Adjacent conductor field corrections to high critical current short sample measurements

We have measured the critical current of several superconductors to 24 kA in magnetic fields to 4.5 T. The samples were hairpin shaped with the straight section under test in a perpendicularly applied field. The proximity of the straight parts causes the field of the current in one leg to change the applied field in the other leg. We have corrected the measured results for this adjacent conductor field (ACF). These corrections result in data for uniform applied fields. One conductor was used in a 300 kJ magnet. This single-layer, edge-wound coil was rated near 9.8 kA using the uniform field data. Under test the magnet reached 12.5 kA giving no sign of approaching the critical current. The "excess current" is due to the field gradient across the conductor in this coil. One contribution is due to the variation of jcwith the field value within the conductor. A second contribution results from the position dependent anisotropy of the filaments. A third possibility is to change the definition of Ic. We have chosen the definition to be the current at which the sample generates an electric field of 3.5 μV/m. If 46 μV/m were chosen, Icwould rise approximately 0.86 kA at 17 kG.