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A 'Cryostat' (cryo=cold and stat=stable) is a vessel, similar in construction to a vacuum flask, or dewar used to maintain cold cryogenic temperatures.
Three common types of cryostat exist, the MRI magnet type, the research magnet type and a biological microtome type.

Contents

MRI and research magnet types

Biological instrument type

MRI and research magnet types

Cryostats used in MRI machines are designed to hold a liquid cryogen typically liquid Helium in a liquid state with minimal boil off (evaporation). The liquid helium bath is designed to keep the superconducting magnets bobbin of superconductive wire in its superconductive state. Superconductivity of the wire bobbin, maintains its magnetism after current is removed. This is dependent on the temperature of helium in a supercooled liquid state. To maintain superconductivity, the bobbin must be immersed in liquid helium. If the helium boils off to quickly, the bobbin reverts to normality. Hence the use of a cryostat.
Typically cryostats are manufactured with two vessels, one inside the other. Those used in MRI are built with an outer evacuated vessel, and an inner or, liquid helium vessel. The outer vessel serves as a thermal insulator as well as a thermal sink or heat removal device. The use of a coldhead and a cryocooler combination, removes thermal energy away from the outer vessel, thereby presenting an even cooler stage to the inner vessel maintaining the cryogenic stability of the inner vessel.
Older cryostats used liquid nitrogen for the outer vessel and liquid helium for the inner vessel. Nowadays only single cryogen units are manufactured, (purely liquid helium), with the trend moving towards cryogenless cryostats.

Biological instrument type

In biology, cryostats are used for freezing specimens for sectioning and staining for microscopical examination, a type of Microtome. specimens that are soft at room temperature need to be frozen before sectioning, (for example at -20 degrees C). Hence the use of a cryostat.