The beams for the Large Hadron Collider LHC will be extracted from the Super Proton Synchrotron SPS in points LSS4 (Long Straight Section) and LSS6 in fast mode during a single turn. For this purpose a new fast extraction will be designed in LSS4. In LSS6 the existing slow extraction channel needs modification such that it can provide fast extracted beam for LHC, while continuing to provide the West Experimental Area with long spills. Both fast and slow extraction must be available from the same point, although in an interleaved mode. This imposes the retention of the thin electrostatic wire septum ZS actually in use. The present article focuses on the effect of total beam loss in the electrostatic septum as it could occur e.g. due to a kicker magnet failure during fast extraction in LSS6. Then the entire SPS beam of 4.13 10^13 protons would impinge on the septum wires. The damage caused by the resulting electromagnetic and hadronic particle showers is determined and the need to protect the ZS is evaluated.<BR

Goddard, B

Knaus, P

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN – SL DIVISIONCERN-SL-2000-035 BTBEAM LOSS DAMAGE IN A WIRE SEPTUMB. Goddard, P. KnausAbstractThe beams for the Large Hadron Collider LHC will be extracted from the Super ProtonSynchrotron SPS in points LSS4 (Long Straight Section) and LSS6 in fast mode during asingle turn. For this purpose a new fast extraction will be designed in LSS4. In LSS6 theexisting slow extraction channel needs modification such that it can provide fast extractedbeam for LHC, while continuing to provide the West Experimental Area with long spills.Both fast and slow extraction must be available from the same point, although in an inter-leaved mode. This imposes the retention of the thin electrostatic wire septum ZS actuallyin use.The present article focuses on the effect of total beam loss in the electrostatic septumas it could occur e.g. due to a kicker magnet failure during fast extraction in LSS6. Thenthe entire SPS beam of  	 protons would impinge on the septum wires. The damagecaused by the resulting electromagnetic and hadronic particle showers is determined andthe need to protect the ZS is evaluated.Presented at EPAC2000, 26-30 June 2000Vienna, AustriaGeneva, SwitzerlandJuly 2000BEAM LOSS DAMAGE IN A WIRE SEPTUMB. Goddard, P. Knaus, CERN, Geneva, SwitzerlandAbstractThe beams for the Large Hadron Collider LHC will be ex-tracted from the Super Proton Synchrotron SPS in pointsLSS4 (Long Straight Section) and LSS6 in fast mode dur-ing a single turn. For this purpose a new fast extractionwill be designed in LSS4. In LSS6 the existing slow ex-traction channel needs modification such that it can providefast extracted beam for LHC, while continuing to providethe West Experimental Area with long spills. Both fast andslow extraction must be available from the same point, al-though in an interleaved mode. This imposes the retentionof the thin electrostatic wire septum ZS actually in use.The present article focuses on the effect of total beamloss in the electrostatic septum as it could occur e.g. dueto a kicker magnet failure during fast extraction in LSS6.Then the entire SPS beam of  	 protons would im-pinge on the septum wires. The damage caused by theresulting electromagnetic and hadronic particle showers isdetermined and the need to protect the ZS is evaluated.1 INTRODUCTIONThe transformation of the SPS into the injector for LHCrequires major modifications to many hardware and soft-ware systems [1]. Among the hardware changes are twonew fast extractions from the SPS to be provided: a com-pletely new extraction channel is required in LSS4 and theexisting resonant extraction in LSS6 must be modified toallow also for fast extraction. Existing LSS6 equipmentwill be reused and is vulnerable to beam induced damage.Here the effect of a high brightness LHC type fast extractedbeam impinging on the ZS wire septum (Figure 1) is stud-ied. The electromagnetic and hadronic cascade provokedafter beam impact and the resulting energy deposition, issimulated by the Monte Carlo code FLUKA [2]. The cor-responding wire heating might lead to material sublimationand destruction of the wire septum. Possible remedies toprotect the ZS are proposed.2 SPS FAST EXTRACTION IN LSS6The reuse of the existing extraction channel in LSS6 of-fers many advantages [3], the most important of which isthe economy involved. However, the LSS6 extraction isdesigned to supply resonantly extracted beams to the WestArea, and as such is more complex than is necessary for astandard fast extraction (Figure 2). During fast