Pulsed-field magnetization of drilled bulk high-temperature superconductors: flux front propagation in the volume and on the surface

We present a method for characterizing the propagation of the magnetic flux in an artificially
drilled bulk high-temperature superconductor (HTS) during pulsed-field magnetization. As the
magnetic pulse penetrates the cylindrical sample, the magnetic flux density is measured
simultaneously in 16 holes by means of microcoils that are placed across the median plane,
i.e. at an equal distance from the top and bottom surfaces, and close to the surface of the
sample. We discuss the time evolution of the magnetic flux density in the holes during a pulse
and measure the time taken by the external magnetic flux to reach each hole. Our data show that
the flux front moves faster in the median plane than on the surface when penetrating the sample
edge; it then proceeds faster along the surface than in the bulk as it penetrates the sample
further. Once the pulse is over, the trapped flux density inside the central hole is found to be
about twice as large in the median plane as on the surface. This ratio is confirmed by modelling.