Cleaning for Ultra-High Vacuum (UHV)

Keith MiddlemanVacuum Science Group

ASTeCCCLRC Daresbury Laboratory

Outline of Presentation

UHV Limiting Factors

Why Do We Need To Clean for Accelerators?

Methods to Reduce Outgassing

SRS Cleaning Procedure

Methods of Testing

Search for New Cleaning Solvents

Limiting Factors for achieving UHV

Material ChoiceGas Sources

Contaminant: prevents vacuum system reaching required base pressure or introduces unwanted species into the residual gas.Outgassing: 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 system.

Why Clean for Accelerator Vacuum Systems?

UHV Total Pressures Required ~ 1x10-9 mbarMaintain Satisfactory Lifetime Stored Electron Beam

Electron Scatter ∝ Atomic Number2

Low presence of high mass speciesHydrocarbons < 0.1% Pump Lubricants < 0.01%

Stimulated desorptionElectron or Ion induced - Major gas sourceDesorption Yield (Number of molecules per incident photon)

Maintain Clean In-Vacuum SurfacesPrevent Particle Target PoisoningMaintain Efficient Optical Properties for EM Radiation Transport

Methods to Reduce Outgassing

Bakeout - If the pressure is not low enough we can reduce the thermal outgassing rates by reducing the surface coverage N or the temperature T.

Beam Cleaning

Passivation Oxide films Nitride coatings (TiN, BN) Active films (NEG coatings)

Methods to Reduce Outgassing

Ultrasonic Cleaning - widely used

Glow discharge

Electropolishing

SRS Cleaning Procedure

EX-SITU BAKEOUT

Leak check (< 10-10

Bake to 250°C for 24 hours

Record RGA scan

Effective pre-treatment ensures outgassingare reduced to < 10l s-1 cm-2

CLEANING PROCEDURE

Hot water jet with detergentSurface stripping with alumina beadsUltrasonic wash in hot Triklone

Vapour Wash in hot TrikloneRinse with de-min waterImmerse in hot alkaline bath of P3-Almeco 36 at 60°CRinse in de-min waterDry in warm, dust free air, bag and seal.

mbar l s-1)

rates -11 mbar

Cleaning FacilitiesWater Jet and Detergent Phase

Cleaning FacilitiesVapour wash

Testing - Is it ‘Clean’?

Surface Analysis Techniques (Detailed Surface Composition)

AES - Auger Electron Spectroscopy

SIMS - Secondary Ion Mass Spectrometry

Scanning Electron Micrograph

Phenomenological Testing (Surface Vacuum Performance)Measure Total Outgassing Rates - Total Pressure Gauge

Measure Partial Pressures - RGA

Stimulated Desorption (Friction, Electron or Ion Induced, Thermal)

Replacement of 1,1,2-Trichloroethylene

Why Change? - Reclassification of 1,1,2-Trichloroethylene (TrikeTM)

What is important to us? - Thermal outgassing and ESD

Throughput method of measuring outgassing rates

CA

PPQ ⋅−

=21

Comparative Tests - existing procedure proven for 20 years

Outgassing Measurement Facility

Thermal Outgassing Results

Experiment Information Thermal Outgassing Rate Q (mbar l s-1cm-2)

Blank Run 9.3E-14 No Contamination Cleaned with Trike 7.9E-13 Full Contamination. Cleaned with Trike 9.3E-13 Full Contamination. Cleaned with Lenium 2.8E-12 Full Contamination. Cleaned with Leksol 9.3E-14 Full Contamination. Cleaned with Novec HFE 7.5E-13 Full Contamination. Cleaned with IPA 7.5E-13 Full Contamination. Cleaned with Micro 90 3.7E-13 Full Contamination. Cleaned with Lancerclean 4.6E-12

RGA Data - Thermal Outgassing

Acceptable UHV ScanHydrocarbon < 0.1%Pump lubricants < 0.01%

Unacceptable UHV ScanHydrocarbon ~ 25%Pump lubricants ~ 5%

RGA Data - Electron Stimulated Desorption

Desorption Yield ~ 10-3 molecules / e-

Desorption Yield ~ 4 molecules / e-

Summary

Accelerators (storage rings) require UHV

Need to minimise thermal outgassing and stimulated desorption

Requires a detailed cleaning recipe

Phenomenological Testing