US20170059214A1 - Compact adiabatic demagnetization refrigeration stage with integral gas-gap heat switch - Google Patents
Compact adiabatic demagnetization refrigeration stage with integral gas-gap heat switch Download PDFInfo
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- US20170059214A1 US20170059214A1 US14/843,711 US201514843711A US2017059214A1 US 20170059214 A1 US20170059214 A1 US 20170059214A1 US 201514843711 A US201514843711 A US 201514843711A US 2017059214 A1 US2017059214 A1 US 2017059214A1
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
- F25B21/00—Machines, plants or systems, using electric or magnetic effects
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
- F25B2321/00—Details of machines, plants or systems, using electric or magnetic effects
- F25B2321/002—Details of machines, plants or systems, using electric or magnetic effects by using magneto-caloric effects
- F25B2321/0023—Details of machines, plants or systems, using electric or magnetic effects by using magneto-caloric effects with modulation, influencing or enhancing an existing magnetic field
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
- Y02B30/00—Energy efficient heating, ventilation or air conditioning [HVAC]
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Abstract
An adiabatic demagnetization refrigeration stage includes a salt pill, a magnet surrounding the salt pill, and a gas-gap heat switch interposed between the salt pill and the magnet. A method of operating an adiabatic demagnetization refrigeration stage includes using a magnet surrounding a salt pill to apply an increasing magnetic field to the salt pill, producing a gas to activate a gas gap heat switch interposed between the magnet and the salt pill to provide a path for heat flow from the salt pill through the magnet to a heat sink, and decreasing the magnetic field applied to the salt pill while adsorbing the gas to de-activate the gas gap heat switch to cool the salt pill to a lower temperature and cool an object attached to a cold tip extending from the salt pill.
Description
- The invention described herein was made by one or more employees of the United States Government, and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.
- The disclosed embodiments generally relate to refrigeration and more particularly to an adiabatic demagnetization refrigeration system.
- Adiabatic demagnetization is a very robust technique for cryogenic cooling, easily producing very low temperatures of approximately 100 milliKelvin and below from relatively high heat sink temperatures, for example, approximately 1-5 Kelvin. Other cooling techniques include dilution refrigeration, helium-3 and helium-4 refrigerators, as well as some more exotic superfluid stirling and Superconductor Insulator Normal metal Insulator Superconductor (SINIS) coolers for producing cooling over some part of these ranges, however, Adiabatic Demagnetization Refrigeration (ADR) covers a wider temperature range, works well in zero-gravity and on the ground, is more efficient, and—having no moving parts—is more reliable.
- ADR utilizes the magnetocaloric effect, which is a phenomenon in which certain materials warm or cool as they are exposed to increasing or decreasing magnetic fields. In paramagnetic materials, the effect originates in the interaction of an external magnetic field with the magnetic moment of unpaired outer-shell electrons of the paramagnetic material, and results in the entropy of the material having a strong dependence on magnetic field and temperature. An ADR stage produces cooling (or heating) by the interaction of a magnetic field with the magnetic spins in a paramagnetic salt. Magnetizing the salt produces heating, and demagnetizing the salt produces cooling.
- A typical ADR stage may include: a capsule of a solid paramagnetic material refrigerant, commonly referred to as a salt pill, an electromagnet, a cold tip, a heat sink and a heat switch. The salt pill may be suspended within the electromagnet without making direct contact using a suspension system providing structural support with minimal thermal conductance. In conventional implementations, the heat switch is external to the magnet and salt pill assembly and interposed between the heat sink and salt pill. A thermal attachment to the heat sink and salt pill is generally implemented with a thermal strap on one or both ends.
- The refrigeration cycle generally includes the following steps: First, the salt pill starts at zero field and is magnetized, causing it to warm up. Second, when its temperature exceeds that of the heat sink, the heat switch is powered into the state. Third, the salt pill continues to be magnetized, generating heat which flows to the heat sink. This continues until full field is reached, which is strong enough to significantly align the spins and suppress the entropy of the salt. In some embodiments, full field may be in a range of approximately 1-4 Tesla. Fourth, at full magnetic field, the heat switch is deactivated to thermally isolate the salt from the heat sink. Fifth, the salt is demagnetized to cool it to the desired operating temperature. In general, the salt will then be receiving heat from components parts. The heat is absorbed and operating temperature is maintained by slowly demagnetizing the salt at a controlled rate. Heat can continue to be absorbed until the magnetic field is reduced to zero, at which point the ADR has run out of cooling capacity.
