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. Cryogenics is often used incorrectly to refer to Cryonics, cryopreserving humans or animals

Cryostat types

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). Cryogenics is often used incorrectly to refer to Cryonics, cryopreserving humans or animals Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical The liquid helium bath is designed to keep the superconducting magnet's bobbin of superconductive wire in its superconductive state. A superconducting magnet is an Electromagnet that is built using superconducting coils Superconductivity is a phenomenon occurring in certain Materials generally at very low Temperatures characterized by exactly zero electrical resistance 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. Transition temperature is the temperature at which material changes from one Crystal to another 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. A Stirling engine is a closed-cycle regenerative Heat engine with a Gaseous working fluid

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. Liquid nitrogen (liquid density at the Triple point is 0707 g/mL is the liquid produced industrially in large quantities by Fractional distillation of 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, a type of Microtome. A microtome is a mechanical instrument used to cut biological specimens into transparent thin sections for microscopic examination Specimens that are soft at room temperature need to be frozen before sectioning (for example at -20 degrees C).

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