The upgrading of the JT-60U to the JT-60SA has started in a joint effort by the Japanese government (JA) and European commission (EU) under the framework of the Broader Approach (BA) Agreement. The superconducting magnet system for the JT-60SA consists of 18 toroidal field (TF) coils, a central solenoid (CS) with four modules, and six equilibrium field (EF) coils. The TF case encloses the winding pack and is the main structural component of the magnet system. The CS consists of independent winding pack modules, which are hung from the top of the TF coils through its pre-load structure. The six EF coils are attached to the TF coil case through supports with flexible plates that allow radial displacement. The CS modules operate at high field and use a Nb3Sn superconductor. The TF coils and EF coils use NbTi superconductors. This paper describes the technical requirements, operational interface and detailed manufacturing design outline of the superconducting magnet system for the JT-60SA.
The flux pinning properties of RE-123 bulk superconductors, RE-123 coated conductors, Bi-2223 tapes and MgB2 superconductors are reviewed. In bulk superconductors, the pinning mechanism of the lower Tc region, such as the Ba sites substituted by RE elements or twin boundaries with oxygen deficiency, is considered to be kinetic energy interaction under a proximity effect with the superconducting matrix. The dependence of the irreversibility field on the superconducting layer thickness is very complicated for RE-123 coated conductors. This is partly attributed to the thickness dependence of the critical current density that arises from the deterioration of the superconducting layer structure in thicker films and partly to the thickness dependence of the flux creep. The reason for the dramatic improvement in the critical current properties is also discussed for Bi-2223 tapes fabricated using the over-pressure sintering technique. Theoretical analysis using the percolation theory clarifies that the low critical current density in conventional MgB2in situ wires is attributed to low electrical connectivity, which is due to the voids and wetting insulating layers between grains. It is clarified that the flux pinning strength of the grain boundaries in MgB2 is significantly strong, and there is a room for a drastic increase in the flux pinning strength by dirtying the MgB2 through C-doping. Finally, the flux pinning mechanism of columnar defects nucleated by heavy ion irradiation is discussed. It is shown that the columnar defects larger than the coherence length are desirable to increase the probability of flux lines being easily captured by the defects.
We fabricated a superconducting level sensor with a magnesium-diboride wire and carried out experiments with liquid hydrogen. First, we observed a normal zone propagation phenomenon in the gaseous hydrogen. We also measured the terminal voltage of the sensor with a constant current depending on the relative change in the liquid level, and recognized that input power during normal operation must be suppressed to realize a level sensor for liquid hydrogen. Operation of the level sensor was numerically simulated based on the experimental results. The time evolution of temperature distribution along the wire was calculated using a heat balance equation including the cooling effects of liquid hydrogen and vaporized gas. The influences of wire size and material properties on minimum propagating current and power consumption in the gaseous hydrogen were evaluated to achieve the optimum design for the level sensor.