- In conventional ADR configurations, the heat flow path from the cold tip to the heat sink involves a number of components whose design is a compromise between keeping mass and size small, and achieving a desired thermal conductance. When heat is being absorbed, it flows from the cold tip into the salt pill via a thermal bus that runs the length of the salt pill and distributes the heat to the refrigerant. When the ADR is being recycled, heat flows back out along the length of the salt pill via this thermal bus, through a thermal strap, through the heat switch, optionally through another thermal strap, and to the heat sink. Because all of the conductors involved (the salt pill, thermal bus, the thermal straps and the heat switch) tend to be long and narrow, it is typically very challenging to achieve sufficiently high thermal conductance for the heat rejection path.
- The size and shape of the magnet and salt pill assembly and the heat switch generally result in a relatively large volume for the ADR assembly in comparison to an implementation where the components are integrated. This is significant for most cryogenic systems because a larger experiment volume translates to a larger cryostat volume, with larger heat loads on the liquid cryogens or mechanical cryocooler. This in turn requires an increased volume of cryogens, or higher power cryocoolers. A larger cryostat size also results in larger, bulkier vacuum vessels which become progressively harder to manipulate.
- The mass of the external heat switch and thermal straps individually is typically larger than an implementation where they are combined. In addition, the externally mounted heat switch and straps are more subject to damage and the externally mounted heat switch adds to the operational complexity of the system.
- It would be advantageous to provide Adiabatic Demagnetization Refrigeration assemblies and techniques that overcome these and other disadvantages.
- The disclosed embodiments are directed to a unique arrangement of adiabatic refrigeration components that reduces the heat flow path to a short, broad flow path with much higher thermal conductance, resulting in better thermal performance of the ADR and faster recycle times.
- According to at least one of the disclosed embodiments, an adiabatic demagnetization refrigeration stage includes a salt pill, a magnet surrounding the salt pill, and a gas-gap heat switch interposed between the salt pill and the magnet.
- The salt pill may be constructed of a paramagnetic material refrigerant in a helium atmosphere enclosed in a copper container.
- The refrigeration stage may include a port in fluid communication with both the gas gap heat switch and a gas source for providing a gas and charge pressure to the gas gap heat switch.
- The refrigeration stage may include a passive gas gap heat switch which may further include a getter thermally coupled to the salt pill and in fluid communication with the gas-gap heat switch.
- The refrigeration stage may include an active gas gap heat switch which may further include a getter in fluid communication with the gas-gap heat switch and having an independent temperature control.
- The gas gap heat switch may include an outer surface of the salt pill substantially concentric with an inner surface of the magnet and a gas confined there between.
- The gas gap heat switch may include radially extending fins of an outer surface of the salt pill interleaved with radially extending fins of an inner surface of the magnet and a gas confined there between.
- The refrigeration stage may include a structure supporting the salt pill within the surrounding magnet, which may further include a standoff positioned around a bellows attached to a first end of the salt pill to constrain axial and lateral motion.
- The refrigeration stage may include a structure supporting the salt pill within the surrounding magnet, which may further include a hub circumscribing a second end of the salt pill and a plurality of stays connecting the hub and a concentric cylindrical sleeve.
- According to at least one other disclosed embodiment, a method of operating an adiabatic demagnetization refrigeration stage includes using a magnet surrounding a salt pill to apply an increasing magnetic field to the salt pill, producing a gas to activate a gas gap heat switch interposed between the magnet and the salt pill to provide a path for heat flow from the salt pill through the magnet to a heat sink, and decreasing the magnetic field applied to the salt pill while adsorbing the gas to de-activate the gas gap heat switch to cool the salt pill to a lower temperature and cool an object attached to a cold tip extending from the salt pill.
- The method may include regulating the magnetic field to maintain the salt pill at the lower temperature.
- The method may also include providing the gas and a charge pressure to the gas gap heat switch using a port in fluid communication with both the gap and a gas source.
