Results of simulations with all movable elements of the LHC collimation system [1] are discussed for various operation modes. Compared to previous results, the placing of additional collimators reduced the beam losses by a factor 10 in the ideal machine case, i.e. nominal collimators settings for both 450 GeV and 7 TeV beam energies. First results for Beam 2 are also reviewed. The sensitivity of the system to free orbit oscillations is addressed. These results show that it is sufficient to use a limited number of beam loss monitors (BLMs) for the setup and optimization of the LHC Collimation System

Assmann, Ralph Wolfgang

Bracco, Chiara

Redaelli, Stefano

Robert-Démolaize, Guillaume

Weiler, Thomas

CERN Document Server

G. Robert-Demolaize, R. Aßmann, C. Bracco, S. Redaelli, T. WeilerEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsCRITICAL HALO LOSS LOCATIONS IN THE LHCResults of simulations with all movable elements of the LHC collimation system [1] are discussed forvarious operation modes. Compared to previous results, the placing of additional collimators reduced thebeam losses by a factor 10 in the ideal machine case, i.e. nominal collimators settings for both 450 GeV and7 TeV beam energies. First results for Beam 2 are also reviewed. The sensitivity of the system to free orbitoscillations is addressed. These results show that it is sufficient to use a limited number of beam lossmonitors (BLMs) for the setup and optimization of the LHC Collimation System.Large Hadron Collider ProjectAbstractLHC Project Report 940CERN,CH-1211 Geneva 23,SwitzerlandGeneva, June 2006CERN, Geneva, SwitzerlandPresented atEPAC'06, Edinburgh, UK, June 26-30, 2006CRITICAL HALO LOSS LOCATIONS IN THE LHCG. Robert-Demolaize, R. Aßmann, C. Bracco, S. Redaelli, T. Weiler, CERN, Geneva, SwitzerlandAbstractResults of simulations with all movable elements of theLHC collimation system [1] are discussed for various op-eration modes. Compared to previous results, the placingof additional collimators reduced the beam losses by a fac-tor 10 in the ideal machine case, i.e. nominal collimatorssettings for both 450 GeV and 7 TeV beam energies. Firstresults for Beam 2 are also reviewed. The sensitivity of thesystem to free orbit oscillations is addressed. These resultsshow that it is sufficient to use a limited number of beamloss monitors (BLMs) for the setup and optimization of theLHC Collimation System.INTRODUCTIONThe LHC collimators located in the two warm insertionsdedicated for beam cleaning are used to intercept beamhalo. A small fraction of the halo leaks out and gets lost atcharacterisitc locations around the ring. The performanceof the system is described by its local cleaning inefficiency:η =number of protons lost in the machine aperturenumber of protons absorbed by the system×ΔL ,with ΔL a given length over which losses are distributed,which will be 10 cm in the following. Critical loss loca-tions are spotted by comparing the local inefficiency val-ues with the magnet quench levels for estimated minimalbeam lifetimes [2, 3]. Simulations are first done for nom-inal machine optics. Afterwards error models are applied.The nominal reference cases are defined with the parame-ters listed in Tables 1 and 2.Table 1: Optics parameters of the simulated nominal cases.Case E [TeV] IR 1 & 5 IR 2 & 8Injection 0.45 β∗ = 17 m β∗ = 10 mCollision 7 β∗ = 0.55 m β∗ = 10 mTable 2: Minimal beam lifetimes τ and corresponding requiredlocal inefficiencies for the simulated nominal cases at the quenchlimit.Case τ [h] ηquench [m−1]Injection 0.1 10−3Collision 0.2 2× 10−5These LHC optics are then used for studies of horizon-tal and vertical halo cases, usually tracked separately. Adescription of the tracking tools can be found in [4, 5].To monitor and control the losses, about 3700 BLMs arebeing installed in the LHC for the two beam lines. In theearly stages of machine commissioning, the full set of BLMinformation may not be required. Identifying the BLMchannels needed for early operation requires the study ofloss locations. This paper presents the results for beamhalo tracking considering various ideal and error scenariosfor both Beam 1 and Beam 2 and for the betatron cleaninginsertion. A list of critical loss locations is introduced asa baseline for a minimum workable BLM system for com-missionning and early operations of the LHC collimationsystem.SCENARIOS FOR HALO TRACKINGIn addition to the ideal case, a horizontal beam halo witha free closed orbit oscillation in the horizontal plane is con-sidered. To take into account all possible cases for the errorboth in phases and amplitudes, two horizontal kickers wereselected, separated by a phase advance of π2 to allow