High Brightness Electron Beam Diagnostics and their ... - CASA
High Brightness Electron Beam Diagnostics and their ... - CASA High Brightness Electron Beam Diagnostics and their ... - CASA
AsforthegeometricemittanceonecandeneTwissparametersfromtherstandsecondorder Thisdenitionofemittanceispracticalsinceitdoesnotvaryiftheforcesarelinear. moments: forthermsemittance: Introducingthermsbeamsize,x,anddivergence,0x,itispossibletohaveasimpleexpression 8>:T=h(xhxi)2i T=hxx0ihxihx0i T=1+2T ~"x ~"x=x0xq1r212=x0x 2T ~"x1+2 (4.11) (4.10) Aswewillsee,bothofthesemeasurementsindeedreducetothemeasurementofabeamtrans- Wherer12isacorrelationcoecientdenedasr12=hxx0i Therefore,measuringatransverseemittance~"xalwaysreducestothemeasurementofbeamdensity alongx-axis(i.e.beamsizemeasurement)andalongx0-axis(i.e.beamdivergencemeasurement). versedensityprole.Thereforeweshallrstconcentrateourdiscussiononthislattertypeof measurement.Wewillthendiscusstheemittancecomputation. x0x;itisameasureofthetracespaceslope. theIRFELdriver-accelerator,thetransversebeamdistributionaremeasuredby: Aswehaveseenintheprevioussection,measurementoftransversetracespacegenerallyrequires measuringthebeamprolesi.e.thetransverseparticledensityalongthehorizontalorverticalaxis. Severaltechniquesarecommonlyusedforsuchapurposedependingonthebeam.Forinstancein 4.2MeasurementofBeamProleUsingTransitionRadiation linearresponseandcansaturateresultinginerroneousbeamdensitymeasurements. measurementofhigheraveragebeamcurrentisnotpossible:theceramicdoesnothavea auorescentscreen:aceramicplateisinsertedinthebeampath,andthelightemittedviathe oftheinjectortoobservelowcurrentbeam.Theuseofthesetypescreensforquantitative ofthespectrum(seeg.4.1b).Thesetypesofbeamprolemonitorareusableforquantitative measurementonlyforextremelylowaveragebeamcurrentoftypically10nA.Itisonlyused asaqualitativebeamtransversesectionmeasurementinthelowenergy(350keV)region uorescenceeectisobservedwithacamerasincetheuorescenceoccursintheopticalregion thepositionofthewiregivesthetransversebeamdensityalongthedirectionperpendicular wirescanthebeaminthetransverseplane,thepotentialacrossitsendsisproportionalto awirescanner:thebeamisinterceptedbyathin(20m)movingtungstenwire.Asthe tothewire(seeg.4.1b).Despitethistypeoftechniquecanachievedveryhighresolution, dependingonthediameterofthewireandthestepsofthescan,itasfewinconvenient:it thebeamcurrentintercepted.Thereforethemeasurementofthiselectricpotentialversus isaveryslowmeasurement,becauseofitsgenerallylargediameter(morethan20m),the
severalsynchrotronradiationmonitors:asthebeamisbentindipolemagnets,itemits componentsandpotentiallydamageselectronicssystem. synchrotronradiation.IntheIRFELthisradiationisemittedintheinfra-redregionofthe wirecanyieldsalargelossofparticlethatcanhitthevacuumchamberorotherbeamline manytransitionradiationmonitors:thinaluminumfoilareinsertedintothebeampath electromagneticspectrumandisimagedonaverysensitiveCCDdetector(seeg.4.1c).This prolemonitorhastheadvantageofbeingnon-invasive(itdoesnotyieldbeamdegradation) anditcanbeusedtomeasurebeamdistributionatveryhighcurrent.Howeveritisnotwell radiationisdetected(seegure4.1d).Thiscongurationrequiresthefoilmaterialtohave andtransitionradiation(seetheIntroductionchapter)isdetectedwithaCCDdetector. Generallythefoilmakesa45deganglewiththebeamdirectionandbackwardtransition suitedforemittancemeasurement:thebeamproleismeasuredinabendi.e.atadispersive agoodreectioncoecient.Sinceverythinfoilareavailable,thistypeofdevicescanstand locationandthereforetheemittancecomputation,requiresasomewhattediousanalysissince highcurrentbeamwithoutyieldingsignicantbeamdegradation. weneedtodeconvolvedthedispersioncontributiontothebeamprole. Suchstudies,experimentallycarriedintheCEBAFmachineatJeersonLab,helpedwiththechoice ofthetypeofscreen(aluminum).Duringthesestudieswealsodevelopedaquasinon-interceptive screenthatwasusedtomeasurethebeamproleofthehighpowerbeamoftheCEBAFaccelerator; choicewasprincipallydrivenbythereliability,thespeedandthelowcostofthistypeofinstrument. BeforetheIRFELwasbuiltwestudymanyaspectofthistypeofapparatus:whataretheaverage beamcurrentlimit,whatarethebeamtransversephasespacedegradationafteranOTRscreen. measurementoftransversedistributionrequiredformeasuringtheemittanceintheIRFEL.This Amongthetechniqueslistedabove,transitionradiationwaschosentoprovidethequantitative alsowedidnotimplementthistypeofmonitorintheIRFELshorttermplan,itmightbesometime implementedtocontinuouslymonitortheelectronbeamqualitywithoutsignicantimpacton AswehavementionedinChapter2,whenwediscussedelectromagneticradiationemittedbymoving diagnosticsandthedevelopmentofthenoninterceptiveprolemonitor. 