In the beam pipe of the positron Main Damping Ring (MDR) of the Next Linear Collider (NLC), ionization of residual gases and secondary electron emission give rise to an electron cloud which can cause the loss of the circulating beam. One path to avoid the

stainlesssteeliscloseto1.1,Fig.1.Withregardtotheguncurrentasafunctionofenergy,itstartsatzeroforzeroenergyandsmoothlyincreasedtoitssaturationvalueat70eV.Wemeasurethemagnitudeandfunctionaldependenceofthebeamcurrentuptosomewhathighervalue(100eV)anduseaconstructedlookuptableofthebeamcurrentforSEYcalculations.Notbiasingthesamplewithahighenoughvoltagewillleadtoanunderestimationofthebeamcurrent.Thisiseasilyunderstoodfromthesecondaryspectrum,Fig.10.Theselected2nAguncurrentismeasuredforagunenergyof0-100eVbyenergystepsof10eV(0-3000eVrange)or2eV(0-300eVrange).

ActualmeasurementoftheSEYisdonebybiasingthesampleto-20V.Thisretarding eldrepelsmostsecondariesfromadjacentpartsofthesystemthatareexcitedbytheelasticallyre ectedprimarybeam.SEYmeasurementsaredonetwice,oncebetween0eVto3000eVwith10eVsteps,thenbetween0eVto300eV,with2eVsteps.The nalenergyoftheprimaryelectronsisrespectively2980eVand280eVbecauseofthebias.TheprimarybeamcurrentfunctionismeasuredandrecordedeachtimebeforeanSEYmeasurement,andwiththesamestepinenergyfortheelectronbeam.Afreshcurrentlookuptableiscreatedwitheachmeasurement.

Thepurposeofthesecondmeasurement,0eVto300eV,istotrytounderstandthestructureoftheSEYcurveatverylowenergy.Severalpointsareimportant,though.

1.Becauseofthenegativesamplebias,primaryelectronsnear0eVatthesampleareassuredtobeleavingthegun(20eVdeparture)andarrivingat(20eV-20V)eV.

2.BecauseofthealgorithmusedtocalculatetheSEYfromtheprimaryandsamplecurrentsat0eVincidentenergy,adivideby0blowupoccurs.Toavoidthisproblem,the rstpointat0eVisforcedtovalueone.The rst”true”datapointisat2eVfor300eVrangeand10eVfor3000eVrange.

3.Theuncertaintyinthesampleelectrometercurrentreadingissetbytheinputoperationalampli erbiasleakage,±20pA.

ingsamplecurrenttodetermineSEYexcludestheelasticallyre ectedelectronsfromthecalculation(theywouldneedtobecollectedbyanexternaltothesamplegridstructure).ThatservestoincreasetheSEY(lesssamplecurrent)by1-3%.Thisisfortuitouslybalancedbythefactthatthe-20Vbiasdoesnotrepel100%ofnearbysurfacesecondaries.

TheconsequenceofallofthesepointsisthattheSEYmeasurementsarejustthat,”secondary”.Whichmeansthatitdoesnotincludetheelastics.TheSEYmeasurementsarehowever,accurate.ItisimportanttonotlookattheSEYatlowprimaryenergyandtrytoconcludesomethingaboutelasticre ectivity.Databelow20eVcomesfromabandstructureandareacombinationofdi ractionfromthecrystallinestructureandenergyabsorptionbythematerial[12].Surfacee ectssuchasroughnesscanalsochangetheSEY.