Abstract:
An electrical interconnect provides a path between cryogenic or cryocooled circuitry and ambient temperatures. As a system, a cryocable  10  is combined with a trough-line contact or transition  20 . In the preferred embodiment, the cryocable  10  comprises a conductor  11  disposed adjacent an insulator  12  which is in turn disposed adjacent another conductor  13 . The components are sized so as to balance heat load through the cryocable  10  with the insertion loss. In the most preferred embodiment, a coaxial cryocable  10  has a center conductor  11  surrounded by a dielectric  12  (e.g. Teflon™) surrounded by an outer conductor  13  which has a thickness between about 6 and 20 microns. The heat load is preferably less than one Watt, and most preferably less than one tenth of a Watt, with an insertion loss less than one decibel. In another aspect of the invention, a trough-line contact or transition  20  is provided in which the center conductor  11  is partially enveloped by dielectric  12  to form a relatively flat portion  28 . The preferred overall geometry of the preferred embodiment of the cable is generally cylindrical, although other geometries are possible (e.g., stripline, microstrip, coplanar or slotline geometries). In a further aspect of the present invention, a push-on connector  120  is provided to facilitate connection and disconnection of the cryocable from an HTS circuit and/or a mating feedthrough  124.

Description:
This is a continuation of application Ser. No. 09/173,339, filed Oct. 15, 1998, which is a continuation-in-part of application Ser. No. 08/638,321,filed on Apr. 26, 1996, now U.S. Pat. No. 5,856,768 issued on Jan. 5, 1999, which is a file wrapper continuation of application Ser. No. 08/227,974, filed on Apr. 15, 1994, now abandoned. The priority of these prior applications is expressly claimed and their disclosures are hereby incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to signal interfaces, particularly coaxial cables and cable-to-circuit transitions (i.e., interconnects) which may preferably be used to interface cryogenic components and ambient-environment components which are at temperature differences of about 50-400 K (or ° C.). The invention is particularly useful in microwave or radio frequency applications of cold electronics or circuits which include high temperature superconductor material. 
     BACKGROUND OF THE INVENTION 
     There are many benefits to having circuitry that includes superconductive material. Superconductivity refers to that state of metals and materials in which the electrical resistivity is zero when the specimen is cooled to a sufficiently low temperature. The temperature at which a specimen undergoes a transition from a state of normal electrical resistivity to a state of superconductivity is known as the critical temperature (“T c ”). The use of superconductive material in circuits is advantageous because of the elimination of resistive losses. 
     Until recently, attaining the T c  of known superconducting materials required the use of liquid helium and expensive cooling equipment. However, in 1986 a superconducting material having a T c  of 30 K was announced. See, e.g., Bednorz and Muller, Possible High T c  Superconductivity in the Ba—La—Cu—O System, Z.Phys. B-Condensed Matter 64, 189-193 (1986). Since that announcement superconducting materials having higher critical temperatures have been discovered. Collectively these are referred to as high temperature superconductors (HTSs). Currently, superconducting materials having critical temperatures in excess of the boiling point of liquid nitrogen, 77 K (i.e., about −196° C. or −321 ° F.) at atmospheric pressure, have been disclosed. 
     HTSs have been prepared in a number of forms. The earliest forms were preparation of bulk materials, which were sufficient to determine the existence of the superconducting state and phases. More recently, thin films on various substrates have been prepared which have proved to be useful for making practical superconducting devices. More particularly, the applicant&#39;s assignee has successfully produced thin film thallium superconductors which are epitaxial to the substrate. See, e.g., Olson, et al., Preparation of Superconducting TlCaBaCu Thin Films by Chemical Deposition, Appl. Phys. Lett. 55, No. 2, 189-190 (1989), incorporated herein by reference. Techniques for fabricating and improving thin film thallium superconductors are described in the following patent and copending applications: Olson, et al., U.S. Pat. No. 5,071,830, issued Dec. 10, 1991; Controlled Thallous Oxide Evaporation for Thallium Superconductor Films and Reactor Design, U.S. Pat. No. 5,139,998, issued Aug. 18, 1992; In Situ Growth of Superconducting Films, Ser. No. 598,134, filed Oct. 16, 1990, now abandoned, and Passivation Coating for Superconducting Thin Film Device, Ser. No. 697,660, filed May 8,1991, now abandoned, all incorporated herein by reference. 
     High temperature superconducting films are now routinely manufactured with surface resistances significantly below 500 μΩ measured at 10 GHz and 77 K. These films may be formed into circuits. Such superconducting films when formed as resonant circuits have an extremely high quality factor (“Q”). The Q of a device is a measure of its lossiness or power dissipation. In theory, a device with zero resistance (i.e., a lossless device) would have a Q of infinity. Superconducting devices manufactured and sold by applicant&#39;s assignee routinely achieve a Q in excess of 15,000. This is high in comparison to a Q of several hundred for the best known non-superconducting conductors having similar structure and operating under similar conditions. 
     A benefit of circuits including superconductive materials is that relatively long circuits may be fabricated without introducing significant loss. For example, an inductor coil of a detector circuit made from superconducting material can include more turns than a similar coil made of non-superconducting material without experiencing a significant increase in loss as would the non-superconducting coil. Therefore, a superconducting coil has increased signal pick-up and is much more sensitive than a non-superconducting coil. 
     Another benefit of superconducting thin films is that resonators formed from such films have the desirable property of having very high-energy storage in a relatively small physical space. Such superconducting resonators are compact and lightweight. 
     Although circuits made from HTSs enjoy increased signal-to-noise ratios and Q values, such circuits must be cooled to below T c  temperatures (e.g. typically to 77 K or lower). In addition, it is desirable to directly interface or connect these cooled HTS circuits to other components or devices that might not be cooled. Most particularly, the signals from the cooled circuits often must be coupled to electronics at ambient temperatures. 
     Furthermore, low temperatures must be maintained when using cryo-cooled electronics and infrared detectors. In such situations an interface to couple signals between cooled and ambient temperatures is needed. 
     Generally, coaxial cables are used as signal interfaces. Coaxial cables are typically made of a central signal conductor (i.e., a center or inner conductor) covered with an insulating material (e.g., dielectric) which, in turn, is covered by an outer conductor. The entire assembly is usually covered with a jacket. Such a cable is “coaxial” because it includes two axial conductors that are separated by a dielectric core. 
     Although coaxial cables are generally used as signal interfaces, when connecting circuits which include HTS material, one end of the connecting coaxial cable might be in contact with a circuit cooled to 77 K, and the other end might be in contact with a device at a much higher temperature (e.g., room ambient temperature is about 300 K). Standard coaxial cables are not manufactured to operate under such conditions. When standard coaxial cables are used under such conditions, the signal losses may be quite high and the heat load by thermal conduction through the cable may be quite large. 
     Minimizing signal losses is important because the ability to transmit signals directly affects the sensitivity and accuracy of the devices. Insertion loss is a measure of such losses due to intermediary components. In equation form, if the output wattage of a circuit is P 1  without intermediary components and P 2  with intermediary components respectively, then the insertion loss L is given by the formula 
     
