The beam centers at the Large Hadron Collider collimators are determined by beam-basedalignment, where both jaws of a collimator are moved in separately until a loss spike isdetected on a Beam Loss Monitor downstream. Orbit drifts of more than a few hundredmicrometers cannot be tolerated, as they would compromise the performance of thecollimation system. Beam Position Monitors (BPMs) are installed at various locations aroundthe LHC ring, and a linear interpolation of the orbit can be obtained at the collimatorpositions. In this paper, the results obtained from beam-based alignment are compared withthe orbit interpolated from the BPM data throughout the 2011 and 2012 LHC proton runs.Louisiana State University (LSU),U.S. Department of Energy, Office of Science,COSYLAB,DIMTEL,Muons, Inc.peer-reviewe

Assmann, Ralph W.

Bruce, Roderik

International Particle Accelerator Conference

Lari, Luisella

Müller, G. J.

Redaelli, Stefano

Rossi, Adriana

Sammut, Nicholas

Valentino, Gianluca

The beam centers at the Large Hadron Collider collimators are determined by beam-based alignment, where both jaws of a collimator are moved in separately until a loss spike is detected on a Beam LossMonitor downstream. Orbit drifts of more than a few hundred micrometers cannot be tolerated, as they would compromise the performance of the collimation system. Beam Position Monitors (BPMs) are installed at various locations around the LHC ring, and a linear interpolation of the orbit can be obtained at the collimator positions. In this paper, the results obtained from beam-based alignment are compared with the orbit interpolated from the BPM data throughout the 2011 and 2012 LHC proton runs

