Cryostat
A cryostat (from cryo meaning cold and stat meaning stable) is a device used to maintain low
Types
Closed-cycle cryostats
Closed-cycle cryostats consist of a chamber through which cold helium vapour is pumped. An external mechanical refrigerator extracts the warmer helium exhaust vapour, 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 cold plate inside a vacuum chamber which is in thermal contact with the helium vapour chamber.
Continuous-flow cryostats
Continuous-flow cryostats are cooled by liquid cryogens (typically liquid helium or nitrogen) from a storage dewar. As the cryogen boils within the cryostat, it is continuously replenished by a steady flow from the storage dewar. Temperature control of the sample within the cryostat is typically performed by controlling the flow rate of cryogen into the cryostat together with a heating wire attached to a
Owing to the scarcity of liquid helium, some laboratories have facilities to capture and recover helium as it escapes from the cryostat, although these facilities are also costly to operate.
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 minimised by shielding the bath with either cold helium vapour, or vacuum shield with walls constructed from super insulator material. The helium vapour which boils away from the bath very effectively cools thermal shields around the outside of the bath. In the older designs there may be additional liquid nitrogen bath, or several concentric layers of shielding, with gradually increasing temperatures. However, the invention of super insulator materials has made this technology obsolete.
Multistage cryostats
In order to achieve temperatures lower than liquid helium at atmospheric pressure, additional cooler stages may be added to the cryostat. Temperatures down to 1 K can be reached by attaching the cold plate to a 1-K pot, which is a container of the He-4 isotope that may be pumped to low vapor pressure via a vacuum pump. Temperatures just below 0.300 K may be achieved using He-3, the rare isotope of helium, as the working fluid in a helium pot. Temperatures down to 1 mK can be reached by employing
Applications
Magnetic resonance imaging and research magnet types
Cryostats used in
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
Cryostats are used in medicine to cut histological slides. They are usually used in a process called frozen section histology (see
See also
References
- ISBN 978-3-540-46356-6