The LHC collimation system requires a high cleaning efficiency in order to prevent magnet quenches due to regular beam diffusion. The cleaning efficiency is significantly reduced due to imperfections of the collimator jaws and the machine optics. Tracking tools have been set up to predict the cleaning efficiency in presence of multiple imperfections. The deterioration of cleaning efficiency is quantified for different errors, including collimator surface non-flatness, collimator alignment errors, beta beating, orbit errors, non-linear field errors, and chromatic effects

Assmann, R W

Jeanneret, J B

Kaltchev, D I

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsEFFICIENCY FOR THE IMPERFECT LHC COLLIMATION SYSTEMR. Assmann1, J.B. Jeanneret1, D. Kaltchev2The LHC collimation system requires a high cleaning efficiency in order to prevent magnet quenches dueto regular beam diffusion. The cleaning efficiency is significantly reduced due to imperfections of thecollimator jaws and the machine optics. Tracking tools have been set up to predict the cleaning efficiencyin presence of multiple imperfections. The deterioration of cleaning efficiency is quantified for differenterrors, including collimator surface non-flatness, collimator alignment errors, beta beating, orbit errors,non-linear field errors, and chromatic effects.1  CERN, Geneva, Switerland2  TRIUMF, CanadaPresented at the Eighth European Particle Accelerator Conference (EPAC)3-7 June 2002 - La Villette, Paris, FranceGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 598Abstract24 July 2002EFFICIENCY FOR THE IMPERFECT LHC COLLIMATION SYSTEMR. Aßmann, J.B. Jeanneret, CERN, Geneva, SwitzerlandD. Kaltchev, TRIUMF, CanadaAbstractThe LHC collimation system requires a high cleaning ef-ficiency in order to prevent magnet quenches due to regularbeam diffusion. The cleaning efficiency is significantly re-duced due to imperfections of the collimator jaws and themachine optics. Tracking tools have been set up to predictthe cleaning efficiency in presence of multiple imperfec-tions. The deterioration of cleaning efficiency is quanti-fied for different errors, including collimator surface non-flatness, collimator alignment errors, beta beating, orbit er-rors, non-linear field errors, and chromatic effects.1 INTRODUCTIONThe high intensity LHC beams require efficient collima-tion at all phases of a beam cycle. The regular proton lossesdue to diffusion and scattering processes will create a pri-mary beam halo with enough intensity to quench and evendamage the super-conducting magnets. The protons of theprimary beam halo must therefore be intercepted at specif-ically designed collimators, so that they cannot reach thecold aperture of the magnets. The detailed requirements ofthe LHC collimation system are described in [1].The particles in the beam halo are characterized by theirnormalized radial amplitudes Ar =√A2x + A2y , where Axis defined with the usual Twiss parameters α, β, γ as:Ax =(γxx2 + 2αxx′ + βxx′2x)1/2(1)The collimation system is then characterized for some cutamplitude Acut by the cleaning inefficiency ηc:ηc(Acut) =Number of protons(Ar > Acut)Number of impacting protons (2)The relevant cut amplitude Acut is given by the availablemechanical aperture downstream of the collimation sys-tem, which is specified to be about 10σ in the LHC. Theavailable aperture is obtained after subtracting tolerancesfor mechanical alignment and closed orbit and its detailsdepend on the installation and operation of the machine.The results have been obtained for nominal LHC parame-ters. The required collimation inefficiency at Acut = 10σis about 10−3 for nominal LHC intensity and top energy,assuming in addition that the escaping protons are lost over50 m of cold aperture [1, 2].Two insertions in the LHC are dedicated to betatron andmomentum collimation [3, 4]. The betatron cleaning sys-tem at top energy is most critical and is considered here. Itincludes 4 primary 0.2 m long Al collimators and 16 sec-ondary 0.5 m long Cu collimators. These collimator mate-rials cannot withstand the expected accidental proton lossesin the LHC. However, until an improved system has beendesigned, the efficiency studies are performed with the ex-isting system. The collimators are set to 6σ for the primaryand 7σ for the secondary jaws. The scattering processes inthe collimator jaws is calculated with the K2 [5] and alter-natively the STRUCT [6] routine.2 LINEAR TRACKINGA simple but fast linear, uncoupled, and non-chromatictracking program was set up. Samples of up to 106 pro-tons are tracked