The performance reach of the LHC depends on the magnitude of beam losses and the achievable cleaning efficiency of its collimation system. The ideal performance reach for the nominal Phase 1 collimation system is reviewed. However, unavoidable imperfections affect any accelerator and can further deteriorate the collimation performance. Multiple static machine and collimator imperfections were included in the LHC tracking simulations. Error models for collimator jaw flatness, collimator setup accuracy, the LHC orbit and the LHC aperture were set up, based to the maximum extent possible on measurements and results of experimental beam tests. It is shown that combined “realistic” imperfections can reduce the LHC cleaning efficiency by about a factor 11 on average

Assmann, R

Bracco, C

Redaelli, S

Weiler, T

Assmann, R W

CERN Document Server

LHC CLEANING EFFICIENCYWITH IMPERFECTIONS

LHC CLEANING EFFICIENCY WITH IMPERFECTIONSC. Bracco, R.W. Assmann, S. Redaelli, T. Weiler, CERN, Geneva, SwitzerlandAbstractThe performance reach of the LHC depends on the mag-nitude of beam losses and the achievable cleaning effi-ciency of its collimation system. The ideal performancereach for the nominal Phase 1 collimation system is re-viewed. However, unavoidable imperfections affect anyaccelerator and can further deteriorate the collimation per-formance. Multiple static machine and collimator imper-fections were included in the LHC tracking simulations.Error models for collimator jaw flatness, collimator setupaccuracy, the LHC orbit and the LHC aperture were set up,based to the maximum extent possible on measurementsand results of experimental beam tests. It is shown thatcombined “realistic” imperfections can reduce the LHCcleaning efficiency by about a factor 11 on average.INTRODUCTIONEach LHC proton beam stores an energy of up to 360 MJ.The superconducting (sc) magnets of the LHC wouldquench after an energy deposition of 5 mJ/cm3 [1]. A so-phisticated system of collimators [2] has been designed toconcentrate beam losses in dedicated cleaning insertionswhere more resistant room-temperature magnets are in-stalled.The performance of the collimation system is character-ized by the local cleaning inefficiency ηc defined as:ηc =NΔslostΔs ·Nabs , (1)where NΔslost is the number of particles lost within a longi-tudinal binning Δs = 10 cm and Nabs is the total number ofparticles absorbed at the collimators. ηc is derived by usingdetailed loss maps calculated with “SixTrack” and a real-istic machine aperture model [4]. Local losses exceedingthe quench limit of the sc magnets could imply a limitationin the maximum allowed beam current Imax [3]. The per-formance of the Phase 1 LHC collimation system [2] andpossible collimator-induced constraints are analyzed, in thefollowing, for the ideal machine and in case of multiple im-perfections.PERFORMANCE REACH FOR THEIDEAL MACHINEThe LHC collimation system must be active during thefull machine cycle, from injection up to physics and extrac-tion. Studies of cleaning efficiency have been performedfor different optics in order to identify limitations in beamintensity for safe machine operation. Peak values of themaximum local cleaning inefficiency ηmaxc are listed in Ta-ble 1 at injection and collision energy for the perfect ma-chine.Table 1: Peak values of ηmaxc for the ideal machine at in-jection and collision energy. The quench limit is evaluatedfor a loss rate corresponding to a beam lifetime of 0.1 h at450 GeV and 0.2 h at 7 TeV.Energy ηmaxc Quench limit[TeV] [10−5 m−1] [10−5 m−1]0.45 19.2±1.8 78.37.00 4.6±0.9 1.7The error on ηmaxc is purely statistical and is given by:σηmaxc =√NΔslostΔs ·Nabs . (2)Losses exceeding the quench limit at 7 TeV (see Table 1)are estimated to reduce Imax to about 40% of its nominalvalue (Inom = 3.22 · 1034 protons) already in the ideal case.While these results are based on conservative assumptions(i.e. 0.2 h beam lifetime and 10 cm loss dilution length),no other margin has been applied. In the LHC, the dumpthreshold of the beam loss monitors (BLM) will be set to30% of the quench limit [5]. Collimation therefore mustbe even more efficient to avoid beam-loss triggered aborts.Other safety margins could further reduce the allowablebeam loss so that these predictions on Imax can be con-sidered as realistic.IMPACT OF IMPERFECTIONS ON THEMACHINE PERFORMANCEIt was seen in other accelerators and always expectedfor the LHC that