Modeling cross-field demagnetization of superconducting stacks and bulks for up to 100 tapes and 2 million cycles

Abstract

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.

Sharp demagnetization of closed-loop HTS coil in first cycle of external AC fields induced by unexpected dynamic resistance

“…In some machinery applications, HTS materials after magnetisation work as permanent magnets by carrying a persistent DC circulating current, and there are mainly three types: bulks, tape stacks, and closed-loop coils. The main issue with this technology is the demagnetization of the HTS permanent magnets caused by the AC fields they experienced in the rotor that transverse to their magnetisation [10,[13][14][15][16]. Dynamic resistance was considered as a mechanism to explain this demagnetization effect [10,13,14,16].…”

“…The main issue with this technology is the demagnetization of the HTS permanent magnets caused by the AC fields they experienced in the rotor that transverse to their magnetisation [10,[13][14][15][16]. Dynamic resistance was considered as a mechanism to explain this demagnetization effect [10,13,14,16]. For closed-loop coils, the effect of dynamic resistance on the acceleration of the decay of persistent current can be expressed as [10]:…”

Numerical modelling of soldered superconducting REBCO stacks of tapes suggests strong reduction in cross-field demagnetization

“…Although the stacks will use low tape-to-tape separation and high number of tapes, the average electromotive force due to the ripple field will be the same. Thus, we expect the same qualitative (and maybe even quantitative) behavior as for our simpler 5-tape stack, but the stationary state in actual stacks will likely require higher number of cycles due to the inductive effects caused by the higher number of tapes 16,39 .…”

“…When the motor is operating, in the air gap of an electric motor there are always excessive harmonics caused by the differences in permeability of the magnetic circuit (slot harmonics) and the discrete distribution of the stator coils (winding harmonics). It has been reported from laboratory testing and computer modeling that the cross-field is of the same order of magnitude as the parallel penetration field in each layer, which could demagnetize the stack in just a few minutes 15,16 . The cross-field that the stacks experience in the rotor causes demagnetization of the trapped field, which reduces the performance of the motor 17,18 .…”

“…It is technically possible to make isolated stacks, which are already used in the ASuMED project 24 . The motivation for studying soldered stacks is that bulks with the same engineering current density are much less sensitive to cross-field demagnetization, as shown in 16 . We expect that soldered stacks will behave as bulk for large enough frequencies of the ripple field.…”