Brüning, Oliver Sim

Hilaire, A

Jeanneret, J B

Mertens, V

Péraire, S

Sala-Ferrari, P

Weisse, E

CERN Document Server

IMPACT OF AND PROTECTION AGAINST FAILURESOF THE LHC INJECTION KICKERSO.Brüning, A.Hilaire, J.B.Jeanneret, V.Mertens#, S.Péraire, P.Sala-Ferrari, E.WeisseCERN, Geneva, SwitzerlandAbstractThe LHC injection systems consist of horizontallydeflecting steel septum magnets and vertically deflectingkickers. A mobile beam stopper is placed downstream ofthe kickers for setting up with single bunches and toprotect the superconducting machine elements duringnormal injection in the event of a malfunctioning of thekickers. The effects of various potential kicker failuresand their impact on the machine have been investigated.The injection parameters, the design principles of thestopper and additional protection measures are discussed.1  INTRODUCTIONTwo new beam transfer lines with a combined length of5.6 km and using over 700 room-temperature magnets, TI2 and TI 8, are being built at CERN to transport450 GeV/c protons from SPS to LHC [1]. An overview ofthese lines is given in [2].TI 2 leads to the injection into LHC ring 1 nearintersection 2 (IP2), TI 8 to the injection into ring 2 nearintersection 8 (IP8). Civil engineering for both lines hasstarted in 1998. First injection tests are foreseen forautumn 2003 (TI 8) and mid 2005 (TI 2).A schematic plan view of an injection region is given inFigure 1: Schematic plan view of IP2 injectionFig. 1. The beam to be injected passes through 5horizontally deflecting steel septum magnets (MSI) with atotal deflection of 12 mrad and a vertically deflectingkicker (MKI), consisting of 4 modules, with a nominaltotal kick strength of 0.85 mrad [3]. A mobile beamstopper (TDI), consisting of 2 absorber blocks positioneda few mm above and below the nominal LHC orbit, willbe placed some 70 m downstream of the MKI, at a phasedistance of ∆µy = 90°. Its main role is to protect theimmediately following (cold) separation dipole D1against  miskicked bunches and to receive intentionally                                                          #  Email: Volker.Mertens@cern.chbeam during commissioning and verification/re-adjustment of the injection before each fill.The main beam and kicker parameters are given inTable 1. The destructive power of the beam imposes highprecision and very good protection when injecting beaminto the small aperture, superconducting LHC. Thequench limit (through  instantaneous energy deposition ina coil) is assumed to 38 mJ/cm3 [4], the damage limit to87 J/cm3 [5].Table 1: Beam and kicker parameters at injection Beam (proton) momentum  450 GeV/c Nominal single bunch intensity  1.1*1011 p Nom. batch intensity (3*81 bunches)  2.67*1013 p Ultim. batch intensity (3*81 bunches)  4.13*1013 p Bunch distance  25 ns Nom. norm’d transverse emittance 3.5 µm rad Nominal kick strength  0.85 mrad Kicker rise time 0.9 µs Kicker flat top length 6.6 µs Kicker fall time 3 µsIn the following the various modes of using theMKI/TDI ensemble, either intentionally or accidentally,will be described. Then, results of simulations arepresented leading to a preliminary design of anappropriate TDI. Finally, the impact of particles escapingthe TDI on the LHC is looked at, discussing the benefitfrom supplementary protection elements.2  OPERATIONAL AND FAILUREMODESVarious circumstances of using the MKI/TDI ensemblehave been investigated and listed in Table 2, grouped into“cases” with the same total kick strength seen by thebeam, in order of decreasing expected occurrence.Operational (intentional) uses (marked shaded) implynormally the use of single bunches, accidental oremergency uses have to proceed from the assumption offull batch intensity.Case 2 (sweep) occurs when the passage of beam to beinjected or circulating beam coincides with the rise or fallslope of the kicker pulse. The latter case results in closeto 100 bunches put on various places of the TDI and closeto 20 bunches escaping the TDI (assuming linear kickslope). One of the reasons for a beam sweep is a possible0-7803-5573-3/99/$10.00@1999 IEEE. 40Proceedings of the 1999 Particle Accelerator Conference, New York, 1999prefire of one of the kicker modules. Special precautionsare foreseen to make