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:
INVENTION BY GOVERNMENT EMPLOYEE(S) ONLY 
       [0001]    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. 
     
    
     ORIGIN OF THE INVENTION 
     Background 
       [0002]    The disclosed embodiments generally relate to refrigeration and more particularly to an adiabatic demagnetization refrigeration system. 
         [0003]    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. 
         [0004]    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. 
         [0005]    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. 
         [0006]    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. 
         [0007]    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. 
         [0008]    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. 
         [0009]    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. 
         [0010]    It would be advantageous to provide Adiabatic Demagnetization Refrigeration assemblies and techniques that overcome these and other disadvantages. 
       SUMMARY 
       [0011]    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. 
         [0012]    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. 
         [0013]    The salt pill may be constructed of a paramagnetic material refrigerant in a helium atmosphere enclosed in a copper container. 
         [0014]    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. 
         [0015]    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. 
         [0016]    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. 
         [0017]    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. 
         [0018]    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. 
         [0019]    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. 
         [0020]    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. 
         [0021]    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. 
         [0022]    The method may include regulating the magnetic field to maintain the salt pill at the lower temperature. 
         [0023]    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. 
         [0024]    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. 
         [0025]    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. 
         [0026]    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. 
         [0027]    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. 
         [0028]    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. 
     
    
     
       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&#39;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.