       
           L (dB)=10log 10 ( P   1   /P   2 ) 
       
     
     Unless such losses are minimized, the benefits of using HTS or cryo-cooled materials may be lost. 
     Minimizing heat load is important because cryogenic coolers used to cool the HTS circuits generally have limited cooling capacity and are relatively inefficient. For example, the best cryocoolers currently available require the supply of approximately forty watts of power to a compressor to remove or lift approximately one watt of heat load. Therefore, it is preferable to limit heat load to 0.1 Watts or less. 
     Although minimizing heat load is important, it is also difficult. Standard coaxial cables are fabricated by extruding or swaging metal tubing (e.g. copper, gold, aluminum, stainless steel, or silver) over a dielectric (e.g., low-loss plastic materials, polyethylene materials, or Teflon™). The thinnest extruded tubing of which applicant is presently aware is about 0.005 inches (about 0.127 mm) thick. 
     In addition, as described above, one of the advantages of using HTS materials in circuits for microwave systems is the elimination of resistive losses. However, the advantage of reduced resistive loss can only be fully exploited if reflection or return losses (i.e., losses due to mismatches in characteristic impedances of the components) are minimized. This is especially true for components to be used at high frequencies (e.g., mm wave). 
     A primary candidate for mismatch problems in circuits including HTS materials is the transition through which a coaxial cable is connected to the circuit. In general, HTS material and circuits containing the same have optimal properties in a planar configuration. However, coaxial cable is cylindrically shielded. The transition between the planar circuit and the cylindrical cable may contribute significant reflection or return losses. 
     The circuit bonding process may also affect the geometry of the transition between the circuit and cable. Typical cables require a transition through which the cable may be attached or bonded to a circuit. Typical coaxial cable transitions use the inner conductor of the cable suspended in air.(e.g., forming a pin) where the air acts as a dielectric. The suspended conductor may be inadvertently slightly bent during a typical bonding process. The geometry of the transition may suffer from unsatisfactory reproducibility problems because of the mechanical stability (or instability) of the pin. A further disadvantage occurs when the contact is wrapped around the inner conductor pin, unnecessarily increasing inductance. 
     In addition, the geometry of the transition between the circuit and cable will directly affect the ease of assembly of the device using such components. To maximize ease of assembly the packaging of HTS circuits that are cooled to cryogenic temperatures must include special input and output leads. As explained above, HTS circuits must be cooled to below T c . Generally, such cooling is achieved by holding the circuits in contact with the cold head of a cryocooler (e.g. enclosed in a vacuum dewar). To connect cooled circuits contained in a dewar, interconnection points must be provided through a wall in the dewar. Such interconnections provide large thermal conduction paths for already inefficient cryocoolers. 
     The prior art has failed to provide a signal interface (including a transmission cable and cable-to-circuit transition) between cryogenic components and ambient-environment components for use in radio frequency applications of cold electronics and high temperature superconductors. The prior art has also failed to provide an interface and transmission cable which exhibit low thermal conduction and low electrical losses (e.g. impedance continuity and low reflection losses), and which work over a frequency range including UHF, microwave, and low millimeter-wave frequencies (e.g. up to 40 GHz). The prior art has further failed to provide such an interface which is also mechanically stable (and, therefore, reproducible) and relatively easy to use. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a signal interface (including a transmission cable and a cable-to-circuit transition) for connecting cryogenic components and ambient-environment components that are to be used in radio frequency applications of cold electronics and high temperature superconductors. In the preferred embodiment, the transmission cable of the present invention comprises an inner conductor positioned within a dielectric which has a thin outer conductor plated on its outer surface. The preferred embodiment of the cable-to-circuit transition of the present invention is also generally cylindrical and comprises an inner conductor positioned within a dielectric which has a thin outer conductor plated on its outer surface. In addition, the transition also preferably includes a semi-circular end area that provides a flat surface at least for ease of bonding the transition to a cryo-cooled circuit and for impedance matching purposes. Preferably, the components are sized so as to balance heat load through the transmission cable and transition with the insertion loss. 
     As is mentioned above, outer conductors for coaxial cables are generally fabricated by extruding or swaging metal tubing over a dielectric. As is also mentioned above, the thinnest extruded tubing of which applicant is presently aware is about 0.005 inches (about 0.127 mm) thick. Such extruded tubing experiences higher heat conduction than would a thinner metal tubing. For example, tubing having a thickness of 0.005 inches (about 0.127 mm) experiences a heat load which is eight times the thermal conduction of a similar tubing having a thickness of about 0.0008 inches (about 20 μ) and twenty times the thermal conduction of a similar tubing having a thickness of about 0.00024 inches (about 6 μ). 
     In the most preferred embodiment, the transmission cable of the present invention comprises a coaxial cryocable having a center conductor surrounded by a dielectric (e.g., Teflon™) surrounded by an outer conductor which has a thickness between about 6 and 20 microns. The heat load is preferably less than one Watt, and most preferably less than one tenth of a Watt, with an insertion loss less than one decibel. The preferred overall geometry of the preferred embodiment of the cable is generally cylindrical, although other geometries are possible (e.g. stripline, microstrip, coplanar or slotline geometries). 
     The present signal interface (i.e., cable and transition) exhibits low thermal conduction, low electrical losses (e.g., impedance continuity and low reflection losses), and works over a frequency range including UHF (300-3000 MHz), microwave, and low millimeter-wave frequencies (e.g., up to 40 GHz). The present signal interface also is mechanically stable, reproducible, and relatively easy to use. 
     In another aspect of the present invention, a push-on connector may be provided at one or both ends of the cryocable. Such push-on connectors have not previously been used in high vacuum cryogenic applications. Mating connectors may also be provided to connect the cryocable to a hermetic feedthrough and/or to the HTS circuit. The push-on connector design allows fast, simple, and repeated connection and disconnection of the cryocable from the feedthrough and/or the HTS circuit. 
     It is a principal object of the present invention to provide an improved signal interface. 
     It is also an object of the present invention to provide a signal interface that exhibits desirable electrical properties (e.g., low electrical reflection, and power losses, and impedance continuity). 
     It is an additional object of the present invention to provide a signal interface that is mechanically stable and readily reproducible. 
     It is a further object of the present invention to provide a signal interface that is easy to assemble. 
     It is another object of the present invention to provide a signal interface for connecting cryogenic components and ambient-environment components that are to be used in radio frequency applications of cold electronics and high temperature superconductors. 
     It is also the object of the present invention to select appropriate materials, thereby providing very low outgassing materials which allows the vacuum integrity to be preserved for several years. 
     It is also an object of the present invention to provide a hermetic feed-through from the vacuum side of a dewar to the warm side of the dewar, which also allows for the vacuum integrity to be preserved for several years. 
     It is yet another object of the present invention to provide a push-on connector that allows easy connection and disconnection of a cryocable from an hermetic feedthrough and/or an HTS circuit. 
     It is also an object of the present invention to provide a clean cryocable with no entrapped contaminants that will compromise the vacuum integrity. 
     It is also an object of the present invention to provide a signal interface that exhibits low thermal conduction. 
     It is yet another object of the present invention to provide a signal interface that exhibits low electrical losses, impedance continuity and low reflection losses. 
     It is still another object of the present invention to provide a signal interface that works over a frequency range including UHF, microwave, and low millimeter-wave frequencies (e.g. up to 40 GHz). 
     It is a further object of the present invention to provide a signal interface that includes a coaxial cryocable having a central conductor surrounded by a dielectric having an outer conductor plated on its surface. 
     It is also a further object of the present invention to provide a signal interface which includes a cable-to-circuit transition having a coaxial connecting end to which a coaxial cable may be attached and a flat bonding surface end to which a circuit may be bonded. 
    
