A good knowledge of the available aperture in the LHC
is essential for a safe operation due to the risk of magnet
quenches or even damage in case of uncontrolled beam
losses. Experimental validations of the available aperture

are therefore crucial and were in the past carried out by either a collimator scan combined with beam excitations or

through the use of local orbit bumps. In this paper, we show
a first comparison of these methods in the same machine
configuration, as well as a new very fast method based on a
beam-based collimator alignment and a new faster variant
of the collimator scan method. The methods are applied
to the LHC operational configuration for 2015 at injection
and with squeezed beams and the measured apertures are
presented.peer-reviewe

7th International Particle Accelerator Conference

Bruce, Roderik

Fiascaris, M.

Garcia, Hector

Giovannozzi, M.

Hermes, Pascal Dominik

Kwee-Hinzmann, Regina

Mereghetti, Alessio

Mirarchi, Daniele

Quaranta, Elena

Redaelli, Stefano

Salvachua, Belen

Valentino, Gianluca

OAR@UM

IMPROVED APERTURE MEASUREMENTS AT THE LHC AND RESULTSFROM THEIR APPLICATION IN 2015 ∗P. D. Hermes†, CERN, Geneva, Switzerland and University of Münster, GermanyR. Bruce, M. Fiascaris, H. Garcia, M. Giovannozzi, R. Kwee-Hinzmann,A. Mereghetti, D. Mirarchi, E. Quaranta, S. Redaelli, B. Salvachua,G. Valentino , CERN, Geneva, SwitzerlandAbstractA good knowledge of the available aperture in the LHCis essential for a safe operation due to the risk of magnetquenches or even damage in case of uncontrolled beamlosses. Experimental validations of the available apertureare therefore crucial and were in the past carried out by ei-ther a collimator scan combined with beam excitations orthrough the use of local orbit bumps. In this paper, we showa ﬁrst comparison of these methods in the same machineconﬁguration, as well as a new very fast method based on abeam-based collimator alignment and a new faster variantof the collimator scan method. The methods are appliedto the LHC operational conﬁguration for 2015 at injectionand with squeezed beams and the measured apertures arepresented.INTRODUCTIONAperture measurements at the CERN Large Hadron Col-lider (LHC) [1] are regularly carried out in the initial com-missioning phase, before high intensity beams are allowedto be injected into the machine [2–8]. The measurements areused to verify the settings of the LHC collimators requiredto protect the machine. The global aperture bottleneck shiftsto the superconducting triplet magnets in the experimentalinsertions, once the beams are squeezed. One of the limitingfactors for small β∗ values and thus for high luminosity isthe available aperture in the triplets.The LHC Collimation System [9] is a multi-stage clean-ing system with primary (TCP) and secondary collimators(TCS) installed in the insertion regions (IR) dedicated tobeam cleaning, IR3 and IR7. In addition, the experimen-tal insertions host tertiary collimators (TCT) to protect thesuperconducting triplet magnets and to reduce machine in-duced background in the experimental detectors.One essential tool for the aperture measurements is theLHC Beam Loss Monitoring (BLM) System [10,11], whichconsists of more than 3000 cylindrical ionization chambers,installed at the outer side of the beam pipes. The BLMsmeasure the energy deposited by secondary shower parti-cles which are produced when beam particles intercept themachine hardware. They can be used to identify preciselywhen beam losses occur at local aperture restrictions. Themeasurements require also transverse emittance blow ups,which are provided by the transverse damper (ADT) [12].∗ Work supported by the German Wolfgang Gentner Programme of theBundesministerium für Bildung und Forschung.† phermes@cern.chThe ADT can induce white noise for the individual planesof both beams in a controlled manner.In this article, we present diﬀerent methods of aperturemeasurements, two of them newly established in 2015. Wecompare the measurement results obtained with the diﬀerentmethods.MEASUREMENT STRATEGIESAperture measurements can be subdivided into two majorclasses: global and local. While local measurements allowthe analysis of the available aperture at speciﬁc locations ofinterest, global aperture measurements probe the global aper-ture bottleneck, i.e. the smallest available beam clearanceper beam and plane.Local MeasurementsLocal aperture measurements [13] are realized by meansof a local orbit bump shaped such that the location of interestcan be touched with the beam. The bump amplitude issuccessively increased