Systematic Behaviors of ab-plane Pinning Variations in a Practical Length Wire Fabricated by the MOD/RABiTS Process
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Systematic Behaviors of ab-plane Pinning Variations in a Practical Length Wire Fabricated by the MOD / RABiTS Process J. Yates Coulter, David W. Reagor, Jeffrey O. Willis, Terry G. Holesinger Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545, U.S.A. ABSTRACT In-field critical current Ic variations, detected using a short sample, angular Ic(77K, H=5.2kOe, Angle) measurement on the ends of a 20 m coated conductor tape fabricated by the MOD / RABiTS process, are shown to be variations in the Ic(H) anisotropy that exist on subcentimeter length scales. A Ic(75 K, H, Angle) study was performed on segments cut from the tape where the power law exponent of the field dependence, D, Ic ~H-D was calculated for Ic(H, Angle) data. Two extrema behaviors, anisotropic and isotropic, were identified. The isotropic material is shown to outperform the anisotropic material for a wide range of fields and angles at T=26 K. INTRODUCTION A component of the Los Alamos National Laboratory and American Superconductor, AMSC, collaboration is the critical current, Ic, characterization of long lengths of coated conductor tape that are measured to determine uniformity and provide process feedback. For this study AMSC identified the existence of Ic(H) variations occurring in a 344B coated conductor fabricated by the MOD / RABiTS process [1]. Information about the length scale and periodicity of Ic variations was desired to correlate process fluctuations to Ic(H) variations. The self field critical current, Ic(H=0), measured as a function of position on long wires that are suitable for applications is insufficient to predict in-field performance because Ic(H=0) merely establishes the existence of superconducting transport currents [2]. While Ic(H||c) has been shown to be tunable by thermal processing, oxygenation, and the introduction of pinning enhancing defects [2-5], recent studies on practical conductors have also shown that H||ab plane pinning is variable and that H||ab variations may occur unintentionally and result from process variations [6-7]. The effect of variable ab-plane pinning along the length of a coated conductor tape is to change the angle at which Ic is a minimum for a given magnetic field and temperature, therefore in addition to Ic(H||c), Ic(H||ab) uniformity is important to for predictive performance. A power law relationship between Ic and the applied magnetic field, H, of the form Ic ~ HD has been predicted from the modified Kim model [7] and been empirically observed [3, 8, 9] to occur for some coated conductors. The exponent, commonly known as the alpha value, has been used to describe the Ic(H) performance of a coated conductor wire in the magnetic field range above the self field and below the irreversibility field which suggests the plausibility of it being a material characteristic, or a control parameter, that can be routinely evaluated. Given a wire with known variations in Ic as a function of position, field, and angle, one goal of this study was to determine the extent to which alpha
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