Valentino, G

Sammut, N

Assmann, R W

Bruce, R

Muller, G J

Redaelli, S

Rossi, A

Lari, L

CERN Document Server

Comparison of LHC collimator beam-based alignment to BPM-Interpolated centers

OAR@UM

COMPARISON OF LHC COLLIMATOR BEAM-BASED ALIGNMENT TOBPM-INTERPOLATED CENTERSG. Valentino∗, N. SammutCERN, Geneva, Switzerland and University of Malta, Msida, MaltaR. W. Assmann, R. Bruce, G. J. Mu¨ller, S. Redaelli, A. RossiCERN, Geneva, SwitzerlandL. Lari,CERN, Geneva, Switzerland and IFIC (CSIC-UV), Valencia, SpainAbstractThe beam centers at the Large Hadron Collider collima-tors are determined by beam-based alignment, where bothjaws of a collimator are moved in separately until a lossspike is detected on a Beam Loss Monitor downstream. Or-bit drifts of more than a few hundred micrometers cannotbe tolerated, as they would compromise the performance ofthe collimation system. Beam Position Monitors (BPMs)are installed at various locations around the LHC ring, anda linear interpolation of the orbit can be obtained at the col-limator positions. In this paper, the results obtained frombeam-based alignment are compared with the orbit inter-polated from the BPM data throughout the 2011 and 2012LHC proton runs.INTRODUCTIONA collimation system protects the Large Hadron Col-lider (LHC) from quenches in the superconducting mag-nets, providing multi-stage hierarchical cleaning [1]. Thecollimators also protect against single-turn losses and ra-diation effects. Maximal cleaning efficiency can only beobtained if the beam centers are known, so that the jawscan be positioned symmetrically around the beam in unitsof σ.In beam-based alignment, the collimator jaws are movedseparately towards the beam from their initial positions instep sizes of 5 µm to 20 µm until a loss spike is observed onthe Beam Loss Monitor (BLM) further downstream of thecollimator. The beam center at each collimator location iscalculated as the average of the two aligned jaw positions.The alignment precision depends on the jaw step size usedto move the jaw. Through the use of a BLM feedback al-gorithm [2] which stops the repetitive jaw movement whenthe losses exceed a pre-defined threshold, the time requiredfor a full alignment of all 86 LHC collimators has been re-duced from ∼29 hours in 2010 to ∼17 hours in 2011.An approximation to the beam centers at the collimatorscan be obtained from an interpolation of the orbit measuredat specific locations by Beam Position Monitors (BPMs).A BPM consists of four button electrode feedthroughsmounted orthogonally in the beam pipe [4]. These mon-itors are placed on each side of the warm quadrupoles, thus∗ gianluca.valentino@cern.chFigure 1: Example of the beam orbit through points 1, Sand 2 (from [3]). With BPMs located at point 1 and 2, theobjective of the interpolation is to find the orbit at point S.providing the minimum configuration that allows a linearinterpolation of the closed orbit, dispersion and β func-tions [5].In a future implementation of the alignment software,the interpolated orbit could be acquired and exploited tospeed up the alignment process, hence the motivation ofthis work. In the paper, the method used to calculate theinterpolated orbit is explained, followed by a quantitativeanalysis of the differences between both sets of measure-ments.BPM-INTERPOLATED ORBITAn example of the LHC beam orbit through variouspoints in the machine is shown in Fig. 1. With BPMs lo-cated at point 1 and 2, the orbit at an intermediate point Scan be calculated using linear transfer matrices. The in-terpolation is done per plane and per segment, which isdefined as the region between two BPMs. The angle canbe calculated from the orbit transfer matrix between a pairof adjacent BPMs. The orbit at point 2 can be establishedfrom point 1 using a transfer matrix:(x2x′2)=M12(x1x′1)=(C12 S12C′12S′12)(x1x′1)where M12 is the transfer matrix between point 1 and 2with elements:−3 −2 −1 0 1 2 302468101214161820Interpolation vs Beam−based Alignment Delta (mm)Number of Alignments(a) Measurements at Injection (450 GeV)−3 −2 −1 0 1 2 3024681012141618Interpolation vs Beam−based Alignment Delta (mm)Number of Alignments(b) Measurements at Flat Top (3.5 TeV)Figure 2: Delta between the BPM-interpolated and the beam-based centers at the LHC collimators in 2011.C12 =√β2β1(cosψ12 + α1 sinψ12) (1)C′12=α1 − α2√β1β2cosψ12 −1 + α1α2√β1β2sinψ12 (2)S12 =√β1β2 sinψ12 (3)S′12=√β1β2(cosψ12 − α2 sinψ12) (4)Similarly for the orbit from point 1 to S:(xSx′S)=M1S(x1x′1)The interpolated orbit at point S can hence be expressed as:xS = C1Sx1 + S1Sx2 − C12x1S12(5)The interpolated orbit is one of the features provided bythe LHC Aperture Meter [6], an application which providesthe operators with real-time information on the current ma-chine bottlenecks. Finally, in order to compare to the beamcenters found from beam-based collimator alignment, theinterpolated