until losses are measured with theBLMs. Typically the bump amplitude is increased in stepsizes between 0.25 σ and 0.5 σ.The beam envelope is deﬁned by exciting the beam withthe ADT until losses are measured at the primary collimator.The beam then ﬁlls the space between the collimator gapsand its normalized beam size is deﬁned by the opening Npof the primary collimator. The remaining collimator settingsare not relevant for the test as long as the bump is not appliedin a region where collimators are present. Otherwise thecorresponding collimators have to be retracted.The normalized aperture Aloc is given by the sum of thebeam envelope Np and the bump amplitude xb at which thelosses occur for the ﬁrst time:Aloc = Np + xb . (1)An intrinsic advantage is that possible asymmetries in theaperture can be identiﬁed by measurements with both signsof the bump. A permanent bump can then be deployed tocenter the beam and gain aperture.In the analysis of the measurement results, the expected or-bit is compared to the orbit measured with the beam positionmonitors (BPM).Global MeasurementsCollimator Scan (CS) The collimator scan method isan iterative measurement in which the global bottleneck isexposed to beam losses in a controlled manner.Proceedings of IPAC2016, Busan, Korea Pre-Release Snapshot 19-May-2016 00:00 UTCTUPMW01401 Circular and Linear CollidersA01 Hadron CollidersISBN 978-3-95450-147-21 Copyright©2016CC-BY-3.0andbytherespectiveauthorsPre-ReleaseSnapshot19-May-201600:00UTC15 20 25 30 35 40 45 50Time [s]01234567LossSignal[Gy/s]/BeamIntensity[a.u.]TCPBottleneckIntensity81012141618TCPhalfgapNp[σ]Raw SignalsNp (σ)9 10 11 12 13 14 15Collimator Half Gap (σ)0.00.20.40.60.81.0Norm.BLMSignal[a.u.]Normalized SignalsBottleneck BLMCollimator BLMFigure 1: Raw data and normalized BLM signals in the post-processing of aperture measurements with the FCS method.The measurement is prepared by retracting all collimatorsbut leaving one dedicated collimator in place.The collimator is retracted in steps of 0.5 σ and the beamis blown up at every step. When the collimator setting issuch that the aperture at the global bottleneck becomes un-protected, the highest measured loss peak in the machinemoves from the collimator to the aperture bottleneck. Themeasurement is continued until all losses have moved to thebottleneck.The aperture is deduced from the collimator half gap atwhich the normalized BLM signal at the bottleneck is higherthan at the collimator. The normalization of the BLM signalsis explained in more detail in the next section.Fast Collimator Scan (FCS) As a complementarymethod to the lengthy procedure with manual beam ex-citation and collimator retraction, a semi-automatic fastermethod for aperture measurements with the collimator scanmethod was established in 2015.The BLM signals, beam intensity evolution and TCP set-ting during a FCS measurement are shown in the upper frameof Fig. 1. The ADT is set up to excite the beam continu-ously over an extended period of time. The collimator isprogrammed to open or close automatically in steps of 0.5 σat a deﬁned frequency. Once the ﬁnal setting (in the shownexample 9 σ) is reached, the measurement is stopped. Theaperture is calculated in the same way as the CS method,but the method is signiﬁcantly faster. In practice, the FCSmethod is applied to conﬁrm the measurement result bythe CS, to ﬁrst identify the global aperture bottleneck in acontrolled and secure way.The frequency of the collimator movement should bechosen low enough, to allow for the extraction of the BLMsignals at each step. This requires a bunch intensity largeenough to provide a suﬃcient loss rate during the wholeexcitation.The BLM response with respect to a deﬁned amount ofbeam loss is in general varying for the diﬀerent loss loca-tions. Furthermore, it depends on the intensity lost at eachstep. This introduces an uncertainty on the aperture deduceddirectly from the collimator setting at which the BLM sig-nal at the bottleneck becomes larger than at the collimator.The measured BLM signals are normalized to abolish theseuncertainties. The peak BLM signals bb (bottleneck) andbc (collimator) at each step are extracted from the measure-ment and normalized by the intensity drop ΔI during theexcitation. In a second step, the signals are normalized totheir respective maximum max(bb ), max(bc ), to take thediﬀerent BLM responses at full exposure into account. Theaperture at the global bottleneck is the interception of thetwo interpolated normalized signal lines (see bottom plotof Fig. 1). A detailed description of the CS method and