This invention relates to an apparatus and method for joining foils used in superconducting magnet tape to provide a superconducting joint.
As is well known, a magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance in the magnetic coils to negligible levels, such that when a power source is initially connected to the coil (for a period, for example, of ten minutes) to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible coil resistance even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter called "MRI").
Considerable research and development efforts have been directed at eliminating the need for a boiling cryogen, such as liquid helium, and in providing superconducting magnets which maintain the magnetic state and do not "quench," or discontinue superconductivity. However, the differential thermal expansion and contraction between materials in superconducting magnets, which are cycled from ambient temperature to temperatures in the range of absolute zero (-270.degree. C.), and the extremely large magnetic forces provided, and utilized, in a MRI lead to conflicting characteristics required of the materials, used in MRI magnet coils. In addition, the desired superconducting magnet coil material such as Nb.sub.3 Sn is often relatively brittle and difficult to handle in winding coils.
However, the manufacture of laminated tape suitable for superconducting use involves the lamination of long lengths of different materials such as niobium tin (Nb.sub.3 Sn) and copper by applying pressure while the Nb.sub.3 Sn and copper foils are fed through a molten solder bath and pinched together. The manufacturing process involves passing the foils and tape through a plurality of manufacturing process stations where the foils and tape are for example cleaned, anodized, unwound from and wound onto spools, passed by idler pulleys, laminated, passed through a solder bath, cut into strips and insulated.
A portion of such a manufacturing process is disclosed in our co-pending U.S. patent application, Ser. No. 07/967,316, now U.S. Pat. No. 5,299,728 entitled "Method and Apparatus For Laminating Foils Into Superconducting Tape For Use In A Superconducting Magnet", assigned to the same assignee as the present invention.
A persistent problem encountered in the manufacture of laminated tape suitable for use in superconducting magnet coils is that the lengths of the foils required to form a magnet does not correspond to the lengths of foil obtainable from foil manufacturers. As a result there is frequently unused portions of expensive foil. In addition, the various manufacturing stations and processes frequently result in the loss of end portions of the foil being processed and the ability to add leaders and trailers, or small portions at the ends of the tape being processed is highly desirable in conserving the foils. It is thus important to be able to suitably join foils used in the manufacture of superconducting tapes. However, it is also extremely important that the joint not only be susceptible of being made superconducting but also minimize any heat generated across the joint which will occur during superconducting current flow since any heat generated will result in the boiling and necessary replacement of the helium. Moreover, in passing through various pinch areas and around pulleys during the manufacturing process, the joint must pass freely through various restricted regions without presenting any loose ends which could bend or get caught in restricted regions of the process equipment. Also, the joints must pass without damage through seals in manufacturing stations which are isolated from the surrounding atmosphere, with such seals and their use in superconducting tape manufacture being disclosed in our co-pending patent application entitled "Seal Assembly for Superconducting Magnet Tape Ovens", Ser. No. 07/923,427, now U.S. Pat. No. 5,332,867 and assigned to the same assignee as the present invention.
The length of foil supplied by foil manufacturers is not uniform and depends on the length obtained during their manufacturing process. Foil lengths vary in the range of from 5,000 to 9,000 feet long while up to 60,000 feet of superconducting tape is typically utilized in a superconducting magnet for MRI use. Joining foils with superconducting joint capabilities enables the manufacture of superconducting tapes for superconducting magnets which are longer than the foils supplied by foil manufacturers.
Existing foil welding apparatus and methods have not proven to be entirely satisfactory. Problems with superconducting ability and critical current flow across the weld for MRI applications are overcome by the present invention.
It thus becomes important to provide satisfactory joints for foils suitable for use in superconducting magnet tapes. It is important that a superconducting joint exhibit minimum heat loss during operation, and minimized overlap without loose or separable joints must be provided. Still further, it is important that the weld region not interfere with subsequent manufacturing operations involving embossing the tape and flowing liquid tin over the embossed surfaces as described, for example, in our co-pending patent application, Ser. No. 08/134,456 entitled "Apparatus For Embossing Superconducting Tape For Use In A Superconducting Magnet", assigned to the same assignee as the present invention. Joints formed by the present invention do not inhibit the proper flow of tin in forming Nb.sub.3 Sn superconducting tape, particularly if the joints are rolled to reduce the thickness of the overlapped joint.
The ability to process longer lengths of foil formed by superconducting joints through use of the present invention at the start of the process of manufacturing laminated superconducting tapes can greatly reduce process cost through reductions in set up times at each step of the process, and increase the overall yield of the process, which in manufacturing Nb.sub.3 Sn tape involves twelve separate serial processes or work stations.