Abstract:
A suspension device for use with a low temperature refrigeration system, such as an adiabatic demagnetization refrigerator is provided. A support ring is provided with three spring-loaded tension assemblies equally spaced about the periphery of the support ring. The tension assemblies each have a pulley, about which is entrained a band of material. Connected to this band is a ring that laterally supports a cylindrical salt pill. Undesired variations in the amount of slack in the band as the salt pill cools are compensated for by the spring loading of the tension assemblies.

Description:
ORIGIN OF THE INVENTION 
       [0001]    The invention described herein was made by an employee of the United States Government, and may be manufactured and used by or for the Government for government purposes without the payment of any royalty thereon or therefore. 
     
    
     TECHNICAL FIELD 
       [0002]    The field of the invention is a suspension device utilized in low temperature environments, and more particularly, one for use with an adiabatic demagnetization refrigerator (ADR) capable of achieving temperatures near absolute zero. 
       BACKGROUND OF THE INVENTION 
       [0003]    Certain astronomical observations are most readily made with a detector array that has been cooled to within a fraction of 1 degree Kelvin. In order to provide and maintain so low a temperature, adiabatic demagnetization refrigeration systems have been developed that can provide such temperatures. They entail the use of salts having suitably large magnetic moments which are subjected to the cooling effects of the adiabatic demagnetization process employed, the details of which are not part of the subject matter of this application but which are further discussed in U.S. Pat. No. 6,959,554, the contents of which are hereby incorporated by reference. This salt is housed in a cylinder called a “salt pill,” which is located in a bore contained within a magnet that is part of the ADR. 
       SUMMARY OF THE INVENTION 
       [0004]    To stabilize at least one end of the salt pill against lateral loads, a suspension device is provided that connects the salt pill to the magnets or other housing elements employed in the adiabatic demagnetization refrigerator apparatus. First, the salt pill is placed within a holder. This holder is attached to an aramid rope or band, which is held in a taught configuration by a series of three spring loaded tension assemblies. The band is made of a material having a low coefficient of thermal conductivity and high strength. An aramid fiber sold under the trademark Kevlar® is suitable for this purpose. The tension assemblies are arrayed equally spaced about a ring-shaped base, and consist of a generally T-shaped tension arm spring loaded to be urged in a generally outwardly radial direction and a pulley about which the band is entrained. Should the band slacken during the cooling process, the spring force provided within the tension assemblies directs the pulley in a generally outwardly direction, thereby taking up any slack and maintaining the tension and stiffness of the system. 
         [0005]    The device provides the end of the salt pill to which it is attached with stability against lateral loads as well as good thermal isolation of the salt pill with respect to the ADR so as to minimize parasitic heating. 
         [0006]    The foregoing and more will be apparent from the following more particular descriptions of exemplary embodiments of the invention as illustrated in the accompanying drawings wherein like reference numbers generally represent like parts of exemplary embodiments of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic view illustrating the general environment of one use to which the embodiments of the device may be put. 
           [0008]      FIG. 2  is a top perspective view of a first embodiment of the device. 
           [0009]      FIG. 3  is a partially exploded view of the embodiment in  FIG. 2 . 
           [0010]      FIG. 4  is a top plan view of the tension arm used in the embodiment depicted in  FIG. 2 . 
           [0011]      FIG. 5  is a top perspective view of a second embodiment. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0012]      FIG. 1  depicts the environment in which the apparatus disclosed herein may be used, namely, as part of an adiabatic refrigeration apparatus employed to cool a detector array to within a few degrees of absolute zero. A heat sink  3  (typically liquid helium) constitutes the high temperature side of the apparatus. It is connected via a heat switch  4  to a so-called “salt pill”  7 , which undergoes adiabatic magnetic cooling in a manner known in the art. The salt pill  7  is contained within a magnet  5 , and is connected at its opposite cold side to a detector array  13  via a thermal strap  9 . On its cold side, the salt pill is axially constrained via a gimble suspension  13 , and on its hot side (nearer the heat sink  3 ) via a lateral load suspension device  100 , the details of which are set forth below. Although shown here as being used in conjunction with a single stage apparatus, multiple ADR stages may be employed, each having its own salt pill leading serially to a low temperature detector array. Moreover, the general arrangement of gimble suspension  13  and lateral load suspension device  100  can be inverted, so that the gimble suspension  13  is utilized at the end of the salt pill  7  nearer the heat sink  3  and the lateral load suspension device  100  is utilized at the other end of the salt pill  7 . 
         [0013]      FIGS. 2-4  illustrate the features of one embodiment of the lateral load suspension device  100  that provides lateral stability against loading. It also provides thermal separation of the salt pill from the magnet and other elements of the ADR apparatus as shall now be more explicitly discussed with respect to  FIGS. 2-4 . 
         [0014]    An outer ring support base  110  serves as a mount for three equally spaced apart and identical spring-loaded tension assemblies  130 , which collectively entrain a band  102  that is made of a strong material having a low coefficient of thermal conductivity. It has been found that a band constructed of aramid fiber sold under the trademark Kevlar® has the suitable thermal and other mechanical properties for this purpose. Other fibers having similar properties of low heat conductivity and high strength may be used. 
