CERN's Large Hadron Collider will store an unprecedented amount of energy in its circulating beams. Beamloss monitoring (BLM) is, therefore, critical for machine protection. It must protect against the consequences (equipment damage, quenches of superconducting magnets) of excessive beam loss. About 4000 monitors will be installed at critical loss locations. Each monitor has 384 beam abort thresholds associated; for 12 integrated loss durations ($40\mu$s to 83 s) and 32 energies (450GeV to 7 TeV). Depending on monitor location, the thresholds vary by orders of magnitude. For simplification, the monitors are grouped in 'families'. Monitors of one family protect similar magnets against equivalent loss scenarios. Therefore, they are given the same thresholds. The start-up calibration of the BLM system is required to be within a factor of five in accuracy; and the final accuracy should be a factor of two. Simulations (backed-up by control measurements) determine the relation between the BLM signal, the deposited energy and the critical energy deposition for damage or quench (temperature of the coil). The paper presents the strategy of determining 1.5 million threshold values

Bocian, D

Böhlen, T T

Dehning, B

Holzer, E B

Kramer, Daniel

Ponce, L

Priebe, A

Sapinski, M

Stockner, M

CERN Document Server

LHC-PROJECT-REPORT-115805Sep2008E.B. Holzer, D. Bocian, T.T. Böhlen, B. Dehning,  D. Kramer, L. Ponce, A. Priebe,M. Sapinski, M. StocknerEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsGENERATION OF 1.5 MILLION BEAM LOSS THRESHOLD VALUESCERN’s Large Hadron Collider will store an unprecedented amount of energy in its circulating beams.Beamloss monitoring (BLM) is, therefore, critical for machine protection. It must protect against theconsequences (equipment damage, quenches of superconducting magnets) of excessive beam loss. About4000 monitors will be installed at critical loss locations. Each monitor has 384 beam abort thresholdsassociated; for 12 integrated loss durations (40μs to 83 s) and 32 energies (450GeV to 7 TeV). Depending onmonitor location, the thresholds vary by orders of magnitude. For simplification, the monitors are grouped in“families”. Monitors of one family protect similar magnets against equivalent loss scenarios. Therefore, theyare given the same thresholds. The start-up calibration of the BLM system is required to be within a factor offive in accuracy; and the final accuracy should be a factor of two. Simulations (backed-up by controlmeasurements) determine the relation between the BLM signal, the deposited energy and the critical energydeposition for damage or quench (temperature of the coil). The paper presents the strategy of determining1.5 million threshold values.Large Hadron Collider ProjectAbstractLHC Project Report 1158CERN,CH-1211 Geneva 23SwitzerlandCERN, Geneva, SwitzerlandPresented atEPAC'08, 11th European Particle Accelerator Conference, Genoa, Italy - June 23-27, 2008GENERATION OF 1.5 MILLION BEAM LOSS THRESHOLD VALUESE.B. Holzer, D. Bocian, T.T. Bo¨hlen, B. Dehning, D. Kramer, L. Ponce, A. Priebe,M. Sapinski, M. Stockner, CERN, Geneva, SwitzerlandAbstractCERN’s Large Hadron Collider will store an unprece-dented amount of energy in its circulating beams. Beam-loss monitoring (BLM) is, therefore, critical for machineprotection. It must protect against the consequences(equipment damage, quenches of superconducting mag-nets) of excessive beam loss. About 4000 monitors willbe installed at critical loss locations. Each monitor has 384beam abort thresholds associated; for 12 integrated loss du-rations (40µs to 83 s) and 32 energies (450 GeV to 7 TeV).Depending on monitor location, the thresholds vary by or-ders of magnitude. For simplification, the monitors aregrouped in “families”. Monitors of one family protect sim-ilar magnets against equivalent loss scenarios. Therefore,they are given the same thresholds. The start-up calibra-tion of the BLM system is required to be within a factor offive in accuracy; and the final