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

Figure12:SEYofdi erentTiN

sampleforelectronenergybe-

tween0-2980

eVFigure13:SEYofdi erentTiNsampleforelectronenergybe-tween0-280eV.

couldbethesuperpositionofthetwoSEYcurves.Asthegunappearsveryreliableinrepetitivemeasurementswedonotbelievetheirregularityisanartifactduetothesystem.OthermeasurementsoftheSEYforunbakedS/S(δ 2at250eV)oranasreceivedNEG,

Fig.14,donotshowstheseshoulders.XPSsurveyofTiN#6showspresenceofmagnesium.Itsexpectedin uenceontheSEYisnotknown.

Finally,analternativecoatingtoTiNissputter-depositedTiZrVgetter(～2µmthick-ness).ThisNEG,whenactivated,showsadrasticreductionofitsSEY,Fig.14andFig.15.Theseresultscon rmwhathasbeeninvestigatedelsewhere[8].ItisalsointerestingtofollowthebehaviouroftheSEYcurveswhenthesampleisjustexposedtoonlyaresidualgasbackgroundof～3.10 10Torrforanextendedperiodoftime.TheSEYoftheTiZrVgoesupwithtimewhenexposedtoevensuchgoodvacuum.Interestinglyenough,itisclaimedin[8]thatthein uenceintheSEYoftheNEGafterexposureto30000L(1L=10 6Torr.s)ofCOorCO2israthersmall.δmaxwillincreasefrom1.1(CERNfullyactivatedNEG)to1.35(max)whileinUHV[8]. (SEY)is0.25usingCERNresultsandis0.3forours,1.3≤δmax≤1.6,Fig.14.

Figure14:SEYofTiZrVafter

di erentprocess,electronenergy

between0-2980eV

Figure15:SEYofTiZrVafterdi erentprocess,electronenergybetween0-280eV