The reduction of AC losses in ultra-fine multifilamentary superconductors is an important task in their application at power frequencies. Sometimes, hysteresis losses in the superconducting wire are much larger than the design values. In many cases, this is a result of proximity coupling in the normal metal layers between the filaments. Determination of the lowest limit of filament spacing and diameter to which hysteresis losses decrease with decreasing filament diameter is necessary in the design of low-loss superconducting wires for AC use. The authors have investigated the proximity coupling in Cu 10% Ni-matrix superconductors with different filament diameters and spacings by measuring hysteresis loss and initial magnetization. The lowest limit of filament spacing, without proximity coupling problem, was found to be 0.13μm for samples given no heat treatment and 0.09μm for heat-treated samples.
