  Superconducting stacks and bulks can act as very strong magnets (more than 17
T), but they lose their magnetization in the presence of alternating (or
ripple) transverse magnetic fields, due to the dynamic magneto-resistance. This
demagnetization is a major concern for applications requiring high run times,
such as motors and generators, where ripple fields are of high amplitude and
frequency. We have developed a numerical model based on dynamic
magneto-resistance that is much faster than the conventional
Power-Law-resistivity model, enabling us to simulate high number of cycles with
the same accuracy. We simulate demagnetization behavior of superconducting
stacks made of 10-100 tapes for up to 2 million cycles of applied ripple field.
We found that for high number of cycles, the trapped field reaches non-zero
stationary values for both superconducting bulks and stacks; as long as the
ripple field amplitudes are below the parallel penetration field, being
determined by the penetration field for a single tape in stacks. Bulks keep
substantial stationary values for much higher ripple field amplitudes than the
stacks, being relevant for high number of cycles. However, for low number of
cycles, stacks lose much less magnetization as compared to bulks.
