Abstract:
A detachable superconducting lead includes a vacuum-sealed thermal transition through which a stabilized conductor passes. Two identical leads are attached and surrounded by a sealed Dewar and allowed to cool either naturally or by way of a cooling element. Detaching the leads requires the joint to be heated up by a heat transfer unit or by a heat gun after the Dewar is removed. Once warmed, the lead can be disassembled with tooling appropriate to the joint. In many instances, regular fasteners can be used. Removable Dewars may be constructed with insulation (including vacuum) using O-rings and flanges.

Description:
This patent claims the benefit of provisional patent application 60/474,326, filed Jun. 2, 2003. 

   BACKGROUND OF THE INVENTION 
   An important issue in large cryogenic power systems is that of maintenance, for example, the ability to easily access, maintain, and repair or interchange electric elements housed inside an evacuated Dewar. In order to achieve this, electronic elements must be removable. That is, one must be able to physically and electrically isolate an electrical element (including superconducting cables) and remove it from the system to maintain or repair the element without disrupting either the vacuum or the temperature control within the remainder of the system. This creates problems when superconducting busses are used to connect the various elements within the system. Superconducting cables currently in use and under development are not easily detachable. Disassembly can compromise the Dewar and vacuum spaces, increasing refrigeration loads (losses).  FIG. 1  shows the current art  10 , which is a completely sealed cryogenic container  28 ,  22 ,  28 ′ including welded sections, flanges or O-rings  11 ,  12 . The cable  21  is continuous. In order to service the system, i.e., to assemble or replace a lead  21 , the cryogenic spaces  23  need to be warmed, the cryogenic fluids or gases in the spaces  23  removed, and the vacuum system brought up to atmospheric pressure. Then, access to the leads  21  is accomplished by removing seals and, in some cases, un-welding some of the metallic vacuum spaces. Reassembly requires replacing the seals and in many cases re-welding the structure. This costs much time and labor. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention a method and apparatus are provided for interconnecting and disconnecting superconducting or cryogenically cooled cables without substantial disturbance to operating conditions; i.e., temperatures, vacuum, refrigeration system loading, in the system where the cables operate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a current state of the art interconnection between modules; 
       FIG. 2  is a detachable lead interconnection in accordance with the invention; 
       FIG. 3  shows a detachable lead interconnection similar to  FIG. 2 , with integral heaters and coolers; and 
       FIG. 4  shows the detached leads of  FIG. 3 , disconnected and with insulating caps. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   This innovation addresses a solution to this assembly/disassembly problem for superconducting or low temperature cables. It can be applied to cryogenic systems with superconducting distribution networks including power splitters, T&#39;s, in-line connectors, etc., that is, anywhere a general-purpose superconducting connection is required. 
   In  FIG. 2 , the method and apparatus  20  is illustrated for attaching and detaching superconducting cables  21 ,  21 ′ without compromising the insulation, the vacuum, or cryogenic spaces, and without requiring that the attached subsystems  28 ,  28 ′ be heated up to do so.  FIG. 2  is a specific example of the detachable superconducting cable concept  20  for high temperature superconductors at 77 K, although it equally applies to all superconductors at any temperature below superconducting transition temperature (T c ) when constructed with the proper insulation systems. The superconducting cables  21 ,  21 ′ are housed in cryogenic dewars  22  and  22 ′ with special sealed terminations  24 ,  24 ′. The cables  21 ,  21 ′ at their ends include two elements, the superconductor  27 ,  27 ′ and a stabilizer element  25 ,  25 ′ in parallel (or another stabilizer construction). 
   The end of the superconducting cable  26  is a copper-stabilized superconducting lead  25 , which passes through the vacuum-sealed thermal transition  24 . The end  26 ′ is similarly constructed. (Copper or any stabilizing metal can be used for the stabilizer  25 ,  25 ′ to prevent the lead  27 ,  27 ′ from burning out if the superconductor quenches. The amount of stabilizing material used depends on an operational/fault assessment.) Externally of the transitions  24 ,  24 ′ the two cables  26 ,  26 ′ are bolted together in such a way that the superconducting element  27  of the first cable  21  is joined to superconducting element  27 ′ in the second cable  21 ′, as shown in  FIG. 2 . Attachment of the exposed ends  26 ,  26 ′ of the cables via the fastener  32  can be made at room temperature even when non-exposed portion of the superconducting cables  21 ,  21 ′ and associated circuits  23 ,  23 ′ are held at low temperature. To do this requires that the exposed ends  26 ,  26 ′ of the cables be warmed. This warming can be accomplished naturally by exposing them to air, by attaching them to a warm heat sink, or by heating them with a heat gun or other heater. 
