第30卷第12期2006年12月高能物理與核物理HIGHENERGYPHYSICSANDNUCLEARPHYSICSVol.30,No.12Dec.,2006PhysicsDesignandStudyoftheBSNSRCSInjectionSystem*TANGJing-Yu1;1)QIUJing1WANGSheng1WEIJie1,21(InstituteofHighEnergyPhysics,CAS,Beijing100049,China)2(BrookhavenNationalLaboratory,Upton,NY11973,USA)AbstractWiththeBeijingSpallationNeutronSource(BSNS)acceleratorindesign,intenseH?beamsare?rstacceleratedbythelinacandtheninjectedintherapidcyclingsynchrotron(RCS)foraccumulationandfurtheracceleration.TheinjectionsystemusesH?strippingandphasespacepaintingmethodto?llthelargeringacceptancewiththelinacbeamofsmallemittance.Themethodiscrucialtomaintainlowbeamlossrateduringtheaccumulationandinitialacceleration.Di?erentfromtheinjectiondesignofsimilarhigh-intensityacceleratorsintheworld,theBSNSringinjectionisaccomplishedbymagneticelementsthatarecompletelycontainedina9meter-longuninterruptedspaceofnear-zerodispersion.Withtheaccumulated1.9×1013particles,spacechargee?ectsplayaveryimportantrole.The3Dsimulationsincludingspacechargee?ectshavebeencarriedouttooptimizetheinjectiondesign.Thispaperpresentsthephysicsdesign,computersimulationresultsanddesignoptimizationoftheinjectionsystem.Keywordsrapidcyclingsynchrotron,H?strippinginjection,phasespacepainting,spacechargee?ects,ORBITcode1IntroductionSince1980’s,spallationneutronsourcesbasedon80/130MeVlinacastheinjectoranda1.6GeVrapidcyclingsynchrotron(RCS)asthemainaccelerator.ThehighpowerprotonbeamextractedfromtheRCSwillbesenttoatargetstationwherespallationpro-cessinaheavymetallictarget(Tungsten)convertstheprotonsintoneutrons.Afterbeingsloweddowninmoderators,theneutronsaretransportedtothespectrometersforuserexperiments.Table1.MainparametersofBSNS.BSNS-Ⅰbeampower/kWrepetitionrate/Hztargetnumberaveragecurrent/μAprotonenergy/GeVlinacbeamenergy/MeV10025162.51.680BSNS-Ⅱ2002511251.6130highpowerprotonacceleratorsandneutronscat-teringtechniqueshavebecomeamajortoolinthestudiesofmaterialstructure.StimulatedfromthegreatsuccessofoperatingsourcesoffromtenskWtohundredskW,USA,JapanandEUaredevelopingsourcesmorepowerfulsothatprotonbeampowerwillreachMWlevel.Atthesametime,hundredsofkWsourcesarealsoingreatdemand.ChinahasproposedtoconstructtheBeijingSpallationNeutronSource(BSNS,formernamed“ChineseSpallationNeutronSource[CSNS]”)ofseveralhundredskW[1—3].Itwillbeconstructedintwophases(BSNS-Ⅰfor100kW,BSNS-Ⅱfor200kW,seeTable1).The?rstphaseoftheprojectisexpectedtobecompletedaround2011.BSNShastwoacceleratorsincascade,withanForhighintensitycircularprotonaccelerators,in-jectionviaH?strippingisactuallytheonlypracticalReceived23April2006,Revised30May2006*SupportedbyNationalNaturalScienceFoundationofChina(10075065),KnowledgeInnovationProgramofCASandundertheAuspicesoftheUSDepartmentofEnergy1)E-mail:tangjy@1184—1189第12期唐靖宇等：北京散裂中子源RCS注入系統(tǒng)物理設(shè)計和研究1185method[4,5].ThedesignoftheRCSinjectionsystemistoinjectthepre-acceleratedH?beamintotheRCSwithhighprecisionandhightransporte?ciency.Atthesametime,asstrongspacechargee?ectsarethemaincausesforbeamlossesinsuchhighintensityac-celerator,itisneededtoincreasethebeamemittanceandbeamuniformityintheRCStocontrolthein-?uenceofspacechargee?ects.Inordertodoso,thephasespacepaintingmethodofinjectingthebeamofsmallemittancefromthelinacintothelargeringacceptancewasdevelopedandusedintheBSNSasinothersimilaraccelerators.2InjectionlayoutSeveralinjectionschemesalongwiththeRCSlat-ticeschemeshavebeenstudied,and?nallythede-signbasedononelongdriftinadispersion-freelongstraightsectionisfavored.Adispersion-freelongstraightsectionotherthanahighlydispersivearcsectionischosenforthedesignoftheinjectionsys-temduetotheadvantages:1)thetransversephasespacepaintingisnota?ectedbytherampingbend-ingmagnets;2)theringpropertiesareessentiallynota?ectedbythelocalorbitbumpingand3)theup-gradingoftheinjectionsysteminfutureismorefea-sible.Atpresent,afour-foldanti-symmetriclatticehasbeenchosenfortheRCS,asshowninFig.1.Fourdispersion-freelongstraightsectionsarefortheRFcavities,theinjectionsystem,theextractionsystemandthecollimationsystem.ThelatticefunctionsoftheRCSringareshowninFig.2.ThefocusingstructureoftheRCSlongstraightsectionsusesDFdoubletsincludingonelongdriftof9minthecenterandtwolongdriftsof6monthesides.ThelinacbeamisinjectedintotheRCSbyus-inghorizontalbendingmagnets;alltheinjectionele-mentsareaccommodatedwithinthelongdriftof9m,seeFig.3.Inboththehorizontalandverticalplanes,phasespacepaintingisperformedbyvaryingthepo-sitionbumpatthestrippingfoil.Twopairsofhor-izontalbumpmagnets(BH1—BH4)areforpaintinginx-xplane,andtwopairsofverticalbumpmagnets(BV1—BV4)areforpaintinginy-yplane.WhereasFig.1.RCSlayoutandfunctions.Fig.2.ThelatticefunctionsforoneRCSsuper-period.Fig.3.LayoutofRCSinjectionsystem.BC1—BC4:closed-orbitbumpmagnets;BH1—BH4:horizontalpaintingbumpmag-nets;BV1—BV4:verticalpaintingbumpmagnets;QDC3&QFC3：quadrupoles;ISEP1&2:injectionanddumpseptummag-nets.twopairsofhorizontalbumpmagnets(BC1—BC4)inthemiddleareforadditionalclosed-orbitshiftof50mm,andthisisimportantforthespaceclearance1186高能物理與核物理(HEP&NP)第30卷oftheinjectionelements.TheBCbumpmagnetswillcollapseafterthebeaminjectiontoreducethepro-tontraversalinthestrippingfoilandtoregainsuper-periodicityforthering.Allthebumpersarepoweredinseriesforthereasonofeliminatingtrackingerrors.Itispossibletouseeitherpositionbumporan-glesweepingoramixtureofthemforphasespacepainting.Thepositionbumpmethodiscarriedoutbymovingtheringacceptanceellipseatstrippingfoilwiththehelpofbumpmagnetswithinthering;theanglesweepingmethodiscarriedoutbysweepingtheinjectionangleatthestrippingfoilwiththehelpofbumpmagnetsintheinjectionline.Twousualpaintingschemesare:tousepositionbumpinboththehorizontalandverticalplanesandtousepositionbumpinthehorizontalplanebutanglesweepingintheverticalplane.Afterthecomparison,the?rstschemeisadoptedfortheBSNS.Themeritsoftheinjectionschemearethattheinjectionsystemisalmostindependentoftheringfocusingstructure,thustheoperationssuchasthetuneadjustmentduringtheinjectiondonota?ectthepaintingprocess,andthateverythinginonelongdriftof9msaveslongitudinalspaceandavoidsadditionalaperturerequirementinthecaseofintercrossingwithquadrupoles.ThedesignisrealizableforbothBSNS-ⅠandBSNS-Ⅱ,consideringtherecentdevelopmentinpulsedpowersuppliesforfastbumpmagnets.Thedesignofdoublewaistsattheinjectionpointisveryusefulindecreasingtheaperturesforthebumpmag-netsandminimizingthein?uenceoftheedgefocusingofthebumpmagnetstotheringlattice.3Transversephasespacepainting3.1RequirementofphasespacepaintingThebeamemittancefromthelinacissmall,about1πmm·mradinrms.IftheH?beamisinjecteddi-rectlyintotheringthroughsimplestripping,aftertensorhundredsofturns,thespacechargee?ectswillblow-upthebeamemittance.Althoughwithalargeringacceptance,thenonlinearpartofthespacechargeforcewillleadtonon-uniformityofthebeamdistribution,thuslargebeamlosseswillproba-blyhappen.Inordertoreducethebeamlossesthatarecriticalinhighpoweraccelerators,paintingintothelargeacceptancewithgooduniformityisusuallyrequired.Oneparameterindicatingthein?uenceofthespacechargee?ectsisthetuneshift(?v).Ac-cordingtothebeamlosstoleranceindi?erentaccel-erators,thetuneshiftiscontrolledatabout?0.3—?0.4forhundredskWacceleratorsandwithin?0.2forMWaccelerators.Inthecaseofuniformdistribu-tion,thetuneshiftduetospacechargee?ectscanbeexpressedby:?v=?rpn2πβ2γ3εB,(1)fwhererp=1.53×10?18mforclassicalprotonradius,nforaccumulatedparticles,εforun-normalizedemit-tance,Bfforlongitudinalbunchingfactor,βandγforLorentzfactors.Table2showsthetuneshiftsduetospacechargee?ectsatBSNS-ⅠandBSNS-Ⅱ.TheaccumulatedparticlesinTable2correspondtobeampowersof120kWand240kWattheextrac-tion,respectively,relativelyhigherthantheprojectgoals.Beamlossesof2%—5%aftertheaccumulationarealsotakenintoaccount.Aftertheaccumulation,thebeambunchingfactorisabout1.0inthecaseofnon-choppinginjectionanddependsonthechop-pingfactorinthecaseofchoppinginjection.How-ever,inallcasesafterinitialacceleration(about1ms),thebunchingfactorwillreachtoabout0.3atBSNS-ⅠwithonlyfundamentalRFcavitiesandtoabout0.4atBSNS-ⅡwithadualharmonicRFsystem.Atthiscriticalmoment,thetuneshiftsreachthemaximum.Fortheringacceptanceof540πmm·mradandthecol-limatedacceptanceof350πmm·mrad,thebeamcoreemittanceischosentobeabout250πmm·mradforthisestimate.Table2.BSNSinjectionparametersandthetuneshiftduetospacechargee?ects.BSNS-ⅠBSNS-ⅡEinj/MeV80130β0.38850.4781γ1.08531.1386n1.9×10133.8×1013Bf0.30.4ε/(πmm·mrad)250250?v?0.33?0.28第12期唐靖宇等：北京散裂中子源RCS注入系統(tǒng)物理設(shè)計和研究11873.2PaintingschemeAsmentionedinSections2and3.1,theinjec-tionwithphasespacepaintingismandatorytore-ducethetuneshiftduetospacechargee?ects.Evenwiththephasespacepainting,spacechargee?ectsstillresultintheemittanceblow-upandthusacare-fuldesignedpaintingschemeisimportanttocontroltheblow-up.Boththecorrelatedpaintingandtheanti-correlatedpaintingschemeshavebeenconsid-eredfortheBSNSinjectionsystem.Withthecor-relatedpaintingscheme,thebeam?llsboththehor-izontalandverticalacceptanceellipsesfrominnertoouterandthe?naldistributionintherealspacex-ywillbealmostrectangular.Withtheanti-correlatedpaintingscheme,thebeam?llsthehorizontalaccep-tanceellipsefrominnertoouterandtheverticalac-ceptanceellipsefromoutertoinnerandthe?naldis-tributioninx-ywillbeelliptical.Thelatterischo-senasthenominalpaintingschemefortheBSNS,andFig.4showsthepositionsoftheRCSacceptanceellipseduringtheinjection.Atthesametime,thecorrelatedpaintingschemeiskeptasanalternative,inwhichtheinjectionpointwillbeliftedbyabout30mmintheverticalbyusingapairofDCcorrectorsintheinjectionline.Fig.4.PositionsoftheRCSacceptanceellipseduringinjection.ByusingORBITcode[6],onecansimulatetheinjectionprocessincludingspacechargeforcesin3-dimension.Thepaintingcurves(orbitbumpvaryingwithtime)canbeoptimizedbyusingthetrialanderrorprocedure.Fig.5showsthesimulatedbeamdistributioninphasespacesattheinjectionendwithnon-chopping,andTable3showsthestatisticalre-sults.Fig.5.Beamdistributioninphasespacesatinjection.Otherfactorsthatin?uencethepaintingresults,includingthepaintedemittance,theinjectionpeakcurrent,thechoppinginjection,theringworkingpoint,andthecomparisonbetweenanti-correlatedandcorrelatedpaintingetc.areunderstudy.Table3.Injectionconditionsandsimulationresults.circumference/m232tunes(Qx/Qy)5.78/5.86βx/βyatinjectionpoint/m5.17/5.99injectionenergy/MeV80injectionbeampeakcurrent/mA15injectionemittanceεx/y/(πmm·mrad,rms)1.0accumulatedparticles1.9×1013paintedemittance