Experimental study of the magnetic shielding performances of a Bi2223 tube closed by a Bi2223 cap.

A superconducting tube can be used as an efficient magnetic shield. However, the shielding factor (defined as the ratio between the applied magnetic induction and the magnetic induction measured inside the tube) decreases towards tube extremities because of the field penetration through the open ends of the tube. To improve the performances at tube extremities, the tube should be closed. This can be achieved by using a superconducting vessel or by closing both extremities with a cap.
The purpose of the present work is to study experimentally the shielding performances of a Bi2223 tube closed by a superconducting Bi2223 cap. The cap is a circular plate with a diameter equal to the outer diameter of the tube and there is no superconducting joint between the cap and the tube. Our interest is to characterize the effect of the cap on the shielding factor distribution along the tube axis when only one extremity of the tube is closed. We also study the effect of the gap size between the cap and the tube in axial configuration. Finally, a tube closed at its both ends is also characterized.
All experiments are carried out at 77K. The tube is subjected to a quasi-static (“DC”) uniform magnetic field. A Hall probe measures the three components of the local magnetic induction along the axis of the tube as a function of the applied magnetic induction.
Results show that the shielding performances in the axial configuration are highly improved at the closed extremity as the cap reduces the penetration through the open end. The shielding factor distribution along the tube axis is affected by the presence of the cap. For an open tube, the shielding factor is maximum near the center and decreases towards both extremities. For the tube closed at one extremity, the shielding factor is maximum at the closed extremity and decreases towards the center. Moreover, the shielding factor distribution between the tube center and the open extremity is not affected by the presence of the cap. The experimental results show that, a small gap between the tube and the cap, which can be necessary for applications, does not strongly affect the magnetic shielding performances of the assembly.