accuracy should be a factor oftwo. Simulations (backed-up by control measurements) de-termine the relation between the BLM signal, the depositedenergy and the critical energy deposition for damage orquench (temperature of the coil). The paper presents thestrategy of determining 1.5 million threshold values.INTRODUCTION TO THE LHC BLMSYSTEMThe BLM system detects and quantifies the amount oflost beam particles. It generates a beam abort trigger whenthe losses exceed predetermined threshold values. Themain detector type is an ionization chamber (IC). About4000 will be installed, mostly around the quadrupole mag-nets, where they probe the transverse tails of the particleshowers through the magnets. The dynamic range of thesemonitors is 108. At certain locations, higher loss ratescould occur due to machine component failures. The high-est continuous loss rates will be in the collimation sections.Secondary emission monitors (SEMs) are added at theselocations. Their sensitivity is approximately 7 · 104 timesless than the one of an IC.BEAM ABORT THRESHOLDSThe BLM interlock limits can be set for each monitor in-dividually. In the arcs they will be set to 30% of the magnetquench levels. They vary with integration time (12 integra-tion time intervals between 40 µs to 83 s) and the energy ofthe beam (32 energy ranges). The BLMs are grouped intofamilies of monitors which, because of their location andloss maps, are expected to have the same thresholds. Thelargest families belong to the arcs where the same config-uration is repeated. The 6 arc families contain more thenhalf of all the monitors, but the remaining configurations onthe Long Straight Sections, injection and dump lines repre-sent a large variety corresponding to an additional numberof 300 families. A factor of 5 and a factor of 2 are thespecified initial and final absolute precisions on the predic-tion of the quench levels respectively. The relative preci-sion for quench prevention is requested to stay below 25%.The dynamic range of the system is given by the calculateddamage and quench levels (and the expected usage frompilot beam intensity to ultimate beam intensity. The obser-vation time range is defined by the fastest possible use ofthe trigger signal by the beam dump (on the side of shortintegration intervals), and the response time of the heliumtemperature measurement system (on the long side). Dam-age and quench limits of sensitive LHC components, i.e.,collimators and cold magnets, are given for transient lossesby a maximal energy density and for steady-state losses bya total energy deposited in the sensitive equipment in a timeinterval.Technical ImplementationThe threshold settings [1] of the BLM system are storedin the LHC Software Architecture (LSA) database, whichis a standard way to manage operational settings. The in-formation about assignment of the monitors to families andfamily thresholds is send to LSA tables by a set of SQLscripts. The threshold tables, in the form corresponding totheir image in the front-end electronics, are created. The ta-bles are created in Master and Applied version. The Mas-ter threshold levels are well below damage levels of theLHC elements, although they exceed the quench level ofcold magnets. The Applied thresholds are scaled down tothe values which prevent from quench. Every monitor canbe scaled separately by applying a monitor factor. To pro-vide safety of the system, the Master thresholds are pro-tected from any change not agreed by Machine ProtectionWorking Group and BLM Experts. The tables in LSA aredoubled in a stage/final mechanism. Loading the thresholdvalues to staging tables allows to compare them with theoriginal values present in the final tables. If the compar-ison is satisfactory the decision can be made to load thestage tables to final ones. The history of changes made tothe tables in LSA is stored and old settings can always bere-loaded. Thresholds plausibility checks are foreseen.COLD MAGNETSAmongst others, the proton loss in LHC Short StraightSection (SSS) made of Main Quadrupole (MQ), OctupoleLSA settings tableBLM Expert ApplicationMonitor tableFamily tableCm − monitor factorelectronic channelF(m   f) − function attributingmonitors to families6400 recordsconversion factorsThreshold table~ 300 recordsTf − threshold tables~ 300 recordsTA(m)=Tf f(m  f) Cm. .Applied < MasterMaster tableApplied tableT(m) = Tf  f(m   f).  