see.Fig. 2 and Table 1). This fa-cilitates the first energy ramps, as collimator settings do notneed to be changed and tolerances are kept constant.43 BunchesThe beam with 43 bunches corresponds to about 0.5%of the nominal beam intensity but it can easily quenchsc magnets and also induce damage on accelerator equip-ment. The stored energy of the beam at 7 TeV can reachthe present state-of-the-art at the TEVATRON collider.In this case a two-stage collimation system is required.Shower absorbers (TCLA) are added to the previous com-plement of collimators, in total 24 collimators per beam areused (8 h for alignment).This configuration allows to reach 5 TeV (nominal top en-ergy for the first years of operation) without beam inten-sity limitations, by scaling collimator gaps with the energyWE6PFP013 Proceedings of PAC09, Vancouver, BC, Canada2508Circular CollidersA01 - Hadron CollidersTable 1: The collimator complements proposed for several beam target intensities foreseen for the LHC commissioningare shown. An estimate of the time needed for the manual setup and of the maximum reachable intensity (for a 0.1 h beamlifetime) at reference top energies are also presented. The best performance is obtained by setting the collimators with thetightest tolerances.Intensity Number of Time Ramp I 5 TeVmax T 5 TeVb I7 TeVmax T 7 TeVb[protons] collimators [h] setting [protons] [mm] [protons] [mm]Pilot Bunch 5.0 · 109 15 5 constant 2.4 · 1010 1.30 2.0 · 1010 1.3043 Bunches 1.7 · 1012 24 8 scaled 1.8 · 1012 0.45 1.4 · 1012 0.3834 11 tol. opt. 7.4 · 1012 1.00 1.7 · 1012 1.00156 Bunches 1.4 · 1013 34 11 scaled 1.3 · 1013 0.30 4.1 · 1012 0.25Nominal 3.2 · 1014 44 15 scaled 1.6 · 1013 0.30 5.0 · 1012 0.25(see Table 1). On the other hand at 7 TeV, either the inten-sity must be reduced or a third collimator family (e.g. sec-ondary collimators, see next section) must be set up. Re-laxed tolerances can then be adopted (see Table 1).156 BunchesFurther stages of cleaning and protection are requiredfor intensities above 43 bunches. Studies for this casewere carried out considering the collimators which wereinstalled in the LHC tunnel for the 2008 run. In partic-ular, secondary collimators are implemented to the two-stage system described above. Only a reduced number ofTCSG (6 out of 11 per beam) was installed in the beta-tron cleaning insertion at that time. Optimization studiesallowed to define the collimators to be installed at first.The foreseen operation up to 5 TeV is possible with thiscomplement but with severe tolerances, as shown in Ta-ble 1. If these tight tolerances cannot be achieved, collima-tor gaps must be increased and the beam intensity shouldbe reduced.Nominal IntensityThe full Phase 1 collimation system will be installed forthe 2009 run of the LHC. This system provides optimalperformance which, however, is expected to be limited be-low nominal intensity (see Table 1). This relies on manyassumptions, as outlined before. It is believed that it willbe possible to increase the beam intensity once some ex-perience of LHC operations is gained, leading to improvedbeam stability (longer lifetime) and reduced machine im-perfections.About 15 h are necessary for setting up the full Phase 1 sys-tem. An automized calibration procedure will be imple-mented to reduce setup time. A new beam based alignmentwill have to be performed after any substantial change inthe beam parameters. If the machine is stable, collimatorswill be moved to reference positions from the last align-ment. High reproducibility of the accelerator and beam pa-rameters are therefore fundamental to avoide frequent col-limator setup and obtain high efficiency of the acceleratorfor physics.CONCLUSIONSThis paper summarizes the main features of a collima-tion commissioning plan optimized according to the in-tensities foreseen for the LHC beam commissioning. Theproposed strategy starts with 30 collimators, for the twobeams, and a tolerance budget that is relaxed by a factorof 5 (at 7 TeV) with respect to the nominal settings. Theachievable performance is then improved in steps until afactor of 250 is gained with the full Phase 1 system and theachievement of tightest tolerances.All the estimates presented here are related to given as-sumption on a peak beam lifetime. The performance reachis indeed improved when loss rates are smaller.Beam commissioning might deviate from the foreseenprocedure for various possible reasons. In this case, the ap-propriate collimator settings must be redefined in a shorttime. A master table has been worked out in order to allowfast reaction time without compromising safety aspects. Itcontains a large variety of collimator settings whose con-sistency and safety have been carefully checked. The casesdescribed above were extracted from this table which is toodetailed for the scope of this paper. The master table willbe used as the reference for collimator setup and will beupdated from the accelerator physics side as the knowledgeand experience of LHC collimation will expand. A releasedversion will be made available for accelerator operation af-ter initial beam commissioning.REFERENCES[1] The LHC Design Report, Vol.1, Chapter 18. CERN-2004-003, pp.467-498.[2] S. Redaelli et al., “Implementation and Performance of the LHC Col-limator Control System”, these proceedings.[3] C. Bracco et al., “LHC Cleaning Efficiency with Imperfections”,these proceedings.[4] C. Bracco, “Commissioning Scenarios and Tests for the LHC Colli-mation System”, CERN-THESIS-2009-031, EPFL, Lausanne, 2009.[5] M. Lamont et al., “A Staged Approach to LHC Commissioning”,LHC-Project-Report 949, 2006.[6] R. Assmann, “Collimators and Cleaning: Could That Limit the LHCPerformance?”, proceedings of the Chamonix XII Workshop, 2003.Proceedings of PAC09, Vancouver, BC, Canada WE6PFP013Circular CollidersA01 - Hadron Colliders 2509

Beam Commissioning Plan For LHC Collimation

The Large Hadron Collider extends the present stateof- the-art in stored beam energy by 2-3 orders of magnitude. A sophisticated system of collimators is implemented along the 27 km ring and mainly in two dedicated cleaning insertions, to intercept and absorb unavoidable beam losses which could induce quenches in the superconducting (sc) magnets. 88 collimators for the two beams are initially installed for the so called Phase 1. An optimized strategy for the commissioning of this considerable number of collimators has been defined. This optimized strategy maximizes cleaning efficiency and tolerances available for operation, while minimizing the required beam time for collimator setup and ensuring at all times the required passive machine protection. It is shown that operational tolerances from collimation can initially be significantly relaxed

Beam Commissioning Plan For LHC Collimation

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