Superconducting materials--A review of recent advances and current problems in practical materials

In the four years since the last International Magnet Technology Conference, there has been a significant increase in both the breadth and scale of superconducting magnet construction. A number of advances have been pursued out of the laboratory into large scale construction so that today we see large scale magnets utilizing He I in free convection boiling, He I in conduit (force flow) and He II. Whereas four years ago filamentary Nb
3
Sn had only just become available in magnet conductor configurations and lengths, now it is being produced in quantity for a number of large projects. The discontinuous route to filamentary Nb
3
Sn has received much laboratory investigation and may soon be available for industrial scale production. Fundamental studies of Nb
3
Sn and other A15 compounds have shown that significant improvements in J
c
and H
c2
are possible, although not all improvements can be achieved with favorable fabrication techniques. There have also been substantial developments in Nb-Ti. Production has increased significantly and experience with large scale production runs has assisted the development of higher J
c
values in both the small diameter, fine filament conductors typical of high energy physics magnets as well as the large monolith, large filament conductors utilized for some MHD and fusion applications. Microstructural-processing studies have begun to reveal some of the crucial steps in the optimization of conductors of arbitrary size and design. At the same time it has proved possible to raise the upper critical field of Nb-Ti by alloying, particularly with Ta, and these alloys may find significant application in magnets of 8-12 Tesla for applications such as fusion, new higher energy accelerators and NMR. This paper reviews these and other important developments in the field of practical superconducting materials.