Figure 1: Schematic view of data organization in LSA database. Themaster table is created from expert input.Figure 2: The longitudinal cross-section of SSS and beam loss locations.Corrector (MO) and Beam Position Monitor (BPM) hasbeen simulated with the Geant4 package [2]. The energydeposition (ED) in MQ coils was scored in cells with di-mensions smaller than the shower scale. Several beam losslocations, as presented in Fig. 2, have been simulated. Thesecondary particles were registered outside the cryostat involumes corresponding to the monitor placement (Fig. 3).The peak of the secondary particle multiplicity is found tobe about one meter after the loss location. As the BPM isone of the most probable loss locations, it can be concludedthat the first BLM is placed in the peak of the cascade,which means that it is placed optimally to detect lossesin BPM. Taking into account the fast loss quench levelof 1.41 mJ/cm3 the critical number of protons to quenchthe magnet has been found to be between approximately0.3·106 and 2·106, depending on the loss location. The flu-ence of particles hitting the monitor is folded with the BLMresponse function to find the generated charge (Fig. 4).This is converted to radiation dose seen by the BLM. Thequench preventing threshold for fast losses in MQ magnetis found to be about 0.1 mGy and about 5 mGy/s for steady-state losses.COLLIMATORSThe LHC has a multi-stage cleaning system consisting ofseveral types of collimators, see Tab. 1. Most LHC collima-tors are located in the momentum and the betatron cleaninginsertion (IR3 and IR7) [5, 6]. Inherent damage thresholdsfor collimators are provided by the LHC collimation work-ing group [7]. Simulations for the assessment of the BLMthresholds focus on the cleaning insertions. Each collima-tor is protected by an IC and a SEM detector. Fig. 5 depictsFigure 3: Correlation between energy deposition inside the coils (redcurve) and the number of secondary particles outside of the cryostat (blackcurve).the typical schematic setup of the BLM detector positionsw.r.t. a collimator. Simulations done with FLUKA [3, 4]address the relation of BLM detector signal to total andmaximal ED in the collimator jaws dependent on differentparameters. ED in the detectors is scored and transformedwith a conversion factor to the corresponding detector sig-nal.Collimator Jaw Length & MaterialPrimary (TCP) 60 cm C reinforced graphiteSecondary (TCSG) 100 cm C reinforced graphiteAbsorber (TCLA) 100 cm W embedded in CuTable 1: Collimator types in cleaning regions. Similar collimators areplaced along the LHC ring.Figure 5: Structural view of a collimator (tilted at 90◦) and its dedicatedBLM detectors, an IC and a SEM, as mounted in the LHC cleaning inser-tions IR3 and IR7.A setup of collimator and BLM detectors as shown inFig. 5 was implemented with interchangeable collimators(TCP, TCSG, TCLA). Focus in geometrical implementa-tion was put on the collimator, the IC and the BLM support.Simulations were run at LHC injection and top energy,450 GeV and 7 TeV, respectively. Changes in response ofthe IC by omitting an implementation of a collimator sup-port were assessed by the insertion of a steel block of 5 cmthickness and resulted in deviation of the IC signal of 7%.Transversal misalignment of the BLM detectors of ±5 cm(vertical) and ±2 cm (horizontal) provoked a deviation ofthe IC signal of maximal 34.1%. Fig. 6 shows the ratio ofsignal seen by the IC to ED in the jaw for selected settingsas function of proton impact depth in the jaw. One can ob-serve that the signal to ED ratio is virtually constant forimpact parameters of up to 1 mm. The signal