    
     Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a preferred embodiment of the cryocable of the present invention. 
     FIG. 2 is a plot of heat load in Watts versus outer conductor upper plating thickness in microns for coaxial cables with various outer diameters. 
     FIG. 3 is a plot of attenuation in decibels per 10 centimeter length versus frequency in gigahertz for coaxial cables with various outer diameters. 
     FIG. 4 is a cross-sectional view of an embodiment of the coaxial cryocable of the present invention having connectors on each end and of a preferred embodiment of the glass feed through of the present invention. 
     FIG. 5 is a cross-sectional view of an embodiment of the coaxial cryocable of the present invention having a similar connector to those shown in FIG. 4 on one end and of an embodiment of the trough line of the present invention that mates to this connector. On the other end of the cable is a fired-in glass feedthrough through which a continuous center conductor passes that continues all the way to the connector that mates with the trough line interface. 
     FIG. 6 is a top view of an embodiment of the trough line launch of the present invention. 
     FIG. 7 is a side view of the trough line launch of FIG.  6 . 
     FIG. 8 is a front view of the trough line launch of FIG.  6 . 
     FIG. 9 is a top view of a fixture for determining the sensitivity of a coaxial line&#39;s impedance. 
     FIG. 10 is a side view of the fixture of FIG.  9 . 
     FIG. 11 is a chart showing an exemplary flow for the production and assembly of a trough line of the present invention. 
     FIG. 12 is a perspective view of a stripline cryocable of the present invention. 
     FIG. 13 is a perspective view of a second embodiment of a stripline cryocable of the present invention. 
     FIG. 14 is a perspective view of a microstrip cryocable of the present invention. 
     FIG. 15 is a perspective view of a balanced microstrip cryocable of the present invention. 
     FIG. 16 is a perspective view of a coplanar slot line cryocable of the present invention. 
     FIG. 17 is a perspective view of a coplanar slot line cryocable of the present invention. 
     FIG. 18 is a perspective view of a first end of a flat cryocable in accordance with the present invention. 
     FIG. 19 is a perspective view of a second end of the flat cryocable of FIG.  18 . 
     FIG. 20 is a perspective view of a push-on connector in accordance with a preferred embodiment of the present invention. 
     FIG. 21 is a cross-sectional view of a push-on connector in accordance with a preferred embodiment of the present invention. 
     FIG. 21A is an end view of the push-on connector of FIG.  21 . 
     FIG. 22 is a cross-sectional view of the push-on connector of FIG. 21 connected to a mating receptacle and feedthrough in accordance with a preferred embodiment of the present invention. 
     FIG. 23 is a cross-sectional view of a feedthrough in accordance with a preferred embodiment of the present invention. 
     FIG. 23A is an end view of the feedthrough of FIG.  23 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 5, the preferred signal interface of the present invention comprises a cryocable  10  and a cryocable transition  20 . Like reference labels appearing in the figures refer to the same elements from figure to figure and may not be explicitly described for all of the figures. The transition  20  is preferably both co-planar and coaxial. The transition  20  may be used to transition circuitry to the cryocable  10  of the present invention or other coaxial cables as are known in the art. 
     The present invention provides a coaxial cryocable  10  which may be used to connect devices held at widely differing temperatures (e.g., up to temperature differences of about 50 to 400 K (° C.) (i.e., temperature differences of about 90 to 720° F.)) while minimizing signal losses and thermal conduction. As shown in FIG. 1, the present invention provides a coaxial cryocable  10  comprising an inner conductor  11 . The inner conductor  11  is a wire, preferably solid, of very low thermal conductivity which is preferably copper, gold or silver plated by electroplating to a thickness which can easily be controlled and/or varied to match the operating frequency of the system. 
     The cryocable  10  also comprises a dielectric  12  that is preferably, made of Teflon™ or other dielectrics that are well known in the art. The dielectric constant of Teflon™ is substantially constant from about 800 MHz through 40 GHz. The dielectric  12  is preferably an extruded tubing such as is available from Zeus Industrial Products, Inc., 501 Boulevard St., Orangeburg, S.C. 29115, U.S.A. The inner conductor  11  should fit inside the dielectric tube  12 . 
     The cryocable  10  further comprises an outer conductor  13 . The outer conductor  13  is preferably a copper, gold, or silver layer which is preferably formed by electroplating the outer surface of the dielectric tube  12  with the desired metal. The thickness of the outer conductor  13  may be accurately controlled by the electroplating process. Electroplating the dielectric may be accomplished by plating firms such as Polyflon Company, 35 River St., New Rochelle, N.Y. 10801, U.S.A. 
     In determining optimal dimensions of the inner conductor  11 , the dielectric  12 , and the outer conductor  13  the following must be considered: (1) the heat load provided by various thicknesses of outer conductor  13  and various diameters of inner conductor  11  (FIG.  2 ); and (2) the attenuation experienced by various diameters of inner conductor  11  at various operating frequencies (FIG.  3 ). 
     FIG. 2 shows the heat load provided by outer conductors having various diameters when the inner conductor has various diameters and when the cryocable is 5 cm long. Table 1 shows the dimensions and materials used for the cryocables from which the information for FIG. 2 was generated. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 INNER CONDUCTOR 
                   