extractionthe electrostatic septum ZS has to be used at nominal field,which is a potential source of problems for operation. TheZS septum is made up of five units each of 3.1 m electrodecathodewiresclearing electrodesreference rodsinsulated anode supportFigure 1: Thin electrostatic wire septum ZS as installed inthe SPS long straight section LSS6.length, with anode elements (septa) made up of arrays of2080 stretched wires at 1.5 mm spacing. These wires aremade from a W74Re26 alloy and are only 25  m in radiusfor the first two ZS to minimise the losses during resonantextraction. In the subsequent septa wires twice as thickare used. The rhenium admixture is added to obtain bettermechanical characteristics of the wires, in particular bet-ter ductility. In theory fast extraction should be a loss-freeprocess, and a certain amount of protection can be obtainedfrom a well conceived interlock system involving the ver-ification of the status of the extraction septa, the bumpers,the kickers, the orbit, etc.Despite these precautions, however, it will not be pos-sible to guarantee that the fast extracted beam will neverstrike the fragile ZS wires, either through equipment fail-ure, software problems or human error. The consequencesof a mis-steered LHC type fast extracted beam impingingdirectly on these wires are likely to involve serious equip-ment damage.3 SPS BEAM TYPESDuring the LHC era three beam types will be extractedfrom the SPS: a 450 GeV/c proton beam for LHC, a400 GeV/c proton beam for CNGS (CERN Neutrino beamto Gran Sasso) or fixed target physics, as well as a177 GeV/u Pb lead ion beam for LHC or fixed targedphysics. From the point of view of expected damage incase of beam loss on the ZS wire septum, the proton beamfor LHC represents the most critical case, because of itshigh intensity and brightness. The present study focuseson this beam whose parameters are listed in Table 1.extractionkicker MKEelectrostaticseptum ZSp+extraction pointextraction septum MSEthin magnetic septum MSTQDA619QFA618QDA617QFA616QD615QF614Figure 2: Schematic layout of fast extraction in LSS6 using existing equipment.Table 1: SPS parameters for ultimate LHC type protonbeam at extraction.Beam parameter ValueMomentum 450 GeV/cRevolution time 23  sNumber of bunches 243aParticles per bunch 1.7 10  pTotal intensity 4.13 10  pTrans. norm. emittance 3.5  radaRecent adoption of the triple bunch splitting technique in the PSlead to a new LHC filling scheme including an increase in the totalnumber of bunches in the SPS from 243 to now 288. This involves anincrease in ultimate beam total intensity from 4.13 10  p to 4.90 10  pand makes the situation for the ZS even worse.4 SIMULATION RESULTS4.1 Energy Deposition CalculationsA small amount of energy  deposited in a volume ﬀof a material with density ﬁ causes a temperature rise ﬂﬃdetermined by  "!$#%&ﬁ'ﬀ(ﬂﬃ . The proportionalityconstant #% is the specific heat of the considered material.The larger the value of #% , the smaller the temperature risecaused by an energy deposit  . For important energy de-positions the specific heat can no longer be considered asconstant, but its temperature dependence must be taken intoaccount. The specific heat # %*) ﬃ,+ for W74Re26 and othermaterial properties can be found in [4]. Now ﬂﬃ must beextracted fromﬀ!-ﬁ.0/21../#%)ﬃ3+*ﬃ (1)by solving numerically for the upper limit of the integral.The conductor temperature before the beam impact is ﬃ24 .4.2 Heating of a W74Re26 Wire SeptumBeam impact on the ZS wires produces a hadronic andelectromagnetic cascade resulting in energy deposition allalong the septum. The total energy deposited was com-puted by FLUKA and, due to the short pulse length of23  s, was assumed instantaneous. The equivalent temper-ature distribution induced on every single wire was deter-µwire nb. 85r=25   mtrans. pos. [mm]250020001500100005001.510.50-0.5-1-1.5T [K]∆Figure 3: The temperature profile along a single W74Re26wire (the hottest) reflects the Gaussian profile of the inci-dent beam. A peak temperature of 2300 5 C is reached.mined and the central 4 mm of every wire, the region cov-ering the lateral width of the particle cascade, were dividedinto 50 cylindrical bins of 25  m radius. For the hottestwire the obtained transverse temperature profile is dis-played in Figure 3. A peak temperature of about 2300 5 C isreached, which is well above the operating limit of 1000 5 C.Similar temperature profiles are obtained all along the sep-tum and the temperature distribution on the septum as awhole is shown in Figure 4. The first 50 cm of the sep-tum heat to very critical temperatures of more than 1500 5 C,with the first 30 cm to even more than 2000 5 C. Clearly, tak-ing also into account the 500 g pretension applied to everywire, the septum does not resist such temperatures and aremedy must be found.0 0.5012-1-22400192014409604801 1.5 2 2.5 3∆500150020001000long. pos. [m]temp.   