   The terminations  26 ,  26 ′, bolted together with the fastener  32 , are covered with electrical insulation and aluminized mylar  29  in a removable Dewar  31 , which clamps around the joint, forming a leak-tight seal with the ambient and with the thermal transitions  24 ,  24 ′. The interface connections between the exposed cable ends  26 ,  26 ′ with the respectively connected thermal transitions  24 ,  24 ′ are leak tight. The Dewar  31  is a removable conventional vacuum wall Dewar, a sealed foam Dewar, or a combination of both. The joint, after connection, cools down naturally with the sealed Dewar  31  around it by thermal conduction. In this process, heat is absorbed from the joint via the thermal transitions  24 ,  24 ′ into the cryogenic space  23 ,  23 ′. The equilibrium temperature of the joint depends on the heat flowing from the outside ambient through the Dewar  31  and thermal transitions  24 ,  24 ′. 
   When disassembling a joint, the Dewar  31  is first removed to allow the joint to heat up at the exposed portions  26 ,  26 ′  32 . 
   The terminations  26 ,  26 ′ of the cables  21 ,  21 ′ can also be outfitted with permanent heating/cooling elements  33 , shown in  FIG. 3 , which are used to control the temperature of the ends  26 ,  26 ′. The heating/cooling elements  33  prepare the joint terminations  26 ,  26 ′ for attachment or detachment. When reattaching the leads, the elements  33  cool down the leads below T c  so they can be put back into service with reduced time delay. Also, if the natural equilibrium temperature reached during cooling ( FIG. 2 ) is above the superconducting transition temperature T c , the joint can be cooled down to a temperature below T c  using the heat exchanger in the element  33 . Cooling the joint below T c  makes the cable and joint almost lossless (superconducting) except for the contact resistance between superconductors  27 ,  27 ′. This operating condition dramatically reduces the overall heat loads of the system, especially for continuous high-current operation. The maximum static heat load occurs during maintenance, when the exposed joint is at room temperature. By proper design, this heat load can be made tolerable in most circumstances. 
   The heat exchangers  33  operate with a heat exchange gas or liquid. For 77 K operation, liquid nitrogen or cold nitrogen gas can be used for cooling, and heated nitrogen gas can be used for heating. Other cryogenic gases or compatible mixed refrigerants may be used in the heat exchangers  33 . The heat exchangers  33  can also contain electric heaters or thermoelectric coolers. Basically, the heat exchangers  33  speed up procedures when connecting and disconnecting the cables  21 ,  21 ′, and can greatly reduce the heat loads on the connected system&#39;s refrigeration unit(s) during transient conditions such as connection and disconnection. Warmed cable ends are available for connection and disconnection as circumstances may require. 
   Detaching the cables  21 ,  21 ′ is straightforward. Once the joint terminations  26 ,  26 ′ are warm, the removable Dewar  31  can be removed and tooling can be applied to detach the joint. Once the cables are detached, the heat losses (loads) to the cryogenic subsystems  28 ,  28 ′ are minimized by covering the ends with an insulating cap or Dewar  35  ( FIG. 4 ). Heat conduction (load) from the terminations  26 ,  26 ′ to the cables  21 ,  21 ′ are reduced by first covering the open terminations, each with a separate insulating cap  25 . Then (if needed) the ends  26 ,  26 ′ are cooled using the heat exchangers  33  as cooling elements. 
   Many types of connections between the superconductor  27 ,  27 ′ can be made using the removable and temporary Dewars  31 ,  35 , including spot welding, soldered connections, multi-pin (coax, triax, etc.) plug/sockets, quick disconnects, etc. One can even envision detachable leads with true superconducting joints that have no resistance. This usually requires special materials technology. For example NbTi low temperature superconductors (LTS) incorporate spot welding. NbSn requires special heat treatment. The key to a good overall design is the vacuum-sealed thermal transition  24 ,  24 , which should provide the minimum heat loss (load) when the joint is cold and yet be strong enough to stand off a vacuum during thermal cycling of the transitions  24 ,  24 ′. Low conductivity ceramics, stainless steel, and epoxies may be employed for the transitions  24 ,  24 . Also provisions must be made for thermal expansion and contraction during thermal cycling (designs allowing flexure, incorporating bellows, braided cables, etc.). Each cable or cryogenic subsystem can be part of a distributed or centralized refrigeration system. For the case of centralized refrigeration, provisions for connecting, disconnecting or bypassing sections using conventional cryogenic plumbing (not shown) is assumed. These issues can be accommodated by good engineering practices. 
   It should be understood that in alternative embodiments (not shown) in accordance with the invention, the stabilizer  25 ,  25 ′ may be omitted from the cables  21 ,  21 ′.