orbit needs to be transformed to the collimatorco-ordinate system:∆inti = xhorS cos θi + xverS sin θi (6)where θi is the azimuthal tilt angle of the collimator i inthe transverse plane. For beam 2 collimators, the sign ofthe horizontal component is inverted. The interpolation ishighly dependent on the BPMs selected, and invalid mon-itors which give erroneous readings need to be removedTable 1: BPM-interpolation and beam-based comparisonstatistics for 2011.Dataset Mean (mm) R.M.S. (mm) SD (mm)Injection 0.553 0.849 0.854Flat Top 0.536 0.811 0.817from the calculation. The interpolation accuracy derivesfrom the linearity of the BPM system (1% of the half ra-dius, corresponding to ∼130µm for arc BPMs).COMPARISON RESULTSThe interpolated orbit at each collimator was extractedfor the same timestamp at which the collimator wasaligned. Both datasets are acquired and logged at a rateof 1 Hz. Comparison results are presented for two stages inthe LHC machine cycle when beam-based collimator align-ment is performed, for data from the 2011 and 2012 runs.Results in 2011Comparison results for 2011 are shown as histogramsin Fig. 2(a) and Fig. 2(b). The deltas generally lie within±1 mm, but large deltas can be observed for the tertiarycollimators (TCTs). The large deltas at these locations arelikely due to a poorer performance of the BPMs located inthis region. The statistics for the 2011 run are presentedin Table 1. The absolute average for the injection datasetcorresponds to 0.553 mm, while for flat top the absolute av-erage is 0.536 mm. The standard deviation is ∼0.840 mm.Results in 2012A similar analysis was performed for the collimatoralignments in 2012 (see Fig. 3(a) and Fig. 3(b)). Once−3 −2 −1 0 1 2 3024681012141618Interpolation vs Beam−based Alignment Delta (mm)Number of Alignments(a) Measurements at Injection (450 GeV)−3 −2 −1 0 1 2 302468101214161820Interpolation vs Beam−based Alignment Delta (mm)Number of Alignments(b) Measurements at Flat Top (4 TeV)Figure 3: Delta between the BPM-interpolated and the beam-based centers at the LHC collimators in 2012.Table 2: BPM-interpolation and beam-based comparisonstatistics for 2012.Dataset Mean (mm) R.M.S. (mm) SD (mm)Injection 0.501 0.758 0.762Flat Top 0.564 0.883 0.885again, large deltas were observed for the TCTs. Thestatistics for the 2012 run listed in Table 2 show an av-erage of 0.501 mm and 0.564 mm for the injection andflat top datasets respectively. The standard deviation is∼0.840 mm.CONCLUSIONIn this paper, a comparison between the beam centersat the LHC collimators obtained from beam-based align-ment to the BPM-interpolated orbit at the collimators waspresented. The analysis was performed for four collima-tor alignments at injection energy and flat top in 2011 and2012. The average difference between the interpolated andthe measured orbit is of∼0.550 mm, with a maximum deltaof 2.8 mm and a standard deviation of ∼0.840 mm. Thestudy proves that beam-based collimator alignment is es-sential for the good performance of the system, as the col-limators cannot be set deterministically from the orbit mea-surements without introducing large errors.FUTURE WORKThe similarity between the two datasets can be exploitedto speed up LHC collimator beam-based alignment, whereall the jaws could be moved in a single step at a move-ment rate of 2 mm/s rather than 10 µm/s around theBPM-interpolated orbit. Another margin in mm could beplaced to take into account the maximum possible differ-ence between the interpolated and the measured beam cen-ter. Moving the jaws immediately to a safe half-gap marginof 5σ would provide a gain in setup time, as all collimatorsexcept the primary collimators are positioned from 6.3σ upto 26σ away from the beam.ACKNOWLEDGMENTSThe authors would like to thank J. Wenninger and K.Fuchsberger for their help in the implementation of the or-bit interpolation.REFERENCES[1] R. W. Assmann et al. Requirements for the LHC CollimationSystem. In Proceedings of EPAC 2002.[2] G. Valentino et al. Semiautomatic Beam-Based LHC Colli-mator Alignment. Phys. Rev. ST Accel. Beams 15, 051002(2012).[3] G. J. Mu¨ller. BPM Interpolation Calculation. Presented at theOM Meeting, 10.02.2011.[4] O. R. Jones. LHC Beam Instrumentation. In Proceedings ofPAC 2007.[5] J.-P. Koutchouk. Measurement of the Beam Position in theLHC Main Rings. LHC Project Document No. LHC-BPM-ES-0004.[6] G. J. Mu¨ller, K. Fuchsberger, S. Redaelli. Aperture Meterfor the Large Hadron Collider. In Proceedings of ICALEPCS2011.View publication stats

Comparison of LHC collimator beam-based alignment to BPM-interpolated centers

https://www.um.edu.mt/library/oar//bitstream/123456789/28239/1/Comparison_of_LHC_Collimator_Beam-Based_Alignment_.pdf

Comparison of LHC collimator beam-based alignment to BPM-interpolated centers

Abstract

Similar works

Full text

Available Versions

CERN Document Server

OAR@UM