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

aluminiumsamplesprovidedbyBNLandaTiZrVsputterdeposited lmonstainlesssteelsubstrate,obtainedfromCERN,

Fig.9.Figure9:Samplesmeasuredthusfar.

SEY(δ)de nitionisgiveninequation2.Inpracticeequation3isusedbecauseitcontainsparametersmeasureddirectlyintheexperiment.

δ=NumberofelectronsleavingthesurfaceIP(3)

WhereIPistheprimarycurrentorthecurrentleavingtheelectrongunandimpingingonthesurfaceofthesampleandITisthetotalcurrentmeasuredonthesample(IT=IP+IS).ISisthesecondaryelectroncurrentleavingthetarget.

Thespectrumofsecondaryelectroncurrentleavingthetargetiscomposedoftruesecondaries(0eVto40eV,byconvention),re-di usedprimaryelectronsexitingaftersu eringlossesinthesample(40eVtoEp)andfromincidentprimaryelectrons(Ep)elasticallyre ectedfromthesurface,Fig.10.Themajorityoftheelectronsleavingthesurfacearetruesecondaries.

Inordertomeasuretheprimarycurrentleavingtheelectrongun,thesampleisbiasedat+150V.Thebiasvoltagepreventsallre-di usedandsecondaryelectronsoflessthan150eVfromleavingthesample.Elasticallyre ectedelectronsarenotcollectedandcouldstrikenearbysurfaces,creatingsecondaryelectronsthatarethencollectedbythesamplebias.Thise ectissmallbecausethere ectivityat100eVisafewpercent.Weestimatetheerrorinprimarybeamcurrentmeasurementtobesmall(1-2%)becausethelossofelasticsisbalancedbythegainofnearbysecondaries.TheSEYaround100eVforbaked