Magnetic shielding of open and semi-closed bulk superconductor tubes: the role of a cap

peer reviewedIn this paper we investigate the magnetic shielding of hollow and semi-closed bulk superconducting tubes at 77 K. We first consider the properties of a commercial Bi-2223 tube closed by a disk-shaped cap placed against its extremity. The results are compared with those obtained on a bulk large grain Y-Ba-Cu-O (YBCO) tube produced by buffer-aided top seeded melt growth. In this process, the disk-shaped pellet and the tubular sample are grown together, resulting in a tube naturally closed at one extremity. The field to be shielded is either parallel or perpendicular to the main axis of the tube. The experimental results are compared with the results of finite element numerical modeling carried out either in two dimensions (for the axial configuration) or three dimensions (for the transverse configuration). In the axial configuration, the results show that the shielded volume can be enhanced easily by increasing the thickness of the cap. In the transverse configuration, the results show the critical role played by the superconducting current loops flowing between the tube and the cap for magnetic shielding. If the tube and the cap are separated by a non-superconducting joint or air gap, the presence of a cap leads only to a small improvement of the transverse shielding factor, even for a configuration where the gap between the cap and the tube contains a 90° bend. The cap leads to a significant increase in the transverse shielding when the cap and the tube are naturally grown in the same process, i.e., made of a continuous superconducting material. The experimental results can be reproduced qualitatively by 3-D numerical modeling

The influence of an initial trapped field on the magnetic shielding performance of bulk high-temperature superconducting tubes

peer reviewedType-II superconducting hollow cylinders can act as efficient passive magnetic shields. Unlike in ferromagnetic materials, the magnetic shielding mechanism in a superconductor occurs through persistent currents flowing on the macroscopic scale. When a superconducting tube is used to shield a sequence of magnetic fields with different orientations, magnetic shielding performance levels are likely to be degraded because of the superconducting currents that are trapped in the superconductor. In this chapter we study experimentally the magnetic shielding configurations where an external magnetic field is applied first along one direction and a second field is subsequently applied along another (perpendicular) direction. In particular, we focus on the effect of an axial trapped field on the transverse shielding performance and on the effect of a transverse trapped field on the axial or the transverse shielding performance. Finally, we show how the pristine state of the tube can be restored by an appropriate magnetic field sequence, i.e., without heating up the shield above its critical temperature Tc

Mesures magnétiques : comment concevoir un magnétomètre adapté aux échantillons de grande taille ?

Les magnétomètres commerciaux traditionnels sont adaptés pour mesurer le moment magnétique d'échantillons de taille relativement réduite, en général < 1cm³. Pour certains matériaux cependant, il est désirable de pouvoir déterminer le moment magnétique d'échantillons plus volumineux de manière non-destructive; c'est le cas notamment des supraconducteurs massifs utilisés comme aimant permanents. Dans ce cas, il est nécessaire de revoir la conception de l'appareillage dans son ensemble, et notamment ta taille des bobines de détection. Dans cet exposé, nous montrerons comment dimensionner les bobines de détection d'un magnétomètre à extraction et comment traiter le signal mesuré de manière à déterminer aussi précisément que possible le moment magnétique d'un échantillon, potentiellement de grande taille. Nous envisagerons également le problème lié à la forme particulière (rapport d'aspect) de l'échantillon sur le signal mesuré. Nos propos seront illustrés par les résultats expérimentaux obtenus sur un magnétomètre conçu sur mesure pour les échantillons de taille importante. Reference : R. Egan et al., "A flux extraction device to measure the magnetic moment of large samples; application to bulk superconductors." Rev Sci Instrum. (2015) 86(2), 025107. doi: 10.1063/1.4907903