T [K]trans. pos. [mm]0Figure 4: Transverse and longitudinal temperature profileon the 3.1 m septum ZS1 with r=25  m W74Re26 wires.4.3 Heating of a Graphite Wire SeptumOne possibility to reduce the expected peak temperature inthe septum is to replace the high-density W74Re26 wiresby a light material of high melting point. Wires madeof carbon fibres appear to be a possible candidate. Dueto their low density of 1.85 g/cm the Bethe-Bloch for-mula predicts a lower 768:9 and the high values of #% of650 J/(kg K) at 20 5 C causes the corresponding temperatureincrease ﬂﬃ to be modest (Figure 5).The maximum heating expected with such wires whenthe ultimate LHC beam of  ;<	=  protons is acciden-tially dumped at 450 GeV/c into the ZS extraction septumis only 60 5 C. Compared to the 2300 5 reached in the tung-sten alloy the advantage is striking. However, graphitewires are not possible due to the high voltage environment.Any graphite deposit on the cathode would pollute the elec-trodes surface and lead to a high sparking rate, and eventu-ally destroy the cathode.0012-1-20.5 1 1.5 2 2.5 3∆10305070long. pos. [m]temp.   T [K]trans. pos. [mm]48362412060Figure 5: Transverse and longitudinal temperature profileon the 3.1 m septum ZS1 with r=25  m graphite wires.4.4 Beam Absorber or DiluterAnother possible and actually favoured approach consistsin designing a mechanical protection element, a beam stop-per or diluter, to be positioned in front of the ZS when fastextraction is requested. In case of mis-steering this elementwould absorb most of the beam such that no or only accept-able energy deposition in the septum will occur. However,this element must be mobile so that it can be withdrawnfor slow extraction where a thin septum is mandatory. ForLHC filling, about three to four times a day, the absorberelement would be brought into position to protect the sep-tum from eventual losses during fast beam transfer to thehadron collider.A preliminary design of such an element is made. Useof a 2.5 m graphite block, followed by 0.5 m aluminumwould significantly decrease the peak temperature in theZS. Use of a few centimetre thick boron carbide plate asbeam spreader in front of the graphite absorber helps to getthe cascade going and reduces the wire peak temperaturesto very acceptable 100 5 C.∆1000 2000 300006005004003002001000absorber length [mm]T[o C]2.5m C Al0.5mFigure 6: Instantaneous temperature increase on the centralaxis in a 3 m C and Al absorber right after beam impact.5 CONCLUSIONSGiven the high cost and radiation constraints associatedwith the fabrication and replacement of ZS septa, a me-chanical protection element will be necessary to protect thewires. This object will have to be moved quickly into placebefore any fast extraction, and removed when resonant ex-traction is requested.Simulations showed that the wire septum in LSS6 needsprotection when, with the start-up of LHC, fast extractionof the 450 GeV/c proton beam will be provided. In case ofan extraction kicker failure the entire SPS proton intensitycould be dumped in the electrostatic septum and deposita sufficient amount of energy to sublimate the W74R26wires. Therefore a protection element in form of an ab-sorber will dilute the beam to such a level that it cannotharm the septum wires. Some concern exists that carbonparticles could be deposited on the high voltage cathodeand, as surface pollution, cause a high sparking rate in theZS. Therefore a beryllium absorber will also be considered.Experimental verification of these results will be pro-vided by a fast wire scanner with W74Re26 wires to bepassed through the beam until destroyed by sublimation.The moment at which the wire breaks and the equivalentenergy deposited by the beam can be determined and com-pared to the simulation.6 REFERENCES[1] P. Collier et al., “The SPS as Injector for the LHC: ConceptualDesign”, CERN SL 97-07 (DI), 1997.[2] A. Fasso`, A. Ferrari, J. Ranft, P. Sala, “Fluka 99 User’s Man-ual”, 1999.[3] B. Goddard, “Fast Extraction”, LEP and SPS Workshop, Cha-monix IX, February 2000.[4] P. Knaus, J. Schinzl, N. Stapley, “MATPROP: TheOn-Line SL Materials DataBase”, WWW access athttp://oraweb01:9000/matprop/owa/accisweb.welcome,Geneva, April 2000.

Beam Loss Damage in a Wire Septum

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