Abstract:
A heat switch includes two symmetric jaws. Each jaw is comprised of a link connected at a translatable joint to a flexible arm. Each arm rotates about a fixed pivot, and has an articulated end including a thermal contact pad connected to a heat sink. The links are joined together at a translatable main joint.  
     To close the heat switch, a closing solenoid is actuated and forces the main joint to an over-center position. This movement rotates the arms about their pivots, respectively, forces each of them into a stressed configuration, and forces the thermal contact pads towards each other and into compressive contact with a cold finger.  
     The closing solenoid is then deactivated. The heat switch remains closed due to a restoring force generated by the stressed configuration of each arm, until actuation of an opening solenoid returns the main joint to its starting open-switch position.

Description:
[0001] The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a heat switch and, more particularly, to a heat switch used in conjunction with a cryogenic refrigerator.  
           [0004]    2. Description of Related Art  
           [0005]    A heat switch is used to conduct heat when closed, and to prevent heat conduction when open. It is used in a variety of cryogenic application where temperature must be controlled and, in particular, comprises an essential component of an adiabatic demagnetization refrigerator (“ADR”). ADRs are discussed in C. Hagmann and P. L. Richards, “Adiabatic demagnetization refrigerators for small laboratory experiments and space astronomy,”  Cryogenics , vol. 35, no. 5, 1995, pp. 303-309. As noted therein, ADRs are capable of reaching operating temperatures below 0.01 K, but typically operate at temperatures near 0.1 K. Such refrigerators are commonly used for small laboratory experiments, space astronomy, and detectors for millimeter waves, X-rays and dark matter. ADRs can also operate in zero gravity, which would make them useful in satellites and space vehicles.  
           [0006]    An ADR typically includes a paramagnetic material suspended in the refrigerator, e.g., a paramagnetic salt such as ferric ammonium alum or chronic cesium alum. The paramagnetic material is in thermal contact with an elongated metal rod called a “cold finger,” which is, in turn, in thermal contact with a cold stage. The cryogenic experiment or instrument that makes use of the cold provided by the ADR is attached to the cold stage. When closed, the heat switch provides for thermal conduction between the paramagnetic material and a heat sink having a temperature of 1° to 4° Kelvin.  
           [0007]    The ADR cycle begins by closing the heat switch to thermally connect the paramagnetic material to the heat sink. A strong magnetic field is then applied to the paramagnetic material to align the magnetic moments of the material. This reduces the entropy of the moments, and the heat of magnetization thereby released is transferred by conduction through the closed heat switch to the heat sink. This process is isothermal.  
           [0008]    The switch is then opened to thermally isolate the paramagnetic material from the heat sink as well as extraneous sources of thermal energy, and the applied magnetic field is decreased. The temperature of the material decreases as magnetic moments in the material lose their alignment and entropy is transferred from the lattice to the magnetic moments. The result is an adiabatic drop in the temperature of the paramagnetic material and thus the cold stage. When the material is partially demagnetized to a desired operating temperature, the temperature can be held constant by very slowly reducing the magnetic field to compensate for heat leakage. By regulation of the magnetic field, a stable temperature can be maintained in the cold stage for many hours, after which the cycle is begun again.  
           [0009]    The requirements for heat switches to be used in conjunction with ADRs include a high ratio of closed to open thermal conductivity, reliability, and low heat emission from the operation of the switch itself. Mechanical, electro-mechanical, and gas-gap heat switches have all been used. Gas-gap heat switches use the thermal conductivity of helium contained between two surfaces. Activated charcoal absorbs the He gas when the switch is open. Because they are always closed at a temperature less than 30° Kelvin, the initial cool-down of the ADR is facilitated. However, this type of switch has the disadvantage of having a finite conductance in the open state due to conduction through the gas tight container. Furthermore, the charcoal must be warmed to absorb the He gas, and this comprises the dominant heat leak for small ADRs. As a result of the two foregoing drawbacks, the mass of paramagnetic salt must be increased in order to obtain a useful cold period, causing concomitant increases in the weight and size of the ADR. C. Hagmann and P. L. Richards, supra, at 306.  
           [0010]    In view of the aforementioned problems endemic to gas-gap heat switches, it has been found that mechanical and electro-mechanical heat switches offer the best performance. An electro-mechanical heat switch is described and illustrated in C. Hagmann and P. L. Richards, supra, at 305. The heat switch shown therein uses a solenoid to force the translation of a plunger that, in turn, forces jaws into normal contact with a cold finger. The drawback attendant to this apparatus is that to keep the jaws closed and in continuous contact with the cold finger, the solenoid must remain actuated for the duration of the isothermal heat transfer to the heat sink, as well as during the lengthy period required to initially cool the ADR down from room temperature. The continuous current generates heat from ohmic resistance that will be conducted into the heat sink and the ADR, thus adversely affecting the ADR&#39;s performance. Furthermore, this continuous actuation reduces the force the solenoid can generate and apply below that available during a short actuation pulse.  