a fullphase scan within [−π,π]. The worst phase is found by ob-serving the most critical loss location, i.e. where the largestnumber of losses occur. Once this phase is found, a scan inerror amplitude for that phase is performed.This whole process (phase + amplitude) is done follow-ing a static situation: all collimators are recentered aroundthe perturbed closed orbit and the amplitude of the errorreaches the estimated tolerances of each operational mode.For the nominal optics described in Table 1, the closed or-bit tolerances correspond to a ± 4 mm perturbation any-where in the machine except in the collision energy casefor which the tolerances are reduced to ± 3 mm in the in-sertion regions1.CRITICAL LOCATIONS FROM BEAMLOSS PATTERNSLoss maps are obtained for the nominal and error sce-narios described above. In the following, locations labeledas cold losses correspond to cold elements of the machinein which protons are lost. Following the same principle,losses happening in warm elements are labeled as warmlosses.By comparing the ideal machine patterns with the per-turbed cases, one can spot the critical loss locations in thesuperconducting regions of the machine.1 For aperture calculations, tolerances are reduced to ± 3 mm every-where in the machine in the collision energy caseClosed orbit error - injection opticsThe observations shown in [6] for Beam 1 are valid forBeam 2 as well. A closed orbit perturbation of 4 mm atinjection optics induces a loss of a factor 2 in cleaning effi-ciency at the worst phase location and the loss locations areidentical between the ideal and the perturbed case down-stream of IR7 (Fig. 1 and 2). The first high loss locationscorrespond to the first two high dispersion locations, at theend of the dispersion suppressor.2.0 2.1 2.2 2.3 2.4 2.5 2.610-610-510-410-3s [ *10 km ]η [ m-1  ]2.0 2.1 2.2 2.3 2.4 2.5 2.610-610-510-410-3s [ *10 km ]η [ m-1  ]cold losseswarm losseslosses in collimatorcold losseswarm losseslosses in collimatorFigure 1: Beam 1 loss patterns for injection optics from the startof IR7 to the end of arc 8-1 for an ideal machine (top) and a ±4 mm horizontal orbit perturbation (bottom). The quench level is10−3 protons.m−1. Beam 1 goes from left to right.1.4 1.5 1.6 1.7 1.8 1.9 2.010-610-510-410-3s [ *10 km ]η [ m-1  ]1.4 1.5 1.6 1.7 1.8 1.9 2.010-610-510-410-3s [ *10 km ]η [ m-1  ]cold losseswarm losseslosses in collimatorcold losseswarm losseslosses in collimatorFigure 2: Beam 2 loss patterns for injection optics from the endof arc 5-6 to the end of IR7 for an ideal machine (top) and a ±4 mm horizontal orbit perturbation (bottom). The quench level is10−3 protons.m−1. Beam 2 goes from right to left.Losses in the dispersion suppressor cannot be avoided:off-momentum halo is generated at the collimators due tosingle-diffractive scattering during the interaction betweenan incoming proton and the collimator material. Protonswith large energy spread are likely to get lost at the firsthigh dispersion location. This defines a characteristic loca-tion for proton losses, and sets at the same time a funda-mental limitation to the betatron cleaning efficiency.Closed orbit error - collision opticsAt collision optics, one can count less loss locationsalong the machine compared to the injection optics casebut at the same time more peaks are getting closer to thequench limit. When comparing the ideal machine case withthe case of an orbit error, it can be seen in figure 3 and 4 thatthe cleaning system looses up to a factor 2.4 in efficiencyfor Beam 2.2 2.1 2.2 2.3 2.4 2.5 2.610-610-510-410-3s [ *10 km ]η [ m-1  ]2 2.1 2.2 2.3 2.4 2.5 2.610-610-510-410-3s [ *10 km ]η [ m-1  ] cold losseswarm losseslosses in collimatorcold losseswarm losseslosses in collimatorquench limit quench limit Figure 3: Beam 1 loss patterns for collision optics from the startof IR7 to the end of arc 8-1 for an ideal machine (top) and a hor-izontal orbit perturbation of ± 4 mm in the arcs and ± 3 mm inthe insertion regions (bottom). Beam 1 goes from left to right.1.4 1.5 1.6 1.7 1.8 1.9 2.010-610-510-410-3s [ *10 km ]η [ m-1  ]1.4 1.5 1.6 1.7 1.8 1.9 2.010-610-510-410-3s [ *10 km ]η [ m-1  ]cold losseswarm losseslosses in collimatorcold losseswarm losseslosses in collimatorquench limit quench limit Figure 4: Beam 2 loss patterns for collision optics from the endof arc 5-6 to the end of IR7 for an ideal machine (top) and a hor-izontal orbit perturbation of ± 4 mm in the arcs and ± 3 mm inthe insertion regions (bottom). Beam 2 goes from right to left.Downstream of IR7, it can be seen that the main loss lo-cations are mostly identical for the ideal and the perturbedcase. It can also be seen that most of the peaks showingup in the