A full-scale prototype of the Large Hadron Collider (LHC) collimator was installed in 2004 in the CERN Super Proton Synchrotron (SPS) and has been extensively used for beam tests, for control tests and also LHC simulation benchmarking during four years of operation. This operational experience has been extremely valuable in view of the final LHC implementation as well as for estimating the LHC operational scenarios, most notably to establish procedures for the beam-based alignment of the collimators with respect to the circulating beam. These studies were made possible by installing in the SPS a first prototype of the LHC beam loss monitoring system. The operational experience gained at the SPS and the lessons learnt for the LHC operation are presented

Aberle, O

Assmann, R

Bracco, C

Dehning, B

Jonker, M

Losito, R

Masi, A

Redaelli, S

Sapinski, M

Weiler, T

Zamantzas, C

CERN Document Server

OPERATIONAL EXPERIENCE WITH A LHC COLLIMATORPROTOTYPE IN THE CERN SPSS. Redaelli, O. Aberle, R. Assmann, B. Dehning, C. Bracco, M. Jonker, A. Masi,R. Losito, M. Sapinski, T. Weiler, C. Zamantzas, CERN, Geneva, SwitzerlandAbstractA full-scale prototype of the Large Hadron Collider(LHC) collimator was installed in 2004 in the CERN SuperProton Synchrotron (SPS) and has been extensively usedfor beam tests, for control tests and also LHC simulationbenchmarking during four years of operation. This oper-ational experience has been extremely valuable in view ofthe final LHC implementation as well as for estimating theLHC operational scenarios, most notably to establish pro-cedures for the beam-based alignment of the collimatorswith respect to the circulating beam. These studies weremade possible by installing in the SPS a first prototype ofthe LHC beam loss monitoring system. The operationalexperience gained at the SPS and the lessons learnt for theLHC operation are presented.INTRODUCTIONIn order to validate for beam operation the design of theLHC collimator [1], a full-scale prototype of the secondarycollimator was installed in the CERN Super Proton Syn-chrotron (SPS) for the 2004 proton run (Fig. 1). Togetherwith robustness tests performed with extracted LHC beamsat 450 GeV [2, 3], the SPS beam tests have provided cru-cial milestones for the validation of the collimator designadopted for 100 collimators of the LHC Phase I system.The SPS collimator prototype has a two 1 m-long Car-bon jaws. It has been kept operational and used for nu-merous collimation studies with LHC-type proton and ionbeams during four years of beam operation (2004 to 2008,except 2005 when the SPS was in shutdown the wholeyear). The SPS is ideal for accelerator physics studies: or-bit and optics are very stable and a broad variety of LHC-type beams can be stored in ‘coasting’ mode. The beamconditions and parameters used during the collimator stud-Figure 1: LHC collimator prototype installed in the SPS.Table 1: Beam Conditions for SPS Collimation StudiesParameter ValueBeam energies 14, 26, 270 GeVTotal beam intensity 3.3×1013 pMaximum stored energy 1.4 MJMinimum bunch spacing 25 nsNumber of bunch trains 1 – 4Number of bunches 1 – 288Normalized beam emittance 1.0 – 3.5 μmBetatron functions (x/y) 24.9 m / 89.9 mTypical beam sizes at 270 GeV (σx/σy) 0.7 mm / 1.3 mmMaximum activation (contact) 780 μSv/h (2007)ies are listed in Table 1. The energy for stored beams is lim-ited to 270 GeV by power constraints on the main magnetcircuits. Other than that, the SPS beams are representativeof the LHC conditions as far as longitudinal and transversedynamics and stored beam energies are concerned.The following studies were performed at the SPS:– beam-based set-up of collimator jaws [4, 5, 6];– verification of the mechanical design (cooling water,mechanical precision, vacuum tightness, ...);– tests of the LHC beam loss monitoring system [7];– halo dynamics and beam shaping by the jaws [4, 8];– validation of electronics (switches, monitoring, mo-tors, ...) and performance of the controls architecturein an operating high-intensity accelerator [9, 10];– beam loss response and LHC thresholds [11, 12];– benchmark of proton [13] and ions [14] simulations;– collimator impedance, trapped modes and tune varia-tion versus collimator gap values [15, 16, 17].The conclusion of these studies was that no hidden weakpoints could be identified and indeed the design choiceswere fully validated for the LHC series production. Here,only aspects related to the system reliability, to the beam-based set-up and to the beam loss monitoring are reviewed.Note also that several Diploma and PhD thesis works haveprofited from the SPS collimator tests [6, 8, 12, 18].OVERALL SYSTEM PERFORMANCEHardware Reliability in Radiation EnvironmentFrom the mechanical point of view, the SPS collimatorprototype did not require any modification and