Importance

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Abstract

A cryogenic apparatus comprises refrigeration means utilizing a two-stage expansion of compressed helium gas that circulates in a closed loop, whereby a first pumping stage can be maintained at a temperature in the range from 50.degree. K. to 80.degree. K. and a second pumping stage can be maintained at a colder temperature in the range from 10.degree. K. to 20.degree. K. The first pumping stage comprises an array of louvers of chevron configuration. These louvers enclose the second pumping stage, and serve as baffles to shield the pumping surfaces of the second stage from thermal radiation that would reduce the usable refrigeration capacity of the second stage for removing gases from a chamber to be evacuated. The chevroned baffles also protect a cryosorbent coating on the interior surfaces of the second stage from becoming iced or plugged by gas species that cryocondense at the higher temperature of the first stage. The assembly comprising the first-stage array of chevroned baffles surrounding the second-stage pumping surfaces may be disposed directly in the chamber to be evacuated, or may be mounted in a housing structure coupled to the chamber. The housing structure preferably has a radially bulging cylindrical configuration that permits substantially unimpeded conductance of gases from the chamber to the first-stage chevroned baffles.

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