Full beam-based alignment of the LHC collimation system was a lengthy procedure as the collimators were setup manually. A yearly alignment campaign has been sufficient for now, although in future this may lead to a decrease in the cleaning efficiency if machine parameters such as the beam orbit drift over time. Automating the collimator setup procedure can allow for more frequent alignments, therefore reducing this risk. This paper describes the design and testing of a semi-automatic algorithm as a first step towards a fully automatic setup. Its implementation in the collimator control software and future plans are described.peer-reviewe

International Particle Accelerator Conference (IPAC11)

Redaelli, Stefano

Sammut, Nicholas

Valentino, Gianluca

Wolfgang Assmann, Ralph

Wollmann, Daniel

OAR@UM

SEMI-AUTOMATIC BEAM-BASED ALIGNMENT ALGORITHM FORTHE LHC COLLIMATION SYSTEMG. Valentino , N. Sammut,∗CERN, Geneva, Switzerland and University of Malta, Msida, MaltaR. W. Aßmann, S. Redaelli, D. Wollmann,CERN, Geneva, SwitzerlandAbstractFull beam-based alignment of the LHC collimation sys-tem was a lengthy procedure as the collimators were setupmanually. A yearly alignment campaign has been sufficientfor now, although in future this may lead to a decrease inthe cleaning efficiency if machine parameters such as thebeam orbit drift over time. Automating the collimator setupprocedure can allow for more frequent alignments, there-fore reducing this risk. This paper describes the design andtesting of a semi-automatic algorithm as a first step towardsa fully automatic setup. Its implementation in the collima-tor control software and future plans are described.INTRODUCTIONCollimators are required to intercept unavoidable beamlosses in the Large Hadron Collider (LHC), which maycause quenches in the super-conducting magnets and ra-diation effects [1]. Beam-based alignment or setup of theLHC collimators is necessary to determine the beam cen-tres and beam sizes at the collimators. This ensures thatthe correct collimator hierarchy is maintained during nor-mal operation. Regular collimator setups are required asthe beam orbit may change over a few months [2], and thetime needed for a full alignment limits the frequency ofthe setups. In turn, the long time intervals between setupsplace constraints on the minimum β* achievable and con-sequently the integrated luminosity reach of the LHC [3].In the 2010 collimator setups, the alignment was per-formed manually using the single collimator control ap-plication [4, 5]. A ‘manual’ setup means that an operatorwould have to intervene for every collimator jaw movementand check every BLM data value to determine whether thejaw is aligned to the beam. A disadvantage of this methodis the setup time required (up to 27 hours [6]), which is non-negligible for LHC operation. Human error may result inincorrect jaw movement, leading to high losses and beamdumps, therefore contributing to the setup time. This pro-cess can therefore be automated in order to reduce manualintervention and optimize the setup time.The first section of this paper describes the collimatorsetup sequence established in the 2010 run. This is fol-lowed by the semi-automatic feedback loop algorithm thathas been developed, tested and commissioned in the LHC.Once this algorithm was operational, it was used as a back-∗ gianluca.valentino@cern.chbone for parallel alignment, which is presented in the thirdsection. In the fourth section, setup parameters developedthrough experience with the software are discussed.COLLIMATOR SETUP SEQUENCEEach collimator is setup in a four-step process [7]. Fig-ure 1 depicts the alignment sequence, where initially bothjaws of the reference collimator are aligned. The referencecollimator is taken as the primary collimator in the sameplane (horizontal, vertical or skew) as the collimator i.Figure 1: Setup Sequence.A collimator jaw is defined to be setup if a jaw move-ment towards the beam produces a satisfactory loss spikein an assigned Beam Loss Monitor (BLM) located down-stream. A loss pattern cannot be attributed to a particularjaw movement unless only that particular jaw was movingin at the time. Therefore, the left and right aligned jawpositions are determined separately. After both jaws of thereference collimator are aligned, the same procedure is per-formed for the collimator i, and finally the reference colli-mator is aligned again. The beam centre and the beam sizecan then be established from the jaw positions of collimatori and the reference collimator [7].The number and types of collimators