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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Alongwiththeseimplementedoperations,somesignalprocessingfunctionssuchasltering,back- thebeamcentroidpositiondenedasx0=RxdxI(x;y)<br />
groundsubtraction,etc...,areavailable. thebeamspotthermswidthdenedas=R(xx0)2I(x;y)dx<br />
Duringourpreliminarymeasurementswehavenoticedthatdirectlycomputingthevarianceofthe beamprolecomputedontherawdatamatrixwasyieldingerroneousvaluesforthermsbeam <strong>and</strong>theHoughtransforms(i.e.projection)alonghorizontal<strong>and</strong>verticalaxis(i.e.P(x)= RI(x;y)dy)<br />
photo-electronemittedasthedrivelaserpulseifo.These\ghostbunches"areindeedduetothe inabilitytohaveaperfectextinctionratioof0betweendrivelaserpulsethatservetocreate\real" bunches.Physicallythisisduetotheelectro-opticscellthatareusedtoswitchthelaserpulseon size.Thiswastracedbacktobeduetotheso-called\ghostpulse"eect:alongwiththemain interestforourmeasurement,thereareparasiticbunchescalled\ghostbunches"thatconsistsof <strong>and</strong>oonthephotocathode.Thereforewemodiedouracquisitionalgorithmtotakeintoaccount thiseectbyusingthefollowingstepsduringameasurement: macropulsethatcontainsaseriesofelectronbunches,whosetransversedensityisthequantityof<br />
5.mathematicallyperformtheoperationMbeam=MghostMbeam+ghost 3.increasethemacropulsetotheappropriatewidth 4.acquirethepixelmatrixMbeam+ghost<strong>and</strong>storeitinthecurrentbuer 1.reducemacropulsewidthto100nstoonlydetect\ghost"pulses, 2.acquirethepixelmatrixMghost<strong>and</strong>storeitinamemorybuer,<br />
methodyieldsreasonableresults. Intheabovestep3,weneedtoclarifythemeaningof\appropriate"macropulseduration:it TheresultsofthisoperationisgraphicallyshowninFigure4.12.Wehavealsoveriedthatthis dependsonthebeamsize,<strong>and</strong>itisexperimentallydeterminedbyinsuringtheCCDarrayisstill operatedinitslineardomain.Typicallythepowersurfacedensity,dP=dS,onthepixelarray,scales 6.computeparameterusingthematrixMbeam<br />
withthetransversermsbeamsizeimage,x<strong>and</strong>y,<strong>and</strong>withthepoweroftheemittedradiation, P,accordingtotherelation:<br />
conditionchangeswecanusethescalinglawofthelatterequationtore-adjustthemacropulse widthdynamically.<br />
determinedfortypicalbeamsizeoneachofthebeamprolemeasurementstation.Itisthenautomaticallyrecalledwhenameasurementisperformed.Alsotoaccommodatepotentialoperating Duringourexperiments,foreachmeasurementstation,themacropulsewidthwasexperimentally dP dS/P1xy (4.23)
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