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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ParameterValue<br />
x -0.178<br />
x(m) 8.331<br />
y -0.124<br />
y(m) 3.979<br />
Table3.1:Twissparametersdownstreamthecryomoduleexpectedfromsimulationswiththecode<br />
parmela.<br />
thatconsistsofsixperiodofaFODOlattice.<br />
ThearcsarebasedontheMIT-Batesacceleratordesign[17];theyprovideeachatotalbendingangle<br />
of180deg.Theyincludefourwedge-typedipoles,eachbendingthebeambyanangleof'28deg<br />
alternatively,installedinpairsymmetricallyarounda180degdipole.Furthermoreprovidingthe<br />
desiredrotation,thearcsarealsousedtoadjustthetotalbeampathlengthoftherecirculated<br />
beaminasuchwaythattheelectronbuncheshavethepropertimingtobeonthedecelerating<br />
phaseoftheSRFlinac,averyimportantparameterforenergyrecovery.Forsuchapurpose,<br />
thearcisinstrumentedwithapairofhorizontalsteererslocatedupstream<strong>and</strong>downstreamthe<br />
180degdipoletovarythereferenceorbitpathlengthinsidethe180degmagnet.Twofamilies<br />
ofquadrupoles(thetrimquadrupoles)<strong>and</strong>sextupolesareinstalledinthearctoprovideboth<br />
linear<strong>and</strong>quadraticenergydependentpathlengthvariationthatarenecessaryinthe\energy-<br />
compression"schemeneededtoproperlyenergyrecoverthebeam[18]i.e.topreciselyadjustthe<br />
momentumcompactionR562(lineardependenceoflongitudinalpositionwithrelativeenergy)<strong>and</strong><br />
thenonlinearmomentumcompactionT566(quadraticdependence).Whenthequadrupoles<strong>and</strong><br />
sextupolesarenotpowered,thearcisoperatinginanon-isochronousmode(R56=13:124cm).<br />
Howeverundernominaloperation,i.e.whentheFELisoperating<strong>and</strong>thelinacisinenergyrecovery<br />
mode,becauseoftheneedofenergycompression,thesextupoles<strong>and</strong>quadrupolesofonefamily<br />
areexcitedtopropervaluesinordertoprovidetherequiredR56betweenthewigglerexitupto<br />
thelinacentrance.<br />
Thebacklegtransportlineconsistsofthirteenquadrupoles.NominallyitisoperatedasaF0D0<br />
latticewithaphaseadvancepercell=90deg,butwehavedemonstrateditsoperationwith<br />
aphaseadvanceof=60degneededduringemittancemeasurementbasedonmulti-monitor<br />
technique.Itstotaltransfermatrixis-Iforbothtransverseplane:itimagesthelatticeoptical<br />
functionsattherstarcexitintotherstarcentranceaccordinglyto!<strong>and</strong>!.<br />
Thereinjectionlineconsistsofatelescopecomposedoffourquadrupolesthatisusedtoadjustthe<br />
beamlatticefunctiontoachievereasonablebeamenvelopeattheentranceoftheSRFlinac.<br />
AstheelectronbunchesgoforthesecondtimesthroughtheSRFlinac,theyaredecelerated<strong>and</strong><br />
inducevoltageinthecavityviabeamloading.Thisinduced-voltageisatthepropermodetoserve<br />
toacceleratethenextbunchintheaccelerationphasewhichisintheneighboringRFbucket.The<br />
wastedbeam,oncedecelerated(i.e.atapproximately10MeV),bifurcatesintoadump.<br />
Typicallatticefunctions,computedwiththesecondorderopticscodedimad,areshownin<br />
gure3.2.Theinitialconditions(seeTable3.2)thatareused,havebeencomputedusingthe<br />
parmelacodesinceitincludespacechargeeectsinthelowenergyregion<strong>and</strong>alsohaveavery<br />
detailedmodeloftheSRFcavities.Forthegurepresented,theundulatormagnetisinstalled.<br />
2Rs0!s<br />
56=Rs<br />
s0=R16(x)<br />
(x)dx,where(x)isthelocalbendingradius.Sosincethepresentlymentionedquadrupole<br />
arelocatedinadispersiveregion,i.e.R166=0,sothatR56canbevaried.
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