COMPACT ADIABATIC DEMAGNETIZATION REFRIGERATION STAGE WITH INTEGRAL GAS-GAP HEAT SWITCH

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.

Claims

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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The foregoing aspects and other features of the embodiments are explained in the following description, taken in connection with the accompanying drawings, wherein:

[0030] FIG. 1 shows a cross section of an embodiment of an ADR stage according to the disclosed embodiments;

[0031] FIG. 2 shows a schematic cross section of a magnet;

[0032] FIG. 3 shows an exemplary embodiment of a gas gap heat switch;

[0033] FIGS. 4A-4C illustrate further embodiments of the gas gap heat switch;

[0034] FIG. 5 illustrates an exemplary embodiment an ADR stage having an active gas gap heat switch;

[0035] FIG. 6 shows exemplary structures for supporting a salt pill;

[0036] FIGS. 7A-7C illustrate an example of a bottom salt pill suspension; and

[0037] FIGS. 8A-8C show exemplary stays of the bottom suspension.

DETAILED DESCRIPTION

[0038] 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.

[0039] 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.

[0040] 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.

[0041] The passive and active gas gap heat switches may be implemented with interleaved fins providing increased surface area and thermal conductance.

[0042] 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.

[0043] FIG. 1 shows a cross section of an embodiment of an ADR stage 100 according to the present disclosure. The ADR stage 100 includes a salt pill 130, a magnet 110 with a center cavity 155, a gas-gap heat switch 115 interposed between the salt pill 130 and the magnet 110, a cold tip 120 extending from the salt pill 130, and a bottom plate heat sink mount 125 for mounting to a heat sink 127.

[0044] The salt pill 130 includes a volume of refrigerant 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. An enclosure 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, the salt pill 130 refers to the enclosure 143 and the refrigerant 105.

[0045] A thermally conductive rod extends from a first end 133 of the salt pill 130 to serve as the ADR's cold tip 120. The thermally conductive rod 120 may comprise copper or other suitable thermally conductive material. A second end 137 of the salt pill 130 may include a cap 135 for filling and sealing in the helium gas, as well as an attachment 140 for a bottom suspension 145.

[0046] 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 gas gap heat switch 115. A getter 150 may be bonded onto the first end of the salt 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.

[0047] FIG. 2 shows a schematic cross section of the magnet 110. In at least one embodiment, the magnet includes one or more conductors 205, such as small diameter NbTi wire, wound onto a mandrel 210. Other exemplary conductors may include aluminum, Nb3Sn, Nb3Al, MgB2, or high temperature superconductors such as YBCO. The mandrel 210 includes a center cavity 215 for enclosing the salt pill 130 and the gas-gap heat switch 115. The mandrel 210 is generally constructed of a heat conductive material, for example, copper.

[0048] Returning to FIG. 1, the salt pill 130 may be suspended within the center cavity 155 of the magnet 110 to maintain a stable gap between an outer diameter of the salt pill 130 and an inner diameter of the magnet 110. The stable gap is used to implement the gas gap heat switch 115.

[0049] The volume inside the center cavity 155 of the magnet 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 the cold tip 120 and the magnet mandrel 210. The bellows may be constructed of stainless steel and one or more solder or braze seals 195 may be made between the bellows 160 and the cold tip 120. The bellows 160 may also be sealed to a top plate 165 which may be further sealed to the magnet mandrel 210 by, for example, an indium seal 185. A bottom plate 170 may also be sealed to the magnet mandrel using an indium seal 190. The bottom plate 170 may include a central post 175 extending to the heat sink mount 125. The top plate 165 and bottom plate 170 may be constructed of a suitable thermally conductive material, for example copper.

[0050] A port 180 may be provided for evacuating the sealed center cavity 155 and for connecting to a gas 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 gas gap 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.

[0051] 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. The bottom plate 170, central post 175, and heat sink mount 125 are shaped to provide a thermal path from the magnet 110 to a heat sink, and for allowing the magnetic shield 183 shield enclose the magnet 110 to limit fringing fields, for example, while the ADR stage is at full field.

[0052] FIG. 3 shows a top view of an exemplary implementation of the gas gap heat switch. In FIG. 3 the gas gap heat switch 300 includes the inner surface 305 of the mandrel 210, the outer surface 310 of the salt pill 130, a gas 315, such as helium-3, confined in a gap 320 between the inner surface 305 of the mandrel 210 and the outer surface 310 of the salt pill 130. The getter 150 is shown for reference and may be positioned elsewhere in some embodiments. In this embodiment, the inner surface 305 of the mandrel 210 and the outer surface 310 of the salt pill 130 are substantially round and substantially concentric with each other.

