Comparison of different configurations of NbTi magnetic lenses

Magnetic lenses are new devices that concentrate magnetic flux by using the diamagnetism of superconductors. Magnetic lenses of two types made from NbTi sheets were constructed; measurements were made on them and they were systematically studied. TypeA was constructed by stacking NbTi rings that had identical outer diameters and increasing inner diameters to form a hollow cone. Each ring had a slit to suppress the circumference current. Three construction methods for typeA were tested: the rings were stacked with their slits aligned but with no insulation between the rings (A-1), with their slits aligned and with insulation between the rings (A-2), and with their slits in different positions and with insulation between the rings (A-3). For typeB, sheets were rolled into hollow cones. Three identical cones were stacked to form a lens (B-1) and a single cone was used as a reference lens (B-2). The lenses were assembled in a cryocooler-cooled cryostat with a NbTi magnet. The quenching behavior, concentration ratio, hysteresis, and decay behavior were measured. Because of its larger dimensions, typeB had a larger concentration ratio (2.49 for B-1) than typeA (1.87 for A-1). Both lenses (typesA and B-1) were quenched when the concentrated flux density reached about 0.64T. The results suggest that quenching was caused by the NbTi sheet itself.