High Brightness Electron Beam Diagnostics and their ... - CASA
High Brightness Electron Beam Diagnostics and their ... - CASA High Brightness Electron Beam Diagnostics and their ... - CASA
6.5.1EmittanceMeasurement Arstexperimentconsistedinvaryingthebunchlengthattheundulatorlocationandmeasuring interaction,ifthosearepresent.Afullyself-consistentcode,writtenbyR.LiofJeersonLab[70], implementedinparmelawefoundthatatsuchpointtheemittanceincreasescomparedtothecase intheundulatorvicinity(asinferredfromthemaximumCTRsignal).Fromasimplisticmodel whereCSRisnotincludedinthecalculationisapproximately10%.Atthetimeofthemeasurement weoperatedthegunwithaverypoorphotocathodeandcouldnotextractmorethat20pCcharge perbunch,atoolowchargetounambiguouslymeasurepotentialeectsfromCSRbunchself ingure6.21.Theemittanceseemstogothroughamaximumforaminimumbunchlength thehorizontalemittanceafterthedecompressorchicane.Atypicalplotobtainedispresented ranfor60pCandanominalemittanceofapproximately7mm-mradyieldeda20%increasein emittance13. Wealsoattemptedtocomparethetransversehorizontalemittancebeforeandafterthearc1.Inthe casecorrespondingtothenominaloperationofthelinac,whichcorrespondstoaminimumbunch lengthintheundulatorvicinity,noemittancegrowthwasobservedwithintheerrorbars(transverse horizontalemittancesmeasuredwereapproximately18mm-mradnormalizedat38MeV). 22 20 18 Before Arc 16 14 Unfortunatelynoparametricstudyhasbeencompletedtothepresentdate Figure6.21:TransverseHorizontalemittanceandtotalpowerCTRsignalmeasuredasafunction ofthelinacgangphase. 13R.Li,privatecommunication,thisincreasewasnoticedtobeverydependentontheopticallatticesetup. 12 10 bunch length CTR signal 8 4 6 8 10 12 14 16 18 20 Linac Phase (RF−deg) Horizontal Norm. Emittance (mm−mrad)
Inthesecondseriesofrun,wewereabletoextracthighchargeperbunchfromthephotocathode (typically60pC)butwewereneverabletoestablishsucientlylowtransverseemittanceinthe 6.5.2EnergySpreadMeasurement IRFEL14toprovidealowenoughsignal-to-noisetoseedenitivelyapotentialemittancegrowth hadremovedalltransverseeectsoftransportfromtheviewerimages,somethingthatneedstobe donetobesureonlyenergyspreadisbeingobservedontheviewers.However,forafewselectcases atthearcexit.Inthecaseofenergyspread,itwasgenerallydiculttoconvinceourselvesthatwe correspondingtoshortbunchlengthsclosetotheundulator,webelievewehavegooddata,and whichindicatestheenergyspreadincreasebetweenthecompressorchicanemidpointandthearc1 exitshouldbeoftheorderof5%15. withintheresolutionofthesemeasurements,theyshownoincreaseinenergyspreadaspicturedin gure6.22.ThisresultisconsistentwithsimulationperformedwiththeJLabselfconsistentcode −0.01 0 0.01 −0.01 0 0.01 −0.01 0 0.01 −0.01 0 0.01 0.5 energyspread(nounits)].Thebottomplotpresentsthermsrelativeenergyspreadcomputedfrom 0.4 thedistributions. Figure6.22:Energydistributionmeasuredalongthebeamline,atthechicanemidpoint(A)and (B)andentranceofthearcs(C)and(D)[thehorizontalaxisoftheseplotrepresenttherelative 0.3 14thereasonisstillnotunderstoodatthepresenttime 15R.Li,privatecommunication 0.2 0.1 0 0 20 40 60 80 Distance from the cryomodule exit (meters) δ (%) Energy Distrib. (A) (B) (C) (D)
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Inthesecondseriesofrun,wewereabletoextracthighchargeperbunchfromthephotocathode (typically60pC)butwewereneverabletoestablishsucientlylowtransverseemittanceinthe 6.5.2EnergySpreadMeasurement IRFEL14toprovidealowenoughsignal-to-noisetoseedenitivelyapotentialemittancegrowth hadremovedalltransverseeectsoftransportfromtheviewerimages,somethingthatneedstobe donetobesureonlyenergyspreadisbeingobservedontheviewers.However,forafewselectcases atthearcexit.Inthecaseofenergyspread,itwasgenerallydiculttoconvinceourselvesthatwe correspondingtoshortbunchlengthsclosetotheundulator,webelievewehavegooddata,<strong>and</strong> whichindicatestheenergyspreadincreasebetweenthecompressorchicanemidpoint<strong>and</strong>thearc1 exitshouldbeoftheorderof5%15. withintheresolutionofthesemeasurements,theyshownoincreaseinenergyspreadaspicturedin gure6.22.ThisresultisconsistentwithsimulationperformedwiththeJLabselfconsistentcode<br />
−0.01 0 0.01 −0.01 0 0.01 −0.01 0 0.01 −0.01 0 0.01<br />
0.5<br />
energyspread(nounits)].Thebottomplotpresentsthermsrelativeenergyspreadcomputedfrom<br />
0.4<br />
thedistributions. Figure6.22:Energydistributionmeasuredalongthebeamline,atthechicanemidpoint(A)<strong>and</strong> (B)<strong>and</strong>entranceofthearcs(C)<strong>and</strong>(D)[thehorizontalaxisoftheseplotrepresenttherelative<br />
0.3<br />
14thereasonisstillnotunderstoodatthepresenttime 15R.Li,privatecommunication<br />
0.2<br />
0.1<br />
0<br />
0 20 40 60 80<br />
Distance from the cryomodule exit (meters)<br />
δ (%)<br />
Energy Distrib.<br />
(A) (B) (C) (D)
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