thenormalization is given in [14]. The method has the advan-tage that the resulting aperture is expressed in terms of thecollimator gap, which is the best way to deﬁne the collima-tion hierarchy. On the contrary, the extensive normalizationis a possible source of uncertainty, especially if the intensitydrop is in the same order of magnitude as the noise of themeasuring device.Beam-Based Alignment (BBA) The global aperturemeasurement via beam-based alignment is a new methodwhich has been established and tested in 2015.11:29:0111:29:0611:29:1111:29:1611:29:21Time0.51.01.52.02.53.0BLMSignal[1e-6Gy/s]BLM SignalTCP half gapBBA Threshold10111213141516TCPhalfgap[σ]Figure 2: Aperture measurement via BBA. The TCP isclosed until the BLM Signal at the collimator exceeds apreviously deﬁned threshold.TUPMW014Proceedings of IPAC2016, Busan, Korea Pre-Release Snapshot 19-May-2016 00:00 UTCISBN 978-3-95450-147-22Copyright©2016CC-BY-3.0andbytherespectiveauthorsPre-ReleaseSnapshot19-May-201600:00UTC01 Circular and Linear CollidersA01 Hadron CollidersIn this type of measurement all collimators are retractedand the beam is excited, such that losses are measured at theglobal aperture bottleneck. The beam has then been scrapedby the aperture bottleneck which deﬁnes the beam envelope.The extension of the beam envelope is measured by per-forming a beam-based collimator alignment with the primarycollimator. In this type of alignment, the collimator is closeduntil the BLM signal at the collimator exceeds a previouslydeﬁned threshold [15] (see Fig. 2). Once it is ensured thatboth collimator jaws have individually touched the beam,the normalized collimator gap can be directly translated intothe available aperture at the global bottleneck.The aperture can be directly deduced without the require-ment for post-processing. The method is particularly suitedfor measurements at the triplet magnets, where the requiredsettings of the local TCT collimators may be concluded fromthe measurement. On the contrary, the beam edge is notsharply deﬁned and the measurement result depends on thesetting of the BLM signal at which the BBA is stopped. Themethod thus risks to underestimate the available aperture.Being still under development, the BBA aperture measure-ment is so far used as a complementary method applied inaddition to the methods mentioned above.RESULTSInjection EnergyFor the aperture measurements at injection energy(450 GeV) in the 2015 commissioning phase, three diﬀerentmethods (local, FCS/CS, BBA) have been used. The mea-surement results are compared in Tab. 1. Note that in thisoperational period, the FCS method was still in its test phase.Only the B1V measurement result could be compared to themanual CS method. The results are in full agreement.The global aperture bottleneck for B1H was found in aseparation dipole. An appropriate bump can not be matchedat this location, such that only global measurements havebeen carried out. The BBA result is smaller than the aperturemeasured with the collimator scan method. For the measure-ment of the vertical plane of B1, the local measurementsshow an aperture larger by 1σ compared to the collimatorscans. This diﬀerence is consistent with the aperture asym-metry of 1σ which was identiﬁed in the local scan. TheBBA measurement returns an aperture close to the resultmeasured with the CS method. The aperture for B2H mea-sured locally and with the CS method agrees within theiruncertainties of 0.5σ and 0.7σ. The BBA measurement re-Table 1: Measured apertures at the global aperture bottleneckat injection energy in 2015.Bottleneck BBA CS/FCS LocalB1H MBRC.4R8 11.6 12.5-13.0 -B1V Q6L4 12.1 11.5-12.0 12.4-12.9B2H Q4L6 12.5 12.8-13.3 13.0-13.7B2V Q4R6 12.0 - 12.7-13.0Table 2: Global aperture measurement results with squeezedand colliding beams at 6.5 TeV in 2015. The measurementwas carried out using the CS method.Squeeze CollisionPlane Aperture [σ] Aperture [σ] BottleneckB1H 16.7-17.7 17.2-18.2 IR5B1V 15.7-16.2 15.7-16.2 IR1B2H - 16.2-16.7 IR1B2V 15.7-16.2 15.7-16.2 IR1sult is slightly below. For B2V, the collimator scan data wascorrupted, so the aperture was evaluated using only localmeasurements and the BBA method. The result from theBBA is below the aperture measured with the local scan.The measurements with the BBA method can potentiallybe improved when the beams are strongly blown up, suchthat the population at the edge of the beam envelope is higherand gives more pronounced loss spikes.Top EnergyThe measurements at top energy were carried out at theend of squeeze and at collision, with proton beams at 6.5 TeVand