imperfections strongly deteriorate clean-ing efficiency. Early qualitative and semi-quantitative es-timates indicated that the performance can be reduced byfactors of 5-10. Here we present the first quantitative studythat estimates in detail the effect of many (still not all) im-perfections.In order to give an evaluation of the “realistic” cleaningperformance of the collimation system and consequent lim-itations to the maximum allowable beam intensity reach,several scenarios with combined imperfections were ana-lyzed for the nominal collision optics, as shown in the fol-lowing.Jaw Flatness ErrorsThe LHC collimators are produced with stringent re-quirements on the flatness of the jaws: ∼40 μm for theWE6PFP012 Proceedings of PAC09, Vancouver, BC, Canada2504Circular CollidersA01 - Hadron Colliders1 2 3 4 5−101234x 10−4scenariosη cmax [m−1 ]1) Idealmachine4) Deformed jaws + tilt +offset+gap error+machine mis−alignment2) Deformed jaws3) Deformed jaws + tilt +offset+gap errorTarget AreaQuench Limit5) Deformed jaws + tilt +offset+gap error+machine mis−alignment+closed orbitFigure 1: ηmaxc for multiple combinations of collimator and machine imperfections at collision energy. The blue pointrefers to machine alignment errors as defined by measurements. The cleaning efficiency gets worse up to a factor of 11by reducing Imax to <5% of Inom.1 m long graphite jaws. Any large deformation of thejaws would create a reduction of the material traversedby the halo particles (active length) and worsen the clean-ing efficiency. However, the achievable flatness is tech-nically limited and measurements performed on differ-ent collimator families show an average deformation of40.3± 22.2 μm [6].Simulations were carried out by using the same systematicnon-flatness, inwards and outwards, for jaws with equallength:• 100 μm for 1 m long jaws (absorbers, secondary andtertiary collimators)• 60 μm for 0.6 m long jaws (primary collimators).A 10% increase in ηmaxc was found with respect to the idealmachine at top energy (see Fig. 1 scenario 2).Collimator Setup ErrorsIdeally, the collimator jaws should be centered andaligned with respect to the beam. In reality the setup ofthe collimators is affected by unavoidable offsets and tiltswith respect to the ideal position.Results of experimental beam tests, performed with a pro-totype collimator installed in the SPS ring [6], were usedto simulate this scenario. Random errors with a Gaussiandistribution cut at 3σ were used and the following imper-fections were combined:1. Jaw flatness as described2. R.m.s. error on gap centre with respect to the beamcentre: 50 μm3. R.m.s. error on gap size: 0.1 σ4. R.m.s. error on jaw angle with respect to the beamenvelope: 200 μradA factor of 2 loss in cleaning efficiency was found withrespect to the perfect case (see Fig. 1 scenario 3).Machine Alignment ErrorsThe collimation simulation package [4] allows takinginto account magnet and beam screen misalignments of theLHC elements (beam screens are first aperture limitations).Table 2: Horizontal and vertical r.m.s magnet misalign-ments at beam screen level for different families of ma-chine elements. The numbers are based on design valuesand measurements performed in the LHC tunnel [7].Design MeasuredDescription σΔx σΔy σΔx σΔy[mm] [mm] [mm] [mm]main dipole 2.40 1.56 1.83 1.10arc quadrupole 2.00 1.20 1.36 0.76triplet quadrupole 1.00 1.00 1.53 1.53warm quadrupole 2.00 1.20 0.67 0.41warm dipole 1.50 1.50 1.96 1.49beam pos. monitor 0.50 0.50 1.36 0.76Starting from the standard aperture model used to derivethe loss maps, up to 20 seeds of misaligned apertures canbe applied to a set of halo trajectories produced by “Six-Track”. An r.m.s. offset in the horizontal (σΔx) and vertical(σΔy) planes has been defined for different families of ele-ments on the basis of design values and measurements per-formed in the LHC tunnel [7]. The applied imperfectionsProceedings of PAC09, Vancouver, BC, Canada WE6PFP012Circular CollidersA01 - Hadron Colliders 2505are listed in Table 2. Misalignment errors are simulatedwith a Gaussian distribution cut at 1.5σ to the geometriccentre of each magnet in the LHC. This is only carried outat the level of the aperture model. Particle tracking is donewith the ideal machine orbit and fields. The reduced aper-ture has different bottlenecks (one-sided) which lead to losspeaks in unusual locations and an increase in the cleaninginefficiency. Fig. 2 summarizes the