Flux compression measurements in sintered bulk Ba2YCu 3O7

Trapping of magnetic fields and subsequent flux compression have been realized in sintered bulk Ba₂YCu₃O₇ superconductors at 77 K. The field is trapped in two interconnected holes of 0.95-cm and 0.52-cm diameter, respectively. By inserting a superconducting plunger with a diameter of 0.93 cm into the larger hole, the trapped flux is compressed into the smaller hole, where the flux density is measured with a Hall probe. The ratio of the areas available for the magnetic flux before and after compression is 3.7. Deviations from this compression ratio are observed to increase as the trapped field is increased, demonstrating penetration of magnetic flux into the interior of the superconductor. Using a simple extension of the critical state model, the authors demonstrate how the flux compression technique can be used to test the validity of the model and also to calculate the critical current density of the superconductor directly from the measurements