setup depends onthe machine configuration. At 450 GeV all 86 ring colli-mators are setup for both beams with 1 nominal bunch perTHPZ034 Proceedings of IPAC2011, San Sebastián, Spain3768Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)01 Circular CollidersT19 Collimationbeam. When the energy is ramped to 3.5 TeV flat top, asetup is performed for all collimators except for the injec-tion protection collimators, which are left at parking afterinjection. After squeezing both beams to the operational β*in the experimental insertion points, the 16 tertiary colli-mators are aligned, since a large change occurs in the beamsizes and beam centres for these collimators. No setup isrequired for the other collimators since the orbit does notchange at these collimators. This procedure is repeated dur-ing collisions.COLLIMATOR JAW ALIGNMENTALGORITHMThe collimator jaw alignment algorithm allows the userto specify four input parameters to move in one or both col-limator jaws to the beam. The four inputs consist of the leftand right jaw step sizes in μm, a BLM loss threshold andthe repetition time of the movement. Figure 2 illustratesthe control panel from which the operator can specify theinput parameters to the algorithm.Figure 2: Semi-Automatic Input GUI.A set of pre-defined possible values exists for each in-put, based on experience with the collimation system in the2010 LHC run. With every jaw step, the algorithm obtainsfeedback from the BLM associated with the collimator be-ing moved, and stops the thread controlling the jaw move-ment if the BLM loss threshold is exceeded. The BLMdata is acquired at a rate of 1 Hz. The algorithm thereforeautomates two key parts of the setup process:• Collimator jaw movement towards the beam with auser-defined step size and time interval• Collimator jaw stopping when the beam losses exceeda user-defined thresholdA flowchart of the semi-automatic alignment algorithmis shown in Fig. 3. The left and right collimator jaw po-sitions are logged automatically, so that the beam centreand beam size can be displayed. The collimator jaw align-ment algorithm was implemented into the top-level colli-mator control software [4], and was tested and commis-sioned during the collimator setup at 450 GeV held at theend of February 2011.Figure 3: Semi- Automatic Algorithm Flowchart.PARALLEL COLLIMATOR SETUPParallel collimator setup was developed in order to opti-mize the setup time, in which the user can select any num-ber of collimators to be aligned in parallel to the beam.This means that the collimator jaw alignment algorithmdiscussed in the previous section is executed for each se-lected collimator. Currently, parallel collimator setup isused to provide a coarse but quick way of positioning aset of collimator jaws at a few micrometers from the beam,after which each collimator is finely aligned in sequence.Parallel collimator setup was tested and commissionedduring the collimator setup at 3.5 TeV flat top held inMarch 2011, as well as the 3.5 TeV setups after squeeze andduring collisions on the 11th March 2011. During the test-ing of this technique, a cross-talk effect was observed, inProceedings of IPAC2011, San Sebastián, Spain THPZ03401 Circular CollidersT19 Collimation 3769 Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)which the loss patterns generated by a particular collimatorwere also being detected on other collimators around theLHC. This prevented the parallel setup method from func-tioning efficiently, and another algorithm was designed toidentify which collimator jaw is at the beam.The parallel setup optimizer algorithm uses a timer taskto check whether any collimators have stopped moving.As soon as a single collimator stops moving due to an ex-ceeded BLM threshold, another timer task is started to de-termine whether any other collimators also stop within apre-defined time period (from experience set to 2 seconds).If this is the case, then all the other collimators moving inparallel are stopped so that the algorithm can concentrateonly on the first collimators. These collimators are thenmoved in one by one depending on options set in the win-dow in Fig. 4.Figure 4: Parallel Collimator Setup Settings.In case the BLM threshold set during the previous move-ment is now too low (i.e. the collimator cannot be moved inagain by a single step), an option allows the user to instructthe program to automatically increase the BLM thresholdin steps up to a maximum amount. If the threshold is ex-ceeded after the second step or thereafter, the collimatorjaw is declared to be aligned to the