TEION KOGAKU (Journal of Cryogenics and Superconductivity Society of Japan)

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Since a low resistive state in high-temperature superconductors is quite stable, the practical application of Ag-sheathed Bi₂Sr₂CaCu₂O₈ (Ag/Bi2212) tape is being considered for current-carrying limitation before thermal runaway. The thermal runaway parameters in relation to the heat transfer coefficient and Ag matrix resistivity for Ag/Bi2212 tape were investigated. In order to determine the maximum current-carrying capacity of Ag/Bi2212 in a magnetic field, we used the zero-dimensional static model. Adopting the linear temperature model for the critical current density of Ag/Bi2212, we can estimate that thermal runaway occurs in electric fields one order of magnitude higher than the critical current criterion of 1x10^-6 V/cm for the Ag/Bi2212 tape. This behavior is in good agreement with experimental results. Further, when the nonlinear temperature and magnetic field dependencies of the critical current density of Ag/Bi2212 were considered to evaluate thermal runaway behavior, we found the existence of the multiple stable states. The E-J and T-J curves may have the discontinuous parts, leading to jumps in the electric field, current and temperature without the transition of the superconducting composite into the normal state. Moreover, an essential increase in the stable branch of the voltage-current characteristics may be observed.

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