           [0011]    There is a need in the art for an electro-mechanical heat switch that remains closed with sufficient normal force on the cold finger to provide thermal conduction to the heat sink, but without generating the increased thermal energy attendant to keeping the solenoid operative throughout this step of the refrigeration cycle. The present invention not only fulfills this need in the art, but also applies a normal force against the cold finger greater than that of the heat switches of the prior art, and thus improves on the thermal conduction provided by the prior art switches.  
         SUMMARY OF THE INVENTION  
         [0012]    Briefly, the present invention is a heat switch this is of particular advantage when used in conjunction with an ADR. The heat switch includes two symmetric jaws. Each jaw is comprised of a link connected at a joint to a flexible arm. The joints are translatable. Each arm rotates about a fixed pivot, and has an articulated end including a thermal contact pad that is thermally connected to a cryogenic heat sink by a metal braid. The links are joined together at a main joint that can move along a path that is collinear with the longitudinal axis of the plunger for a closing solenoid.  
           [0013]    To close the switch, the closing solenoid is actuated and its plunger forces the main joint to an over-center position, beyond an unstable center position where the two links are aligned with their respective arms at the joints. This movement rotates the arms, bends them into a stressed configuration, and forces the thermal contact pads towards each other and into compressive contact with a cold finger that lies between them. This contact provides for the exhaust to the heat sink of the heat of magnetization released during the application of a magnetic field to the paramagnetic material inside the ADR. It also provides thermal contact for cooling the paramagnetic material and cold stage when the heat switch is closed during the initial cooling from room temperature.  
           [0014]    Once the over-center position of the main joint is achieved, the closing solenoid is deactivated. The heat switch remains closed by virtue of the restoring force applied by the stressed arms. When the isothermal heat exhaust step is completed, actuation of an opening solenoid opens the heat switch by pushing the main joint up and beyond the center position, and the heat switch is returned to its starting open-switch position. With the switch open, the cold stage is thermally isolated from everything except the paramagnetic material. The cold stage is cooled by gradually decreasing the applied magnetic field. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a side view of a heat switch of the present invention.  
         [0016]    [0016]FIG. 2 is a front view of the heat switch in the open position.  
         [0017]    [0017]FIG. 3 is a top view of the heat switch.  
         [0018]    [0018]FIG. 4 is a front view the heat switch, wherein the switch is in the closed position.  
         [0019]    [0019]FIG. 5 is a front view of the articulated arm ends and attached thermal contact pads of the heat switch with the heat switch in the open position.  
         [0020]    [0020]FIG. 6 is a front view of the arm ends and attached thermal contact pads shown in FIG. 5, with the heat switch in the closed position.  
         [0021]    [0021]FIG. 7 is a schematic drawing of one of the jaws of the heat switch in the open position.  
         [0022]    [0022]FIG. 8 is a schematic drawing of one of the jaws of the heat switch in the closed position.  
         [0023]    [0023]FIG. 9 is a graph showing an example of the translation of the end of one of the arms versus the displacement of the plunger on the closing solenoid.  
         [0024]    [0024]FIG. 10 is a graph showing an example of leverage versus the rotation angle of one of the arms about its fixed pivot, for both opening and closing the heat switch.  
         [0025]    [0025]FIG. 11 is a graph showing an example of force versus the rotation angle of one of the arms about its fixed pivot, for both opening and closing the heat switch. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    Turning to the drawings, FIGS. 1, 2 and  3  show side, front and top views, respectively, of heat switch  13  of the present invention. Heat switch  13  is shown in the open, non-conducting position. FIG. 4 is a front view of switch  13  wherein the switch is shown in the closed, thermally conducting position.  
         [0027]    Heat switch  13  includes casing  14 , closing solenoid  15  having plunger  17 , opening solenoid  19  having plunger  21 , arms  23  and  25 , and links  27  and  29 . Arm  23  and link  27  are rotatably attached by joint  31 . Arm  25  and link  29  are rotatably attached by joint  33 . Links  27  and  29  are rotatably attached to each other at main joint  35 . Arm  23  rotates about pivot  37 , and arm  25  rotates about pivot  39 .  
         [0028]    Arm  23  includes articulated end  41  having thermal contact pad  43  attached thereto. Arm  25  includes articulated end  45  having thermal contact pad  47  attached thereto. Thermal contact pad  43  thermally communicates with heat sink  49  via metallic braid  51 . Thermal contact pad  47  thermally communicates with heat sink  49  via metallic braid  53 . Heat sink  49  is maintained at a temperature of 1° to 4° Kelvin.  