4.2.1ThelimitationofTransitionRadiationMonitor thebeamitself.Inthefollowingsectionswediscussthelimitationsoftransitionradiation-based methodallowstheobservationoftheTRproducedasthebeamcrossestheboundaryvacuum/foil (backwardTR)orfoil/vacuum(forwardTR).Asthebeamisinterceptedbythefoilsomeconcerns mightarise: FirstlybecauseofthedE=dxofthematerialthebeamdepositssomeenergyintheTRradiator therebyincreasingitstemperature.Thereforewemuststudythethermaleectofthebeamonthe chargedparticles,thattransitionradiationcanbeobservedwheneverachargedparticleexperience TRradiator. adiscontinuityintheelectricpropertiesofitsenvironment.Acommonwayofobservingtransition Secondlywhentheelectronsthatconstitutethebeampassthroughthefoilmaterial,theyundergo radiationistointerceptthebeamtrajectorywithathinmetallicfoil(orTRradiator).Such resultinanon-interceptivediagnostics.Ontheotherhand,thedivergenceinducedviascattering scatteringonthenucleithatcanpotentiallydegradesthebeamemittanceandthereforewillnot
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Asforthegeometricemittanceonec<strong>and</strong>eneTwissparametersfromtherst<strong>and</strong>secondorder Thisdenitionofemittanceispracticalsinceitdoesnotvaryiftheforcesarelinear. moments:<br />
forthermsemittance: Introducingthermsbeamsize,x,<strong>and</strong>divergence,0x,itispossibletohaveasimpleexpression 8>:T=h(xhxi)2i T=hxx0ihxihx0i T=1+2T ~"x<br />
~"x=x0xq1r212=x0x 2T ~"x1+2<br />
(4.11) (4.10)<br />
Aswewillsee,bothofthesemeasurementsindeedreducetothemeasurementofabeamtrans- Wherer12isacorrelationcoecientdenedasr12=hxx0i Therefore,measuringatransverseemittance~"xalwaysreducestothemeasurementofbeamdensity alongx-axis(i.e.beamsizemeasurement)<strong>and</strong>alongx0-axis(i.e.beamdivergencemeasurement). versedensityprole.Thereforeweshallrstconcentrateourdiscussiononthislattertypeof measurement.Wewillthendiscusstheemittancecomputation. x0x;itisameasureofthetracespaceslope.<br />
theIRFELdriver-accelerator,thetransversebeamdistributionaremeasuredby: Aswehaveseenintheprevioussection,measurementoftransversetracespacegenerallyrequires measuringthebeamprolesi.e.thetransverseparticledensityalongthehorizontalorverticalaxis. Severaltechniquesarecommonlyusedforsuchapurposedependingonthebeam.Forinstancein 4.2Measurementof<strong>Beam</strong>ProleUsingTransitionRadiation<br />
linearresponse<strong>and</strong>cansaturateresultinginerroneousbeamdensitymeasurements. measurementofhigheraveragebeamcurrentisnotpossible:theceramicdoesnothavea auorescentscreen:aceramicplateisinsertedinthebeampath,<strong>and</strong>thelightemittedviathe oftheinjectortoobservelowcurrentbeam.Theuseofthesetypescreensforquantitative ofthespectrum(seeg.4.1b).Thesetypesofbeamprolemonitorareusableforquantitative measurementonlyforextremelylowaveragebeamcurrentoftypically10nA.Itisonlyused asaqualitativebeamtransversesectionmeasurementinthelowenergy(350keV)region uorescenceeectisobservedwithacamerasincetheuorescenceoccursintheopticalregion<br />
thepositionofthewiregivesthetransversebeamdensityalongthedirectionperpendicular wirescanthebeaminthetransverseplane,thepotentialacrossitsendsisproportionalto awirescanner:thebeamisinterceptedbyathin(20m)movingtungstenwire.Asthe tothewire(seeg.4.1b).Despitethistypeoftechniquecanachievedveryhighresolution, dependingonthediameterofthewire<strong>and</strong>thestepsofthescan,itasfewinconvenient:it thebeamcurrentintercepted.Thereforethemeasurementofthiselectricpotentialversus isaveryslowmeasurement,becauseofitsgenerallylargediameter(morethan20m),the
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