This invention is related to methods for forming superconductive joints; more particularly, the present invention is directed to methods for forming superconductive joints between superconductors having as many as thousands of wire filaments.
In whole body, nuclear magnetic resonance (NMR) imaging systems, it is necessary to provide a high strength, uniform magnetic field, typically between about 0.04 and 1.5 Tesla, or more. A desirable means for providing this field employs superconductive coil circuits operating in the persistent mode. This method for producing such magnetic fields is desirable in that continual supply of electrical energy to the coils is not required. However, such circuits must typically carry current at a high level of current density, typically 10,000 amperes/cm.sup.2, or more. To insure that such circuits do not spontaneously exhibit localized resistive (or ohmic) regions which leads to quenching of the current, it is necessary to provide sound superconductive joints on various portions of the current loop. Furthermore, because of the high levels of current employed, it is highly desirable to employ superconductive conductors having a composite structure. The composite structure typically takes a form in which a large plurality of superconductive strands or filaments are disposed within an overlaying or surrounding matrix of normal, resistive material such as copper or aluminum. Typically the copper or aluminum matrix provides an alternate, localized current path in the event that a small portion of the superconductive wire exhibits a transition to the resistive (ohmic) state. Such a composite structure is desired because of the relatively large current density desired in NMR imaging. It should also be understood that the superconductors and composite superconductors described herein are typically disposed within a coolant bath, such as liquid helium, so as to keep their temperature in the vicinity of 4.2.degree. K. to maintain the superconductive material in the zero resistance state.
Two methods have heretofore been used to produce superconductive joints. In a first of these methods, individual superconductive filaments, after stripping of the surrounding matrix, are individually spot welded to an intermediate niobium foil. However, this method is impractical when hundreds or thousands of filaments are to be joined. Unfortunately, modern composite superconductive conductors often employ thousands of filaments in their construction. Since such conductors are desirable in the coils for NMR imaging systems, this method of superconductive joint formation is therefore highly undesirable from a manufacturing view point. In a second method, bundles of superconductive filaments from which the surrounding matrix has been stripped, are crimped together in a copper sleeve. However, this method of joint formation has not been demonstrated to work with a strand of more than one or two hundred filaments. Thus, this method is also seen to be undesirable for the construction of the main magnetic field coils for NMR imaging systems.
Accordingly, it is seen that it is highly desirable to be able to join together superconducting conductors having between 200 and 2,000 filaments (or more) together in a way in which a sound truly superconductive joint is formed. It is further seen that this joint should be capable of long term operation in a low temperture coolant bath and be constructed so that it does not exhibit a tendency for transition to the normal, resistive state particularly as a result of microscopic filament motion.