Cryostat

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.

Magnetic Resonance Imaging and Research magnet types
Cryostats used in MRI machines are designed to hold a cryogen, typically helium, in a liquid state with minimal evaporation (boil-off). The liquid helium bath is designed to keep the superconducting magnet's bobbin of superconductive wire in its superconductive state. In this state the wire has no electrical resistance and very large currents are maintained with a low power input. To maintain superconductivity, the bobbin must be kept below its transition temperature by being immersed in the liquid helium. If, for any reason, the wire becomes resistive, i.e. loses superconductivity, a condition known as a "quench", the liquid helium evaporates, instantly raising pressure within the vessel. A burst disk, usually made of carbon, is placed within the chimney or vent pipe so that during a pressure excursion, the gaseous helium can be safely vented out of the MRI suite. Modern MRI cryostats use a mechanical refrigerator (cryocooler) to re-condense the helium gas and return it to the bath, to maintain cryogenic conditions and to conserve helium.

Typically cryostats are manufactured with two vessels, one inside the other. The outer vessel is evacuated with the vacuum acting as a thermal insulator. The inner vessel contains the cryogen and is supported within the outer vessel by structures made from low-conductivity materials. An intermediate shield between the outer and inner vessels intercepts the heat radiated from the outer vessel. This heat is removed by a cryocooler. Older helium cryostats used a liquid nitrogen vessel as this radiation shield and had the liquid helium in an inner, third, vessel. Nowadays few units using multiple cryogens are made with the trend being towards 'cryogen-free' cryostats in which all heat loads are removed by cryocoolers.

Biological microtome type
In biology, cryostats are used for freezing specimens for sectioning and staining for microscopical examination. The cryostat-microtome is a type of Microtome where the device is located completely within a refrigerated chamber with glass window to enable viewing during the sectioning. To minimize unnecessary warming all necessary mechanical movements of the microtome can be achieved by hand via a wheel mounted outside the chamber. Specimens that are soft at room temperature need to be frozen before sectioning (for example at -20 degrees C).

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