Stabilized Nb-Zr strip and its use in large magnets

The three main obstacles to the use of Nb-Zr strip superconductor in magnets have been overcome in recent work with this material. These three problems are as follows: 1) Anisotropy of critical current with magnet field direction: Nb-Zr strip in its as rolled state displays extreme anisotropy with the peak critical current occurring when the field is parallel to the rolling plane, making it difficult to use in anything but long solenoids. This effect has been drastically reduced by certain annealing procedures so that the minimum critical current is about 50 percent of the peak value instead of less than one percent for as rolled strip. 2) Truncation of the anisotropy curves with wide strips: With strips in excess of 1 mm in width the peak of the anisotropy curve becomes progressively more truncated or chopped off as the width is increased. By stabilizing with a copper backing, however, truncation can be completely eliminated so that the strip can be used with equal effectiveness in any width. 3) Coil degradation effect: The failure of superconductors in general to operate at their short sample critical currents when wound into a magnet is well known. Strip Nb-Zr is no exception and is in fact, worse than wire. However, when stabilized with copper to eliminate truncation, the coil degradation is also eliminated provided sufficient cooling passages are provided to remove the Joule heating released by flux jumps and eddy currents. Two actual coils (12.7 cm ID, 1.25 Wb/m², and 2.54 cm ID, 3 Wb/m²) constructed with this stabilized material operate at the short sample critical currents. These developments have made it possible to construct large magnets from Nb-Zr strip and predict their performance in advance from short sample data. The high critical currents afforded by operation at the short sample critical currents permit designs using only a fraction of the material necessary in unstabilized coils.