Magnetic Shielding with Bulk High Temperature Superconductors: Improvement of the Shielded Volume in Hollow Cylinders

The superconducting HTS hollow cylinder has been one of the first geometries to demonstrate its utility as an efficient passive magnetic shield, and particularly for DC magnetic shielding. The shielding performances can be characterized by two parameters: (i) the shielding factor SF, defined as the ratio between the applied magnetic induction Bapp and the magnetic induction inside the shield Bin and (ii) the threshold induction Blim for which the shielding factor becomes lower than a given level.
The main drawback of the hollow cylinder geometry is that the shielding is only effective in the central region of the tube and decreases towards the tube extremities because of the field penetration through the open ends. In order to improve the performances at tube extremities and hence increase the shielded volume, the tube should be closed. This can be achieved by using a superconducting vessel or by closing one or both extremities with a cap.
This chapter describes the experimental and the numerical analysis of the DC shielding performances of a Bi-2223 tube closed by a superconducting Bi-2223 cap, for an axial configuration. The first part is dedicated to the experimental study of two tubes of different lengths when they are closed at one extremity or closed at both extremities. We also study the effect of the gap size between the cap and the tube. In the second part, we use numerical simulations to highlight the possible improvements and to optimize the design. In particular we study the influence of the critical current density and of the thickness of the cap.