                 OUTER CONDUCTOR 
                   
               
             
          
           
               
                 LINE 
                 DIAMETER 
                 MATERIAL 
                 DIAMETER 
                 MATERIAL 
               
               
                   
               
             
          
           
               
                 A 
                 0.010″ 
                 COPPER* 
                 0.0335″ 
                 COPPER 
               
               
                 B 
                 0.012″ 
                 COPPER* 
                 0.040″ 
                 COPPER 
               
               
                 C 
                 0.017″ 
                 COPPER* 
                 0.057″ 
                 COPPER 
               
               
                 D 
                 0.020″ 
                 COPPER* 
                 0.067″ 
                 COPPER 
               
               
                   
               
             
          
         
       
     
     Copper Plated CRES (Corrosion Resistant Steel) 
     As explained above, it is preferable to keep the heat load below 0.10 Watts. Therefore, an extrapolation of line A of FIG. 2 indicates that a cryocable  10  having an inner conductor  11  about 0.010 inches thick, should have an outer conductor  13  which is preferably no more than about 20 microns thick to keep the heat load to no more than about 0.10 Watts. As indicated by line D of FIG. 2 the maximum thickness for the outer conductor  13  of a cryocable  10  having an inner conductor  11  about 0.020 inches thick for a heat load of 0.1 Watt is preferably no more than about 7.5 microns thick. 
     FIG. 3 shows the attenuation or insertion loss experienced by various cryocables operating at various operating frequencies. Table 2 shows the dimensions and materials used for the cryocables which were tested for FIG.  3 . In all examples the copper plating is about 6 microns thick (i.e., 3 skin depths). 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 INNER CONDUCTOR 
                   
                 OUTER CONDUCTOR 
                   
               
             
          
           
               
                 LINE 
                 DIAMETER 
                 MATERIAL 
                 DIAMETER 
                 MATERIAL 
               
               
                   
               
             
          
           
               
                 E 
                 0.020″ 
                 COPPER 
                 0.067″ 
                 COPPER 
               
               
                 F 
                 0.017″ 
                 COPPER 
                 0.057″ 
                 COPPER 
               
               
                 G 
                 0.012″ 
                 COPPER 
                 0.040″ 
                 COPPER 
               
               
                 H 
                 0.012″ 
                 COPPER 
                 0.040″ 
                 CRES 
               
               
                 I 
                 0.0045″ 
                 SPCW** 
                 0.015″ 
                 CRES 
               
               
                   
               
             
          
         
       
     