[0053] FIGS. 4A-4C illustrate other embodiments of the mandrel 405 and the salt 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, the mandrel 405 as shown in FIG. 4A, and the salt pill 410 as shown in FIG. 4B, both include a splined configuration in which the mandrel 405 has radial fins 415 and the salt pill 410 also has radial fins 420 positioned to interleave with each other. The interleaved radial fins 415, 420 provide an increased surface area, in some embodiments by as much as a factor of three depending on the dimensions of the radial fins. FIG. 4C shows a top view of the mandrel 405 and the salt pill 410 assembled together with the interleaved radial fins 415, 420. In at least one embodiment, fasteners 425 are radially spaced around the mandrel 405 for fastening the top plate 165 to the mandrel 405. In some embodiments, the gas gap heat switch 430 may be implemented with an odd number of fins which is one less than twice the number of fasteners 425. This number of fins allows the salt pill 410 to be rotated through different radial fastening positions to find the best spacing for the fins.

[0054] FIG. 5 illustrates an embodiment of an ADR stage 100 having an active gas gap heat switch 515. In this embodiment, the getter 505 may be external to the center cavity 155 and includes getter material 510, for example charcoal, and an independent heating mechanism 520, for example a resistive heater. In this exemplary embodiment, port 180 may be used for evacuating the sealed center 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 the heating mechanism 520, providing a selectable temperature range where the heat conducting and non-conducting states occur.

[0055] As shown in FIG. 6, the disclosed embodiments include structures for supporting or suspending the salt pill 130 within the magnet 110. At the first end 133 of the salt pill 130, a top suspension 600 may include a standoff 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 braze seals 195 between the bellows 160 and the cold tip 120. The standoff 605, which may be constructed of a temperature stable material, for example, a polyimide-based plastic such as Vespel, may be positioned around the bellows 160 and attached to the magnetic shield 183 as well as the top plate 165. With the bellows 160 and standoff 605, axial motion, as well as motion in the two lateral degrees of freedom is constrained.

[0056] FIGS. 7A-7C illustrate an example of the bottom suspension 145 utilized at the second end 137 of the salt pill 130. In this embodiment, the bottom suspension 145 includes a wheel and hub type suspension including a wheel 610 and a hub 615 connected by a number of stays 620. While the bottom 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 the salt pill 130 is controlled within the center cavity 155. As shown in FIG. 6, the wheel portion may be implemented by a cylindrical protrusion 610 integral to the bottom 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.

[0057] Turning to FIG. 7A, the wheel portion of the bottom suspension 145 may include cylindrical sleeve 710. Cylindrical sleeve 710 may include a plurality of through holes 725 which may extend radially through the sleeve and may be equally spaced around the perimeter of the sleeve 710. The cylindrical sleeve 710 may also include a groove 730 around its outside circumference in which the through holes may be located. An exemplary hub 715 is shown in FIG. 7B. Hub 715 includes a groove 735 around its outside circumference and has an inner diameter 740 sized to receive the bottom suspension attachment 140 of the salt pill 130. FIG. 7C shows an exemplary embodiment of the bottom suspension 145 showing the position of the cylindrical sleeve 710, hub 715, and stays 720 as assembled together.

[0058] As shown in FIGS. 8A-8C, the stays may be formed by individual loops 820 A, 820 B, 820 C positioned around the circumference of the cylindrical sleeve 710 in the groove 730, threaded through one of the holes 725 and positioned in the groove 735 around the outside circumference of the hub 715. The loops 820 A, 820 B, 820 C have lengths such that the loops in tension, securely center the hub 715 concentrically with the cylindrical 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 the salt pill 130 is assembled within the center cavity 155 of the magnet 110, the bottom suspension attachment 140 engages the hub to provide constraint in two degrees of freedom, securing the salt pill 130 within an inner diameter of the magnet 110.

[0059] The materials and construction techniques of the top suspension 600 and bottom suspension 145 significantly limit any parasitic heat leak into the salt pill 130.

[0060] 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 gas gap 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.

[0061] 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 the magnet 110 causing the salt pill 130 to increase in temperature. In passive gas gap heat switch embodiments, the temperature increase of the salt pill causes the gas gap heat switch 115 to turn fully on by releasing helium gas from the getter 150 into the center cavity 155 between the salt pill 130 and magnet 110. In active gas gap heat switch embodiments, the heating mechanism 520 is used to apply heat to the getter material 510, releasing helium gas from the getter 505 into the center cavity 155 between the salt pill 130 and magnet 110. The helium gas conducts heat from the salt pill 130 to the magnet mandrel 210, and heat flows through the bottom plate 170 and central post 175 to the heat sink mount 125 and to the heat sink 127.

[0062] As the magnet 110 is ramped to full field, the salt pill 130 approaches its peak charge and begins to re-equilibrate with the heat sink. The magnet current can then be ramped down, cooling the salt pill 130. For embodiments utilizing a passive gas gap heat switch, the cooling causes the helium gas to re-adsorb onto the getter 150 from the center cavity 155 and the gas gap heat switch 115 to turn off. In active gas gap heat switch embodiments, the heating mechanism 520 is disabled, allowing the getter material 510 to cool and re-adsorb the helium gas from the center cavity 155, also causing the gas gap heat switch 115 to turn off. As the salt pill 130 cools, the magnetic field may be regulated to achieve a stable low temperature for operation.

[0063] 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.

[0064] In ADRs, a common design consideration is eddy current heating, relevant because of the use of copper in some embodiments of the magnet mandrel 210, 405 and 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.

[0065] 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.