Trapped magnetic field distribution above a superconducting linear Halbach array

Abstract

In applications requiring a large magnetic force, permanent magnets with non-parallel magnetization directions can be assembled in a Halbach array to generate a large gradient of magnetic flux density. The saturation magnetization of permanent magnets, however, brings a fundamental limit on the performance of this configuration. In the present work, we investigate experimentally the assembly of cuboid bulk, large grain melt-textured YBa2Cu3O7−x superconductors (∼14×14×14 mm3) with orthogonal c-axes so as to form a basic unit of Halbach array. The experiments are carried out at 77 K. The experimental distribution of the magnetic flux density above the array of trapped-field superconductors is compared to a similar array made of permanent magnets. A simple analytical model is developed and is shown to accurately reproduce the main experimental observations. The results suggest that a redistribution occurs in the current flowing in the central sample when the distance between the superconductors is reduced, whereas the neighbouring superconductors are unaffected. It is shown that this current redistribution yields a reduced contribution of the central sample to the magnetic flux density above the centre of the array and a new negative contribution associated with stray fields to the magnetic flux density at this location. This interpretation is confirmed by modelling of the distribution of transport currents in the superconductor using a 3D finite element model.

How to overcome the demagnetization of superconducting Halbach arrays?

“…In this work, two sets of experiments are carried out with the experimental rig presented in our earlier work [8]. This experimental rig enables three magnetized superconductors to be moved in reproducible way and to be assembled as a linear Halbach array at 77 K. To do so, the samples are initially magnetized sequentially at 77 K using a field cooling process starting from 1.2 T and gradually removing the magnetic field at a rate of 1 mT s −1 .…”

“…Since they do not suffer from any saturation magnetization [9][10][11][12], superconducting magnets are attractive candidates for replacing permanent magnets in Halbach arrays. Hull et al investigated theoretically this idea for a circular Halbach array [13] and in our work we investigated experimentally and numerically a linear Halbach array made of bulk superconductors [8]. Unlike permanent magnets, however, bulk superconducting trappedfield magnets are prone to partial demagnetization when they are subjected to a time-varying magnetic field component perpendicular to their permanent magnetization.…”

“…In this work, we use an analytical model based on Biot-Savart law, as formulated in [8], to compute the distribution of the magnetic flux density ⃗ B generated by a fully magnetized cubic bulk superconductor. This model essentially assumes that the superconductor is in the critical state (Bean model [32]) and that the sample exhibits a critical current density J c that is independent of the applied magnetic field.…”

“…In order to overcome this limitation, we have proposed assembling a Halbach array from superconducting trappedfield magnets [8]. Such trapped-field magnets consist of centimetric-size bulk superconductors in which persistent current loops are induced and trapped permanently.…”

“…In these studies, a magnetized superconductor is subjected to a time-varying external magnetic field perpendicular to its main axis of magnetization, which is analogous to the conditions encountered when assembling a linear superconducting Halbach array. For a Halbach array made of superconducting trapped-field magnets, we showed experimentally [8] that this partial demagnetization results in a reduction of 13% of the maximum magnetic flux density generated by the Halbach assembly and thus limits its performance.…”