Dry dilution refrigerator
Encyclopedia
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.
In the traditional design of a dilution refrigerator
(Figure 1a) a liquid helium
bath is used. It mainly serves the following purposes:
In a dry dilution refrigerator a cryocooler
is integrated in the design of the refrigerator in order to replace the liquid helium
bath (see Figure 1 below). The first experimental machines were built in the 1990s, when (commercial) cryocoolers
became available, capable of reaching a temperature lower than that of liquid helium
and having sufficient cooling power (on the order of 1 watt at 4.2 K). At present the most commonly used cryocooler is a pulse tube cooler
.
The dry dilution refrigerator design shown in Figure 1b is the easiest to realize from an engineering point of view, but not the easiest in operation. The design incorporates an inner vacuum
can, which is used to initially precool the machine from room temperature down to the base temperature of the pulse tube cooler
(using heat-exchange gas). However, it implies that every time the refrigerator is cooled down, a vacuum
seal that holds at cryogenic temperatures needs to be made. It also implies complications (low temperature vacuum feed-through) in the experimental wiring.
A design as shown in Figure 1c is possible, but technically more demanding to realize. It requires the use of heat switches, which are necessary for precooling. The heat switches and thermal radiation
shields are not shown in the figure. This type of refrigerator is in the SCUBA-2 sub-millimetre camera on the James Clerk Maxwell Telescope
.
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...
that does not require the use of any external cryogens, in particular liquid helium
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...
, to operate.
In the traditional design of a 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...
(Figure 1a) a liquid helium
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...
bath is used. It mainly serves the following purposes:
- It initially precools the unit from room temperature to 4.2 K.
- It acts as a thermal radiationThermal radiationThermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
shield. - It cools the returning incoming 3HeHelium-3Helium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...
and helps to condense it.
In a dry dilution refrigerator a cryocooler
Cryocooler
Cryocoolers are the devices used to reach cryogenic temperatures by cycling certain gases.A cryostat is likely to be used to reach and/or maintain similar conditions or keep some environment in cryogenic stasis...
is integrated in the design of the refrigerator in order to replace the liquid helium
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...
bath (see Figure 1 below). The first experimental machines were built in the 1990s, when (commercial) cryocoolers
Cryocooler
Cryocoolers are the devices used to reach cryogenic temperatures by cycling certain gases.A cryostat is likely to be used to reach and/or maintain similar conditions or keep some environment in cryogenic stasis...
became available, capable of reaching a temperature lower than that of liquid helium
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...
and having sufficient cooling power (on the order of 1 watt at 4.2 K). At present the most commonly used cryocooler is a pulse tube cooler
Pulse tube refrigerator
The pulse tube refrigerator or pulse tube cryocooler is a developing technology that emerged largely in the early 1980s with a series of other innovations in the broader field of thermoacoustics...
.
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
can, which is used to initially precool the machine from room temperature down to the base temperature of the pulse tube cooler
Pulse tube refrigerator
The pulse tube refrigerator or pulse tube cryocooler is a developing technology that emerged largely in the early 1980s with a series of other innovations in the broader field of thermoacoustics...
(using heat-exchange gas). However, it implies that every time the refrigerator is cooled down, a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
seal that holds at cryogenic temperatures needs to be made. It also implies complications (low temperature vacuum feed-through) in the experimental wiring.
A design as shown in Figure 1c is possible, but technically more demanding to realize. It requires the use of heat switches, which are necessary for precooling. The heat switches and thermal radiation
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
shields are not shown in the figure. This type of refrigerator is in the SCUBA-2 sub-millimetre camera on the James Clerk Maxwell Telescope
James Clerk Maxwell Telescope
The James Clerk Maxwell Telescope is a submillimetre-wavelength telescope at Mauna Kea Observatory in Hawaii. Its primary mirror is 15 metres across: it is the largest astronomical telescope that operates in submillimetre wavelengths of the electromagnetic spectrum...
.