         [0015]    The ring support base  110  may be made of a metal such as aluminum, magnesium, or other metal or metal alloy (typically non-ferromagnetic). Where the apparatus is to be launched into space and weight is at a premium, then light weight metals and alloys are preferred. 
         [0016]    The ring support base  110  has three identical angle support brackets  112  arrayed at equal intervals about its general periphery. As seen in  FIGS. 2 and 3 , these brackets  112  have a first portion  114  parallel to the face of the ring  110 ; a through-hole  116  to accommodate bolts or other hardware for attaching the device to magnets or other hardware within the ADR apparatus; and a vertically oriented bracket portion  118  which is provided with a through-hole  120 . Adjacent but spaced apart from each of these brackets  112  is a cylindrical stem  122  which contains a tapped, threaded hole  124 . 
         [0017]    During cooling, the band  102  may become lax due to the behavior of the material as it cools to near absolute zero. Compensation for this laxity is provided by the three spring-loaded tension assemblies  130 , which provide for a generally outwardly directed force that helps keep the band  102  taut. In the preferred embodiment, three tension assemblies are employed, as this provides the optimal degree of mechanical stability (three points suffice to determine a plane). However, in some settings it may be that more than three tension assemblies may be mechanically optimal. 
         [0018]    Each tension assembly  130  is composed of a generally T-shaped tension arm  132  (see  FIG. 4 ) having a left portion  133  having a through hole  138 , a central stem  134  having a tapped hole  136  and a pair of tapped holes  144 ; and a right portion  135  that is further provided with a vertically extending portion  140  having a tapped hole  142 . Hole  138  is sized so that it can contain stem  122 , about which the tension arm  132  can pivot. The tension arm  132  is kept axially in place via threaded bolt  152  which screws into the tapped hole  124  in the stem  122 , with washers  153  and  154  located above the tension arm  132 , and washer  155  serving to space the tension arm  132  above the support base  110  (see  FIG. 3 ). Thus constrained, the tension arm is free to rotate about the cylindrical stem  122  but not otherwise move axially. The tension arms are further provided with a pair of grooves  192  to accommodate the band  102  with a minimum of friction between the two. 
         [0019]    Bracket  162  is attached to the tension arm  132  via threaded bolts  164 , which pass through bracket through holes  165  to screw into the tapped holes  144  located in the central portion  134  of the tension arm  132 . The bracket  162  also axially bounds a pulley  170  having a lower protruding portion  172  and an upper protruding portion  174 . The upper protruding portion  174  fits into a through hole  169  in bracket  162 , and the lower protruding portion  172  fits into the tapped hole  136  of the tension arm  132 . Washers  167  and  168  are provided as shown in  FIG. 3 . The pulley is thus axially bound by the tension arm  132  and the bracket  162 . 
         [0020]    At the right portion  135  of tension arm  132 , a Bellville washer spring assembly is provided. Threaded bolt  182  passes through a fitting  184  into tapped hole  142 , and provides a stem onto which are placed the Bellville washers  188 . Alternatively, other mechanical elements providing a compressive force, such as a compression spring, may be used in place of a Bellville washer. The result is that the spring, which can be adjusted by adding nestable Bellville washers to one another and/or by adjustments to the bolt  182 , generates a compression force that directs the tension assembly  130  to pivot generally radially outwardly about the cylindrical stem  122 , and thereby take up any slack that should develop in the band during cooling. 
         [0021]    The salt pill  7  is held within a split ring salt pill collar  104  ( FIG. 2 ) that terminates in three arms  105 , each of which is provided with a through-hole  106  to accommodate the passage of the band  102 . 
         [0022]    When the suspension device is assembled, it is pretensioned as noted above to provide a level of spring force via the Bellville washer assembly sufficient to keep the band  102  taut during its subsequent cool-down and use within the ADR. 
         [0023]      FIG. 5  illustrates an embodiment of a lateral load suspension device  200  in which each tension assembly  230  has two compression springs  280 . 
         [0024]    The ring-shaped support base  210  is provided with three angle support brackets  212 , each of which has two wing portions  213  that each have a tapped through hole  214  to receive a threaded bolt  282  that screws into a correspondingly tapped threaded hole in the left and right wing portions  235 ,  236  of a generally T-shaped tension arm  232 , which also has a central stem  234 . Held within a slot inside the central stern is a pulley  270 . Both the left and right wing portions  235 ,  236  are further provided with grooves  292  to accommodate the band  204  so as to minimize friction therebetween. 
         [0025]    A pair of compression springs is employed to urge the tension assembly  230  in a generally radially outwardly direction. For example, an assembly of compression spring  280 , washer and threaded bolt  282  is provided at each side of tension arm  232  to bias the tension arm in a radially outward direction to remove any slack in the band  204 . The level of bias can be adjusted by adjustments to the bolts  282  or to the stiffness of the springs employed. 
         [0026]    A split ring salt pill collar similar to the one depicted in  FIG. 2  (not shown in  FIG. 5 ) provides for the connection of the salt pill to the band  102 , which as in the previous embodiment is made of an aramid polymer. 
         [0027]    Each embodiment provides a mechanically stable and thermally isolated mounting for a salt pill in a low temperature apparatus such as in an ADR and allows for the salt pill to be maintained in a correct position with respect to the ADR, even when the device is subjected to temperatures within a few degrees of absolute zero.