- The method may further include producing the gas using a getter thermally coupled to the salt pill and in fluid communication with the gas-gap heat switch.
- The method may additionally include producing the gas using a getter in fluid communication with the gas-gap heat switch and having an independent temperature control.
- The method may likewise include providing an increased surface area for heat transfer through the gas gap heat switch by interleaving radially extending fins of an outer surface of the salt pill with radially extending fins of an inner surface of the magnet.
- The method may similarly include supporting the salt pill within the surrounding magnet using a standoff positioned around a bellows attached to a first end of the salt pill to constrain axial and lateral motion.
- The method may still further include supporting the salt pill within the surrounding magnet using a hub circumscribing a second end of the salt pill and a plurality of stays connecting the hub and a concentric cylindrical sleeve.
- The foregoing aspects and other features of the embodiments are explained in the following description, taken in connection with the accompanying drawings, wherein:
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FIG. 1 shows a cross section of an embodiment of an ADR stage according to the disclosed embodiments; -
FIG. 2 shows a schematic cross section of a magnet; -
FIG. 3 shows an exemplary embodiment of a gas gap heat switch; -
FIGS. 4A-4C illustrate further embodiments of the gas gap heat switch; -
FIG. 5 illustrates an exemplary embodiment an ADR stage having an active gas gap heat switch; -
FIG. 6 shows exemplary structures for supporting a salt pill; -
FIGS. 7A-7C illustrate an example of a bottom salt pill suspension; and -
FIGS. 8A-8C show exemplary stays of the bottom suspension. - The disclosed embodiments are directed to a compact, self-contained Adiabatic Demagnetization Refrigeration (ADR) stage that incorporates a gas-gap heat switch into the internal structure. This results in a reduction in the size, mass and complexity of the refrigeration stage. The unit may have two thermal interfaces: a cold tip, and a heat rejection plate that also serves as the mounting surface. The latter feature allows the unit to be mounted directly to a heat sink, which could be a liquid helium tank or a mechanical cryocooler.
- The disclosed embodiments include the ability to vary gas gap heat switch gas and gas-gap heat switch charge pressure to selectively change the transition temperature between heat conducting and non-conducting states. Thus, the gas gap heat switch may be adjusted to turn on and off at different temperatures. The internal gas gap heat switch is tuned or controlled so that the magnetic refrigerant in the salt pill positioned in a core of a magnet becomes thermally coupled to the heat sink as it is magnetized and it warms above the heat sink temperature. Continued magnetization charges the refrigerator as heat flows to the heat sink. Subsequent demagnetization causes the refrigerant to cool while the gas gap heat switch is opened and the refrigerant is thermally decoupled from the heat sink. With proper choice of refrigerant, the unit can operate at temperatures well below 1 Kelvin, using heat sinks of up to 5 K.
- In embodiments with a passive gas gap heat switch, a simple power control may be used to ramp the magnet current up and down with no power control required for the gas gap heat switch. In embodiments with an active gas gap heat switch, a simple power control may be used to ramp the magnet current up and down and to apply or remove power for the active gas gap heat switch.
- The passive and active gas gap heat switches may be implemented with interleaved fins providing increased surface area and thermal conductance.
- The disclosed embodiments also include suspension components that constrain the movement of the salt pill within the magnet and minimize heat leak at cold temperatures.
