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

Figure16:XPSCpeakofaTiZrV lmaftertheendofactivation

fortendaysthestickingcoe cientis0.04,theNEGwouldhavepumpedanadditional～0.6ML.AccordingtotheroughcalculationforthetotalamountofCOpumpedbytheNEG,theXPSof11daysand20daysshouldbesimilar,astheNEGisbasicallysaturated.ThisincreaseoftheC-Opeak,Fig.16,suggeststhatthesurfacestillhadsomeremainingpumpingspeedafterelevendaysofvacuumexposure.HencehavingaroughnessR>1.NEGprovidesanicesolutionforadistributedpumpinginsideavacuumchamber,whenactivated.Oursmallsample,afewcm2,beingat300KpumpedalloftheresidualgasinthechamberexceptCH4.InafewmeterslongNEGcoatedchamberthecontaminationcomesfromoutgassingsurfacesoutsidethischamber,sincetheNEG lmisadi usionbarrierforoutgassingmoleculesofthesubstrate.DependingoftheaveragevacuuminthemachineandthelengthoftheNEGchambers,therecontaminationofthesurfacemighttakelongerthanforoursample.Hence,theSEYmightnotincreaseasrapidlyasmeasuredhere,Fig.14andFig.15.

Also,ithastobetakenintoaccountthatthepumpinglifetimeofthin lmNEGdependsonitsthicknessandthenumberofactivationcycles[11].Thein uenceofthethicknessofthe lmshouldalsobetakenintoaccount,whencalculatingtheimpedancefortheimagecurrentonthevacuumchamberwall,duetoapassingparticlebeam.Therequirementontheconductivityandthegoodmechanicalperformanceafteranin-situbakeofthesubstrateisofimportance.

Forthemaindampingrings,currentdesignwillusealuminium-alloychamber.Aluminiumloosesmechanicalstrengthwhenheatedabove150 C.AnadequatesubstrateforTiNorTiZrVwhichful llsthemechanicalrequirementsisthealloy,Al6061,orAl6060whichiseasiertoextrude.Theirconductivityis～1.4timeslowerthanforpurealuminium.SEYmeasurementsonaTiN lmdepositedonAl6061hasalreadybeencarriedoutatSLAC

[13].