This lecture discusses the longitudinal magnetic field effect that appears in current-carrying superconductors in a parallel magnetic field. The peculiar electromagnetic phenomena in this magnetic field configuration are directly associated with the rotationally sheared structure of flux lines that appears in the force-free state in which the current flows locally parallel to the flux lines. It is shown that a driving torque, named force-free torque, works to reduce the shear in a similar manner to the Lorentz force. Since no Lorentz force works in this configuration, the force-free torque is independent of the Lorentz force. This is a large difference from the dynamics where torque appears as the moment of force. The critical current density is determined by the balance between the force-free torque and the pinning torque, and the electromagnetic phenomena are caused by the rotational flux motion driven by the force-free torque that exceeds the pinning torque. The difference of the electromagnetic phenomena from those in usual transverse magnetic fields is discussed.
Pulse-tube refrigerators are suitable for cooling systems of onboard high-temperature superconducting (HTS) magnets. This is because pulse-tube refrigerators have no moving parts in the cryogenic area and their maintenance cycle is longer than that of other cryocoolers. A novel pulse-tube cooling system that consists of a single compressor and multiple pulse-tubes has been investigated. This system allows uniform cooling of large HTS magnets and minimizes maintenance costs for the compressor. In addition, the refrigeration cycle timing of each pulse-tube can be controlled individually. We examined the same-phase and reverse-phase operations. The COP of the reverse-phase operation was 50 % higher than that of same-phase operation. Numerical simulations indicated a similar result. In the case of the reverse-phase operation, the compressor load was flat, and total energy consumption was reduced. These results show that higher performance was achieved for the reverse-phase operation.