Screening of axial DC magnetic fields by using superconducting loops of second generation coated conductors without joints

The performances of second generation (2G) YBCO coated conductors for screening DC magnetic fields at 77 K are investigated experimentally. An uninterrupted superconducting path is achieved by splitting two sections of a slit 2G tape in order to obtain a loop without joint. Several loops are stacked together to obtain a coil acting as a magnetic screen. Persistent currents in this coil are shown to shield efficiently magnetic fields that are applied at very low sweep rates (500 @T/s). The shielding properties are characterized by measuring the shielding factor of the screen, i.e. the ratio between the magnetic field with and without screen. In a coil made of a single layer of coated conductors, the shielding factor at the center of the coil is shown to exceed 10 for an applied DC magnetic flux density of 5 mT. The uniformity of the shielding factor along the axis of the screen is also investigated; results show the magnetic field is attenuated by a factor larger than 3 near the open ends. Both the maximum field amplitude that can be shielded and the shielding factor of the coil can be enhanced by increasing the number of concentric coils. The key advantages of this shielding technique are (i) the moderate weight compared to magnetic screens made of bulk materials and (ii) the possibility of shielding larger volumes by using longer sections of YBCO coated conductors.