ION TRAPPING IN HIGH INTENSITY LINEAR ACCELERATORS ADissertation PresentedtotheFacultyoftheGraduateSchool ofCornellUniversity inPartialFulfillmentoftheRequirementsfortheDegreeof DoctorofPhilosophy by StevenFull May2017 (cid:13)c 2017StevenFull ALLRIGHTSRESERVED IONTRAPPINGINHIGHINTENSITYLINEARACCELERATORS StevenFull,Ph.D. CornellUniversity2017 Achargedparticlebeamwillrapidlyionizeanyresidualgasinanaccelerator’svacuum chamber,and,ifthatbeamisnegativelycharged,theresultingpositiveionscanbecome trappedwithin thebeam. Ion trappinghasoften beenobservedin circular accelerators, buthasneverbeforebeenseeninsingle-passlinearaccelerators. Howeveranewclassof highintensitylinacswillbethefirstsuchlinearacceleratorsthatexperienceiontrapping. In theCornell photoinjector, we have recentlyobserved this phenomenonfor the first time,andwewillshareourexperiencesconductingexperimentstostudyiontrapping,as wellastheoryandsimulationsmodelingthephenomenon. We start by outlining theories to determine whether or not ion trapping will occur in ff anaccelerator. Wedescribeindetailsomeofthee ectsthationscanhaveonabeam, includingemittancegrowth,opticalerrors,beamlossesandevenbeaminstabilities. The ff severity ofthese e ects varieswidely dependingon the accelerator in question,so we ff o erupseveralsimulationtechniquesthatcanbeusedtopredicttheiroccurrence,as wellasionsignaturesthatcanbeobservedexperimentallytoconfirmthepresenceof ions. Weshareresultsfromexperimentsthattestedthreemajorionclearingmethods: ionclearingelectrodes,bunchgaps,andbeamshaking. Resultsobtainedfromthese experimentsaresupportedbyvarioustheoriesandsimulationcodes. Finally,because takingbeampropertymeasurementsintheregimewhereiontrappingoccurscanbe ffi ff di cult,weo erupanewdesignforarotatingwirescannercapableofobtainingbeam profilesathighbeamintensity. iographical ketch B S StevenFullwasborninMcKeesport,PA(justoutsidethecityofPittsburgh)onJune11, 1988. HeattendedGatewayseniorhighschoolandgraduatedin2006. Stevenreceiveda bachelorsdegreeinphysicsfromthePennsylvaniaStateUniversityin2010,andhe represented the physicsdepartment assalutatorianduring graduationceremonies. Inthe summerof2009,heparticipatedinanundergraduateresearchprogramatUCLA,where hefirstfell inlovewiththefieldof acceleratorphysics. Thisexperience ultimatelyled himtopursueaPh. D.inacceleratorphysicsatCornell. iii Thisthesisisdedicatedtomyfamily,foralloftheirsacrificesthatallowedmetoreach thispoint. iv cknowledgments A Acceleratorphysicsisafieldthatreliesonahugenumberofpeopletogetthings + done,andthisthesisisnoexception. Overthepast6 yearsI’vebeenhelpedbysomany ff ffi di erentpeoplethatit’sdi culttokeeptrackofeverybody,butI’llgiveitashot. ff I‘llstartbythanking alloftheERLsta . Iprobablywouldn’thave completedhalfof thisthesiswithoutthehelpofAdamBartnikandColwynGulliford(especiallyAdam, whohelpedimmenselywiththebeamshakingsimulationsandempiricalmodelsforthe bunchgapdata). Theytaughtmetheropesofbeingagradstudentandhowtonotbe suchanamateur. TheywerealwaysthereinthecontrolroomwhenIneededhelp,and arethesourceoftonsofinsidejokesandstoriesthatwe’llprobablyremember10years fromnow(thatpizza). TobeyMoore,mechanicalwizard,wastheonewhocameupwiththegreatwire scannerdesigninchapter4. HebuiltandputtogethersomanyofthedevicesI’veused overtheyears,andIwouldn’thavegottenjacksquatdonewithouthim. I’llmissallthe Tobey-isms too(I’m stillworking