Magnetic Imaging of Superconducting Tapes to Determine Current Flow
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Magnetic Imaging of Superconducting Tapes to Determine Current Flow
G.W. Browna, M.E. Hawleya, S.R. Foltynb, and F.M. Muellerb a Structure/Property Relations (MST-8) , b Superconductivity Technology Center (MST-STC), Materials Science & Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545 ABSTRACT We have developed a magnetic imaging system that uses magnetoresistive read heads from computer hard disk drives to map the transport-current-induced magnetic field at the surface of superconducting tapes at liquid nitrogen temperature. Transport current pathways are determined from the 2-dimensional magnetic field maps using established inversion schemes. We examined the current flow in pulsed-laser-deposited YBa2Cu3O7- films patterned on single crystal SrTiO3 substrates and on a textured yttria-stabilized-zirconia layer deposited on an Inconel ribbon by ion beam assisted deposition. The transport current densities in all cases were consistent with the Critical State Model. For the Inconel-based sample, the transport current density maps have allowed us to observe defects and determine the region that limits the current carrying capacity of the structure. INTRODUCTION A variety of electrical power applications could be improved by replacing conventional conductors with superconductors operating at liquid nitrogen temperature. YBa2Cu3O7- films on metal tape substrates are promising candidates for these applications because of the large Jc (and Ic) attainable at liquid nitrogen temperatures over moderate lengths of material.1 In order to produce longer YBCO coated conductors, current limiting defects must be eliminated, since even one region of low Jc limits the performance of the entire tape. It is therefore important to be able to map the transport current to see where these regions of low critical current density are located. To address this need, we have developed an imaging apparatus capable of mapping the normal component of the transport-current-induced magnetic field at the surface of a superconducting sample at 75 K (immersed in liquid nitrogen).2 Our sensors are magnetoresistive devices scanned over the surface of the sample, making this a type of scanning magnetoresistance microscopy3. The magnetic field maps are numerically inverted, using the Biot-Savart law, to provide the spatial dependence of the transport current density J. These maps are useful since they reveal the current paths under conditions similar to those encountered in actual use. Mapping transport current by measuring and inverting the self-field has previously been applied to superconducting samples using either magneto-optical imaging4,5 or scanning Hall probe instruments.6,7 Our implementation combines the benefits of negligible sample preparation, very little sensor preparation, flexible imaging conditions, high resolution data, and straightforward sensor response.
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EXPERIMENTAL We obtain magnetic field maps by scanning a magnetic sensor over the sample surface. A schematic of the instrument is shown in Fig.
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