Identifying the current penetration depth in high-Tc superconducting bulks through levitation force measurements

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Abstract High-Tc superconducting REBa2Cu3O7−x (REBCO) bulks are actively researched
for use in superconducting magnetic bearings. Superconducting magnetic bearings are
attractive in applications which require zero friction, ultra high speeds, or integration
within crygoenic environments. However, superconducting magnetic bearings are
limited by low stiffness values compared to mechanical bearings, prohibiting their use
in turbulent environments such as rocketry or aviation. To improve the performance
of superconducting magnetic bearings, a comprehensive understanding of the precise
distribution of currents and magnetic fields within the material during magnetic
suspension is required. Hence, the results of vertical and lateral levitation force
measurements between a single directional melt growth REBCO bulk and an NdFeB
magnet are reported. The measurements are carried out between 77 K and 90 K and
for bulk thickness between 0.5 mm to 4.5 mm. By reducing the bulk thickness, the
depth to which the induced currents penetrate the bulk is identified in zero-field cooling
conditions. At 77 K, the current and magnetic field occupy the top 1 mm, closest to
the magnet. By reducing the bulk thickness further, the levitation force profiles vary
significantly. Field-cooling results are similarly impacted; reducing the bulk thickness
below 1 mm dramatically reduces the lateral stiffness. The above results are interpreted
using finite element simulations employing experimentally measured J c(B,θ) data
from the bulk, following the completion of the levitation force measurement routine.
These results are relevant for future bearing designs, helping to inform the optimal
distribution of superconducting material for a given application.