An internal-tin route Nb3Sn superconducting wire that has both remarkably low hysteresis loss (Qh) and high critical -current density (Jc) was developed with a new design idea. The wire was constructed by arranging the filaments in a radial layout, enlarging the outer filaments along the radial direction, intentionally narrowing the filament spacing in the radial direction and enlarging the filament spacing in tangential direction. Thus, the electromagnetic combination among the filaments in tangential direction due to the bridging and/or proximity effect was suppressed without decreasing the volume fraction of Nb. As a result, excellent properties such as Jc(12 T)= 1.15×103 A/mm2 and Qh=301 mJ/cm3 (for 1 cycle of B=±3 T) were obtained. We also evaluated the transition temperature (Tc) and upper critical field (Bc2) of the wire. The values for Tc and Bc2 were 17.3 K and 24.1 T, respectively, which were much better than those of usual internal-tin route wires. Electron probe micro-analyses confirmed that the good Tc and Bc2 were the result of the qualitative improvement in the Nb3Sn compound based on the effects of arranging the Nb filaments radially, increasing the ratio of Sn-to-Nb and shortening the diffusion length for Sn due to the increase of the module number. This wire is promising for use with conduction-cooled high-field magnets, in which there is a need to decrease the load of the cryocooler, and also for the strands of fusion coils.
We have analyzed the state of the superconducting coil system in a LHD at NIFS (National Institute of Fusion Science) using a fuzzy logic theorem to detect quenching at an early stage. In this method, the “warning coefficient" of the coil system is calculated. As for the fuzzy variables, “effective stored heat" in the coil is introduced in addition to the voltage signal in order to improve quench detection and state estimation. The “effective stored heat" is an integrated value of the heat generated in the coil on the assumption that instantaneous heat in the conductor is continuously cooled by liquid helium. Experiments conducted using the LHD coils confirmed that quench alarm signals can be issued with sufficient lead time before quenching. On the other hand, in the case of small local disturbances, the system shows only a small increase in the caution level.
We have studied the possibility of utilizing inversely connected pancake coils in series for fault current limiters. Fundamentally, pancake coils have various merits, including 1) a high winding packing factor, 2) high mechanical strength, 3) good electric insulation characteristics, 4) good current balance between the coils, and 5) high impedance. Therefore, the authors introduced an inversely connected pancake coil system. In this paper, the impedance characteristics of inversely connected pancake coils are studied. The change in coil resistance between normal operating conditions and under fault conditions is shown, and the leakage inductance of the coils was measured. The results are compared to calculations obtained using FEM.