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1、4.1.3PumpsandHolders4.1.3.1.VACUUMS4.1.3.2.ROUGHINGPUMPSThemostcommonroughingpumpisamechanical(rotary)pumpinwhichabelt-driven,eccentricallymountedreciprocatingmechanismsucksairthroughaninletvalveintoachamberandexpelsitthroughanexitvalveSuchpumpsareveryreliable,relativelyinexpen
2、sive,noisyanddirty,andonlylowerthepressureto~10-1Pa(~10-3Torr).Figure4.1.3.1.Amechanicalpumpforroughingvacuums.TheeccentricmotionofthepumpcreatesavacuumintheRHsidewhenitrotatesandthevacuumsucksairintotheinletvalve.Asthecylinderrotatesfurther,itcutsofftheinletandforcestheairthro
3、ughtheoutletontheLHside,creatingavacuumagainontheinletsideasitdoesso.Becauseoftheconstantcontactbetweentherotatingcylinderandtheinsideofthepump,oilisneededtoreducefrictionalheating.4.1.3.3.HIGH/ULTRA-HIGHVACUUMPUMPS4.1.3.3.A.DiffusionPumpsFigure4.1.3.2.Principlesofdiffusionpump
4、operation.Aheaterplateatthebaseofthepumpboilssyntheticoil.Theexpansionoftheoilvaporonboilingcreatesapressure,whichforcesthevaporupthecentralcolumnandoutofseveralholes.Thestreamofoilvaporpullsgasmoleculesoutofthetopofthepumpdowntothebase,wheretheoilcondensesandtheairispumpedouto
5、fthebasebyamechanicalbackingpump.Diffusionpumpsarecapableofveryefficientpumpingfrom~10-1to~10-9Pa(10-11Torr)and,ifproperlytrapped,willprovideacleanUHVsystemthatisveryreliable.4.1.3.3.B.TurbomolecularPumpsTurbomolecularpumps,orturbopumps,asthenameimplies,useaturbinetoforcegasesf
6、romthemicroscope.Theyhavemanypartsmovingathighspeeds(inexcessof20,000-50,000rpmiscommon),sotheyaremoreliabletofailthandiffusionpumps.Figure4.1.3.3.Aturbopump(withandwithoutitscasing)whichisnothingmorethanasmallturbinethatrotatesathighspeed.Likeajetturbineitpullsairinatthefronte
7、ndandforcesitoutoftheback.Thebladesaredesignedlikeairfoilstoenhancetheflowofgasthroughthesystem.4.1.3.3.C.IonPumpsIonpumpsdonotcontainoil,sotheycannotcontaminatetheTEMcolumn.Theyalsohavenomovingparts,relyingsolelyontheionizationprocesstoremoveair.Theionpumpemitselectronsfromaca
8、thode,andthesespiralinamagneticfieldandionizeairmolecu
