High Brightness Electron Beam Diagnostics and their ... - CASA
High Brightness Electron Beam Diagnostics and their ... - CASA
High Brightness Electron Beam Diagnostics and their ... - CASA
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expansionoftheBFFderivedinthischapter,thespectrumcanbeextrapolatedusingtheequation: I()=a0+a22+O(3).Thepointofattachmentofthisparabolaischosentobeintheneighbor- thefrequency;theCTRspectrumhasthesamedependence.Hence,usingthegeneralpolynomial hoodofthelowfrequencycuto.Thecoecienta2<strong>and</strong>a0arecomputedbyusingthecontinuity conditionsatthecutopoint:weassumeboththespectrum<strong>and</strong>itslocalderivativewithrespect towavelengtharecontinuous.Ingure5.19weperformdierentlowfrequencyextrapolationsof forlargefrequency(i.e.lowwavelength),thebunchformfactorhasaquadraticdependenceon<br />
ontherecoveredlongitudinalbunchdistributionisshowningure5.20.Itisinterestingtonote thatonewaytorejectunphysicaldistributionistorejectalltheparabolicextrapolationsthatgive asignicantnumberofnegativevaluesinthebunchlongitudinaldistributions. atthecut-opointtocomputethecoecienta0.Theinuenceofthesedierentextrapolations theenergyspectrumbyvaryingtheparametera2<strong>and</strong>usingonlythecontinuityofthespectrum<br />
5.6ZerophasingTechniqueforBunchLengthMeasurement<br />
awaythatthebunchcentroidcoincideswithazeroacceleratingelectriceld.Hencethecavities investigatedthepossibilityofitsapplicationtomeasurethebunchlengthintheIRFELaccelerator. ThezerophasingmethodusesRFacceleratingcavitiesphased90degreesocresti.e.insuch Ithasbeendemonstratedtoresolvebunchlengthinthesubpicosecondregime[52].Thereforewe Theso-calledzerophasing(orbackphasing)techniquehasproventobeaverypowerfulmethod. 5.6.1BasisoftheMethod<br />
inducealongitudinallydependentenergyrampalongthebunch.Then,bymeansofaspectrometer, theenergydistributionismappedintothetransversedirection,<strong>and</strong>thebeamtransversedensity thedispersioninthechicaneatthebeamprolemeasurementstationisabout2timeslessthan anoptimumchoice:themaximumbeamenergythatcanbedeectedintothedumpisabout readilyavailabletoperformsuchmeasurements:wecanusetheenergyrecoverydumplineorthe thismethodweonlyneedacceleratingcavities<strong>and</strong>spectrometers.Therearetwospectrometers theoneattheOTRprolemonitorlocatedintheenergyrecoverydump. Despitethefactthattheenergyrecoverydumphasbeenchosenasaspectrometer,itisnot ismeasuredwithabeamprolestationlocatedinthedispersiveregion.Thereforetoimplement<br />
24MeV5whichimpliesthatthefourlastcavitiesofthecryomodulemustbeturnedo<strong>and</strong>/or usedaszerophasingcavities.Thenon-zerophasedcavitiesareoperatedunder<strong>their</strong>nominalsettings (acceleratinggradient7.33MV/m,phase-9.6deg)givingabeamenergyof23.74MeV.Atsuchan rst4-bendchicane.Preliminaryconsiderationshaveshownthatthelatterisnoteasilyworkable:<br />
intermediateenergy,thedynamicsof60pCbunchesisnotemittancedominated,requiringastudy ofspacechargeeectsonthemeasurement. IntheFEL,theenergyrecoverydumplineconsistsofaquadrupole<strong>and</strong>anOTRprolemonitor. ThedispersionattheOTRlocationwhenthequadrupoleisturnedoisexpectedtobe=75cm; thislattervaluecanbereduced,ifneeded,usingtheupstreamquadrupole. Itshouldbestressedthatthemeasuredbunchlengthisthebunchlengthattheexitofthefourth cavityi.e.inthemiddleoftheSRF-linac(theparmelapredictedbunchlength<strong>and</strong>phasespace slopeatthislocationare370m<strong>and</strong>-48.74MeV/mrespectively). FollowingnotationofReference[52],wewritethehorizontalpositionxonthebeamprole 5PrivatecommunicationfromR.Legg,January1998
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