Longitudinal Magnetic Field Effect

Abstract

Chapter 4 covers various phenomena observed when the transport current is applied to a long superconducting cylinder or tape in a longitudinal magnetic field. It is empirically known that the force-free model, which assumes a current flow parallel to the flux lines, explains some of such phenomena. Although it is assumed that the force-free state is intrinsically stable in this model, the observed critical current density in a longitudinal magnetic field depends on the flux pinning strength, similarly to the case in a transverse magnetic field, indicating that the force-free state is unstable without the pinning effect. The restoring torque is derived from the energy increase when a distortion is introduced to the flux lines by the parallel current. This predicts that the critical current density is determined by the balance between this torque and the pinning torque. The rotational motion of flux lines driven by to the restoring torque that exceeds the pinning torque explains the observed break in Josephson’s formula on the induced electric field. A peculiar helical structure of the steady electric field with a negative region in the resistive state can also be explained by this flux motion.