Table Of ContentION 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
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anaccelerator. Wedescribeindetailsomeofthee ectsthationscanhaveonabeam,
includingemittancegrowth,opticalerrors,beamlossesandevenbeaminstabilities. The
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severity ofthese e ects varieswidely dependingon the accelerator in question,so we
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o erupseveralsimulationtechniquesthatcanbeusedtopredicttheiroccurrence,as
wellasionsignaturesthatcanbeobservedexperimentallytoconfirmthepresenceof
ions. Weshareresultsfromexperimentsthattestedthreemajorionclearingmethods:
ionclearingelectrodes,bunchgaps,andbeamshaking. Resultsobtainedfromthese
experimentsaresupportedbyvarioustheoriesandsimulationcodes. Finally,because
takingbeampropertymeasurementsintheregimewhereiontrappingoccurscanbe
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di cult,weo erupanewdesignforarotatingwirescannercapableofobtainingbeam
profilesathighbeamintensity.
iographical ketch
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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.
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Thisthesisisdedicatedtomyfamily,foralloftheirsacrificesthatallowedmetoreach
thispoint.
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cknowledgments
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Acceleratorphysicsisafieldthatreliesonahugenumberofpeopletogetthings
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done,andthisthesisisnoexception. Overthepast6 yearsI’vebeenhelpedbysomany
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di erentpeoplethatit’sdi culttokeeptrackofeverybody,butI’llgiveitashot.
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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
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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
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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
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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
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at7-8amwithwordsofencouragementoverco ee. Itmeantalottomeatthetime.
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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
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group. They’re far better grad students than Iever wasat theirage, andI knowthey will
dogreatiftheykeepitup.
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Sharingano cewiththeCHESSgraduatestudents,DavidAgyeman-Budu,Howie
Joress,XinHuangandRohitGarg,wasgreatfuntowardstheend,andtheytaughtme
quiteabitaboutthesortsofprojectsthatlightsourcesareactuallyusedfor(something
thatisrarelytalkedaboutinacceleratorphysics).
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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
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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
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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.
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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
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