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Daresbury Laboratory Study of NEG Coated Tubes and Flanges

Daresbury Laboratory Study of NEG Coated Tubes and Flanges. Keith J. Middleman ASTeC Daresbury Laboratory. Outline. NEG Experimental Set-up Calibration of Total Pressure Gauges and RGA’s NEG Activation Procedures NEG Coated Flanges - Results NEG Coated Tube Monte Carlo Simulations

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Daresbury Laboratory Study of NEG Coated Tubes and Flanges

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  1. Daresbury Laboratory Study of NEG Coated Tubes and Flanges Keith J. Middleman ASTeC Daresbury Laboratory NEG Workshop - Catania, Sicily

  2. Outline • NEG Experimental Set-up • Calibration of Total Pressure Gauges and RGA’s • NEG Activation Procedures • NEG Coated Flanges - Results • NEG Coated Tube Monte Carlo Simulations • NEG Tube set-up (Old and New) • NEG Coated Tubes - Results • Summary • Discussion NEG Workshop - Catania, Sicily

  3. NEG Experimental Set-up • Static Expansion Method • Gas injection system consisting of H2, CO, CO2 and CH4 • High accuracy Baratron Gauge used in the injection system • Extractor gauge and RGA used in the UHV systems NEG Workshop - Catania, Sicily

  4. NEG Experimental Set-up NEG Workshop - Catania, Sicily

  5. Calibration of Extractor Gauge and RGA’s • Secondary calibrations of all Extractor gauges • Determine the correction factor for each gauge • All RGA’s are calibrated in-situ against the Extractor gauge • Importance of this procedure is crucial to the validity of the results NEG Workshop - Catania, Sicily

  6. Calibration of Extractor Gauge and RGA’s • Probability of Ionisation H2 = 0.44 CO = 1.05 CH4 = 1.6 CO2 = 1.4 Ar = 1.2 • Compared to our measurements H2 differs by a factor of 4.3 CO differs by a factor of 2.4 CH4 differs by a factor of 3.0 CO2 differs by a factor of 4.7 Ar differs by a factor of 3.3 NEG Workshop - Catania, Sicily

  7. NEG Coated Flanges NEG Workshop - Catania, Sicily

  8. Old NEG Activation Procedure During original NEG activation the main chamber was approximately 75C due to thermal conduction from the NEG flange. Monitoring RGA showed H2 and CO were the main gases. Partial pressure of CO ~ 3 x 10-8 mbar Area with transitional temperature Cooling channel TNEG TVC NEG Flange NEG Workshop - Catania, Sicily

  9. New NEG Activation Procedure • In new NEG activation the main chamber is cooled with compressed air • RGA shows partial pressure of CO is ~ 4 x 10-10 mbar. Main gas is H2 At a partial pressure of 10-9 mbar NEG Workshop - Catania, Sicily

  10. Results - NEG Coated Flanges NEG Workshop - Catania, Sicily

  11. Results - NEG Coated Flanges NEG Workshop - Catania, Sicily

  12. Results - NEG Coated Flanges NEG Workshop - Catania, Sicily

  13. Summary of NEG Coated Flanges • Initial Activation usually failed • Best results obtained after at least 2 or 3 activations • Sticking coefficients are lower than expected • Sticking coefficients and surface capacity are typically a factor of 10 less than those observed with NEG coated tubes • Best surface capacity observed was ~ 0.1 monolayer. Questions to consider: • Why do planar samples not perform as well as expected? • How crucial is the activation procedure? • Is the ratio of NEG coated surface to uncoated surface critical? • Why are sticking coefficients and surface coverage reduced by a factor 10? NEG Workshop - Catania, Sicily

  14. NEG Coated Tubes NEG Workshop - Catania, Sicily

  15. Monte-Carlo Simulations NEG Workshop - Catania, Sicily

  16. Original NEG Tube Set-up • Original measurement of NEG tube had an Extractor gauge, RGA and Inverted Magnetron gauge on top • Partial pressure of CH4 increases by least an order of magnitude when IMG switched off. CH3 and CH2 also increase due to cracking of CH4 in the Ext gauge and RGA • Combined outgassing rate of the 3 gauges and the large surface area was believed to contaminating the NEG surface during activation NEG Workshop - Catania, Sicily

  17. New NEG Tube Set-up • Extractor gauge operates with a low emission current resulting in a low outgassing rate but it offers no information on the gases present • RGA has a higher outgassing rate but provides information on what gases are present • IMG has strongest influence on pumping CH4 • Result was to use the RGA as our diagnostic tool NEG Workshop - Catania, Sicily

  18. Monte-Carlo Simulations NEG Workshop - Catania, Sicily

  19. NEG Activation Procedure NEG Workshop - Catania, Sicily

  20. Results – SAES NEG Coated Tubes • H2 Injection • Maximum ratio is 26 • Sticking coefficient of 7 x 10-3 NEG Workshop - Catania, Sicily

  21. Results – SAES NEG Coated Tubes 0.5m Tube • CO Injected • Maximum ratio we’re able to measure between RGA’s is 87.5 after gas injection of typically 10-15 minutes • RGA noise limits our sensitivity to mid 10-11 mbar range. • Sticking coefficient of >> 0.05 for CO NEG Workshop - Catania, Sicily

  22. Results – CERN 1m NEG Coated Tubes • CO Injected • Maximum ratio between RGA’s is 500 • This corresponds to a sticking coefficient of >>0.05 for CO NEG Workshop - Catania, Sicily

  23. Results – CERN 1m NEG Coated Tubes • CO2 Injected • Maximum ratio between RGA’s is 1400 • This corresponds to a sticking coefficient of >>0.1 for CO2 NEG Workshop - Catania, Sicily

  24. Results – CERN 1m NEG Coated Tubes (16mm diameter) • CO Injected • Maximum ratio between RGA’s is 9000 • RGA using SEM • This corresponds to a sticking coefficient of 0.45 for CO NEG Workshop - Catania, Sicily

  25. Summary of NEG Coated Tubes • RGA used as the main diagnostic tool on top of the NEG tube • IMG and Extractor gauge do not provide information on gas species • H2 sticking coefficient measured was close to figures quoted in the literature • Due to problems with the RGA sticking coefficients were less than expected using the pressure ratio between the top and bottom RGA’s. • This pressure ratio was only measurable after gas injection for 10-15 minutes at 10-8 mbar, we believe we were saturating the NEG tube at the lower end thus reducing the number of active sites. • Surface capacity of the NEG tube was measured at 1 monolayer as expected. • New RGA with use of the SEM shows a much larger pressure ratio across the NEG tube with the sticking coefficient of 0.45 for CO which is similar to those quoted in the literature. NEG Workshop - Catania, Sicily

  26. Discussion • Why do NEG flanges not perform as well as NEG tubes? • How crucial is activation procedure? • Is the ratio of uncoated surface to coated surface critical? • How do we minimise contamination of the surface during activation? • Any general comments and suggestions? NEG Workshop - Catania, Sicily

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