to ED ratiofor a secondary shower is about 50% lower than the ratiofor beam protons on the collimator. For critical failures, theratio of IC signal to maximal energy density within a TCPkinetic energy [MeV]-310 -210 -110 1 10 210 310 410 510 610detector response function [aC/p]-210-110110210310protonneutrongammae-e+mu+/mu-pi+/pi-energy [MeV]-310 -210 -110 1 10 210 310 410 510 610]-2/dE [cmφd×E-510-410-310-210-110protonneutrongamma-e+emu+/-pion+/-LHC MQY 7TeVenergy [MeV]-310 -210 -110 1 10 210 310 410 510 610integrated detector signal [aC/p]-210-110110210310protonneutrongammae-e+muon+/-pion+/-LHC MQY 7TeVFigure 4: Left: GEANT4 simulated LHC BLM detector response functions for particle impact direction of 60◦. Center: Secondary particle fluencespectrum on the outside recorded in a 3.4 m long stripe, lethargy representation, MQY magnet, protons with 7 TeV impacting on the beam screen. Right:Integrated detector signal.jaw is of the order of 10−14 Gy/(GeV/cm3). This relationis still subject to more detailed studies. 0 5e-14 1e-13 1.5e-13 2e-13 2.5e-13 3e-13 1e-05  0.0001  0.001  0.01  0.1  1IC dose/Tot. Energy Deposition in Jaw (Gy/GeV)Depth in Right Jaw (cm)Signal in IC for Pencil Beam impacting on TCP/TCSGTCSG at 450 GeVTCSG at 7 TeVTCP at 450 GeVTCP at 7 TeVFigure 6: Ratio of dose deposition in the IC to total energy deposition(ED) in the right collimator jaw vs. the impact depth of a pencil protonbeam for the TCP and the TCSG collimator.An experimental setup consisting of a prototype TCSGcollimator and a set of BLM detectors is mounted in theSPS, see Fig. 7. Responses of the BLM detectors weretaken at 26 GeV with up to 1.3 · 1013 protons impactingon the collimator. This setup was implemented in FLUKAusing a similar approach as above. Systematic deviations ofthe detector signals due to misalignment and simplificationin geometry were assessed and found to be smaller than5% each. A preliminary comparison of dose per protonon collimator between experiment and simulation shows anagreement within 18% for the IC and an agreement within25% for the SEM signal. More measurements for detailedcomparison are planed.SUMMARYThe start-up of the LHC being now imminent, the 1.5million beam abort threshold values are being currently fi-nalized. Presented are the two most copious cases, the arc(and dispersion suppressor) cold magnets and the BLMs atthe collimators (and absorbers). The simulations involvedin determining the thresholds as well as the technical im-plementation of their deployment are described.ACKNOWLEDGMENTSWe would like to thank P. Sala and the FLUKA teamfor providing the detailed FLUKA input of the IC detec-tor as well as the collimators and R. Assmann, M. Jonker,Figure 7: The upper figure shows the prototype TCSG collimator asmounted for test purposes in the SPS. The schematic drawing shows atop view of the arrangement. The collimator is equipped with two ICs(IC1A and IC1B) and a SEM.S. Redaelli and Th. Weiler for the support during the BLMtests at the SPS.REFERENCES[1] M. Sapinski et al., “Specification of the Database Structureand the Software Needed to Configure the Beam Loss Moni-tor System in the LHC”, under preparation.[2] Geant4 - A Simulation Toolkit, S. Agostinelli et al., NuclearInstruments and Methods A 506 (2003) 250-303.[3] A. Fasso`, A. Ferrari, J. Ranft, P.R. Sala, “FLUKA: a multi-particle transport code”, CERN-2005-10, INFN/TC 05/11,SLAC-R-773, 2005.[4] A. Fasso`, A. Ferrari, J. Ranft, P.R. Sala, “The physics mod-els of FLUKA: status and recent developments”, Comput-ing in High Energy and Nuclear Physics 2003 Conference(CHEP2003), USA, 2003.[5] CERN, “The LHC Design Report, vol. I: The LHC MainRing”, Technical Report CERN-2004-003, 2003.[6] R. Assmann et al., “The Final LHC Collimation System”,CERN-LHC-PROJECT-REPORT-919, EPAC 06.[7] R. Assmann, Private Communications, CERN, 2008.

Generation of 1.5 Million Beam Loss Threshold Values

Generation of 1.5 Million Beam Loss Threshold Values

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