M. Takeo

T. Kiss

T. Mito

V.S. Vysotsky

National Institute for Fusion Science (NIFS-Repository)

§ 11 . Current Transport Properties and Stabi I ity in High Tc Superconductors Kiss, T., Vysotsky, V.S., Takeo, M. (Dept. Electrical & Electronic Sys. Eng. Kyushu Univ.), Mito, T. Based on a simulation study in a random pin medium, we have proposed a new method I) to describe current transport properties in high Tc superconductors (HTS). The theoretical results are compared with measured electric field (E) vs. current density (.I) curves in a YBCO film under magnetic fields up to 12 T. It has been shown that the nonlinear E-J curves can be characterized by a power function [(J -Jcm )/ Jot with three parameters, i.e., the minimum value of critical current density (Jc) denoted by Jcm' the variance of the distribution, Jo, and the exponent, m, which determines the shape of Jc distribution. Magnetic field (B) and temperature (n dependencies of the E-J curves can be attributed to the thermodynamic properties of Jcm and the typical value of Jc denoted by Jk( == Jcm + Jo) through the corresponding macroscopic pinning force densities JcmB and JkB, respectively, as In the similar form in low Tc stability margin of HTS against a local disturbance was very large, quench current was limited by a catastrophic temperature nse originated from the nonlinear characteristic of Joule heating in HTS. Using the theoretical expression of E-J characteristics mentioned above and the heat balance equation, we can calculate the dynamic characteristics of the quench and the value of the quench current.2) Results for a Bi-based HTS coil was shown in Fig. 2. -,e J=1x107 E -----,A J=2x1 07 10 ---- ,. J=5x107 "C CD -----,A J=2x10' iI: ----, . J=5x10' 0 ~ C 0> 5 ----, . J=5x10· co ::l: OL..:.:.:.:..::-L.......................JL................:.::l~...;"o;"I~~ ........... ...........J 60 65 70 75 80 85 90 Temperature [K] Fig. 1 Iso-Jlines defined by E=10-3 V/m. The lines are the analysis based on the statistic model whereas the points are measured results. superconductors. 55 f.--.!!!""""'!!!!!!!!""""'!!!!!!!!"""""!!!!Ll-F--r-\+-+-----I The present method allows us to estimate the statistic Jc distribution from E-J curves. Moreover, B- and T-dependencies of the E-J curves can be predicted in a wide (B, 1)-plane based on the scaling properties of the pinning force densities. The results obtained in a YBCO film was shown in Fig. 1. These results are useful not only to estimate critical current properties in HTS materials, but also to design HTS based devices by taking into account operation conditions such as B, T and power dissipation. As an example, we studied the quench dynamics in a YBCO HTS film as well as in a Bi-based small HTS coil. It has been shown that the nonlinear transport properties are crucial to determine the quench properties in HTS. While the II) 5 50~-+-~-+-41--r+-+-~ +' ('II "-II) ~ ~ 45~-+-~L-~~--r~~~ 100 200 Time [5] 300 Fig. 2 Temperature rise for different bias currents in a Bi-based HTS coil. Reference 1) Kiss, T. et aI., ICEC17 (lOP Publishing Ltd, London, UK 1998) 427 2) Kiss, T. et aI., Physica C 310, (1998)372 101 

Current Transport Properties and Stability in High Tc Superconductors

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