(εpx/εpy)/(πmm·mrad)255/178emittanceatinjectionend(turn152)287/330(99%)(εx/εy)/(πmm·mrad)255/252(95%)230/210(90%)114/89(50%)4Otherconsiderationsanddiscus-sions4.1In?uenceoftheinjectionmagnetstotheRCSlatticeBecauseallinjectionelementsareinalongdriftofdoublewaistsandtheyarearrangedalmostsymmet-rically,theedgefocusinge?ectofthebumpmagnetshasnegligiblein?uenceontheRCSfocusingstruc-ture.Themaximumchangeinβ-functionisabout±0.5m,andalmostinvisibleattheinjectionpoint.Theresidualdispersionoutsidethesectionisalsoneg-ligible.Asallbumpmagnetswillcollapseafterthe1188高能物理與核物理(HEP&NP)第30卷injection,thesuper-periodicityoftheringwillbere-covered.4.2PartialstrippingofH?beamandstrippedelectronsTheH?strippinginjectionrequiresthattheac-celerationandtransportfromtheionsourcetotheinjectionpointbewithanH?beam.DuetotheLorentzstrippingofH?beaminmagnetic?eld,themagnetic?eldofthemagnetsinthehighenergypartofthelinac,theinjectionbeamlineandtheinjectionsystemisdesignedatarelativelylowlevel.Astheinjectionenergyis80/130MeVattheBSNS,evenforpossibleupgradingto230MeV,themaximummag-netic?eldof0.7Tisacceptable.Itisconsideredtouseacarbonoranaluminafoilof50μg/cm2forBSNS-Ⅰ.ISISusesthesamethicknessandobtainsthestrippinge?ciencyofabout98%[7].BSNS-Ⅱcanuseathickerfoilof100μg/cm2.Amongthosenon-orpartiallystrippedparticles,theoverwhelmingmajorityisH0andverysmallpartisstillH?.ItisplannedtostriptheH0particlesintoprotonswithathickerstrippingfoilandsendthemtotheinjectionbeamdump.TheremainedH?beamcanbestoppeddirectlybyanabsorber.SomeH0particlesinexcitationstatesarestrippedbymagnetic?eldwhenpassingthoughBC3magnetandbecometheringbeamhalo.Theywillberemovedbytheringtransversecollimators.EveryH?particlewillproducetwoelectronswhenstrippedintoproton.Iftheelectronsareleftfreelyinthevacuum,theymayenhancethepossibilityofe-pinstability.Ontheotherhand,theelectronsarealsoharmfulifbentbackintothefoil.Heretheedge?eldofBC3magnetisconsideredtobendtheelectronstoanelectroncatcher.Thebeampoweroftheelectronsisbelow20WevenatBSNS-Ⅱ,soitissuitablefornaturalcooling.4.3ProtontraversalinthestrippingfoilBesidesalleviatingthespacechargee?ects,thephasespacepaintingcanreducethenumberoftraver-salsofthecirculatingprotonsonthestrippingfoil,whichmightbeacriticalissueforthefoillifetimeandbeamlosses.Evenwiththephasespacepainting,thecirculatingprotonsstillhavegoodprobabilitytocrossthestrippingfoilduringthebeaminjection.Depend-ingonthepaintingscheme,theprotontraversalcanbefromabouttwotoseveraltens.Ontheonehand,theprotontrasversalincreasesthedamagerateofthestrippingfoilorreduceitslifetime;ontheotherhand,thenuclearelasticscatteringandthemulti-scatteringprocessbymultiplecrossingofprotonsthroughthefoilwillproducemorebeamhaloandresultintheincreaseofbeamlosses.Therefore,thephasespacepaintingisdesignednotonlytoobtainagoodbeamdistributionbutalsotoreducetheprotontraversal.Withtheactualpaintingscheme,theprotontraversalisabout3.4andthemaximumtemperatureofthefoilisabout1193K.5ConclusionsBSNS/RCSemploysH?strippinginjectionmethod.Thewholeinjectionsystemisaccommo-datedinalongdriftof9minoneofthedispersion-freelongstraights.Withinthering,itconsistsoffourclosed-orbitbumpmagnets,fourhorizontalpaintingbumpmagnetsandfourverticalpaintingbumpmag-nets,allpoweredinseries.TheORBITsimulationsshowthattheanti-correlatedpaintingschemegivesagooddistributioninthetransversephasespaces.TheinjectionsystemcansatisfytherequirementsforbothBSNS-ⅠandBSNS-Ⅱ.TheauthorswouldliketothankQ.Qin