this very unlikely, or to reduce itsconsequences, e.g. applying the high voltage only veryshortly before the trigger pulse to the thyratron switchesor  firing rapidly the other three kicker modules if oneproduces a prefire.Table 2: Operational (shaded) and failure modes of theMKI/TDI ensembleCase Kick [%] Reason(s) [expected rate]1 0 Setting-up [during commissioning]Verification/re-adjustment [before eachfill]SPS extraction launched but LHC notready [occasional]Trigger missing (MKI internal orexternal) [rare]2 0-100 Beam sweep:Wrong timing (MKI internal orexternal) [occasional]Prefire of one MKI module, followedby firing the other 3 [rare]3 75 One MKI module full fault [rare]4 75-125 One MKI module flashover or equiv.[extremely rare] (grazing case ≈ 86 %)Fig. 2 shows a simplified side view (left) of theinjection region (not to scale), with an enlarged frontview (right) of the upper TDI block, showingschematically the beam impacts for the different cases.Figure 2: Schematic side view (left) of injection area(case 1 shown); enlarged beam view (right) of upper TDIblock with schematic beam impacts for various cases(case 4 shown for grazing impact).Case 1 corresponds to an impact distance from thebottom edge of about 30 mm, case 3 to about 3.5 mm.The lower TDI block (not shown) is foreseen to receivemiskicked circulating beam in mirrored positions. Atinjection the TDI blocks are supposed to be ± 8.5 σ(tangential to the machine aperture) or less distant fromthe LHC orbit (corresponding to about ± 4.3 mm). 3  TDI/D1 SIMULATION RESULTS The TDI/D1 ensemble has been simulated using FLUKA[6]. For the most frequent case 1 the length and thecomposition of the TDI was varied. The results are shownin Fig. 3, plotted as maximum energy deposition in theD1 coil against the number of interaction lengths of theTDI. The transverse dimensions were kept fixed at 8*8cm. Even though the counter-rotating beam imposes aspace limitation in the horizontal plane, these dimensionsreveal to be sufficient with a contribution from lateralleaking of only ≈5*10-7 GeV/cm3p.  1.E-071.E-061.E-051.E-041.E-031.E-021.E-011.E+000 5 10 15 203, 4, 5 m full graphite TDIdamage level full batchInteraction lengths [ ]Energy deposition in D1 coil [GeV/cm3 *p]2.5 m graphite, 1 m Al, 1 m Cu2.5 m graphite, 1 m Al, 0.5 m Cu, 0.3 m W2.5 m graphite, 1 m Al, 0.8 m Cu, 0.5 m Wquench level single bunchquench level full batch Figure 3: Energy deposition in D1 coil as a function ofthe number of interaction lengths of the TDI (case 1).  Sandwich constructs of appropriate materials have thebenefit over full graphite to stay below the quench limiteven for full batches, at a comparable length. A sequenceof 2.5 m graphite, 1 m aluminium, 0.5 m copper and 0.3m tungsten (marked with arrow) has been chosen as“reference TDI”. In a next step it has been investigatedwhat protection this TDI procures to D1 in the variouscases. The results are given in table 3. Table 3: Energy deposition in D1 coil (preliminary) forvarious cases (reference TDI) Case Error Energy deposition in D1 coil [J/cm3][%] 1.1*1011 p 2.67*1013 p 4.13*1013 p1 50 1.8*10-5 4.3*10-3 6.7*10-32 50 6.8 10.53 50 2.3*10-2 5.6 8.74 25 0.25 60.3 93.3  Case 1 leads to no quench, even with highest intensi-ties. Full batches in cases 2 and 3 will, without additionalmeasures, quench D1. Case 4 (values given for grazingimpact) approaches the damage limit for a nominal fullbatch and surpasses it slightly for the ultimate intensity.To test the effect from additional shielding a coppercylinder (25 ≤ r ≤ 140 mm, 1m long) has been introducedin the simulation 3 m in front of D1. This reduced theenergy deposition by about a factor 120, thus excludingdamage to D1 under all circumstances. Whereas such ashield would only be mandatory for case 4 with highestintensity, it is also beneficial in the other cases. TheMKI TDI D1orbit1234TDI(upper block)41Proceedings of the 1999 Particle Accelerator Conference, New York, 1999figure for the sweep case at nominal batch intensity isthen close to the quench level. Some further shieldoptimisation will probably allow to fall short of thequench level for this case.A perspective sketch of the preliminary design of thereference TDI is given in Fig. 4. Each TDI block has 2servo motors allowing a vertical adjustment with aprecision of better than 0.1 mm. The enlargement inFig. 