Cleaning for Ultra- High Vacuum (UHV)

1 Cleaning for Ultra- high Vacuum (UHV)Keith MiddlemanVacuum Science GroupASTeCCCLRC Daresbury LaboratoryOutline of PresentationzUHV Limiting FactorszWhy Do We Need To Clean for Accelerators?zMethods to Reduce OutgassingzSRS Cleaning ProcedurezMethods of TestingzSearch for New Cleaning SolventsLimiting Factors for achieving UHVzMaterial ChoicezGas SourceszContaminant: prevents Vacuum system reaching required base pressure or introduces unwanted species into the residual :The outgassing rate is the time-dependent rate at which gases and vapours are released under Vacuum . This limits the ultimate pressure achievable and can introduce contamination into the Vacuum Clean for Accelerator Vacuum Systems?zUHV Total Pressures Required ~ 1x10-9mbarzMaintain Satisfactory Lifetime Stored electron BeamzElectron Scatter Atomic Number2zLow presence of high mass specieszHydrocarbons < Pump Lubricants < desorptionzElectron or Ion induced - Major gas sourcezDesorption Yield (Number of molecules per incident photon)zMaintain Clean In- Vacuum SurfaceszPrevent Particle Target PoisoningzMaintain Efficient Optical Properties for EM Radiation TransportMethods to Reduce OutgassingzBakeout-If the pressure is not low enough we can reduce the thermal outgassing rates by reducing the surface coverage N or the temperature CleaningzPassivation Oxide films Nitride coatings (TiN, BN) Active films (NEG coatings)

2 Methods to Reduce OutgassingzUltrasonic Cleaning -widely usedzGlow dischargezElectropolishingSRS Cleaning Procedure EX-SITU BAKEOUTzLeak check (< 10-10zBake to 250 C for 24 hourszRecord RGA scan Effective pre-treatment ensures outgassingare reduced to < 10l s-1cm-2 Cleaning PROCEDUREzHot water jet with detergentzSurface stripping with alumina beadszUltrasonic wash in hot TriklonezVapour Wash in hot TriklonezRinse with de-min waterzImmerse in hot alkaline bath of P3-Almeco 36 at 60 CzRinse in de-min waterzDry in warm, dust free air, bag and l s-1)rates -11 mbar Cleaning FacilitiesWater Jet and Detergent PhaseCleaning FacilitiesVapour washTesting - Is it Clean ?zSurface Analysis Techniques(Detailed Surface Composition)zAES - Auger electron Spectroscopy zSIMS - Secondary Ion Mass Spectrometry zScanning electron MicrographzPhenomenological Testing(Surface Vacuum Performance)zMeasure Total Outgassing Rates - Total Pressure GaugezMeasure Partial Pressures - RGAzStimulated Desorption (Friction, electron or Ion Induced, Thermal)Replacement of 1,1,2-TrichloroethylenezWhy Change?

3 -Reclassification of 1,1,2-Trichloroethylene (TrikeTM)zWhat is important to us? -Thermal outgassing and ESDzThroughput method of measuring outgassing ratesCAPPQ =21zComparative Tests -existing procedure proven for 20 yearsOutgassing Measurement FacilityThermal Outgassing Results Experiment InformationThermal Outgassing Rate Q (mbar l s-1cm-2) Blank No Contamination Cleaned with Trike Full Contamination. Cleaned with Trike Full Contamination. Cleaned with Lenium Full Contamination. Cleaned with Full Contamination. Cleaned with Novec HFE Full Contamination. Cleaned with IPA Full Contamination. Cleaned with Micro 90 Full Contamination. Cleaned with RGA Data - Thermal OutgassingAcceptable UHV ScanHydrocarbon < lubricants < UHV ScanHydrocarbon ~ 25%Pump lubricants ~ 5%RGA Data - electron Stimulated DesorptionDesorption Yield ~ 10-3 molecules / e-Desorption Yield ~ 4 molecules / e-SummaryzAccelerators (storage rings) require UHVzNeed to minimise thermal outgassing and stimulated desorptionzRequires a detailed Cleaning recipezPhenomenological Testing