     Silver Plated Copper Clad Steel 
     FIG. 3 shows that as the conductors of the cryocables get smaller and smaller the attenuation gets larger and larger. Therefore, although smaller conductors are preferred to minimize heat load (see FIG.  2 ), smaller conductors may also lead to unacceptably high insertion losses. 
     For microwave and radio frequency operations of cold electronics or circuits that include high temperature superconductor material a preferred operating frequency range is up to about 40 GHz. In addition, for such applications it is preferable that the attenuation amount to no more than about 0.7 dB for a 10 cm length of cryocable. Cryocables represented by lines E, F, and G, in FIG. 3, have no more than 0.7 dB attenuation when operating at 40 GHz. As explained above, the smaller cryocables have smaller thermal conduction. Therefore, the preferred cryocable is the smaller cryocable such as that represented by line G. 
     In addition, the ratio of the outer diameter of the inner conductor  11  (i.e., the inner diameter, ID, of the dielectric  12 ) and the inner diameter of the outer conductor  13  (i.e., the outer diameter, OD, of the dielectric) is relatively fixed, by formula, depending on the range of operating frequencies of the cryocable  10 , the impedance of the cryocable  10 , and on the dielectric constant of the dielectric  12 . For example, for an impedance of 50 Ω, the ratio of OD to ID is approximately 3.35. The desired ratio is easily calculated by those skilled in the art according to the known formula: 
       Z   0 =(138/E r) log   10 ( OD/ID ) 
     wherein Z 0  is the characteristic impedance of the coaxial cable and E r  is the dielectric constant. Furthermore, the sum of the ID and OD relate to the maximum voltage of operation. For example, if the sum of an ID and OD amounts to 0.12 inches, the signal will start deteriorating at about 40 GHz. 
     Taking into consideration all of the above, the features of the cryocable  10  of the present invention having the following dimensions. The inner conductor  11  preferably has a diameter of about 0.012 inches (i.e., 0.30 mm), and the plating on the inner conductor  11  is preferably no thicker than 20 microns. The dielectric tubing  12  preferably has an inner diameter of about 0.012 inches (i.e., 0.30 mm) and an outer diameter of about 0.040 inches (1.02 mm). To reduce thermal conductivity, the outer conductor  13  is preferably on the order of between about twenty and about six microns thick. This thickness should allow for at least a few skin depths. For example, if the plating is copper, it is preferably at least about 0.00024 inches (i.e., 6μ) which is about three skin depths thick at 1 GHz. 
     The coaxial cryocable  10  comprising the structure and materials described above is semirigid and can be bent slightly to facilitate connecting the cryocable  10  to components. In addition, a service loop may be provided to allow for thermal contraction of the cryocable  10  when it is cooled from a room ambient temperature of about 300 K (i.e., about 27° C. or 80° F.) to a cryogenic temperature of 77 K (i.e., about −196° C. or −321° F.). 
     As is explained above, a typical coaxial cable requires a transition and a typical transition comprises an inner conductor suspended in air (e.g. forming a pin) where the air acts as a dielectric for the inner conductor. As is also explained above, wire bonding reproducibility may be affected where the suspended conductor is bent during the process of attaching or wire bonding the cable to a circuit. Mechanical stability of the pin is greatly increased if the dielectric material under the pin were solid, rather than air. Bonding to the pin is easier when the pin has a flat surface to which to bond. The present invention utilizes these structures. 
     As shown in FIGS. 4 and 5, it is preferred that the coaxial cryocable  10  of the present invention be connectable at each end. One end of the cryocable  10  should be connectable to cold electronics or circuits containing high temperature superconductors, preferably through the cable transition  20  of the present invention which is described below and shown in FIG.  5 . The other end of the cryocable  10  should be connectable to ambient environment electronics, preferably through a connection which would maintain an hermetic vacuum seal so the cryocable  10  may be positioned within a dewar holding cooled components without providing a vacuum leak as is described below and shown in FIGS. 4 and 5. 
     Generally, as is explained above, circuits which must be held at cryogenic temperatures (e.g., 77 K, −196° C., −321° F.) are placed in contact with a cold plate in a vacuum dewar or similar holding device. The cryocable  10  of the present invention must be connectable through the dewar to ambient environment while maintaining the vacuum within the dewar. 
     As shown in FIGS. 5-8, the present invention includes a cable transition  20  that has a cylindrical portion  21  and a semi-cylindrical portion  22 . The cylindrical portion  21  includes a cylindrical inner conductor  23 , a cylindrical solid dielectric  24 , and an outer conductor  25  on the curved outer surface of the cylindrical dielectric  24 . 
     Also shown in FIGS. 5-8, the semi-cylindrical portion  22  includes a semi-cylindrical inner conductor  26  and a semi-cylindrical solid dielectric  27 . The semi-cylindrical inner conductor  26  and dielectric  27  form a flat exposed surface  28 . The semi-cylindrical portion  22  includes a semi-cylindrical surface  29  and an outer conductor  30  preferably plated on the curved outer semi-cylindrical surface  29  of the semi-cylindrical dielectric  27 . The outer conductors  25  and  30  provide metal surfaces that may be soldered to a metal circuit housing  31  as shown in FIG.  5 . The dielectric  24  and  27  could be made of any suitable material and is preferably made from a hard plastic such as PEEK available from Victrex® of ICI Advanced Materials, 475 Creamery Way, Exton, Pa. 19341, U.S.A. 
     Because the outer conductor  30  is located only on the semi-cylindrical surface  29  of the dielectric  27 , the outer conductor  30  does not completely shield the semi-cylindrical inner conductor  26  electrically. In addition, the overall dielectric constant of the dielectric surrounding the inner conductor  26  (solid dielectric  27  on one side and air on the other) will no longer be uniform. Therefore, the transition  20  will have an impedance which is a function of a dielectric constant which is somewhere between that of the two dielectrics around the inner conductor  26  (solid dielectric  27  and air). 
     Because air (with a dielectric constant of 1) is the dielectric for about one-half of the semi-cylinder inner conductor  26 , the effective dielectric constant of the transition  20  will be lower at the semi-cylindrical portion  22  than it is at the full cylindrical portion  21 . Therefore, it is preferable that the diameter d (shown in FIGS. 6 ) of the semi-cylindrical portion  22  be smaller than the diameter D (also shown in FIGS. 6) of the full cylindrical portion  21 . The portion of the transition  20  which is semi-cylindrical will be referred to as the cable trough line or CTL  22 , as is shown in FIGS. 6 and 7. 
     A small number of variables have been used to describe the transition  20  of the present invention for the purposes of devising a model. A simple model has been devised to find the impedance of each segment of the transition  20  so that dimensions could be determined for experimentation purposes. D 1 , D 2 , and D 3  respectively represent the diameters of the semi-cylindrical dielectric  27  at the cable trough line  22 , the coaxial inner conductor  23 , and the coaxial outer conductor  25  (shown in FIG.  8 ). E r  represents the dielectric constant of the solid dielectric  24  in the cylindrical portion  21  and the solid dielectric  27  in the stabilized half of the semi-cylindrical or cable trough line portion  22 . 
     A number of dielectric materials have been considered for use as the solid dielectric  24  and  27 . There are many good candidates. The solid dielectric  24  and  27  must bond to the inner conductor  23  and  26 , and be suitable for production to small tolerances (possibly 0.001 inches or less (i.e., 0.025 mm or less)). The material is preferably grindable with conventional grinding equipment. Other requirements further narrow the list of possible dielectrics. These requirements include frequency of operation, the nature of the connection cable (and its impedance), vacuum compatibility, temperature exposures, and stability through thermal cycling. Although many materials may be used for the dielectric  24  (e.g. hard plastic such as PEEK), Table 3 below illustrates the output of the model using dense Teflon™ as the dielectric  24 . 
     
       
         
               
             
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 TROUGH/COAX LINE EVALUATION 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 TROUGH COAX LINE OUTER DIA, D 1   
                 0.0258″ 
               
               
                 COAX INNER DIA, D 2   
                 0.0120″ 
               
               
                 COAX OUTER DIA, D 3   
                 0.0402″ 
               
               
                 1ST SECTION COAX REL DIEL CONST, E r   
                 2.100 
               
               
                 1ST SECTION COAX LINE IMPEDANCE 
                 50.00Ω 
               
               
                 IMPEDANCE OF TROUGH LINE 
                 50.00Ω 
               
               
                 TOTAL CAP/UNIT L OF TROUGH LINE 
                 0.8959E − 10 F/m 
               
               
                 EFFECTIVE DIEL CONST OF TROUGH LINE 
                 1.806 
               
               
                 TROUGH LINE RELATIVE PHASE VELOCITY 
                 0.7442 
               
               
                   
               
             
          
         
       
     