hardat hardlyworking atworking hard). Hopefully thenextplaceIworkathas“aTobey”tokeepmeontrack. Nick Agladze also helped considerably with the wire scanner project. He was involved in setting it up, testing it and writing the paper. He taught me so many techniqueswithopticsanddetectorsthatwillbeinmybagoftricksforalongtimeto come. JohnDobbins wasresponsiblefor manyof theexperimentalsetups foundin this paper. Whenweneededanyelectronicswork,hewasjustacallaway. Hesavedus multipletimeswhenwewerescramblingtosetupdiagnosticsinthemiddleofan v experiment,andweprobablyneverwould’vefinishedthebeamshakingexperimentsifit weren’tforhislastminutehelp. LucaCultreragrewplentyofwonderfulhighQEcathodesformetodestroyinmyion experiments. He also provided some pretty cool artistic photos of photocathodes wreckedbyionbackbombardmentthatareshowninchapter2. BruceDunhamletmetestmycrazyideasintheaccelerator,despitethefactthat leakinggasintotheacceleratorsoundedlikeahorribleideaatthetime. Iguessitturned outOKthough. Also,weneverwould’vehithighcurrentwithouthisvastexpertise. I ffi don’t thinkI realizedhow incrediblydi cult itis tohit highcurrent untilafter heleft, andhe’sdefinitelymissedhereatCornell. YulinLihelpedwithallvacuumrelatedissues,andespeciallywitheverythingrelated totheclearingelectrode. Hewastheonewhocameupwiththecurrentdesign,and testeditlongbeforeIwasinvolvedinthisproject. Chris Mayes and Dave Sagan taught me how to use BMAD, and many other simulationtricks. The simulationworkhereprobably wouldn’thave gottenfinished withouttheirhelp. ValKostrounprovidedmewithsomeoftheionizationcrosssectiondata,andsome interestingadviceonvacuumtubesandiontrapping. RickRyan,thecommunicationsguru,willonedaymakeamoviethat will crushthe LCLS-IIpromovideo. Itwillbeawesome. KarlSmolenskiandRitchiePattersonweregreatleadersandamazingpeopletowork for. We’reprettyluckyatCornelltohavethosetwoattheheadofthings. I’dalsoliketothankseveralmembersoftheSRFgroup. MatiasLiepe,headofthe vi ff SRFgroup,o eredmesomereallyusefuladviceonthesiswriting,andI’mthankfulfor that. PeterQuigley,VadimVeshcherevich,JohnReilly,RogerKaplan,FumioFuruta,Ralf Eichhorn,JamesSears,EricSmithandtherestoftheSRFcrewsavedtheSRFsystemsand myexperimentswhenthecryosystemscrashed(twice!) duringmyexperiments. They sacrificedso muchtime togetthe injectorback onlineas quickly aspossible, andthe experimentsneverwould’vegottendonewiththem. Bytheway-sorryif,inafewyears, itturnsoutIdamagedtheHTCbyleakinggasintothephotoinjector. While I was writing my thesis, all of the Newman Lab equipment technicians, including John Kaufman, Terri Gruber and Holly Conklin, keptme going every morning ff at7-8amwithwordsofencouragementoverco ee. Itmeantalottomeatthetime. ff Thereareplentyofpeopleontheadministrativesta thatsavedmeatthelastminute whenI was panicking aboutfunding or getting importantforms signed. Thanks to Katerina Malysheva, Monica Wesley and Kacey Acquilano in particular, although there areplentyofothers. Thankssomuchtoallmyfellowgraduatestudentsforallthegoodtimeswehad,and forpickingmeupwhenthingsgotrough. HyeriLeeandIstartedgradschoolatthe sametime, andwent throughitall together thickandthin. I thinkwecan bothagreethat wegotreallyluckybyendingupatCornell,andIcan’tbelievewemadeitthroughalive. Oh,andStephen’sprettycooltoo–Hyerididalrightthere,evenifhe’salittletoointothe