Cryopump

A cryopump is a vacuum pump that traps gases and vapours by condensing them on a cold surface. They are only effective on some gases, depending on the freezing and boiling points of the gas relative to the cryopump's temperature. They are sometimes used to block particular contaminants, for example in front of a diffusion pump to trap backstreaming oil, or in front of a McLeod gauge to keep out water. In this function, they are called a cryotrap or cold trap, even though the physical mechanism is the same as for a cryopump. Cryotrapping can also refer to a somewhat different effect, where molecules will increase their residence time on a cold surface without actually freezing. There is a delay between the molecule impinging on the surface and rebounding from it. Kinetic energy will have been lost, the molecules slow down. For example, hydrogen will not condense at 8 kelvin, but it can be cryotrapped. This effectively traps molecules for an extended period and thereby removes them from the vacuum environment just like cryopumping.

Operation

Cryopumps are commonly cooled by compressed helium though they may also use dry ice, liquid nitrogen, or stand-alone versions may include a built-in cryocooler. Baffles are often attached to the cold head to expand the surface area available for condensation, but they also increase the radiative heat uptake of the cryopump. Over time, the surface eventually saturates with condensate and the pumping speed gradually drops to zero. It will hold the trapped gases as long as it remains cold, but it will not condense fresh gases from leaks or backstreaming until it is regenerated. Saturation happens very quickly in low vacuums, so cryopumps are usually only used in high or ultrahigh vacuum systems.

Regeneration of a cryopump is the process of evaporating the trapped gases. This can be done at room temperature and pressure, or the process can be made more complete by exposure to vacuum and faster by elevated temperatures. Best practice is to heat the whole chamber under vacuum to the highest temperature allowed by the materials, allow time for outgassing products to be exhausted by the mechanical pumps, and then cool and use the cryopump without breaking the vacuum.

Some cryopumps have multiple stages at various low temperatures, with the outer stages shielding the coldest inner stages. The outer stages condense high boiling point gases such as water and oil, thus saving the surface area and refrigeration capacity of the inner stages for lower boiling point gases such as nitrogen. As cooling temperatures decrease when using dry ice, liquid nitrogen, then compressed helium, lower molecular-weight gases can be trapped. Trapping nitrogen, helium, and hydrogen requires extremely low temperatures (~10K) and large surface area as described below. Even at this temperature, the lighter gases helium and hydrogen have very low trapping efficiency and are the predominant molecules in ultra-high vacuum systems.

Cryopumps are often combined with sorption pumps by coating the cold head with highly adsorbing materials such as activated charcoal or a zeolite. As the sorbent saturates, the effectiveness of a sorption pump decreases, but can be recharged by heating the zeolite material (preferably under conditions of low pressure) to outgas it. The breakdown temperature of the zeolite material's porous structure may limit the maximum temperature that it may be heated to for regeneration.

Sorption pumps are a type of cryopump that is often used as roughing pumps to reduce pressures from the range of atmospheric to on the order of 0.1 Pa (10^-3 Torr), while lower pressures are achieved using a finishing pump (see vacuum).

References

• Van Atta, C. M.; M. Hablanian (1991) [1990]. "Vacuums and Vacuum Technology". In Ed. by Rita G. Lerner and George L. Trigg. Encyclopedia of Physics (2nd edition ed.). New York: VCH Publisher. pp. 1330–1334. ISBN 0-89573-752-3. 
• Strong, John (1938). Procedures in Experimental Physics. Bradley, IL: Lindsay Publications. , Chapter 3

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