Losses of NbTi multifilament superconductors in a copper matrix in alternating magnetic fields were measured using a specially developed sensitive calorimetric method. The investigated samples have filament diameters in the micrometer range and exhibit a strong proximity effect. The measured losses at fixed ac frequency and field amplitude showed an unexpected time dependence on a very long time scale of hours, compared to the ac field cycle time, which was in the millisecond range. This time dependence is probably a consequence of slowly decaying proximity currents.
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