beams squeezed to β∗ = 0.8 m in IP1 and IP5, β∗ =10 m in IP2 and β∗ = 3 m in IP8. In collision, additionalbumps are applied to control the luminosity, which can movethe orbit and either improve or worsen the available aperture.The aperture measured with the CS method are listed forthe two conﬁgurations in Tab. 2. Apparently, the measuredaperture was similar in the two conﬁgurations, while in thecollision mode the measurement is even compatible with alarger global aperture than at squeeze.SUMMARY AND CONCLUSIONSAperture measurements in the LHC play an essential rolefor the achievement of safe operation and small β∗ valueswith high beam intensities. Diﬀerent techniques can be em-ployed to probe the available aperture in the machine. In thelast years of LHC operation, aperture measurements haveshown to provide reproducible and reliable results. In thisnote we presented well-established and novel measurementstrategies. Besides local aperture measurements with orbitbumps, two global measurement strategies have been pre-sented: collimator scan and beam-based alignment. Thecollimator scan method can be applied manually or in a newfast and semi-automatic manner.The apertures measured at injection, squeeze and withcolliding beams were presented. As expected, the measure-ments via beam-based alignment have the tendency to pre-dict slightly smaller apertures than the other methods. Themethod is still under development. With the upcoming im-provements we aim to reach an accuracy similar to the CSmethods. The results from local scans and the CS methodagree within their uncertainties.Proceedings of IPAC2016, Busan, Korea Pre-Release Snapshot 19-May-2016 00:00 UTCTUPMW01401 Circular and Linear CollidersA01 Hadron CollidersISBN 978-3-95450-147-23 Copyright©2016CC-BY-3.0andbytherespectiveauthorsPre-ReleaseSnapshot19-May-201600:00UTCREFERENCES[1] O. S. Brüning et al.(Eds.), “LHC design report v.1 :The LHC main ring”, CERN-2004-003-V1 (2004).[2] C. A. Pons et al., Proc. of IPAC 2010, Kyoto, Japan,(MOPEC010), pp. 477-479, (2010).[3] C. A. Pons et al.CERN-ATS-Note-2011-110 MD, (2011).[4] R. Assmann et al.. Proc. of IPAC 2011, San Sebastian, Spain,(TUPZ006), pp. 1810-1812, (2011).[5] S. Redaelli et al., Proc. of IPAC 2012, New Orleans,Louisiana, USA, (MOPPD062), pp. 508-510, (2012).[6] C. A. Pons et al., CERN-ATS-Note-2012-017 MD, (2012).[7] R. Bruce et al., CERN-ATS-Note-2013-026, (2013).[8] R. Bruce et al., CERN-ACC-Note-2013-0011 MD, (2013).[9] R. W. Assmann et al., Proc. of EPAC 2006, Edinburgh, UnitedKingdom , TUODFI01, pp. 986-988, (2006).[10] E. B. Holzer et al., Proc. of EPAC 2008, Genoa, Italy,(TUPC037), pp. 1134-1136, (2008).[11] E. B. Holzer et al., IEEE Nuclear Science Symposium Con-ference Record, vol. 2, pp. 1052–1056, (2005).[12] W. Höﬂe et al., Proc. of IPAC 2012, New Orleans, Louisiana,USA, (THPPRO039), pp. 4059-4061, (2012).[13] R. Bruce et al., CERN-ATS-Note-2013-026 MD, (2013).[14] R. W. Assmann et al., Proc. of IPAC 2011, San Sebastian,Spain, (TUPZ006), pp. 1810-1812, (2011).[15] G. Valentino et al., Phys. Rev. ST Accel. Beams 15, 051002,(2012).TUPMW014Proceedings of IPAC2016, Busan, Korea Pre-Release Snapshot 19-May-2016 00:00 UTCISBN 978-3-95450-147-24Copyright©2016CC-BY-3.0andbytherespectiveauthorsPre-ReleaseSnapshot19-May-201600:00UTC01 Circular and Linear CollidersA01 Hadron CollidersView publication stats

Improved aperture measurements at the LHC and results from their application in 2015

A good knowledge of the available aperture in the LHC is essential for a safe operation due to the risk of magnet quenches or even damage in case of uncontrolled beam losses. Experimental validations of the available aperture are therefore crucial and were in the past carried out by either a collimator scan combined with beam excitations or through the use of local orbit bumps. In this paper, we show a first comparison of these methods in the same machine configuration, as well as a new very fast method based on a beam-based collimator alignment and a new faster variant of the collimator scan method. The methods are applied to the LHC operational configuration for 2015 at injection and with squeezed beams and the measured apertures are presented

Hermes, Pascal

Fiascaris, Maria

Giovannozzi, Massimo

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Improved Aperture Measurements at the LHC and Results from their Application in 2015

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Improved aperture measurements at the LHC and results from their application in 2015

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