NSfora30yearold. JaredMaxsonshowedusallwhatittakestobeatopresearcherinthisfield. Istill can’tbelievehe’sgoingtobeAdam’sbossinayear. I’llalwaysrememberthegoodtimes weallhadinthecontrolroom,andespeciallythecrazyERL2015roadtrip. Thanksto WilliamLou andNilanjan Banerjee,the othergraduate studentsin Georg’s vii group. They’re far better grad students than Iever wasat theirage, andI knowthey will dogreatiftheykeepitup. ffi Sharingano cewiththeCHESSgraduatestudents,DavidAgyeman-Budu,Howie Joress,XinHuangandRohitGarg,wasgreatfuntowardstheend,andtheytaughtme quiteabitaboutthesortsofprojectsthatlightsourcesareactuallyusedfor(something thatisrarelytalkedaboutinacceleratorphysics). ff TheSRFgraduatestudents,JamesManiscalco,PeteKoufalis,ThomasOsero and “TheDans”(HallandGonnella),werealsogoodfriends,andwehadsome fun times hostingvisitorsinvolvedwiththeCenterforBrightBeamsproject. Itwould’vegone horriblywithouttheirhelp. I’vehadtheopportunitytosuperviseseveralundergraduatesovertheyears,including Seth Pree, Bryce Fore, Kristina Smith and Incheoul Chung. These projects were a great experience,andIlearnedalotaboutleadershipintheprocess. Ihopetheyallpicked somethingupfromittoo. I’dalsoliketothankmanyofthecollaboratorswhohelpedonthisproject. Iwas ff lucky enough to get to perform experiments at many di erent accelerators, and it providedinsightsthatIotherwiseneverwould’vegotten. First,thankstoallofthefolksatCESR–DaveRubin,thesageMikeBilling,Avi Chatterjee,andKelvinBlaser–forlettingmesitinontheirfastionexperiments,and givingmesometipswithdatatakingandsimulations. Ispent5weeksabroadinGermany,andmetmanycollaboratorsovertherewho contributedsomuchtomydevelopmentas agraduatestudent. I’dliketo first thank Atoosa (“Ati”) Meseckfor inviting me to Germanyand hosting me. I was so fortunateto havehergivemethisopportunityoutofnowhere,anditwasoneofthebestexperiences viii I’veever had asa graduatestudent. I’llnever forgetit. Ati alsohelped agreat dealwhen designingourinitialexperimentsinthephotoinjector,andI’mnotsureifwewouldhave discoveredanythingwithoutherinsights. WhileIwasinGermany,thepeopleatHZBtreatedmesowell,asifIwereoneoftheir own. Theirhospitalitywasmorethananyonecouldaskfor(theyreallywentaboveand beyond),andIwouldprobablyhavetolistanother30peoplejusttocovereveryone. But I’dliketosingleoutStephiRädel,Ati’sgraduatestudent,whoshowedmearoundBerlin andhelpedmesurviveonadailybasis. Berlinwould’vebeenprettyboringwithouther andherfriends. I’dalsoliketothankJörgFeikesandhisgraduatestudentsforbeingsoaccommodating by lettingus observeions inthe MLSon short notice. Their insightsand observations formedthebasisformuchofthisthesis. Theyalsotooksomewonderfuldataduring theirownsetsofexperimentswhichwasveryilluminating. WealsoconductedseveralexperimentsatELSAinBonn,thankstotheapprovalof Wolfgang Hillert. There Igottoconduct experimentswithDennisSauerland andother graduatestudentsatELSA(Iwonderifyouallhavestillkeptyoursecret?). Itwas especiallynicetomeetDennis,afellowtraveleronthepathofions–ararityoverthelast 6years. ff Lastbutnotleast,I’dliketothankmyadvisorsGeorgHo staetterandIvanBazarov forsupportingmefinancially(eventhroughthatroughpatchthelabfacedafewyears ago)andgivingmeadviceovertheyears. Theytaughtmetoomuchtolisthere,and there are too many thingsthat I owe them both. I think most importantly theytaught me howtobeexactandprecisewithwhatIsay,toformulatequestionscarefully,andtowork ix