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FIG. 1 shows a cross section of an embodiment of anADR stage 100 according to the present disclosure. TheADR stage 100 includes asalt pill 130, amagnet 110 with acenter cavity 155, a gas-gap heat switch 115 interposed between thesalt pill 130 and themagnet 110, acold tip 120 extending from thesalt pill 130, and a bottom plateheat sink mount 125 for mounting to aheat sink 127. - The
salt pill 130 includes a volume ofrefrigerant 105, for example, a single crystal or polycrystalline gadolinium compound, chromium potassium alum, ferric ammonium alum, or any other suitable refrigerant. The refrigerant 105 may be encapsulated in a helium gas atmosphere, for example, helium-3 or helium-4, depending on the temperature range of operation. Anenclosure 143 may be used to encapsulate the refrigerant 105, and in some embodiments, may be constructed of thermally conductive material such as copper or other suitable substance. For purposes of this disclosure, thesalt pill 130 refers to theenclosure 143 and the refrigerant 105. - A thermally conductive rod extends from a
first end 133 of thesalt pill 130 to serve as the ADR'scold tip 120. The thermallyconductive rod 120 may comprise copper or other suitable thermally conductive material. Asecond end 137 of thesalt pill 130 may include acap 135 for filling and sealing in the helium gas, as well as anattachment 140 for abottom suspension 145. - In this exemplary embodiment, the gas
gap heat switch 115 is a passive gas gap heat switch where the transition temperature between heat conducting and non-conducting states is dependent on the type of gas and the number of atoms of gas within the gasgap heat switch 115. Agetter 150 may be bonded onto the first end of thesalt pill 130 for effecting the transition between heat conducting and non-conducting states. In an exemplary embodiment, the getter of the passive gas gap heat switch may comprise a quantity of sintered stainless steel powder. -
FIG. 2 shows a schematic cross section of themagnet 110. In at least one embodiment, the magnet includes one ormore conductors 205, such as small diameter NbTi wire, wound onto amandrel 210. Other exemplary conductors may include aluminum, Nb3Sn, Nb3Al, MgB2, or high temperature superconductors such as YBCO. Themandrel 210 includes acenter cavity 215 for enclosing thesalt pill 130 and the gas-gap heat switch 115. Themandrel 210 is generally constructed of a heat conductive material, for example, copper. - Returning to
FIG. 1 , thesalt pill 130 may be suspended within thecenter cavity 155 of themagnet 110 to maintain a stable gap between an outer diameter of thesalt pill 130 and an inner diameter of themagnet 110. The stable gap is used to implement the gasgap heat switch 115. - The volume inside the
center cavity 155 of themagnet 110 is hermetically sealed to retain the gas used to implement the gas gap heat switch. A bellows 160 may be used as a seal between thecold tip 120 and themagnet mandrel 210. The bellows may be constructed of stainless steel and one or more solder or brazeseals 195 may be made between thebellows 160 and thecold tip 120. Thebellows 160 may also be sealed to atop plate 165 which may be further sealed to themagnet mandrel 210 by, for example, anindium seal 185. Abottom plate 170 may also be sealed to the magnet mandrel using anindium seal 190. Thebottom plate 170 may include a central post 175 extending to theheat sink mount 125. Thetop plate 165 andbottom plate 170 may be constructed of a suitable thermally conductive material, for example copper. - A
port 180 may be provided for evacuating the sealedcenter cavity 155 and for connecting to agas source 182, for example, helium-3 or helium-4, for the gas-gap heat switch 115. The transition temperature between heat conducting and non-conducting states is dependent on the type of gas and the number of atoms of gas within the gasgap heat switch 115, and the port provides the ability to vary the gas gap heat switch gas and the gas-gap heat switch charge pressure to selectively change the transition temperature. - A
magnetic shield 183 may enclose the ADR system to prevent fringing fields. In at least one embodiment, the magnetic shield may be made of a ferromagnetic material or any other suitable magnetic shielding material. In some embodiments, the magnetic shield may be assembled from a number of component pieces. Thebottom plate 170, central post 175, andheat sink mount 125 are shaped to provide a thermal path from themagnet 110 to a heat sink, and for allowing themagnetic shield 183 shield enclose themagnet 110 to limit fringing fields, for example, while the ADR stage is at full field. -
FIG. 3 shows a top view of an exemplary implementation of the gas gap heat switch. InFIG. 3 the gasgap heat switch 300 includes theinner surface 305 of themandrel 210, theouter surface 310 of thesalt pill 130, agas 315, such as helium-3, confined in agap 320 between theinner surface 305 of themandrel 210 and theouter surface 310 of thesalt pill 130. Thegetter 150 is shown for reference and may be positioned elsewhere in some embodiments. In this embodiment, theinner surface 305 of themandrel 210 and theouter surface 310 of thesalt pill 130 are substantially round and substantially concentric with each other. -
