Sorbs

I have typed up a summary of important information about sorbs:

• Charcoal as a Helium Cryopump, Version 1.0

Here is a summary of key points:

• The bonding agent does indeed have a very significant effect on the pumping abilities of a sorb. Data shows that poor thermal conductors like SS or ZrO2 cements have little or no pumping speed, while Ag-Sn braze bonds and copper cements have the best pumping speed. This would suggest that using Stycast as a bonding agent is not ideal, since it has very poor thermal conductivity (~1/5 that of 316 SS at 4K).

• Baking out sorbs is always a good idea, but heating them too much can be bad. Experiments have shown that charcoal heated to very high temperatures (~900 Celcius) in preparation had a reduced pumping speed. However, heating to moderate temperatures (<250 Celsius) seems to be fine.

• Contamination of sorbs by pump oil is significant enough that after a few weeks under vacuum, pumping speed can drop by almost an order of magnitude. Sorbs should be baked out and cooled in the presence of a cold surface onto which contaminants can condense, for example a clean metal surface at or below LN2 temperatures.

This is a first draft. There is more that I would like to add to it, but there is enough in it for now. The references are listed below. -- Nick Hutzler, 28 October 2008

• 1. Sedgley et al, Characterization of charcoals for helium cryopumping in fusion devices. J. Vac. Sci. Technol. A 5(4) (1987).
• 2. Tobin et al, Evaluation of charcoal sorbents for helium cryopumping in fusion reactors. J. Vac. Sci. Technol. A 5(1) (1987).
• 3. Sedgley et al, Development of Charcoal Sorbents for Helium Cryopumping. 10th Symposium on Fusion Engineering (1983), pp. 454–458.
• 4. Wes Campbell's Thesis, sec. 3.2.5
• 5. Hseuh et al, Cryosorption Pumping of He by Charcoal. 8th Symposium on Engineering Problems of Fusion Research (1979), pp. 1578–1562.
• 6. Hands, Recent developments in cryopumping. Vacuum 32(10/11) (1982).
• 7. Lessard, Cryogenic Adsorption of Noncondensibles in the High-Vacuum Regime. J. Vac. Sci. Technol. A 7(3) (1989).
• 8. Gurevich et al, Helium Cryopumping. Plasma Devices and Operations 1 (1990).
• 9. Sedgley et al, Cryopumping for Fusion Power Applications. 11th Symposium on Fusion Engineering (1985), pp. 958–962.
• 10. Coupland et al, Experimental Performance of a Large-Scale Cryopump. J. Vac. Sci. Technol. A 5(4) (1987).

Some other good papers that did not yet make it into the summary:

• Halama et al, Cryopumping of Hydrogen and Helium