Magnetic screening with bulk high-temperature superconducting hollow tubes in the “emission” configuration

Due to their unique ability to trap magnetic fields exceeding the saturation limit of
conventional ferromagnetic materials, bulk superconductors can be used successfully in
motors and generators. In several topologies however, the stray magnetic field produced by
the superconducting magnet or coils needs to be strongly reduced outside the device. The
screening can be performed using either active or passive magnetic screens. In this respect, a
hollow tube made of bulk high temperature superconducting (HTS) material provides a strong
attenuation of external low frequency magnetic fields and acts as a very efficient passive
screen. The situation where the stray magnetic field generated by an electromagnetic device
(e.g. a motor or a generator) needs to be attenuated corresponds to an “emission” problem.
In the present work, we report the magnetic behaviour of a bulk HTS hollow cylinder in the
“emission” configuration, i.e. a coil is placed inside the cylinder and the magnetic field is
recorded outside the cylinder. We study the configuration where the coil is placed in the
central part of the superconducting cylinder, either along its axis (axial configuration) or
perpendicular to it (transverse configuration). The distribution of magnetic field at several
distances of the generating coil is measured by miniature Hall probe mapping. Results show
the significant attenuation of the magnetic field provided by the superconductor as well as the
dependence with respect to the sweep rate. The experimental data are analyzed by comparing
the behaviour of the superconductor in the inhomogeneous and homogeneous magnetic field
configurations. It is shown that a simple analytical model is able to reproduce the behaviour
of the superconductor in the axial configuration