worked re-liably without failures. From 2004 to 2007, the collimatorProceedings of PAC09, Vancouver, BC, Canada WE6RFP023Accelerator Technology - SubsystemsT19 - Collimation and Targetry 2835was equipped with 4 old LEP stepping motors. One failedduring the cold-checkout at the beginning of 2008 and itwas then decided to replace all the motors with the newLHC type. The LEP motor failure is not representative ofthe LHC case.The design of jaw cooling circuits and the vacuum layout[1] were successfully validated with high beam intensitiesand high losses induced at the collimator jaws (maximumlosses of up to 1012−1013 p at 270 GeV and some 1014p at26 GeV). The maximum activation at the end of the run wasmeasured on Nov. 23rd, 2007: up to 800 μSv/h on contactand 110 μSv/h at 40 cm from the collimator tank. In theother years the activation was at least 2 to 3 times lower. Inno case we could see failures of components (mechanicalor electronics) induced by radiation. The high activationof 2007 was induced by losses of 26 GeV proton beamsgenerated a few days the end of the run for BLM calibrationstudies [12]: ≈ 3×1014 p were lost on the collimator onNov. 8th and ≈10×1014 p on Nov. 12th.Controls oftware erformanceThe SPS tests also played an important role as test-benchfor the LHC collimator controls. The first 2004 imple-mentation relied on a local system derived from LEP, con-trolled from the control room via remote desktop connec-tions. This was adequate for the first accelerator physicsstudies but not for the LHC system with about 100 colli-mators. The LHC final architecture based on PXI platformsby National Instruments [19] was deployed and validate forthe accelerator controls in 2006, together with the first pro-totype of the top-level applications developed in the LHCSoftware Applications (LSA) environment [10] for genera-tion, orchestration and maintenance of the collimator set-tings. The architecture proved to be reliable during thebeam tests and was up and running without significant fail-ures during three years of operation. The LHC system willneed to exceed this achievement by far but the first resultsof remote commissioning are very promising [20].BEAM-BASED ALIGNMENTOne of the main purposes of the SPS collimator studieswas the validation with beam of the commissioning pro-cedures for the collimator jaw set-up. The procedure toestablish the local beam centre and size at the collimator,required to define the normalized collimator settings for op-timum system performance, is now well established thanksto the extensive studies carried out at the SPS. This pro-cedure takes full profit of the two-sided collimator design(Fig. 2): a sharp edge of the beam halo is produced withone jaw and the other jaw is moved until it becomes closerto the beam. This is seen on the signals of BLMs placedclose to the collimator (2 ionization chambers in the exam-ple). The step size of collimator movements determines theaccuracy of this procedure. After the first results [4], sys-tematic studies have been performed [6] with the conclu-sion that (1) the resolution of this method is below 20 μm;Circulating beamRight collimator jawLeft collimator jaw"sharp" edge3.6 3.65 3.7 3.75 3.8 3.85103104105Beam loss signal [ a. u. ]  BLMI-1BLMI-20 0.05 0.1 0.15 0.2 0.25-10-50510Jaw position [ mm ]Time [ h ]Left jawRight jawBeam position = 2.82 mmFigure 2: Example of collimator centring at the SPS: BLMsignal (centre) and collimator positions (bottom) versustime. Loss spikes highlighted with circles indicate whenone of the jaw goes closer to the beam the the other jaw.(2) the absolute accuracy, taking into account reproducibil-ity and comparison with other alignment methods, is about50 μm [6]. The beam size can be inferred precisely fromfull beam scraping, which also provides information on thebeam orbit. This is a precise (within 10% of wire scannermeasurements) but destructive method.What remains to be improved is the beam-based adjust-ment of the collimator jaw angle with respect to the beamenvelope (the jaws shall be parallel to the envelope to max-imize the material seen by the beam). The approach similarto that of the centring, i.e., moving one jaw corner into thebeam and then moving in the other in steps until it getscloser, did not provide reliable and reproducible data forangle set-up below 100 μrad. Under these conditions, themost reliable solution is probably to calculate the anglefrom beam-based measurements of the machine optics.BEAM LOSS MONITORINGThe SPS studies of collimator-induced beam losses pro-vided a crucial benchmark for the development of the LHCbeam loss monitoring system. The LHC ionization cham-S PWE6RFP023 Proceedings of PAC09, Vancouver, BC, Canada2836Accelerator Technology - SubsystemsT19 - Collimation and Targetry0 0.5 1 1.5 2100101102103104105106Time [ s ]Beam