perturbed case correspond to critical locations al-ready spotted in the injection case.SummaryScanning all phases and amplitudes, it was possible toidentify 43 critical loss locations for injection and 29 forcollision considering the betatron cleaning insertion only.This sums up to 59 different locations that systematicallyshow loss peaks, 13 elements being critical for both ener-gies. These 13 locations correspond to the end of the dis-persion suppressor of IR7 and the arc downstream. Tables3 and 4 summarize the predicted locations for ”golden”BLMs, i.e. characteristic loss locations due to collimation,to which locations of collimators should be added. It is sug-gested that a possibly reduced BLM system should at leastcover these locations for an efficient quench protection invarious machine operation with error scenarios.BLMs at similar locations around IR3 and at the tripletmagnets must be added. Energy deposition studies are per-formed in parallel to check the influence of particle showersoriginating from the tertiary collimators (TCT) protectingthe triplet magnets close to the experimental insertions.Table 3: Critical loss locations at injection optics withoutmomentum cleaning.Beam 1 Beam 2Q11.R3 Q28.R3 Q11.R6DFBA.R6 Q18.L4 Q31.L7MB9.R7 Q10.L4 Q27.L7MB11.R7 Q22.R5 Q23.L7Q11.R7 Q28.L6 Q19.L7MB13.R7 MB28.L6 MB19.L7Q13.R7 Q25.L6 Q15.L7Q23.R7 Q20.L6 MB15.L7Q27.R7 MB20.L6 Q11.L7Q31.R7 MB16.L6 MB11.L7Q33.L8 MB14.L6 MB9.L7Q29.L8 MB12.L6 Q8.L7Q25.L8 MB9.L6 MB8.L7Q2.R8 MB8.L6Q6.R8 Q4.L6Table 4: Critical loss locations at collision optics without mo-mentum cleaning.Beam 1 Beam 2Q6.L3 Q21.R7 Q11.R6 Q9.L7Q8.R7 MB34.L8 MB12.R6 MB9.L7MB9.R7 Q33.L8 Q25.R6 Q8.L7Q9.R7 Q25.L8 Q33.R6 MB8.L7Q10.R7 Q17.L8 Q19.L7MB11.R7 Q16.R8 Q13.L7Q11.R7 Q30.R8 MB13.L7Q13.R7 Q22.L1 Q11.L7MB21.R7 MB11.L7The tracking tools also allow checking the longitudi-nal distribution of losses over any magnetic element. Theplanned positions of the BLM system for each element isshown in Fig. 5, which also includes the longitudinal dis-tribution of losses along the considered magnet. It can beseen that it would be sufficient to use the channels from thefirst 2 BLMs at a quadrupole since the losses appear to beconcentrated at the beginning of the element (see also [7]for more details).CONCLUSIONThe response of the LHC collimation system to free or-bit oscillations for Beam 1 and Beam 2 has been reviewed.MBSEXTBPMOCTUMQ MCBH/VDECAMBxx x xx xz=248z=243z=154 z=0z=353z=325Figure 5: Shower development in the cryostat of a quadrupole.The positioning of the detectors (blue boxes) has been optimizedto catch losses (colored curves) and to minimize uncertainty ofratio of energy deposition in coil and detector (courtesy of L.Ponce).With the specified LHC orbit errors, critical locations alongthe machine can be identified and used to define a minimumworkable BLM system for the commissioning and set-up ofthe collimators during the early stages of operation. Somelocations are critical for both 450 GeV and 7 TeV. The dis-persion suppressor immediately downstream of IP7 is themost critical region of the machine, with many losses con-centrated over a few elements (the equivalent of two cells ofthe lattice). Energy deposition studies are ongoing for par-ticle showers generated by inelastic proton-matter interac-tion in the tertiary collimators (close to the triplet magnets)and downstream of the beam dump protection equipment(TCDQ).Further studies are planned to obtain loss maps forbeta-beating errors, tables of magnetic field errors for thedipoles, alignment errors for magnets in the aperture mod-els and error scenarios for the mechanical parameters of thecollimators (e.g. longitudinal tilt angle of one jaw).REFERENCES[1] ”LHC Design Report”, Volume I, Ch. 18, CERN-2004-003.[2] R. Aßmann: ”Collimators and cleaning: could this limitthe LHC performance ?”, Proc. Chamonix 2003, CERN-AB-2003-008, pp. 163-170.[3] J.B. Jeanneret, D. Leroy, L. Oberli, T. Trenckler: ”Quenchlevels and transient beam losses in the LHC magnets”,CERN-LHC-PROJECT-REPORT-44, 1996.[4] G. Robert-Demolaize, R. Aßmann, S. Redaelli, F. Schmidt:”A new version of SixTrack with collimation and apertureinterface”, Proc. PAC 2005, CERN-AB-2005-033.[5] S. Redaelli: ”LHC aperture and commissioning of the col-limation system”, Proc. Chamonix 2005, CERN-AB-2005-014, pp. 268-277.[6] G. Robert-Demolaize, R. Aßmann, C. Bracco, S. Redaelli,T. Weiler: ”Critical beam losses during commissioning andinitial operation of the LHC”, Proc. Chamonix 2006, CERN-AB-2006-014, pp. 109-116.[7] B. Dehning: ”Commissioning of beam loss monitors”, Proc.Chamonix 2006, CERN-AB-2006-014, pp. 117-119.

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