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ST411 - Measurement and Analysis of Critical Current of 100 -kA Class Simply-Stacked HTS Conductors (Contest winner)

Measurement and Analysis of Critical Current of 100 -kA Class Simply-Stacked HTS Conductors
Y. Terazaki1, N. Yanagi2, S. Ito3, Y. Seino3, S. Hamaguchi2,
H. Tamura2, T. Mito2, H. Hashizume3,  and A. Sagara2
1The Graduate University for Advanced StudiesStudies, 322-6 Oroshi-cho,
Toki 509-5292, Japan
2National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
3Department of Quantum Science and Energy Engineering,
Graduate School of Engineering, Tohoku University,
6-6-01-2 Aramaki-Aza-Aoba, Aoba-ku, Sendai, Miyagi, 980-8579, Japan
Abstract  Design activities of the LHD-type helical fusion reactor FFHR-d1 are progressing at NIFS. To produce the toroidal magnetic field of 4.7 T at the center of the continuously wound helical coils (major radius: 15.6 m), a 100 kA current capacity is required for the winding conductor under the maximum magnetic field of ~12 T. The high-temperature superconductor (HTS) is a promising option for the helical coil conductor. Magnets for fusion reactors using HTS conductors are supposed to have high cryogenic stability and low refrigeration power at elevated temperature operations at >20 K. Furthermore, it is proposed that the helical coils of large-diameter and complex-shape be constructed by connecting conductor segments exploiting the advantage of HTS. For the development of such a HTS conductor suitable for the helical fusion reactor, we fabricated and tested 30 kA-class and 100 kA-class prototype HTS conductors, which used 20 and 54 GdBCO tapes, respectively, simply stacked in a stabilizing copper jacket,. The copper jacket was installed into a rigid stainless-steel jacket, which was assembled by bolts. The conductor sample formed a one-turn short-circuit coil with a race-track shape having a bridge-type mechanical lap joint. The transport current of the sample was induced by changing the external magnetic field, which was generated by a 9-T split coil. The critical current of the sample was measured at various temperature and magnetic field: e.g., 45 kA at 20 K, 6.1 T and 68 kA at 4.2 K, 1.2 T for the 30 kA-class sample, 100 kA at 20 K, 5.3 T and 81.7 kA at 33 K, 0.3 T for the 100 kA-class sample. A numerical analysis of the critical current is being performed by self-consistently solving the spatial distributions of the current density and magnetic field among the simply-stacked HTS tapes to verify the measured critical current of the samples. The critical current characteristics of a single HTS tape is evaluated by a percolation transition model, using parameters given by literature. The obtained results are in good agreement with the measured critical currents at the self-magnetic field, but there is some discrepancy under the bias magnetic field. For performing more accurate calculations, an experiment is being carried out to obtain the suitable parameters for the HTS tape used in the present experiment.
Keywords (Index Terms)  Critical current, FFHR, fusion reactor, GdBCO, high temperature superconductor
IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), January 2015.
Received August 18, 2014; Selected October 2, 2014.  Reference ST411; Category 5.
ASC 2014 manuscript 3LPo2I-06 published online in IEEE Trans. Appl. Supercond.
(IEEE XPLORE) DOI: 10.1109/TASC.2014.2377793, on December 5, 2014.