Superconducting Magnet Materials

A promising new development in practical high-field superconductors is the technique developed by Tachikawa of the Institute of Metals, Tokyo, Japan, for fabrication of V3Ga in flexible wire or strip form. This can also be made as a composite with copper. The short-sample characteristic obtained is approximately that of present day commercially manufactured Nb3Sn at magnetic fields of up to 150 kG and is much higher beyond this field. The U.S.S.R. and Japan are manufacturing three element systems of NbZrTi which are comparable in properties with currently available NbTi in the U.S.A., but such systems do not, as yet, appear to offer any significant advantages over NbTi. At present magnets are being fabricated from NbTi, Nb3Sn, or both, usually as composite conductors in intimate contact with copper and frequently with the addition of reinforcing where the operating stress levels are high enough to merit this. The properties of these basic superconductors are well known. NbZr is now obsolescent and current practice favors the use of the cheaper, lighter, and more ductile NbTi. Several recent and highly successful large coils have been fabricated from standard superconductors but improvements in the manufacture of composites of copperclad NbTi with one or more superconducting strands and almost any desired current carrying capacity or size render such stranded conductors obsolescent. Experience with cables and composites has shown that metallurgical-type diffusion bonds between the superconductors and co-extruded copper give best performance and are mandatory when superconductors of large cross section are used. Mechanical-type bonds are being used with success in composities containing small-cross-section superconductors. When the highest current density is desired the diffusion-type bond permits the use of much less copper in the cross section of the composite for the same performance. The techniques of adding additional copper and reinforcement are also being applied to the 1.25 cm wide Nb3Sn ribbons now available so Nb3Sn conductors of almost any strength and current carrying capacity can also be fabricated. In this case, soldering techniques are used to add the additional copper. The most advanced heavy-section composite conductor yet supplied and tested for large low-field magnets is the copper-clad NbTi strip of width 2 in. and thickness 0.1 in. produced for the Argonne National Laboratory 4.8 m i.d. hydrogen bubble-chamber magnet. This contains six NbTi strips and has been operated in a 60 cm i.d. test coil to 4600 A, while developing maximum fields of 30 kG parallel to the longer dimension of the strip cross section and 19 kG perpendicular to this direction. More advanced large conductors are in production. The most advanced NbTi composite conductors for use in high current density smaller coils of up to 60 cm i.d. at central fields of about 60 kG are of the type being produced for the outer sections of the 88 kG, 50 cm i.d. coils being constructed for the NASA Lewis Research Center by the Avco-Everett Research Laboratory. The conductors can be wound into stable magnets if suitably cooled. They contain a considerably lower ratio of copper to superconductor than earlier conductors of this type. The increased stability is attained by limiting the temperature rise in the superconductor when sudden movements of flux occur. This is accomplished by using a larger number of superconducting strands for a given total cross section of superconductor so as to maximize the surface-to-volume ratio for the superconductor and hence obtain the best possible cooling. Extra superconductor is also being incorporated in the conductors where necessary as a simple and cheap means of providing extra reinforcing in highly stressed conductors. Proposals have been made for hollow copper-clad conductors in which high-pressure helium in the dense fluid phase can be circulated so that high heat-transfer rates can be obtained. Tests on samples of this type of conductor have been made at the Stanford Linear Accelerator Center, and it is hoped that this will allow high current densities to be obtained in larger coils without the necessity for a liquid-helium tank encasing the complete coil. Present Nb3Sn composite conductors are both more rugged than formerly, more stable in operation, and have a higher current-carrying capacity. The most advanced conductor of this type has recently been developed by General Electric to meet the Avco-Everett Research Laboratory requirement for the inner sections of the previously mentioned 50 cm. i.d. 88 kG magnet. The 1.25 cm wide conductor contains outer layers of 0.075 mm thick stainless steel for reinforcing and the superconductor is sandwiched between two layers of 0.05 mm thick copper to provide the electrical protection. The conductor is rated at 300 A for 100 kG and recent tests with 18 cm i.d. counterwound pancakes of this material in Argonne National Laboratory show it to be stable in operation with a smooth reversible transition into the resistive region. Our experiments in Argonne with larger-size pancaketype coils also show that higher operational stability in 1.25 cm wide copper-coated Nb3Sn strips can be obtained if a number of narrower parallel superconducting strips are used instead of one superconducting strip occupying the whole width of the superconductor. It is still true at the moment that NbTi composite conductors provide the cheapest means of providing fields of from 60 to 80 kG in medium-size coils of the order of 20 cm i.d. or larger and that Nb3Sn composite conductors are necessary for the higher fields. The techniques for using Nb3Sn strips in pancake-type windings to provide the highest fields and highest current densities in coils greater than 5 to 10 cm i.d. are still in their infancy and require considerable development. Although no significant new practical conductors have appeared recently, a great deal has to be done before the potential performance of present materials, as measured by the attainable current densities and maximum attainable magnetic fields reached in short samples, can be realized in practical magnets.