     Some of the benefits of using a material such as PEEK or Teflon™ as the dielectric include that these materials may be produced by injection molding or conventional machining and grinding of a solid piece. In addition, precise dimensions may be obtained. Thus, a transition  20  made with a PEEK or Teflon™ dielectric is easy and inexpensive to produce. The flat surface  28  of the cable trough line  22 , shown in FIGS. 5-8, provides a bonding surface which may also be produced inexpensively and in large numbers despite its small size. Therefore, the preferable material for the dielectric  24  and  27  for the transition  20  is a material such as PEEK or Teflon™. 
     The degree of precision necessary for the dimensions of the transition  20  must be determined for the particular material used for the dielectric  24  and  27 , with consideration of the methods used for constructing the cable trough line  22 . FIGS. 9 and 10 show a fixture  40  that may be used to determine the sensitivity of a coaxial line&#39;s impedance to the dimensions of the cable trough line  22 . K-connectors™, which are well known in the art, may be used to interface the fixture  40  with test equipment. The return loss of the fixture  40  is monitored as a fixture-trough  41  (which is to become the cable trough line  22 ) is ground down. The depth of the fixture trough  41  will be monitored as the grinding progresses so that voltage standing wave ratio (VSWR) at a given frequency can be measured as a function of depth of the trough  41  and used to prove the design dimensions. The dimensions of the fixture  40  may be determined using information such as that in Table 3. 
     Once dimensional specifications are determined for the dielectric  24  and  27  and inner conductor  23  and  26  (see FIG.  9 ), a method of manufacturing the transition  20  can be determined. For a solid dielectric material with a strong interface to the inner conductor  23  and  26  (such as sealing glass), a grinding process could be used once the dielectric  24  and  27  is attached to a housing. For a softer dielectric material, such as Teflon™ or PEEK, the dielectric  24  and  27  could be manufactured separate from the inner conductor  23  and  26  and used as a standard part for any variety of housings. 
     The transition  20  may be manufactured through a process similar to that described above for the cryocable  10 . However, before the outer conductors  25  and  30  (shown in FIGS. 5-8) are plated on the cylindrical surfaces of the dielectric  24  and  27 , the transition  20  is turned to form the portion with the smaller diameter d (see FIG.  6 ). After the portion having the smaller diameter d is formed, the outer conductors  25  and  30  may be plated on the exterior surfaces of the dielectric  24  and  27 . After the plating is completed, the portion of the transition  20  with the smaller diameter d is then ground down or chopped to form the semi-cylindrical portion  22  and the flat surface  28  of the semi-cylindrical portion  22  (shown in FIGS.  5 - 8 ). 
     FIG. 11 provides an exemplary flow chart for the production and assembly of a transition  20  including a cable trough line  22  using Teflon™ as the dielectric  24  and  27  material. First, as is described above, a designed is used in which a model of the transition  20  may be tested for its impedance at various dimensions. Then, the particular components may be designed. Next, the inner conductor  23  and  26  and the dielectric  24  and  27  are manufactured. Then, the inner conductor  23  and  26  and the outer curved surfaces of the dielectric  24  and  27  are plated. Finally, the inner conductor  23  and  26  is positioned in the dielectric  24  and  27  and glued, bonded, epoxied, soldered, or held by friction in place. The transition  20  is now ready to be assembled in a housing and bonded to a circuit as shown in FIG.  5 . 
     Coaxial connectors enable the cryocable  10  to connect to the transition  20  and/or to electronics held at ambient temperatures. FIGS. 4 and 5 show an exemplary cold housing connector  50  that provides an appropriate coaxial connection between the cryocable  10  and the transition  20 . The cold housing connector  50  includes an end receptacle or sleeve  51  which accepts both the inner conductor  11  from the cryocable  10  and the inner conductor  23  from the transition  20  (see FIG.  5 ). The inner conductors  11  and  23  may be soldered together within the end receptacle  51 . The end receptacle  51  may be provided with a spring finger contact  52  to provide a snug fit between the inner conductor  23  and the end receptacle  51 . 
     As shown in FIGS. 4 and 5, axially surrounding the end receptacle  51  is a dielectric  53  and axially surrounding the dielectric  53  is a metal connector housing  54 . The dielectric  53  must be sized to provide the cold housing connector  50  with the appropriate impedance (i.e., with an impedance which matches that of the cryocable  10  and the transition  20 ). One would expect that to provide the cold housing connector  50  with the appropriate impedance, the dielectric  53  would be of a larger diameter than the dielectric  12  of the cryocable  10  due to the end receptacle  51  having a larger diameter than the inner conductor  11 . The connector housing  54  is preferably made from metal and preferably acts as an outer conductor for the connector  50 . 
     FIGS. 4 and 5 each show an embodiment of an exemplary warm housing connector  55  that may provide an appropriate coaxial connection between the cryocable  10  and electronics held at ambient temperatures. The warm housing connector  55  shown in FIG. 4 includes an end receptacle or sleeve  56  which accepts both the inner conductor  11  of the cryocable  10  and a feed through inner conductor  57 . As is mentioned above, it is preferable that the connection between the cryocable  10  and ambient temperature electronics have a vacuum seal so, for example, the connection may extend through the wall of a vacuum dewar. The feed through inner conductor  57  shown in FIG. 4 is provided with a soldered in glass bead  58  surrounding the inner conductor  57  and thereby providing a vacuum seal. The glass bead  58  may then be attached to the wall of the dewar to provide a vacuum tight seal. The glass bead  58  has a metal outer coating to enable the glass bead  58  to be soldered into the dewar wall to thereby provide a vacuum tight seal. The inner conductors  11  and  57  may be soldered together within the end receptacle  56 . The end receptacle  56  may be provided with a spring finger contact  59  (see FIG. 4) to provide a snug fit between the inner conductor  57  and the receptacle  56 . 
     The warm housing connector  55  shown in FIG. 4 also includes a dielectric  60  axially surrounding the end receptacle  56  and a metal connector housing  61  axially surrounding the dielectric  60 . As with the dielectric  53  of the cold housing connector  50  described above, the dielectric  60  of the warm housing connector  55  must be properly sized to provide the connector  55  with the appropriate inductance. As with the connector housing  54  of the cold housing connector  50  described above, the connector housing  61  of the warm housing connector  55  is preferably made from metal and is preferably gold plated so it acts as an outer conductor for the connector  55 . 