loss signal [ a. u. ]  Full beam scrapingBLMI-1BLMI-2BLMS-1BLMS-20 0.02 0.04 0.06 0.08100101102103Time [ s ]Beam loss signal [ a. u. ]  50μm step into the beamBLMI-1BLMI-2Figure 3: BLM measurements at 2.56 ms and 40 μs trig-gered by hardware when the jaw is moved into the beam.bers (BLMIs) as well as the secondary emission monitors(SEMs, or BLMSs) were installed in the vicinity of the col-limator prototype. The LHC BLM acquisition electronics,with 12 integration times from 40 μs to 89 s, was deployedin 2006. The software for the data concentration, necessaryat the LHC to collect efficiently the measurements from4000 monitors in the machine, was tested in 2007. In 2008all the LHC acquisition types were available at the SPS:(1) standard acquisition at 1 Hz for all 12 integration times;(2) acquisition of buffers of 2048 points at a sampling rateof 40 μs or 2.56 ms (see Fig. 3); (3) acquisition for fastcollimator set-up, triggered by dedicated synchronizationsignals from the collimator gateways [10]; (4) post-mortembuffer of 43000 points at 40 μs. Possible trigger eventswere timing, collimator movements and loss levels.SPS collimator studies represented the main test-benchfor evaluating the LHC BLM system and most of its func-tionality. It has helped to identify early potential problemsin the acquisition chain (like cross-talk between channels)as well as post-processing transmission rate limits. Thewhole infrastructure including the expert applications wassystematically evaluated before deploying it into the LHC.CONCLUSIONSThe studies with a full scale LHC collimator prototypein the SPS were a great success. Four years of beam op-eration in the SPS accelerator have provided a very impor-tant feedback for the LHC collimator design, have giventhe possibility to perform many accelerator physics studiesrelated to high-intensity beam collimation, have allowedthe benchmark of important systems related to collima-tion, such as the LHC beam loss monitoring system andhave provided a playground for training the LHC collima-tor commissioning team. This experience is invaluable inview of the preparation for the commissioning of the colli-mation system with LHC beams.The collimator prototype is still in the SPS for beamstudies in 2009. In the future it will be upgraded to en-able studies of specific aspects of the Phase II collimationsystem, such as the integration of beam position monitorsin the jaws, and will continue to support the .The authors would like to thank the many colleagueswho participated to the SPS collimation studies. The mostimportant contributions can be consulted in the references.REFERENCES[1] A. Bertarelli et al., Mechanical design for robustness of theLHC collimators, PAC2005.[2] R. Assmann et al., LHC collimation: design and resultsfrom prototyping and beam tests, PAC2005.[3] A. Bertarelli et al., Permanent deformation of the LHC col-limator jaws induced by shock beam impact: An analyticaland numerical interpretation, EPAC2006.[4] S. Redaelli et al., LHC aperture and commissioning of thecollimation system, LHC Workshop Chamonix 2005.[5] T. Weiler et al., Beam loss response measurements with aLHC prototype collimator in the SPS, PAC2007.[6] C. Bracco, CERN-Thesis-2009-031.[7] E. B. Holzer et al., CERN-AB-2006-009 BI (2006).[8] G. Robert-Demolaize, CERN-Thesis-2006-069.[9] M. Jonker et al., The control architecture for the LHC colli-mation system, ICALEPCS2005, 2005.[10] S. Redaelli et al., The LHC collimator controls architecture– design and beam tests, PAC2007.[11] B. Dehning et al., Energy Deposition Simulation and Mea-surements in a LHC Collimator and Beam Loss Monitors,these proceedings.[12] T. T. Boehlen, CERN-Thesis-2008-092.[13] S. Redaelli et al., Comparison between measured and simu-lated beam loss patterns in the CERN SPS, EPAC2006.[14] R. Bruce et al., Phys. Rev. ST Accel. Beams 12 (2009)011001.[15] H. Burkhardt et al., Measurements of the LHC collimatorimpedance with beam in the SPS, PAC05.[16] F. Zimmermann et al., Tune shift induced by nonlinear re-sistive wall wake field of flat collimator, EPAC2006.[17] E. Me´tral et al., Transverse impedance of the LHC collima-tors, PAC2007.[18] D. Kramer, CERN-Thesis-2008-090.[19] A. Masi and R. Losito, “LHC Collimator Lower Level Con-trol System,” 15th IEEE NPSS Real Time Conference 2007.[20] S. Redaelli et al., Final implementation and performance ofthe LHC collimator controls system, these proceedings.Proceedings of PAC09, Vancouver, BC, Canada WE6RFP023Accelerator Technology - SubsystemsT19 - Collimation and Targetry 2837

Operational Experience with a LHC Collimator Prototype in the CERN SPS

OPERATIONAL EXPERIENCE WITH A LHC COLLIMATOR PROTOTYPE IN THE CERN SPS

Operational Experience with a LHC Collimator Prototype in the CERN SPS

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