FIGS. 4A-4C illustrate other embodiments of themandrel 405 and thesalt pill 410 having an increased surface area for heat transfer through the gas gap heat switch between the mandrel and the salt pill. The increased surface area provides an increase in the thermal conductance of the gas gap heat switch. In this embodiment, themandrel 405 as shown inFIG. 4A , and thesalt pill 410 as shown inFIG. 4B , both include a splined configuration in which themandrel 405 hasradial fins 415 and thesalt pill 410 also hasradial fins 420 positioned to interleave with each other. The interleavedradial fins FIG. 4C shows a top view of themandrel 405 and thesalt pill 410 assembled together with the interleavedradial fins fasteners 425 are radially spaced around themandrel 405 for fastening thetop plate 165 to themandrel 405. In some embodiments, the gasgap heat switch 430 may be implemented with an odd number of fins which is one less than twice the number offasteners 425. This number of fins allows thesalt pill 410 to be rotated through different radial fastening positions to find the best spacing for the fins. -
FIG. 5 illustrates an embodiment of anADR stage 100 having an active gas gap heat switch 515. In this embodiment, thegetter 505 may be external to thecenter cavity 155 and includesgetter material 510, for example charcoal, and anindependent heating mechanism 520, for example a resistive heater. In this exemplary embodiment,port 180 may be used for evacuating the sealedcenter cavity 155 and filling it with gas, for example, helium-3 or helium-4, for the gas-gap heat switch 115. The transition temperature between heat conducting and non-conducting states is controlled by applying or removing power from theheating mechanism 520, providing a selectable temperature range where the heat conducting and non-conducting states occur. - As shown in
FIG. 6 , the disclosed embodiments include structures for supporting or suspending thesalt pill 130 within themagnet 110. At thefirst end 133 of thesalt pill 130, atop suspension 600 may include astandoff 605 and bellows 160. The bellows 160 provides a rotational constraint on salt pill motion, at least by way of the one or more solder or brazeseals 195 between thebellows 160 and thecold tip 120. Thestandoff 605, which may be constructed of a temperature stable material, for example, a polyimide-based plastic such as Vespel®, may be positioned around thebellows 160 and attached to themagnetic shield 183 as well as thetop plate 165. With thebellows 160 andstandoff 605, axial motion, as well as motion in the two lateral degrees of freedom is constrained. -
FIGS. 7A-7C illustrate an example of thebottom suspension 145 utilized at thesecond end 137 of thesalt pill 130. In this embodiment, thebottom suspension 145 includes a wheel and hub type suspension including awheel 610 and ahub 615 connected by a number of stays 620. While thebottom suspension 145 is shown as having a wheel and hub configuration, it should be understood that any suitable suspension may be used so long as the position of thesalt pill 130 is controlled within thecenter cavity 155. As shown inFIG. 6 , the wheel portion may be implemented by acylindrical protrusion 610 integral to thebottom plate 170. In other embodiments, the wheel portion may be separately constructed and may be made of other materials so long as the wheel portion is capable of functioning as described herein. - Turning to
FIG. 7A , the wheel portion of thebottom suspension 145 may includecylindrical sleeve 710.Cylindrical sleeve 710 may include a plurality of throughholes 725 which may extend radially through the sleeve and may be equally spaced around the perimeter of thesleeve 710. Thecylindrical sleeve 710 may also include agroove 730 around its outside circumference in which the through holes may be located. Anexemplary hub 715 is shown inFIG. 7B .Hub 715 includes agroove 735 around its outside circumference and has aninner diameter 740 sized to receive thebottom suspension attachment 140 of thesalt pill 130.FIG. 7C shows an exemplary embodiment of thebottom suspension 145 showing the position of thecylindrical sleeve 710,hub 715, and stays 720 as assembled together. - As shown in
FIGS. 8A-8C , the stays may be formed byindividual loops cylindrical sleeve 710 in thegroove 730, threaded through one of theholes 725 and positioned in thegroove 735 around the outside circumference of thehub 715. Theloops hub 715 concentrically with thecylindrical sleeve 710. In some embodiments, the loops may be constructed of Kevlar®, however, it should be understood that any other suitable material may be used. When thesalt pill 130 is assembled within thecenter cavity 155 of themagnet 110, thebottom suspension attachment 140 engages the hub to provide constraint in two degrees of freedom, securing thesalt pill 130 within an inner diameter of themagnet 110. - The materials and construction techniques of the