     The warm housing connector  55  shown in FIG. 5 incorporates the inner conductor  11  of the cryocable  10  as a continuous inner conductor. The inner conductor  11  extends through a fired in glass bead  62 . The fired in glass bead  62  provides a vacuum seal between the inner conductor  11  and a metal connector housing  63 . The metal connector housing  63  may then be directly attached to the dewar housing  64  via, for example, electron beam or laser welded. 
     As shown in FIGS. 4 and 5, the cryocable  10  is preferably connected to the cold housing connector  50  and the warm housing connectors  55  via separate protective jacket  65  and a threaded collar  66  arrangements. The protective jackets  65  are preferably provided over a portion of the outer conductor  13  of the cryocable  10  that is to be covered by the threaded collars  66 . The protective jackets  65  protect the thin outer conductor  13  from being damaged by the connection. The threaded collars  66  preferably fit over the protective jackets  65  and by pressure contact caused by the collar  66  threadedly screwing into the housing  54 , connect the cryocable  10  to the cold housing connector  50  and the warm housing connector  55 . The threaded collars  66  provide mechanical rigidity and electrical integrity to the cryocable  10  at the connections. 
     The cold housing connector  50  and the warm housing connectors  55  may be provided with bolt apertures  67  (shown in FIGS. 4 and 5) to enable the cold housing connector  50  to be bolted to the circuit housing  31  and the dewar housing  64  respectively. However, as is explained above, the warm housing connector  55  shown in FIG. 5 may be directly connected to the dewar housing  64  by means other than bolting (i.e., by soldering, gluing, electron beam welding or laser welding). 
     Embodiments of interconnects other than a coaxial cable geometry may be used to accomplish the present invention. Specifically, the cryocable  10  may be produced as a stripline (with or without side grounds) as shown in FIGS. 12 and 13 respectively. Such stripline cryocables  10 , as are shown in FIGS. 12 and 13, would include a center conductor  11 , a surrounding dielectric  12 , and an outer conductor  13  which may completely surround the dielectric  12  as is shown in FIG. 12 or which may exist only on two sides of the dielectric  12  as is shown in FIG.  13 . 
     In another variation of the stripline configuration, the cryocable may be configured as a flat cryocable  100  as shown in FIG.  18 . The flat cryocable  100  is very similar to the cryocable  10  shown in FIG.  13  and likewise includes a center conductor  11  surrounded by a surrounding dielectric  12 . The dielectric  12  may be formed by two strips of dielectric, such as PTFE sandwiching the center conductor  11 . Outer conductors  13  are attached to two sides of the dielectric  12 . 
     One or both ends of the flat cryocable  100  may be configured as shown in FIG. 18 for attachment to a warm housing connector and /or a cold housing connector. A slot  102  is cut out of the conductor  13  and through the dielectric to expose the center conductor  11  from the top and/or bottom of the cryocable  100  (only a top slot  102  is shown in FIG. 18, with the understanding that a similar slot may be formed in the bottom of the cryocable  100 ). The method of attachment to a housing connector is described below in detail in conjunction with the description of a push-on connector. 
     The opposite end of the flat cryocable  100  may also be configured as shown in FIG. 18, and may additionally be fitted with a T-shaped connector  104  as shown in FIG.  19 . The T-shaped connector  104  has a bottom-plate  106  which is bonded to the conductor  13 . The T-shaped connector  104  has an access hole  108  to provide access for a connecting HTS circuit to the center conductor  11 . Two mounting holes  110  are provided for bolting the T-shaped connector  104  to a structure such as the circuit housing  31  (see FIG.  5 ). 
     In addition, the cryocable  10  may be produced in a microstrip configuration or a balanced microstrip configuration as is shown in FIGS. 14 and 15 respectively. Such microstrip cryocables  10 , as are shown in FIGS. 14 and 15, would include a first conductor  11  which acts as a center conductor, a dielectric  12 , and a second conductor  13  which acts as an outer conductor. The first conductor  11  of the microstrip cryocable  10  shown in FIG. 14 is smaller in size than that second conductor  13 . As shown in FIG. 15, the first and second conductors  11  and  13  of the balanced microstrip crypcable  10  are of approximately the same size. 
     Furthermore, the cryocable  10  may be produced in a coplanar waveguide or a coplanar slotline configuration as are shown in FIGS. 16 and 17 respectively. Such coplanar cryocables  10 , as are shown in FIGS. 16 and 17, would include a first conductor  11  which acts as a center conductor, a dielectric  12 , and a second conductor  13  which acts as an outer conductor. These cryocables  10  are coplanar because both conductors  11  and  13  are positioned on the same side of a planar dielectric  12 , as is shown in FIGS. 16 and 17. The coplanar waveguide cryocable  10 , as shown in FIG. 16, includes two-second conductors  13  that are positioned on the dielectric  12  on either side of the first conductor  11 . As shown in FIG. 17, the first and second conductors  11  and  13  of the coplanar slotline cryocable  10  are singular and lie next to each other on the dielectric  12 . 
     The use of stripline, microstrip, or coplanar or slotline transmission lines instead of coaxial cables does not change the mode of operation of the cryogenic cables. The basic change is that the stripline interconnects, the microstrip interconnects, and the coplanar or slotline interconnects are rectangular (rather than round as for the coaxial case described above). This means that the stripline, the microstrip, or the coplanar or slotline realization can be manufactured from standard circuit patterning and etching of thin copper conductors on a dielectric substrate (for example, RT Duroid from Rogers Corporation, 100 S. Roosevelt Ave., Chandler, Ariz. 85226, U.S.A.). 
     In another embodiment of the cryocable  10  shown in FIGS. 4 and 5, the warm housing connector and/or the cold housing connector may be replaced by push-on connectors  120  as shown in FIGS. 20,  21 ,  21 A,  22 . Instead of the threaded connectors  50  and  55 , a push-on connector  120  may be provided at one or both ends of the cryocable  10 . The push-on connector  120  of the present invention allows faster and simpler assembly and disassembly of the cryocable  10  to the HTS circuit and/or the feedthrough than the threaded connectors  50  and  55  described above or bonded connections such as soldering or adhesive. 
     The push-on connector  120  disconnectably mates with a receptacle  122  as shown in FIGS. 22,  23 ,  23 A. At the warm housing side of the cryocable  10 , the receptacle  122  may be housed in an ultrahigh vacuum hermetic feedthrough  124 . On the cold housing side of the cryocable  10 , the receptacle  122  may be integrated with the transition  20 , or alternatively, the receptacle  122  may be configured with another connection (not shown) which mates with the transition  20 . In the still another embodiment (not shown), an interface connector may be provided which connects the receptacle  122  to the transition  20 . 