4 (upper left corner) shows the front face of the TDIin more detail. The main absorber material is shrink-mounted into an aluminium frame, attached to an ironbeam which in turn is moved by the motors. The requiredvertical movement is relatively small in IP8 but in IP2 itmust take into account the opening requirements of theALICE Zero Degree Calorimeter [7].Figure 4: Perspective sketch of the TDI assembly(preliminary).4 OTHER IMPACTS THAN ON D14.1 Triplets / Dispersion Suppressors / Arcs The effect of injected bunches missing the TDI on otherparts of the LHC than D1 has also been looked at. Twoworst cases are considered: Firstly, the case 2 where close to 20 bunches could beswept between the orbit and the TDI edge, starting tooscillate around the orbit. The mean particle density isabout 2*1010 p / 0.1 σ. The damage level is estimated tobe 1012 p lost per m, the quench level 109 p lost per m.Damage seems therefore excluded, but to avoid a quenchthe TDI must be set such that it covers entirely themachine aperture of 8.5 σ. Secondly, the case 4 with a full batch just missing theTDI edge. Here the worst case particle density is about1.6*1012 p / 0.1 σ (peak). Excluding machine damagewith certainty would again require a sufficient closure ofthe TDI. Two additional collimators, positioned at a phaseadvance ∆µ ≈ ± 20° from the TDI (at around Q6/Q7 onthe other side of the injection insertions), with the sameaperture, would provide the same protection as the TDI inthe presence of phase errors.4.2 Cleaning Sections Badly injected particles oscillating around the LHC orbitbetween  8.5 σ (TDI) and 7 σ (primary cleaning collima-tors) will end up in the LHC cleaning sections. This doesnot cause problems for a few bunches, but if a full batchis lost in this area, the collimators are likely to bedamaged.4.3 ExperimentsSince the aperture of the experimental vacuum chambersis large compared to the machine aperture, it seemsexcluded that parts of the detectors can be hit directly bymisinjected bunches. However particles leaking out of theTDI or supplementary protection elements or scatteredparticles may reach the experiments in IP2 and IP8. Theirimpact is however at present estimated to be insignificantcompared to the radiation from normal operation. Moredetailed studies are required to confirm this assumption.5 CONCLUSIONSThe destructive beam power and the LHC characteristicsas superconducting, small-aperture machine requirehighest care at injection. Mishaps can have severeconsequences. Although the injection kickers are beingbuilt for utmost reliability, failures are not entirelyexcluded. Simulations of these failures reveal that a beamstopper with supplementary shielding and collimatorscan, appropriate setting assumed, provide sufficientprotection, except in very rare cases where the warmaperture limiting cleaning collimators can be affected.6 ACKNOWLEDGEMENTSThe fruitful discussions with members of the CERN SL-BT kicker team are gratefully acknowledged. M. Goujonis thanked for the sketch of the TDI.7 REFERENCES[1] The LHC Study Group, “The Large Hadron Collider ConceptualDesign”, CERN/AC/95-05 (LHC) (1995).[2] A. Hilaire, V. Mertens, E. Weisse, “Beam Transfer to and Injectioninto LHC”, Proc. EPAC’98, Stockholm (1998) and CERN/LHCProject Report 208 (1998).[3] L. Ducimetière et al., “Design of the Injection Kicker MagnetSystem for CERN's 14 TeV Proton Collider LHC”, Proc. IEEEPulsed Power Conference, Albuquerque, USA, July 10-13, 1995.[4] J. B. Jeanneret et al., CERN/LHC Project Report 44 (1996).[5] O. Brüning, J. B. Jeanneret, CERN/LHC Project Note 141 (1998).[6] A. Fassò, A. Ferrari, J. Ranft, P. Sala, “FLUKA: Present Status andFuture Developments”, Proc. Int. Conf. on Calorimetry in HighEnergy Physics, La Biodola (Is. d’Elba), Italy, September 20-25,1993, Eds. A.Menzione and A.Scribano, World Scientific, p.493.[7] ALICE Technical Design Report of the Zero Degree Calorimeter(ZDC), CERN/LHCC 99-5 (1999).42Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

Impact of and Protection against Failures of the LHC Injection Kickers

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