10-04-2017, 09:29 PM
Prepared by:Ch. Sridhar
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Introduction
Cryostat is a vessel, similar in construction to a vacuum flask used to maintain cold cryogenic temperatures.
Low temperatures may be obtained either by using mechanical refrigeration method or using cryogenic fluids(LHe).
Working
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.
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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.
Types
Closed cycle cryostats
Continuous flow cryostats
Bath cryostats
Closed-cycle cryostats
Closed-cycle cryostats consist of a chamber through which cold helium vapor is pumped. An external mechanical refrigerator extracts the warmer helium exhaust vapor, which is cooled and recycled. Closed-cycle cryostats consume a relatively large amount of electrical power, but need not be refilled with helium and can run continuously for an indefinite period. Objects may be cooled by attaching them to a metallic coldplate inside a vacuum chamber which is in thermal contact with the helium vapour chamber.
Continuous-flow cryostats
Continuous-flow cryostats are cooled by liquid helium from a storage dewar. As the liquid helium boils within the cryostat, it is continuously replenished by a steady flow of liquid helium from the storage dewar. No electrical power is required by continuous-flow cryostats, but large quantities of expensive liquid helium are consumed during operation. Some laboratories have facilities to capture and recover the helium as it escapes from the cryostat, although these facilities are also costly to operate. The length of time over which cooling may be maintained is dictated by the volume of the storage dewar.
Bath cryostats
Bath cryostats are similar in construction to vacuum flasks filled with liquid helium. A cold plate is placed in thermal contact with the liquid helium bath. The liquid helium may be replenished as it boils away, at intervals between a few hours and several months, depending on the volume and construction of the cryostat. The boil-off rate is minimized by shielding the bath with either cold helium vapor or with liquid nitrogen. In the former case, the helium vapor which boils away from the bath is used to cool thermal shields around the outside of the bath. The boil-off rate may be reduced further by using several concentric layers of shielding, with gradually increasing temperatures. In the latter case, the thermal shields are placed in contact with a second container filled with much cheaper liquid nitrogen.
Applications
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.
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.