     Returning to FIGS. 20,  21 ,  21 A, the preferred embodiment of the push-on connector  120  will be described in detail. The push-on connector  120  comprises an outer shell  126 , which is made of an electrically conductive material, preferably BeCu as shown in FIG.  21 . The outer shell  126  has a spring-loaded locking portion  128 . The locking portion  128  preferably comprises a flared cylinder having longitudinal slots thereby forming a plurality of flexible detents  130 . For example, four slots will form four detents  130  (see FIG. 21) as shown in the end view of FIG.  21 A. The number of slots may be varied to adjust the flexibility or stiffness desired. A raised lip  132  is provided at the end of the locking portion  128  and is shaped to fit within a recess  134  (see FIGS. 22,  23 ) of the receptacle 
     The end of the outer shell  126  opposite the locking portion  128  is a cable connection  136 . The cable connection  136  on the push-on connector embodiment shown in FIGS. 20,  21 ,  21 A,  22  is configured for attachment to the flat cryocable  100  as shown in FIGS. 18-19. It is to be understood, however, that the cable connection  136  may be configured for a coaxial cryocable as shown in FIGS. 4-5, or any other suitable cable, for example, the cables shown in FIGS. 12-15. 
     The cable connection  136 , as shown for the flat cryocable  100 , comprises a solid section of a cylinder  138 , the section cut just below the center axis  140  of the cylinder to create a flat ledge  142 . The flat ledge  142  effectively receives the flat cryocable  100 . 
     A dielectric  144  is inserted into the locking portion  128  and extends to the edge of the ledge  142 . The dielectric  144  can be made of any suitable material and is preferably made from PTFE. The dielectric  144  has a center bore which accommodates a center conductor  146  and a spring contact  148  (as shown in FIG.  21 ). The center conductor  146  and the spring contact  148  are electrically conductive and are electrically connected to each other. A portion of the center conductor  146  extends out of the dielectric  144  to form a pin  150  which is easily accessible so it can be connected to the center conductor  11  of the flat cryocable  100 . 
     Referring to FIGS. 22,  23 ,  23 A, the push-on connector  120  is connected mechanically and electrically to the flat cryocable  100  by sliding the slotted end of the cryocable  100  onto the ledge  142 . The pin  150  of the push-on connector  120  fits into the slot  102  of the cryocable  100  such that the pin  150  sits on or over the cryocable center conductor  11  that is exposed through the slot  102 . 
     The cryocable center conductor  11  may be attached to the pin  150  via a ribbon wire by ultrasonic bonding, gap welding or any other suitable method. Alternatively, it may be attached directly with solder or conductive adhesive. The cryocable center conductor  11  of the cryocable  100  is attached to ledge  142  by solder or conductive adhesive. 
     Returning to FIG. 22, the push-on connector  120  is shown connected to a mating receptacle  122  which is shown integrated with a vacuum feedthrough  124 . Although the receptacle  122  is shown in FIGS. 22 and 23 and described herein as integrated within a vacuum feedthrough  124 , it is contemplated that the receptacle  122  may be a stand alone connector without the vacuum feedthrough  124 . For example, a similar receptacle may be used to connect the cold side of the cryocable  10  to the HTS circuit wherein there is no need for a hermetically sealed feedthrough. 
     As is shown in FIGS. 23 and 23A, the receptacle  122  has a body  152 , preferably formed of Kovar. The body  152  has a substantially cylindrical cavity sized to receive the locking portion  128  of the push-on connector  120 . The receptacle  122  further includes a lead-in chamfer  154  and the recess  134  shaped to receive the raised lip  132  of the locking portion  128 . Another chamfer  156  is provided to facilitate removal of the locking portion  128  from the receptacle  122 . The chamfers  154  and  156  bias the detents  130  upon insertion and removal of the push-on connector  120  from the receptacle  122 . 
     The feedthrough  124  further comprises a dielectric  158  bonded to the body  152  in a manner which provides a high vacuum tight seal between the dielectric  158  and the body  152 . The dielectric is preferably made of glass, for example Corning 7052. Suitable glass-to-metal (e.g., Kovar to Corning 7052) sealing techniques are described in E. B. Shand,  Glass Engineering Handbook , 2nd Edition, McGraw-Hill Book Co., copyright 1958, which is hereby incorporated herein by reference. Such techniques have not previously been applied in high frequency electronics applications. A feedthrough center conductor  160  is bonded within the dielectric.  158  using a vacuum tight sealing method. 
     The feedthrough  124  may be attached to the dewar housing  64  in a manner providing a vacuum tight seal between the body  152  and the housing  64 , via, for example, electron beam welding, laser welding, or other known suitable methods. The body  152  of the receptacle  122  may be provided with a groove  162  to facilitate welding of the feedthrough  124  to the wall of the dewar housing  64 . Suitable sealing methods are well-known in the art and therefore, they are not described in detail herein. In a preferred embodiment, the feedthrough  124  has a leak rate of less than 1.0×10 −14  cc/second for Helium. 
     As with the threaded connectors  50  and  55  described above, the components of the push-on connector  120  are configured to be impedance matched to the cryocables  10  and  100 , the transition  20 , and the feedthrough  124 , as the case may be. This may be accomplished by approximately matching the ratios of the diameters of the respective conductors and dielectrics at each of the interfaces between the push-on connector  120 , the cryocables  10  and  100 , and the feedthrough  124 . For example, at the interface between the push-on connector  120  and the feedthrough  124 , the diameter of the dielectric  144  of the connector  120  should be larger than the diameter of the dielectric  158  of the feedthrough  124  because the spring contact  148  has a larger diameter than the feedthrough center conductor  160 . 
     The method of connecting the push-on connector  120  to the receptacle  122  and feedthrough  124  is quite simple. The lip  132  of the locking portion  128  of the connector  120  is first aligned with the lead-in chamfer  154  of the receptacle  122 . As the connector  120  is pushed into the receptacle  122 , the lead-in chamfer  154  forces the flexible detents  130  inward, thereby allowing the connector  120  to be further inserted. As the connector  120  is further inserted, the spring contact  148  receives the feedthrough center conductor  160 . Upon full insertion, the raised lip  132  reaches the recess  134  and the detents  130  expand outward radially such that the raised lip  132  locks into the recess  134  as shown in FIG.  22 . The connector is disconnected by simply pulling the connector  120  out of the receptacle  122 . 
     While embodiments of the present invention have been shown and described, various modifications may be made without departing from the scope of the present invention, and all such modifications and equivalents are intended to be covered.