Lead-in module for the supply of a low critical temperature superconducting electric load
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a lead-in module for the supply of a low critical temperature superconducting electric load. 
     The invention particularly applies to currents ranging from a few ten&#39;s to a few hundred amperes. 
     2. Description of the Related Art 
     Lead-in modules are known of which consist of metal conductors, made of copper, for example, connected to the electric load located within liquid helium, with the connection between the load and the lead-in conductors submerged in the helium. However, this type of arrangement results in losses through the Joule effect in the lead-in conductors and also through thermal conduction, leading to a significant helium consumption. It is also necessary to provide a support for the load, which can also result in losses through conduction. 
     SUMMARY OF THE INVENTION 
     The purpose of this invention is to eliminate these disadvantages and its object is a lead-in module for the electrical supply of a low critical temperature superconducting electric load, said module being located inside a cryostat and fixed to its sealing cover, characterized in that it comprises a pair of metal conductors which pass through said cover and whose lower ends are connected to the upper end of a high critical temperature superconducting module comprising two conductors electrically connected to said pair of metal conductors and separated by an insulating core making up a mechanical reinforcement, with an insulating structure whose upper end is fixed to the bottom of said cover of the cryostat, surrounding said pair of metal conductors in a sealed manner until its junction with said high critical temperature superconducting module, said structure extending in an unsealed manner until at least the lower end of said superconducting module, said structure comprising at its lower end fastening means to support said electric load. 
     According to another characteristic, the sealed part of said insulating structure is partly filled with liquid nitrogen. 
     According to another characteristic, said electric load is electrically connected to the lower end of the two conductors of the high critical temperature superconducting module, and said cryostat is partly filled with liquid helium to a level reaching at least said electrical connection between said load and said conductors of the superconducting module. 
     Advantageously, both said structure and the insulating core separating the two conductors of the superconducting module are made of loaded epoxy resin, with the two conductors made of superconducting ceramic. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A description will now be provided of an example of the invention&#39;s implementation referring to the attached drawing in which: 
     FIG. 1 is a diagram of a lead-in module according to the invention located inside the cryostat. 
     FIG. 2 is a cross-section along II--II of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the figures, a cryostat 1 is shown with its sealing cover 2 to which is fixed a lead-in module 3 according to the invention for the electrical supply of a low critical temperature superconducting electric load 4, such as a coil. This electric load 4 is submerged in liquid helium 5 at 4.2° K. 
     The lead-in module 3 comprises a pair of metal conductors 6 and 7, made of copper for example, connected at their lower ends to the upper end of a high critical temperature superconducting module 8 comprising two conductors 9 and 10 made of superconducting ceramic with a critical temperature Tc≧80° K and separated by an insulating core 11 made of loaded epoxy resin which makes up a mechanical reinforcement for conductors 9, 10. 
     The superconducting ceramic conductors 9, 10 are electrically connected to copper conductors 6, 7 by means of a process known per se. 
     At their lower ends, superconducting ceramic conductors 9, 10 are also electrically connected to the two ends 12, 13 of loading coil 4 in a manner known per se. This, for example, can consist of a solder which can be easily undone in order to possibly change the load 4. 
     The metal conductors 6, 7 are surrounded by an insulating structure 14 made of loaded epoxy resin, for example, which is fixed to the bottom of the cryostat&#39;s cover 2 by means of a flange 15. 
     This structure 14 makes up a sealed enclosure until the junction 16 between the metal conductors 6, 7 and the conductors 9, 10 of the superconducting module 8. This junction 16 makes up the bottom of this sealed enclosure. The lower part of this sealed enclosure contains liquid nitrogen 22 at 77° K. 
     A tube 17 which passes through the cover 2 ensures the supply of nitrogen. This tube is equipped with a plug 18. 
     Below the junction 16, the structure 14 made of loaded epoxy resin extends until beneath the lower end of the superconducting module 8 by means of a plurality of struts 19 whose lower ends form a flange 20 to fasten the load 4 which is thus suspended on to the structure 14. 
     The helium level 21 in the cryostat is such that it lies above the electrical connection between the conductors 12, 13 of the load 4 and the ceramic superconducting conductors 9, 10. 
     A lead-in is thus obtained which creates only a weak cryogenic load at the temperature of the liquid helium. Indeed, the copper conductors 6, 7 are not submerged in the helium. 
     The nitrogen 22 keeps the upper end of the superconducting module 8, i.e. the junction 16, at a temperature of 77° K, a temperature below the critical temperature. 
     The intermediate temperature of the junction 16 can also be obtained by placing a heat exchanger within the insulating structure 14 in its sealed part. This exchanger is in contact with the junction 16 and a stream of cold gas flows through it. 
     The structure 14 made of loaded epoxy resin, molded as a single piece, is sturdy and ensures both the role of a nitrogen tank 22, making it possible to obtain, from the lower end of the module 8 at 4.2° K until the junction 16 at 77° K, a temperature gradient which, as mentioned above, keeps the module 8 at a temperature below its critical temperature throughout its entire length as long as the working current is not seriously exceeded and, at the same time, this structure 14 ensures the role of a mechanical support for the electric load 4.