         [0029]    Plungers  17  and  21  share axial centerline  55 . When actuated, plungers  17  and  21  move in opposite direction along centerline  55 . Centerline  55  intersects the center of main joint  35 . Main joint  35  translates linearly along centerline  55 . Pivots  37  and  39  are attached to casing  14 , and thus remain stationary relative to casing  14 . Joint  31  translates along a fixed radius of curvature having a center at pivot  37 . Joint  33  translates along a fixed radius of curvature having a center at pivot  39 .  
         [0030]    An ADR (not shown) includes cold finger  57  and a salt pill (not shown). A cold stage (not shown) is attached to and thermally communicates with cold finger  57 . The salt pill is composed of a paramagnetic salt, and also thermally communicates with the cold finger, and therefore the cold stage. FIG. 2 shows heat switch  13  in the open, non-conductive position. Thermal contact pads  43  and  47  are spaced apart from cold finger  57  when heat switch  13  is in the open position, and the cold stage and salt pill thus are isolated from heat sink  49 .  
         [0031]    To close heat switch  13 , closing solenoid  15  is actuated to force plunger  17  downward along centerline  55 . This pushes main joint  35  over its center position, i.e., the position where links  27  and  29  are aligned and horizontal. More particularly, the force generated by closing solenoid  15  and the stroke of plunger  17  are sufficient to force main joint  35  to an over-center position in contact with plunger  21 . Closing solenoid  15  is deactivated when this position is reached.  
         [0032]    The closed position of heat switch  13  is shown in FIG. 4. As may be discerned by comparing FIGS. 3 and 4, the downward motion of plunger  17  and main joint  35  causes the rotation of arms  23  and  25  about pivots  37  and  39 , respectively, until thermal contact pads  43  and  47  abut cold finger  57 . As shown in FIGS. 5 and 6, the articulation of ends  41  and  45  is facilitated by compressed springs  59  and  61 , respectively.  
         [0033]    Arms  23  and  25  are flexible, so that they bend when heat switch  13  is closed and act as a pair of compressed springs. Arms  23  and  25  continue to apply a compressive force against links  27  and  29  to keep the switch in the closed, over-center position, as well as against both sides of cold finger  57 . Thus the switch remains in the closed position even after closing solenoid  15  has been deactivated.  
         [0034]    To open heat switch  13 , opening solenoid  19  is actuated to push plunger  21  upward along centerline  55 . The force generated by solenoid  19  is sufficient to overcome the compressive load applied by arms  23  and  25 , and push main joint  35  out of its over-center position. Opening is completed by spring  63  (shown only in FIG. 4), which is attached at its ends to casing  14  and main joint  35 , respectively, and is in tension that is increased when heat switch  13  is closed.  
         [0035]    The refrigeration cycle of the ADR is begun by closing heat switch  13  and applying a magnetic field to the salt pill. As the magnetic moments of the salt become aligned, the heat of magnetization is generated and transferred by conduction through cold finger  57 , thermal contact pads  43  and  45 , and braids  51  and  53 , to heat sink  49 . The foregoing process is isothermal.  
         [0036]    After a pause to achieve thermal equilibrium, heat switch  13  is opened and the magnetic field is adiabatically decreased. The temperature of the salt falls as entropy is transferred from the salt lattice to the magnetic moments. The salt pill is partially demagnetized to a desired operating temperature, after which the magnetic field is isothermally reduced to compensate for incidental heat conduction to the salt pill. The cryogenic experiment or instrument using the cold temperature obtained by demagnetizing the salt pill is mounted to the cold stage, which is in thermal communication with the salt pill via cold finger  49 . By regulating the magnetic field, a stable temperature can be maintained for hours, after which the ADR must be cycled again.  
         [0037]    [0037]FIGS. 7 and 8 are schematic drawings of the right half of heat switch  13  in the open and closed positions, respectively, and are provided to facilitate a better understanding of the present invention in conjunction with the following discussion. The angle φ denotes the angle of rotation of arm  25 , where φ= 0 ° when arm  25  is vertical. Of particular significance is the relationship between:  
         [0038]    K, the distance between cold finger  57  and thermal contact pad  45 ; and  
         [0039]    L, the distance between the center of main joint  35  and origin  65  on centerline  55 , with L&gt;0 being above origin  65  and L&lt;0 being below origin  65 ; where  
         [0040]    origin  65  is the location of main joint  35  when α=0°; and  
         [0041]    α is the angle between link  27  and the horizontal.  
         [0042]    The variables a, b, c, d, f, g and h are defined in FIG. 8 as they pertain to the illustrated elements comprising heat switch  13 .  
         [0043]    Both K and L are related to the angle φ by the rotational matrix R:  
             R   =     (           cos                 φ             -   sin                   φ               sin                 φ           cos                 φ           )             (   1   )                               
 