ηmaxc values found for20 different seeds of machine alignment errors (design val-ues): the local cleaning inefficiency varies by up to a factorof four for different seeds. The cleaning efficiency is de-0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21012345x 10−4machineη cmax [m−1 ]  ideal machinemeanFigure 2: Cleaning inefficiency ηmaxc for 20 different seedsof machine alignment errors (design values). ηmaxc is in-creased on average by a factor of 9.5 with respect to theideal machine.graded by about one order of magnitude.The calculated maximum cleaning inefficiency averagedover the simulated seeds is:1. ηmaxc = (3.0±0.2)·10−4 m−1 for machine alignmenterrors from design values.2. ηmaxc = (2.7±0.2)·10−4 m−1 for machine alignmenterrors from measurements.Results are in agreement within the simulation error. Bothvalues are indicated in the summary plot shown in Fig. 1(scenario 4: red point for design values, blue point for mea-sured values).Non Ideal Closed OrbitThe effect of a non-ideal horizontal closed orbit is addedfor the study of collimation system performance.Previous in-depth studies on orbit perturbations [8] de-fined a conservative scenario in compliance with the spec-ified LHC orbit for the collision optics. On the basis ofthese results two kickers were used to generate a static hor-izontal closed orbit oscillation with maximum amplitude of±4 mm in the arcs. The orbit was corrected to the specified±3 mm maximum amplitude in the straight sections. Sim-ulations were carried out by using the standard initial halodistributions and the errors defined above. Tracking wasperformed with collimators centered on the non-ideal or-bit. This resulted in a further increase in the local cleaninginefficiency by a factor of 1.16 (see Fig. 1).To date this scenario (scenario 5 in Fig. 1) is considered asthe most realistic one, and could limit the maximum inten-sity reach to <5% of Inom.CONCLUSIONSThe LHC collimation system shall provide a cleaning ef-ficiency (absorption of losses) of better than 99.99% for the7 TeV LHC beams.This work extends the earlier studies on the achievablecleaning efficiency taking into account realistic static im-perfections. Simulations include, in particular, manufac-turing errors on flatness of collimators, collimator setup er-rors, design orbit errors and magnet alignment errors. Asexpected, it was found that combining these imperfectionshas a strong effect on the achievable cleaning efficiency.The performance loss was simulated for the first time formultiple imperfections, finding a factor of 11 reduction incleaning efficiency if compared to the ideal performance.The LHC beam intensity reachable with the Phase 1 col-limation system is therefore predicted to be below 5% ofthe nominal value. This prediction is based on critical as-sumptions for the sc magnet quench limit and a minimalbeam lifetime of 0.2 h. Nevertheless, since several imper-fections are not yet taken into account (namely effects frombeta-beat, coupling and non-linearities in the LHC), we ex-pect these estimates to be representative of the LHC op-eration. It is then concluded that these are strong hints atpossible limitations of the LHC intensity from cleaning oflosses. Higher beam intensities would require lower peakloss than specified, better collimator setup, reduced ma-chine imperfections or collimation upgrade.REFERENCES[1] J.B.Jeanneret et al., LHC project report 44 (1996).[2] The LHC Design Report, Vol.1, Chapter 18. CERN-2004-003, pp. 467-498.[3] R.W. Assmann, ”Collimators and Cleaning: Could ThatLimit the LHC Performance?”, proceedings of the ChamonixXII Workshop, 2003.[4] G.Robert-Demolaize et al., “A new version of SixTrackwith collimation and aperture interface”, proceedings of PAC2005, CERN-AB-2005-033.[5] E.B. Holzer et al., “Generation of 1.5 Milion Beam LossThreshold Values”, proceedings of EPAC08, Genoa, Italy,2008.[6] C. Bracco, “Commissioning Scenarios and Tests for the LHCCollimation System”, CERN-THESIS-2009-031, EPFL,Lausanne, 2009.[7] J.B. Jeanneret, “Geometrical Tolerances for the Qualificationof LHC magnets”, LHC-Project-Report-1007, 2006.[8] G. Robert-Demolaize, “Design and Performance Optimiza-tion of the LHC Collimation System”, LHC-Project-Report981, 2006.WE6PFP012 Proceedings of PAC09, Vancouver, BC, Canada2506Circular CollidersA01 - Hadron Colliders

LHC Cleaning Efficiency with Imperfections

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