Cryostat Dewars

Abstract

There are two distinct ways of designing a cryostat for work at very low temperatures, which we shall take as meaning liquid-helium temperature, leaving the reader to make the appropriate simplifications for himself if he is working at higher temperatures. For straightforward experiments there may be a case for enclosing a container for liquid helium in a radiation shield and an outer vessel within which a high vacuum can be maintained. (See Croft and Thomas¹ for a small-scale cryostat built on this principle.) However, most experimental work at low temperatures is so complicated that it is usual to use a separate metal dewar vessel which can be raised up round the complete experimental assembly so that the latter is readily accessible when setting up experiments. The design requirements of such vessels are basically similar to those for the storage vessels described in Chapter 2, but low evaporation rate has to take second place to demands set by the experimental apparatus and its environment. For example, the neck tube has to be wide enough to pass over whatever is to be submerged in liquid helium and its length is usually determined by the distance between the floor and the ceiling of the laboratory — unless it happens that a pit can be made in the floor. (This point should be remembered in laboratory design; a 10-ft ceiling height can be a serious limitation.)