         [0044]    It follows that  
           K ( d, f, g, h, φ )= d  sin φ− f  cos φ− g+h   (2)  
         [0045]    and  
                     L        (     a   ,   b   ,   c   ,   f   ,   g   ,   φ     )       =                    -   b                   sin                 φ     +     c        (       cos                 φ     -   1     )       +     a                 sin                 α                   =                    -   b                   sin                 φ     +       c        (       cos                 φ     -   1     )       ±                                    a   2     -       (     h   -     b                 cos                 φ     -     c                 sin                 φ       )     2                       (   3   )                               
 
         [0046]    Note that L is symmetric only around the origin if a+b=f+g=h, which is clearly not the case of interest. Further, φ=0° does not necessarily correspond to heat switch  13  being closed.  
         [0047]    The derivatives of K and L with respect to φ are:  
                    K          φ       =       d                 cos                 φ     +     f                 sin                 φ               (   4   )                      L          φ       =         -   b                   cos                 φ     -       c                 sin                 φ     ±         (         -   b                   sin                 φ     +     c                 cos                 φ       )          (     h   -     b                 cos                 φ     -     c                 sin                 φ                 a   2     -       (     h   -     b                 cos                 φ     -     c                 sin                 φ       )     2                       (   5   )                               
 
         [0048]    From the structure shown in FIGS. 7 and 8, a minimum and maximum for the angle φ may easily be deduced:  
               φ   min     =       arcsin        (       h   -   a           b   2     +     c   2           )       -     arctan        (     b   c     )                 (   6   )                 φ   max     =       arcsin        (     h     a   +         b   2     +     c   2             )       -     arctan        (     b   c     )                 (   7   )                               
 