top suspension 600 andbottom suspension 145 significantly limit any parasitic heat leak into thesalt pill 130. - After assembly, the
center cavity 155 of the ADR stage is evacuated and filled with a controlled amount of helium gas, for example, helium-3 or helium-4. In embodiments utilizing a passive gas gap heat switch, the transition temperature between heat conducting and non-conducting states is dependent on the type of gas and the number of atoms of gas within the gasgap heat switch 115. In embodiments utilizing an active gas gap heat switch, the transition temperature range where the heat conducting and non-conducting states occur is dependent on when and how much power is applied to the heating mechanism 520 (FIG. 5 ). The target on and off temperatures are generally at or slightly above the expected heat sink temperature. - After the gas gap heat switch is filled with gas, the
ADR stage 100 equilibrates to the heat sink temperature. Current may be increasingly applied to themagnet 110 causing thesalt pill 130 to increase in temperature. In passive gas gap heat switch embodiments, the temperature increase of the salt pill causes the gasgap heat switch 115 to turn fully on by releasing helium gas from thegetter 150 into thecenter cavity 155 between thesalt pill 130 andmagnet 110. In active gas gap heat switch embodiments, theheating mechanism 520 is used to apply heat to thegetter material 510, releasing helium gas from thegetter 505 into thecenter cavity 155 between thesalt pill 130 andmagnet 110. The helium gas conducts heat from thesalt pill 130 to themagnet mandrel 210, and heat flows through thebottom plate 170 and central post 175 to theheat sink mount 125 and to theheat sink 127. - As the
magnet 110 is ramped to full field, thesalt pill 130 approaches its peak charge and begins to re-equilibrate with the heat sink. The magnet current can then be ramped down, cooling thesalt pill 130. For embodiments utilizing a passive gas gap heat switch, the cooling causes the helium gas to re-adsorb onto thegetter 150 from thecenter cavity 155 and the gasgap heat switch 115 to turn off. In active gas gap heat switch embodiments, theheating mechanism 520 is disabled, allowing thegetter material 510 to cool and re-adsorb the helium gas from thecenter cavity 155, also causing the gasgap heat switch 115 to turn off. As thesalt pill 130 cools, the magnetic field may be regulated to achieve a stable low temperature for operation. - It should be noted that heat is conducted from the refrigerant through the thin wall of its copper container. That is, the heat is conducted through a large surface area across a very small thickness of copper, so the thermal conductance is exceedingly large and the heat transfer is very efficient.
- In ADRs, a common design consideration is eddy current heating, relevant because of the use of copper in some embodiments of the
magnet mandrel salt pill enclosure 143. For typical magnet ramp rates, for example, 4 Tesla in 2-5 minutes, the eddy current heating in the copper salt pill enclosure and the copper magnet mandrel may each be approximately 100 microwatts, however, the heat is transferred immediately to the heat sink, where that amount of heat is generally negligible. The heat generated in the copper salt pill enclosure on the increasing ramp at the start of recycling is also dumped directly to the heat sink which has sufficient capacity to accommodate this. The heat generated on the decreasing ramp after the gas gap heat switch turns off must be absorbed by the refrigerant, however, in the disclosed embodiments, the eddy current heating generated after the gas gap heat switch turns off represents approximately 1% of the available cooling capacity, representing an acceptable inefficiency in the system. - The disclosed embodiments utilize a gap between the salt pill and the magnet mandrel to create a gas-gap heat switch. As a result, sensitive components are located on the interior of the adiabatic stage and are protected and surrounded by the magnet windings, the magnetic heat shield, and the top and bottom plates, thus providing rugged exterior surfaces. Control for the ADR stage is simplified, requiring simple controls for ramping the magnetic field up and down for embodiments with passive gas gap heat switches, and optional controls for applying or removing power from a getter heating mechanism in embodiments with active gas gap heat switches. The disclosed embodiments provide the ability to vary the transition temperature between heat conducting and non-conducting states by varying refrigerants and gas-gap heat switch charge pressures, or by varying an amount of power applied to a getter heating mechanism. The disclose embodiments further provide simple, robust suspension components that constrain the movement of the salt pill within the magnet and minimize heat leak at cold temperatures.