up to several hundred turns, when mostparticles have been lost in the collimators. The impact pa-rameter was set to 1-2 µm. The normalized populationsof secondary and tertiary beam halos have been calculatedwithout imperfections. As shown in Fig. 1, the secondaryhalo extends to just below 10 σ. For calculating the ineffi-ciency the tertiary halo is integrated above 10 σr. An idealcleaning inefficiency of 6 × 10−4 is obtained. The phasespace distribution in the tertiary halo is highly non-uniform,as particles originate at specific phase space locations.1e-0071e-0061e-0050.00010.0010.010.116 8 10 12 14 16 18 20Normalized populationAmplitude [σr]Secondary haloTertiary halo-0.15-0.1-0.0500.050.10.15-2 -1.5 -1 -0.5 0 0.5 1 1.5 2Particle angle y’ [mrad]Particle offset y [mm]Figure 1: Transverse distribution of secondary and tertiarybeam halos, normalized to the number of impacting protons(top). Vertical phase space of the tertiary halo (bottom).Transient orbit errors and beta beat The collimatorsettings are adjusted to the values required from static orbitand beta beat errors. A transient change in the machine set-tings or natural drifts will then change the orbit or the betabeat without the collimators being re-optimized. The lossof cleaning inefficiency is shown in Fig. 2 for a transientorbit error and in Fig.3 for a transient beta beat. The worstphase of orbit and beta beat error is assumed. It is seen thatthe inefficiency is increased by 50% for transient beta beatof 8% or an orbit change of 0.6 σ (120 µm).00.0020.0040.0060.0080.010.0120 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6Inefficiencyy orbit error [σy]Figure 2: Dependence of the collimation inefficiency on anuncontrolled change in vertical orbit. The impact parame-ter at the primary vertical collimator was kept constant andthe orbit change was supposed to be out of phase with theprimary collimator.0.00050.0010.00150.0020.00250.0030.00350.0040 5 10 15 20Inefficiencyδβ/β [%]Figure 3: Dependence of the collimation inefficiency onan uncontrolled transient beta beat, considering the worstphase. Beta beat was modelled as a modulation of collima-tion depth.Required accuracy for collimator adjustments Theinefficiency is shown in Fig. 4 for different collimator set-tings. A shallow minimum exists and the settings of colli-mation depth need to be accurate on the level of ± 0.5-σor ± 100 µm (secondary settings relative to primary set-tings). This is a rough estimate, as the dependence is quitenon-linear and there is more room for some cases.Collinearity errors and surface flatness The effi-ciency of the collimation system depends strongly on theactive length of collimator jaws. This is the length of mat-ter that the beam traverses. The active length of a jaw canbe shorter than its real length, for example due to angularmisalignment or flatness errors. The dependence of inef-ficiency on the rms angle between the jaw surface and thelongitudinal beam direction is illustrated in Fig. 5. Angleerrors were randomly assigned with several seeds per point.There is a quite steep dependence with a tolerance of about50 µrad for a 50% increase in inefficiency.00.0050.010.0150.020.0250.030.0350.04Inefficiencyn1 = 5σn1 = 6σn1 = 7σ00.0020.0040.0060.0080.015 5.5 6 6.5 7 7.5 8 8.5 9Inefficiencyn2 [σ]n1 = 5σn1 = 6σn1 = 7σAbove 10 σAbove 8 σFigure 4: Dependence of the collimation inefficiency onthe collimator settings for n1 (primary collimation depth)and n2 (secondary collimation depth).00.00050.0010.00150.0020.00250.0030.00350.0040.00450.0050 50 100 150 200 250 300 350 400InefficiencyRms tilt of sec coll [µrad]Figure 5: Dependence of the collimation inefficiency onthe rms angle between jaw surface and longitudinal beamdirection.The small impact parameter at the collimator imposesstrict tolerances on the surface flatness of the jaws. Thetypical impact parameter at primary collimators is expectedto be about 1-2 µm. Errors in surface non-flatness can re-duce the effective length of the primary collimator. Thishas two effects: 1) The cleaning half time (time until halfthe impacting protons are lost) is dramatically increased,being ten times longer (≈100 turns), if the active length isonly 10% of the actual length. 2) The cleaning inefficiencyis increased, as shown in Fig. 6. Note, that only the sec-ondary collimators show a strong dependence of cleaninginefficiency on the active jaw length. This is expected andto some extent compensated by the larger impact parameterof about 200 µm.Preliminary estimate of tolerances Based on the re-sults shown above, preliminary tolerances on the differentimperfections were defined, for each with a 50% increasein cleaning inefficiency. The tolerances are summarized inTable 1. Note that the flatness tolerances were determinedto be equivalent to a 50 µrad angle error between beam00.0020.0040.0060.0080.010.0120.0140.0160 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5InefficiencyActive jaw length [m]Secondary collimatorsPrimarycollimatorsFigure 6: Dependence of the collimation inefficiency on theactive jaw length for primary and secondary collimators.The lengths of jaws of one type were reduced together.Table 1: Preliminary tolerances, each for a 50% increasein cleaning inefficiency. The setting accuracy of secondarycollimators is relative to the primary collimators. The flat-ness tolerances assume impact parameters of 10 µm and200 µm. Interdependencies between errors are not yettaken into account.Error ToleranceOrbit 0.6 σBeta beat 8%Longitudinal angle 50 µrad∆L/L (prim) 75%Surface flatness (prim) 10 µm∆L/L (sec) 20%Surface flatness (sec) 25 µmSetting accuracy (prim) -1.0/+0.5 σSetting accuracy (sec) ≥ ± 0.5 σand jaw surface (in terms of active jaw length). Further re-finements are required and a full model must simulate alleffects concurrently.3 CHROMATIC EFFECTSThe off-momentum nature of the beam halos requires forthe cleaning inefficiency to include the dispersive offset atlocations of non-zero dispersion and for the tracking to in-clude full chromatic effects. The simple and fast trackingapproach was not designed to include all chromatic effects.However, the maximum arc dispersion can be included intothe definition of cleaning inefficiency. This was done andFig. 7 shows the modified normalized distributions of sec-ondary and tertiary halo. Comparing the result to the on-momentum halos in Fig. 1 it is seen that the tertiary halo isessentially unchanged. However, the secondary halo is nolonger cut at 10 σ but extends much further. The protonsbeyond 10 σ now originate in roughly equal numbers fromthe secondary and tertiary halo. The cleaning inefficiencyis about doubled.The collimation scattering routines were implementedinto the standard tracking programs DIMAD (STRUCTscattering routine) and SIXTRACK (K2 scattering routine).1e-0071e-0061e-0050.00010.0010.010.116 8 10 12 14 16 18 20Normalized populationAmplitude [σr]Secondary haloTertiary haloFigure 7: Transverse distribution (in normalized units) ofsecondary and tertiary beam halos, normalized to the num-ber of impacting protons. This is like Fig. 1 but here takesinto account the dispersive horizontal offsets due the off-momentum nature of halo particles.A preliminary comparison of the DIMAD result with thefast linear tracking showed that the results are basicallycomparable with a factor of five disagreement at 10 σ. Thisis true even for non-chromatic tracking in DIMAD, whenthe momentum loss in collimators is suppressed. The ori-gin of this disagreement is under study.4 CONCLUSIONThe efficiency of the LHC betatron collimation systemwas studied for top energy including imperfections, as theyare expected in the LHC. Different tools were employed,showing results different by about a factor of five. As thestudies involve the detailed understanding of proton-matterinteraction at 7 TeV and the accelerator physics of largeamplitude halos, the observed disagreement seems reason-able. Nevertheless work is continuing to understand theorigin of discrepancies and to further improve the predic-tive power of the numerical tools.The existing tools were used to calculate the expectedsecondary and tertiary beam halos at locations of zero andmaximum dispersion. The off-momentum secondary beamhalo was found to extend far beyond 10 σ, doubling thecleaning inefficiency. For a number of imperfections pre-liminary tolerances were specified with the criterion of a50% increase of cleaning inefficiency for each error.The future work will aim at further improving the pre-dictive power of the numerical tools and at establishing acomplete and fully interdependent error model for the LHCcollimation system.5 REFERENCES[1] R. Aßmann et al. These proceedings.[2] J.B. Jeanneret, Proc. Chamonix X, CERN SL 2000-007 DI,R. Le Roux et al. Eds, February 2000, p. 162.[3] J.B. Jeanneret, Phys. Rev. ST-AB, 1,081001, 1998.[4] D.I. Kaltchev et al., Proc. PAC99. CERN LHC Project Re-port 305 (1999).[5] N. Catalan Lasheras et al., CERN LHC Project Report 156(1998).[6] I. Baishev et al., SSCL-MAN-0034, Dallas, 1994.

Efficiency for the imperfect LHC collimation system

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