         [0049]    The maximum for the angle φ, φ max , is calculated assuming that heat switch  13  is opened by moving main joint  35  upwards. For main joint  35  moving downwards the maximum angle, φ max , is larger:  
               φ   max     ≅       arcsin        (       h   -     0.1   ″             b   2     +     c   2           )       -     arctan        (     b   c     )                 (   8   )                               
 
         [0050]    [0050]FIG. 9 is a graph showing the opening K as a function of the translation of main joint  35 , L, for the following example comprised of a set typical parameters (in inches).  
             a   =   0.5               b   =   0.5               c   =   1.0               d   =   1.365               f   =   0.85               g   =   0.15               h   =   1.0                               
 
         [0051]    Obviously, the curve is quite asymmetric. For a large opening, K, of heat switch  13 , it is optimal to have the open state at L&gt;0, whereas for a large closing force it would be advisable to have L&lt;0 when in the open state.  
         [0052]    The force, F K (x), that can be exerted parallel to the x axis by thermal contact pad  47  on cold finger  57  is given by the following equation:  
                     F   K          (   x   )       =       F   L          (   y   )         )                 L          φ       /          K          φ                 (   9   )                               
 
         [0053]    where:  
         [0054]    F L (y) is the force parallel to the y axis applied by plunger  17  to main joint  35 .  
         [0055]    [0055]FIG. 10 shows the force translation curve  
                   L          φ       /          K          φ                                  
 
         [0056]    as a function of φ for the aforementioned typical set of parameters, for both opening and closing heat switch  13 . Clearly the leverage becomes infinite for φ=φ min .  
         [0057]    The force F L (y) is given by the specifications for closing solenoid  15  and opening solenoid  19 . To close heat switch  13  having the aforementioned typical set of parameters, a Ledex low profile linear solenoid type 4EC having a maximum stroke of about 12 mm, or 0.47″ was used for closing solenoid  15 . To open heat switch  13 , a Ledex 4EF solenoid, having a shorter stroke but larger force than the type 4EC, was used for the opening solenoid  19 .  
         [0058]    In order to make optimum use of the opening force provided by opening solenoid  19 , spring  63  is used to completely open heat switch  13 . With spring  63  applying a 2 lbs. tensile force to main joint  35  at φ=φ min , and a 1 lb. tensile force when heat switch  13  is completely open, the force F L (y) is given by:  
           F   L,   open ( y )= F   19 ( L (φ min )− L (φ))+ F   63   (10)  
           F   L,   close ( y )= F   15 ( L (φ min )+ L   0 ))− F   63   (11)  
         [0059]    where:  
         [0060]    L 0  is the distance main joint  35  travels between origin  65 , i.e., L(φ min ), and the closed position, and  
               F   63     =     2   -       L   -     L        (     φ   min     )           S   -     L   0                   (   12   )                               
 
         [0061]     and  
         [0062]    S is the maximum stroke of plunger  21  of opening solenoid  19 .  
         [0063]    [0063]FIG. 11 is a graph showing the force applied to thermal contact pad  47  by closing solenoid  15  and opening solenoid  19  as a function of φ with L&gt;0 for the open state using the aforementioned typical parameters and spring  63 .  
         [0064]    Arm  25  has a finite elasticity, given by a spring constant k d  or a sliding modulus G d =k d ×d. Thus, when heat switch  13  is closed, arm  25  slightly bends. This provides for an added normal force against cold finger  57 , i.e., K&lt;0 in the closed position. The bending of arm  25  also applies a downward force against main joint  35  to keep it locked in the closed, over-center position until opened by opening solenoid  19 .  
         [0065]    The maximum force that spring  63  can exert on main joint  35  in the closed switch position is:  
         Min(F L, close(L(φ min )−L 0 ), F L, open(L(φ min )−L 0 ))−2  (13)  
         [0066]    where 2 lbs. have been subtracted to allow for possible friction.  
         [0067]    It is to be understood that the foregoing description relates to an embodiment of the invention, and that modifications may be made thereto without departing from the spirit and scope of the invention as set forth in the following claims.