Claims (20)
1. An adiabatic demagnetization refrigeration stage comprising:
a salt pill;
a magnet surrounding the salt pill; and
a gas-gap heat switch interposed between the salt pill and the magnet.
2. The refrigeration stage of claim 1 , wherein the salt pill comprises a paramagnetic material refrigerant in a helium atmosphere enclosed in a copper container.
3. The refrigeration stage of claim 1 , comprising a port in fluid communication with both the gas gap heat switch and a gas source for providing a gas and charge pressure to the gas gap heat switch.
4. The refrigeration stage of claim 1 , comprising a passive gas gap heat switch.
5. The refrigeration stage of claim 4 , comprising a getter thermally coupled to the salt pill and in fluid communication with the gas-gap heat switch.
6. The refrigeration stage of claim 1 , comprising an active gas gap heat switch.
7. The refrigeration stage of claim 6 , comprising a getter in fluid communication with the gas-gap heat switch and having an independent temperature control.
8. The refrigeration stage of claim 1 , wherein the gas gap heat switch comprises an outer surface of the salt pill substantially concentric with an inner surface of the magnet and a gas confined there between.
9. The refrigeration stage of claim 1 , wherein the gas gap heat switch comprises radially extending fins of an outer surface of the salt pill interleaved with radially extending fins of an inner surface of the magnet and a gas confined there between.
10. The refrigeration stage of claim 1 , comprising a structure supporting the salt pill within the surrounding magnet.
11. The refrigeration stage of claim 10 , wherein the structure comprises a standoff positioned around a bellows attached to a first end of the salt pill to constrain axial and lateral motion.
12. The refrigeration stage of claim 10 , further comprising a hub circumscribing a second end of the salt pill and a plurality of stays connecting the hub and a concentric cylindrical sleeve.
13. A method of operating an adiabatic demagnetization refrigeration stage comprising:
using a magnet surrounding a salt pill to apply an increasing magnetic field to the salt pill;
producing a gas to activate a gas gap heat switch interposed between the magnet and the salt pill to provide a path for heat flow from the salt pill through the magnet to a heat sink; and
decreasing the magnetic field applied to the salt pill while adsorbing the gas to de-activate the gas gap heat switch to cool the salt pill to a lower temperature and cool an object attached to a cold tip extending from the salt pill.
14. The method of claim 13 , comprising regulating the magnetic field to maintain the salt pill at the lower temperature.
15. The method of claim 13 , comprising providing the gas and a charge pressure to the gas gap heat switch using a port in fluid communication with both the gap and a gas source.
16. The method of claim 13 , comprising producing the gas using a getter thermally coupled to the salt pill and in fluid communication with the gas-gap heat switch.
17. The method of claim 13 , comprising producing the gas using a getter in fluid communication with the gas-gap heat switch and having an independent temperature control.
18. The method of claim 13 , comprising providing an increased surface area for heat transfer through the gas gap heat switch by interleaving radially extending fins of an outer surface of the salt pill with radially extending fins of an inner surface of the magnet.
19. The method of claim 13 , comprising supporting the salt pill within the surrounding magnet using a standoff positioned around a bellows attached to a first end of the salt pill to constrain axial and lateral motion.
20. The method of claim 13 , comprising supporting the salt pill within the surrounding magnet using a hub circumscribing a second end of the salt pill and a plurality of stays connecting the hub and a concentric cylindrical sleeve.
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US20190063794A1 (en) * | 2017-08-24 | 2019-02-28 | United States Of America As Represented By The Administrator Of Nasa | Lateral suspension device for mechanical support of spaceflight adr salt pill |
US20190063795A1 (en) * | 2017-08-25 | 2019-02-28 | Astronautics Corporation Of America | Drum-type magnetic refrigeration apparatus with improved magnetic-field source |
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US11210601B2 (en) * | 2015-12-29 | 2021-12-28 | Iqm Finland Oy | Circuit assembly, a system and a method for cooling quantum electric devices |
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US11125477B2 (en) * | 2017-08-25 | 2021-09-21 | Astronautics Corporation Of America | Drum-type magnetic refrigeration apparatus with improved magnetic-field source |
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CN114909818A (en) * | 2022-07-18 | 2022-08-16 | 南方科技大学 | Tin separating and combining device for nuclear heat insulation demagnetization refrigeration system |
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