Magnetic shielding with YBCO coated conductors: influence of the geometry on its performances.

A superconducting magnetic screen can be built as a stack of several sections of milled 2G coated conductors. Each section consists of a closed loop where persistent currents can flow and provide a strong attenuation of external dc magnetic fields. The purpose of the present work is to study experimentally several geometries of such magnetic screens made out of YBa2Cu3O7 (YBCO) coated conductors from SuperPower. Our aim is to investigate in details the influence of the aspect ratio and the number of layers of the assembly on the magnetic shielding properties.
In order to do so, the magnetic screen is subjected to an axial quasi-static (“dc”) magnetic field ramped slowly a fixed sweep rates. A Hall probe is used to measure the local magnetic induction inside the assembly as a function of the applied magnetic induction.
Results show that the threshold magnetic induction (above which magnetic shielding ceases) increases with the number of layers and that the shielding factor (defined as to ratio between the applied magnetic induction and the magnetic induction measured inside the screen) is improved for increasing aspect ratios of the global coated conductor assembly. Using a double layer of 18 sections at T = 77 K, dc magnetic fields up to 56 mT can be shielded by a factor larger than 10. The experimental results are compared to theoretical predictions for a perfect (type-I) magnetic shield with the same aspect ratio. Finally, the effect of an air gap of constant width between coated conductor sections is also characterized.