beam, and the algorithmterminates. A flowchart of the parallel collimator setup al-gorithm is shown in [8]. Testing was carried out duringan LHC Machine Development (MD) time slot on the 2ndJuly 2011 and during a beam recovery period.SETUP PARAMETERSDuring the setups at 450 GeV and 3.5 TeV with semi-automatic alignment, it was necessary to change the algo-rithm inputs depending on the beam energy. For example,at injection energy step sizes of at least 20μm were requiredto be able to observe a significant loss spike, while at 3.5TeV step sizes of 5μm to 10μm were sufficient.A detailed comparison of the collimator setup perfor-mance with manual and semi-automatic alignment is givenin [6]. With the latter method, a reduction in the setup timeby a factors of around 1.5 and 5 in setup time at the “flattop” and “squeezed” modes was registered. Additionally,an elimination of setup-induced beam dumps shows thatthe usage of a BLM threshold as a stopping value for thesemi-automatic algorithm is safe.The BLM threshold is usually set to 1× 10−6 Gy/s be-fore any of the jaws of a particular collimator have touchedthe beam, and is then raised manually according to the“background” loss signal observed after each loss spike, upto a maximum value of 1× 10−4 Gy/s. The period of themovement is initially set to 1 second, and is then increasedto 2 or 3 seconds after the first loss spike. This caters for“loss spike overshoot”, which occurs when losses continueto increase after the jaw movement has stopped. It is prefer-able to generate a significant loss spike, part of which isovershoot, with a single jaw movement than to cut furtherinto the beam with multiple jaw movements.CONCLUSIONThe development and implementation of the semi-automatic algorithm is part of a phased transition frommanual to fully automated collimator setup. Its achieve-ments in decreasing the setup time and reducing the needfor manual intervention has opened the door to fully auto-matic collimator setup. The use of a user-defined BLMthreshold as a stopping value for the jaw alignment al-gorithm has proven to be safe and efficient in terms ofcollimator-triggered beam dumps and setup time.FUTURE WORKPattern recognition of loss spikes to determine whichloss patterns indicate that a jaw is aligned to the beam iscurrently ongoing. Future work will concentrate on the de-velopment of a collimator setup simulator, which would al-low new algorithms to be tested offline.REFERENCES[1] R. W. Aßmann et al. Requirements for the LHC CollimationSystem. In Proceedings of EPAC 2002.[2] R. J. Steinhagen. LHC Stability and Feedback Control - Orbitand Energy. CERN-THESIS-2007-058.[3] R. Bruce, R. W. Aßmann, W. Herr. Calculation Method forSafe Beta* in the LHC, these proceedings.[4] S. Redaelli, R. W. Aßmann, M. Jonker, M. Lamont. Appli-cation Software for the LHC Collimators and Movable Ele-ments. CERN-EDMS document LHC-TCT-ES-0001.[5] S. Redaelli, R. W. Aßmann, R. Losito, A. Masi. Final Imple-mentation and Performance of the LHC Collimator ControlSystem. In Proceedings of PAC09.[6] G. Valentino et al. Comparison of LHC Collimation Setupswith Manual and Semi-Automatic Collimator Alignment,these proceedings.[7] D. Wollmann et al. First Cleaning with LHC Collimators. InProceedings of IPAC 2010.[8] G. Valentino et al. Improving Collimator Setup Efficiency at3.5 TeV, CERN-ATS-Note-2011-062.THPZ034 Proceedings of IPAC2011, San Sebastián, Spain3770Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)01 Circular CollidersT19 CollimationView publication stats

Semi-automatic beam-based alignment algorithm for the LHC collimation system

Full beam-based alignment of the LHC collimation system was a lengthy procedure as the collimators were setup manually. A yearly alignment campaign has been sufficient for now, although in future this may lead to a decrease in the cleaning efficiency if machine parameters such as the beam orbit drift over time. Automating the collimator setup procedure can allow for more frequent alignments, therefore reducing this risk. This paper describes the design and testing of a semi-automatic algorithmas a first step towards a fully automatic setup. Its implementation in the collimator control software and future plans are described

Valentino, G

Assmann, R W

Redaelli, S

Sammut, N

Wollmann, D

CERN Document Server

https://www.um.edu.mt/library/oar//bitstream/123456789/26514/1/Semi-automatic_beam-based_alignment_algorithm_for_2011.pdf

Semi-automatic beam-based alignment algorithm for the LHC collimation system

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