appendix a vacuum systems附录a真空系统

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1、AppendixA:TheVacuumSystemforExperiment6I.IntroductionWithpropercare,mostvacuumsystemscanoperatetrouble-freeforlongperiodsoftime:notonlydoescontinuousoperationnotharmthembutitinfactcleansesthemofsomecontaminants.Itshouldbeunderstoodthattheultimatevacuum(withinthecapabilityofthesystem)andthetime

2、ittakestoreachthatvacuumaredeterminedbythecleanlinessofthesystem.Ittakesonlyone“lunkhead”openingavalveatthewrongtimetocontaminateavacuumsystem,slowingthepumpingspeedandruiningitsvacuumcapability.Thoseoperatingvacuumsystemsshouldreadandunderstandoperatinginstructionsbeforestartingtheexperiment,

3、andaskforassistanceiftheinstructionsarenotunderstood.II.VacuumSystemsVacuumsystemsintheAdvancedLaboratoryconsistofamechanicalrotarypumpforlowvacuum,anair-cooledoildiffusionpumpforhighvacuum,andthenecessaryvalvesandtrapstointerconnectthem.Lowvacuumismeasuredbythermocouplegaugesandhighvacuumbyco

4、ld-cathodeiongauges.A-6FigureA-1Amechanicalrotarypumpisusedforbothforelineandroughingpumping.(SeeFig.A-1).Asolidsteelcylindricalrotorismountedeccentricallyinalargersteelcylinder.Contactbetweenthetwoiseffectedbytwospring-loadedvanesindiametricalslotsintherotor.Astherotorturns,itpullsairparticle

5、sthroughtheintakeportandexpelsthemthroughtheoutletvalve.Vacuumsealingissuppliedbytheprecisemachiningofthepartsandspecialmechanicalpumpvacuumoil.Highvacuumissuppliedbyathree-stageoildiffusionpump.Aspecialsiliconeoilisboiledbyelectricheaters.Astheoilturnsintoagasitnaturallyincreasesinvolume.Thee

6、xpandinggasjetsuptheconcentrictubesandisdeflecteddownwardpullingairparticlesfromabove.Whenthegashitsthecooledwallsofthepump,itcondensesandreturnstothebottomofthepumpforreheating,whiletheairparticlesarepulledfromthediffusionpumpthrougha“foreline”bythemechanicalpump.Adiffusionpumpcannotoperatewi

7、thoutthisforelinepumping.Sincetherearegaseousoilparticlesinthelowerpartofthediffusionpumpandahighvacuumabove,somemethodmustbeA-6usedtopreventtheoilfrom“backstreaming”intothehighvacuumarea.Onesuchmethodistheuseofbaffleplateswhich,beingat