The addition of Ta to Nb-Ti alloys suppresses the param­agnetic limitation of Hc2 by the large orbital moment of the alloys (53). Although Ta is only of benefit below 4.2 K (54), it has a relatively long history of study because it should extend the useful field range of ductile superconductors by 1 T or more (55). So far, however, improved Hc2 has not translated effectively into improvements in Jc, except very near to Hc2 (above 11 T). Lazarev et al. (56) were able to

Figure 12. Partial cross-section of a strand designed for the Large Hadron Collider at CERN by IGC Advanced Superconduc­tors (now Luvata Waterbury, Inc.). 250,000 km of Nb-Ti strand were required in order to produce magnets for the 27 kilome­ter LHC ring, including 1232 dipoles and 858 quadrupoles. Each dipole was 15 m in length and weighed 35 tonnes. The LHC uses 1.9 K operation to push the Nb-Ti based magnets beyond 8 T. In­set is the full strand cross-section showing the individual filament stacking units. Each LHC strand has 6425 or 8800 filaments of 6 or 7 xm diameter respectively.

attain a critical current density 1000 A/mm2 at a field of

11.5 T (2.05 K) using an Nb-37 wt. % Ti-22 wt. % Ta alloy. Ta has an even higher melting point than Nb, making the fabrication of chemically homogeneous ternary alloys par­ticularly difficult. The behavior of Nb-Ti-Ta alloys under the conventional process is similar to that of binary alloys, but the precipitates do not appear to pin as efficiently (57).