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.
Cryogenic slush fluids, such as slush hydrogen and slush nitrogen, are two-phase, single-component fluids containing solid particles in a liquid. Since their density and refrigerant capacity are greater than those of liquid-state fluids alone, there are high expectations for use of slush fluids as functional thermal fluids in various applications, such as fuels for spacecraft engines, clean energy fuels to improve the efficiency of transportation and storage, and as refrigerants for high-temperature superconducting equipment. In this research, a three-dimensional numerical simulation code (SLUSH-3D), including the gravity effect based on the thermal non-equilibrium, two-fluid model, was constructed to clarify the flow and heat-transfer characteristics of cryogenic slush fluids in a horizontal circular pipe. The calculated results of slush nitrogen flow performed using the numerical code were compared with the authors' experimental results obtained using the PIV method. As a result of these comparisons, the numerical code was verified, making it possible to analyze the flow and heat-transfer characteristics of slush nitrogen with sufficient accuracy. The numerical results obtained for the flow and heat-transfer characteristics of slush nitrogen and slush hydrogen clarified the effects of the pipe inlet velocity, solid fraction, solid particle size, and heat flux on the flow pattern, solid-fraction distribution, turbulence energy, pressure drop, and heat-transfer coefficient. Furthermore, it became clear that the difference of the flow and heat-transfer characteristics between slush nitrogen and slush hydrogen were caused to a large extent by their thermo-physical properties, such as the solid-liquid density ratio, liquid viscosity, and latent heat of fusion.