Measurement of time constants for superconducting cables with Hall probes

The time constants of coupling losses have been evaluated for the composite-type aluminum stabilized superconducting cables used for the pool-cooled helical coils of the Large Helical Device (LHD). A new method using Hall probes was incorporated to measure the local field change generated by the screening currents in the conductors. During an exponential discharge of the external magnetic field, the current decay was found to be well fitted according to a simple magnetic diffusion equation with a geometrical shape factor. Measurements in the magnetic fields both perpendicular and parallel to the wide surface of the flat cable were performed. The shielding effect of the pure aluminum stabilizer was confirmed by removing it from the cable, which then revealed a longer time constant.