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Beam Position Monitors in the TESLA Test Facility Linac

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BeamPositionMonitorsintheTESLATestFacilityLinac

R.Lorenz,TUBerlin,Einsteinufer17,10587Berlin

Abstract

ThetransversepositionofthebeamintheTTFLinacwillbe

measuredusingtwodifferenttypesofmonitors.Forthealign-

mentofthequadrupolesacircularcavitywasdesignedbecause

ofthedesiredresolutionof10mandthelimitedlongitudinal

space.TheamplitudeoftheTM-modewillbemeasuredina

homodynereceiver,usingasignalfromthetimingsystemasa

reference.Stainlesssteelprototypesweretested.Furthermore,

thesinglebunchbehaviourwasmeasuredattheCLICTestFa-

cility.

CoaxialStriplineswillbeinstalledintheexperimentalarea,hav-

ingaresolutionofbetterthan100m.Theaveragedpositionof

thewholebunchtrainorthepositionofanindividualbunchwill

bemeasuredusingtheamplitude-to-phaseconversion.

I.Introduction

Inordertoestablishatechnicalbasisforasuperconducting

linearcollidertheTESLATestFacilityisanessentialpartofthe

developmentofinjectors,acceleratingcavities,cryostatandnew

diagnostictechniques.

Thetransversepositionofthebeamaftertheinjectorwillbe

measuredusingtwodifferenttypesofmonitors.Forthealign-

mentofthequadrupolesasinglecircularcavitywasdesigned

becauseofthelimitedlongitudinalspaceandthedesiredreso-

lutionofabout10minacoldenvironment(seealso[3]).The

amplitudeoftheTM-modewillbemeasuredinahomodyne

receiver,usingtheseventhharmonicofa217MHztimingsig-

nalasareference.

StriplineBPMswillbeinstalledintheexperimentalarea,having

aresolutionofabout100m.ForInjectorI(bunchspacingof

4.6ns)theaveragedpositionofthebunchtrainwillbemeasured

usingtheamplitude-to-phaseconversion.

II.TM-Cavity

TheamplitudeoftheTM-modeexcitedinthecavityby

anoff-centerbeamyieldsasignalproportionaltothebeamdis-

placementandthebunchcharge.Itsphaserelativetoanexternal

referenceyieldsthesignofthedisplacement.Bothpolarizations

ofthismodehavetobemeasuredtogetthedisplacementinx

andy,respectively.ForInjectorIthissystemcanmeasureonly

anaverageoverthebunchtrain.

A.PrototypeDesign

ThecavityparametersgiveninTableIwerecalculatedwith

URMEL,andthemeasurementswereperformedonastainless

steelprototypeatroomtemperature(Fig.1).CrNiwaschosenas

thecavitymaterialtomeasureindividualbunchesspacedat1s

(InjectorII).

Aftercoolingdownthestructure,theseventhharmonicof216.7

MHzhastobewithinthecavitybandwidthtoavoidanactive

tuningsysteminsidethecryostat.TheantennaeconsistingofKyocera-feedthroughsweldedintoaspecialflange,arereplace-

abletoallowapre-tuning(byadjustingthecouplingafterweld-

ing).parameterat290Ktargetsensitivity

dimensionmm

radius115.2mm114.77mmKHz

length52.0mm51.80mmKHz

beampipe39.0mm37.85mmKHz

theoret.lossfactor[VpC],

theoreticalunloadedQ

measuredfrequ.[GHz],

measuredcoupling,

TableICavitydesignandmeasuredparameters

Figure.1.CavityBPM

B.EstimatedSignals

Theresolutionneartheelectricalcenterofthecavityislim-

itedbythethermalnoiseoftheelectronicsandtheexcitationof

commonmodes.Foracavitywithoutbeampipes,thevoltageoftheTMexcitedbyabeamatapositionvs.thenoisevolt-

agecanbeestimatedas

noisemaxmax

noisemaxTtrNFTB

whereisthefirstrootof,Ttrthetransittimefactor,

pCthebunchcharge,thelong.lossfactorandthecavity

radius(TableI).ForanoisefigureofNF,theS/N-ratioina

bandwidthMHzat293Kisaboutm.

Sincethefieldmaximumofthecommonmodesisonthecav-

ityaxis,theywillbeexcitedmuchstrongerthantheTMby

abeamneartheaxis.ThevoltageoftheTMwithrespectto

theTMandtheratioofthespectraldensitiesatcanbe

estimatedas([1])

mmmm

givestherequiredfrequencysensitivecommon-moderejec-

tion-about69dBforadisplacementofm.Butthe

minimumdetectablesignalneartheelectricalcenterofthecav-

ityisstilllimitedbyresidualsignalsat().Witha

combinationoftwoantennaeinahybridonegetsafieldselec-

tivefilterandarejectionofunwantedcommonfieldcomponents

at,limitedonlybythefiniteisolationofthehybridbetween

the-andthe-port.Finally,wegetatheoreticalresolutionof

lessthan6mfor20dBofisolation.

C.SignalProcessing

Weadoptedahomodynereceiverscheme(Fig.2),wherethe

amplitudeoftheTMandareferencearemixeddowntoDC.

Thereference-signalisgeneratedbymixinga217MHz-signal

fromthetimingsystemandamplifyingtheseventhharmonic.

Whenthebeamisontheright,thesystemcanbesetuptogive

positivevideopolarity.Thesignalchangesthephaseby

whenthebeammovestotheleft,andforacenteredbeamitbe-

comeszero.25dB

ADCADCADC(sameas in x)

yxcurrenty-polarisationx-polarisation

PD

timingsystem216.7MHzφPD

Figure.2.SignalprocessingschemeforthecavityBPMs

Duetothelimitedspace,thecombinationoftwooppositeanten-

naewasrealizedoutsidethecryostat.Thetubularbandpassfil-

terhasabandwidthof100MHzandastopbandattenuationof

morethan70dB,upto8GHz.Togetherwiththehybridandthe

couplingfactorsthisgivesafrequencysensitivecommonmode

rejectionofabout100dB.Becauseofthefiniteisolationofthe

hybridandbetweenbothpolarizationsoftheTM(asymme-

triesinthecavity),thefullaperturewasdividedintotwomea-

surementranges.

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