Technique for Studying Overcurrent Behavior in YBCO Coated Conductors Using a Localized Magnetic Field
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Technique for Studying Overcurrent Behavior in YBCO Coated Conductors Using a Localized Magnetic Field J. Yates Coulter, Stephen P. Ashworth, Paul C. Dowden, and Jeffrey O. Willis Superconductivity Technology Center, Los Alamos National Laboratory, Los Alamos, NM, 87545 U.S.A. ABSTRACT Over current stabilization of YBa2Cu3Ox (YBCO) coated conductor high temperature superconductor tape is required in most applications. The conductor must carry currents in excess of the critical current, Ic, without damage during over current events. Conductor damage is the result of joule heating and excessive temperature rise in regions with low Ic. We have developed and applied a measurement technique using a locally applied magnetic field with a high spatial gradient to define a small area over which the Ic is depressed. By measuring the voltage and temperature as a function of current, power dissipation and temperature rise were determined. Unstabilized conductors experienced thermal runaway and are easily damaged. Copper stabilizers applied by electroplating decreased dramatically the temperature rise and increased the level of power dissipation compared with the unstabilized conductor. INTRODUCTION At currents in excess of the critical current, Ic, a superconductor, and YBCO coated conductor tape in particular, generates a voltage that can be approximated as V=Vc(I/Ic)n [1], where V is the voltage, I is the current, Vc is the voltage criterion for determining the critical current Ic, and n is the power law exponent commonly called the n value. Power dissipation in the conductor in the form of joule heating is generated according to P=VI. Ic measurements of coated conductors are most commonly made in a bath of liquid nitrogen at temperatures 77 K to 64 K. At low power levels convection cooling results in relatively inefficient heat transfer to the cryogen. At higher power levels, nucleate boiling occurs with small nitrogen bubbles generated at the heated surface, which then move away into the liquid. This results in very good heat transfer. Eventually, film boiling and poor heat transfer returns. At these high power levels, the temperature of the conductor can rise rapidly, and conductor damage may result. This damage has been documented, and a technique for the nondestructive characterization of the positional dependence of Ic has been developed [2] and is currently in use in our laboratory. Within the last several years, practical coated conductors have been fabricated with an integral stabilizer of copper applied by electroplating [3, 4] or by soldering [5] on top of the typically much thinner silver overcoating. This has greatly improved the ability of the conductor to withstand overcurrent conditions without damage. How the addition of stabilizer material affects the voltage / current and the thermal characteristics at high currents is not well known. In this paper, we present a measurement technique that allows the determination of the temperature rise from overcurrents in a small section of conductor defined by th
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