Cryostat
Encyclopedia
A cryostat is a device used to maintain cold cryogenic temperatures. Low temperatures may be maintained within a cryostat by using various refrigeration methods, most commonly using cryogenic fluid bath such as liquid helium
. Hence it is usually assembled into a vessel, similar in construction to a vacuum flask
or Dewar
. Cryostats have numerous applications within science, engineering, and medicine.
s filled with liquid helium. A coldplate 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 so-called 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 obsoleted this technology.
Temperatures down to 1K can be reached by attaching the coldplate to 1-K pot
, which is a container of He-4 isotope which is connected to vacuum pump.
Temperatures down to 1mK can be reached by employing dilution refrigerator
or dry dilution refrigerator
typically in addition to the main stage and 1K pot.
Temperatures below that can be reached using magnetic refrigeration
.
, 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.
Cryostat are used in medicine to cut histological slides. They are usually used in a process called frozen section histology (see Frozen section procedure
). The cryostat is essentially an ultrafine "deli-slicer", called a microtome
, placed in a freezer. The cryostat is usually a stationary upright freezer, with an external wheel for rotating the microtome. The temperature can be varied, depending on the tissue being cut - usually from minus 20 to minus 30 degree Celsius. The freezer is either powered by electricity, or by a refrigerant like liquid nitrogen. Small portable cryostats are available and can run off generators or vehicle inverters. To minimize unnecessary warming all necessary mechanical movements of the microtome can be achieved by hand via a wheel mounted outside the chamber. Newer microtomes have electric push button advancement of the tissue. The precision of the cutting is in micrometres. Tissue are sectioned as thin as 1 micrometre. Usual histology slides are mounted with a thickness of about 7 micrometres. Specimens that are soft at room temperature are mounted on a cutting medium (often made of egg white) on a metal "chuck", and frozen to cutting temperature (for example at -20 degrees C). Once frozen, the specimen on the chuck is mounted on the microtome. The crank is rotated and the specimen advances toward the cutting blade. Once the specimen is cut to a satisfactory quality, it is mounted on a warm (room temperature) clear glass slide, where it will instantaneously melt and adhere. The glass slide and specimen is dried with a dryer or air dried, and stained. The entire process from mounting to reading the slide takes from 10 to 20 minutes, allowing rapid diagnosis in the operating room, for the surgical excision of cancer. The cryostat can be used to cut histology and tissue slide outside of medicine, but the quality of the section is poor compared to standard fixed section wax mounted histology.
Liquid helium
Helium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atmosphere is approximately 0.125 g/mL Helium-4 was first liquefied...
. Hence it is usually assembled into a vessel, similar in construction to a vacuum flask
Vacuum flask
A vacuum flask is an insulating storage vessel which keeps its contents hotter or cooler than its surroundings. Invented by Sir James Dewar in 1892, the vacuum flask consists of two flasks, placed one within the other and joined at the neck...
or Dewar
Vacuum flask
A vacuum flask is an insulating storage vessel which keeps its contents hotter or cooler than its surroundings. Invented by Sir James Dewar in 1892, the vacuum flask consists of two flasks, placed one within the other and joined at the neck...
. Cryostats have numerous applications within science, engineering, and medicine.
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 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 flaskVacuum flask
A vacuum flask is an insulating storage vessel which keeps its contents hotter or cooler than its surroundings. Invented by Sir James Dewar in 1892, the vacuum flask consists of two flasks, placed one within the other and joined at the neck...
s filled with liquid helium. A coldplate 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 so-called 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 obsoleted this technology.
Multistage cryostats
In order to achieve temperature lower than liquid helium additional cooler stages may be added to the cryostat.Temperatures down to 1K can be reached by attaching the coldplate to 1-K pot
1-K pot
A 1-K pot is a cryogenic device used to attain temperatures down to approximately 1 kelvin.The 1-K pot is a small vessel in a cryogenic system that is filled with liquid helium. Usually it is a few cubic centimeters in size with a pickup-tube extending into the primary liquid helium bath of the...
, which is a container of He-4 isotope which is connected to vacuum pump.
Temperatures down to 1mK can be reached by employing dilution refrigerator
Dilution refrigerator
A dilution refrigerator is a cryogenic device first proposed by Heinz London. Its refrigeration process uses a mixture of two isotopes of helium: helium-3 and helium-4...
or dry dilution refrigerator
Dry dilution refrigerator
A dry dilution refrigerator is a type of dilution refrigerator that does not require the use of any external cryogens, in particular liquid helium, to operate....
typically in addition to the main stage and 1K pot.
Temperatures below that can be reached using magnetic refrigeration
Magnetic refrigeration
Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique can be used to attain extremely low temperatures , as well as the ranges used in common refrigerators, depending on the design of the system.The effect was first observed by the German physicist Emil...
.
Magnetic Resonance Imaging and Research magnet types
Cryostats used in MRI machines are designed to hold a cryogen, typically heliumHelium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, in a liquid state with minimal evaporation (boil-off). The liquid helium bath is designed to keep the superconducting magnet
Superconducting magnet
A superconducting magnet is an electromagnet made from coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. In its superconducting state the wire can conduct much larger electric currents than ordinary wire, creating intense magnetic fields...
'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
Transition temperature
Transition temperature is the temperature at which a material changes from one crystal state to another. There are seven crystal systems and every crystallized material exists in one of them....
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
Quenching (disambiguation)
A quench is a rapid cooling.Quench or quenching may also refer to:* Quench , an album by The Beautiful South* Quench , a British rock band...
", 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
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...
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
Frozen section procedure
The frozen section procedure is a pathological laboratory procedure to perform rapid microscopic analysis of a specimen. It is used most often in oncological surgery. The technical name for this procedure is cryosection....
). The cryostat is essentially an ultrafine "deli-slicer", called a microtome
Microtome
A microtome is a sectioning instrument that allows for the cutting of extremely thin slices of material, known as sections. Microtomes are an important device in microscopy preparation, allowing for the preparation of samples for observation under transmitted light or electron radiation...
, placed in a freezer. The cryostat is usually a stationary upright freezer, with an external wheel for rotating the microtome. The temperature can be varied, depending on the tissue being cut - usually from minus 20 to minus 30 degree Celsius. The freezer is either powered by electricity, or by a refrigerant like liquid nitrogen. Small portable cryostats are available and can run off generators or vehicle inverters. To minimize unnecessary warming all necessary mechanical movements of the microtome can be achieved by hand via a wheel mounted outside the chamber. Newer microtomes have electric push button advancement of the tissue. The precision of the cutting is in micrometres. Tissue are sectioned as thin as 1 micrometre. Usual histology slides are mounted with a thickness of about 7 micrometres. Specimens that are soft at room temperature are mounted on a cutting medium (often made of egg white) on a metal "chuck", and frozen to cutting temperature (for example at -20 degrees C). Once frozen, the specimen on the chuck is mounted on the microtome. The crank is rotated and the specimen advances toward the cutting blade. Once the specimen is cut to a satisfactory quality, it is mounted on a warm (room temperature) clear glass slide, where it will instantaneously melt and adhere. The glass slide and specimen is dried with a dryer or air dried, and stained. The entire process from mounting to reading the slide takes from 10 to 20 minutes, allowing rapid diagnosis in the operating room, for the surgical excision of cancer. The cryostat can be used to cut histology and tissue slide outside of medicine, but the quality of the section is poor compared to standard fixed section wax mounted histology.