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Lee, Wongi,Lee, Jhinhwan,Youm, Dojun,Yoo, Jaeun IOP 2016 Superconductor science & technology Vol.29 No.6
<P>The relaxation of magnetic flux in high Tc superconducting films was investigated. After the samples were cooled in the applied magnetic fields, the magnetic field was turned off and the changes of the remaining magnetic flux distribution were observed by using the magneto-optical image method. The induced current density was examined which varies with the logarithmic-time dependence associated with the creep motions of vortices. The overall magnitude of the induced current density is observed to decrease as the external magnetic field applied during cooling is increased. The range of external fields examined was 30–50 mT. This could be explained by taking into account the formation of meandering shapes of vortices which develop during the period of transition to the creep mode. The results of the numerical simulation for this effect are in good agreement with the experimental results.</P>
Yoo, Jaeun,Lee, SangMoo,Jung, YeHyun,Kwak, Kisung,Rhee, Joonkyu,Youm, Dojun,Kim, Hosup,Ha, Hongsoo,Oh, SangSoo,Oh, Sangjun IOP Publishing Ltd 2009 Superconductor science & technology Vol.22 No.4
<P>The hysteresis loss in a Sm<SUB>1</SUB>Ba<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7−δ</SUB> coated conductor was estimated from magnetic field profiles measured by the scanning Hall probe method. Current, <I>I</I><SUB>a</SUB>, and magnetic field, <I>B</I><SUB>a</SUB>, were applied simultaneously; <I>B</I><SUB>a</SUB> was applied in the normal direction with respect to the tape surface. <I>B</I><SUB>a</SUB> and <I>I</I><SUB>a</SUB> were varied from <I>B</I><SUB>peak</SUB> to −<I>B</I><SUB>peak</SUB> and from <I>I</I><SUB>peak</SUB> to −<I>I</I><SUB>peak</SUB>, respectively, with the ratio α = <I>I</I><SUB>a</SUB>/<I>B</I><SUB>a</SUB> fixed during the variation. Three values of α were taken for the three load lines. The values of <I>B</I><SUB>peak</SUB>/<I>I</I><SUB>peak</SUB> were varied from 0 mT/0 A to 10.7 mT/116 A, 99.1 mT/50 A, and 298.2 mT/46.1 A, respectively, for the three load lines. From the measured values of magnetic field profiles, the current profiles were calculated by the iterative inversion method. From the current profiles, the flux density profiles and the hysteresis loss, <I>Q</I>, were then calculated for various values of <I>I</I><SUB>peak</SUB>(= α<I>B</I><SUB>peak</SUB>) in each load line. The results were compared with theoretical calculations based on Brandt’s model. When <I>B</I><SUB>peak</SUB> was about 300 mT, the estimated values of <I>Q</I> were several times smaller than the theoretical values of <I>Q</I> with the self-field <I>I</I><SUB>c0</SUB>. The low value of <I>Q</I> in this case is due to the field dependent <I>I</I><SUB>c</SUB> and the saturation effect of the current profiles, which results in significant reduction of the induced magnetic moment, <I>M</I>. </P>
Lee, SangMoo,Jung, YeHyun,Kwak, Kisung,Rhee, Joonkyu,Yoo, Jaeun,Youm, Dojun,Kim, Hosup,Ha, Hongsoo,Oh, SangSoo IOP Publishing Ltd 2010 Superconductor science & technology Vol.23 No.4
<P>A simple model for estimating the hysteresis energy loss of coated conductors under a general load line was studied. We took advantage of the characteristic line <I>I</I><SUB>b</SUB>(<I>H</I><SUB>a</SUB>) to determine the major parameters used in this model. The value of <I>I</I><SUB>b</SUB>(<I>H</I><SUB>a</SUB>) was based on the scanning Hall probe measurements (SHP) on a Sm<SUB>1</SUB>Ba<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7 − δ</SUB> coated conductor. During SHP measurement, a magnetic field (<I>H</I><SUB>a</SUB>) and current (<I>I</I><SUB>a</SUB>) were applied simultaneously and were varied along 11 different load lines. From the values of SHP measurements, the current density profiles, <I>J</I>(<I>x</I>, <I>H</I><SUB>a</SUB>, <I>I</I><SUB>a</SUB>), were calculated using a numerical inversion method. We define the quantity <I>I</I><SUB>b</SUB> = ∫ <SUB> − <I>w</I></SUB><SUP><I>w</I></SUP>|<I>J</I>(<I>x</I>, <I>H</I><SUB>a</SUB>, <I>I</I><SUB>a</SUB>)| d<I>x</I> and we calculated <I>I</I><SUB>b</SUB> at many points (<I>H</I><SUB>a</SUB>, <I>I</I><SUB>a</SUB>) in every load line. We found that when <I>I</I><SUB>a</SUB> is less than <I>I</I><SUB>b</SUB> and the flux trap regions are absent, the values of <I>I</I><SUB>b</SUB> for all points (<I>H</I><SUB>a</SUB>, <I>I</I><SUB>a</SUB>) constitute a single line <I>I</I><SUB>b</SUB>(<I>H</I><SUB>a</SUB>), which can be easily extrapolated to a high field. This line provided a major parameter for our model. </P>
Scanning Hall probe measurements of field distributions of a coated conductor under applied fields
Yoo, Jaeun,Jung, Yonghwan,Lee, Jaeyoung,Lim, Sunme,Lee, SangMoo,Jung, YeHyun,Youm, Dojun,Kim, Hosup,Ha, HongSoo,Oh, Sangsoo IOP Publishing Ltd 2006 Superconductor science & technology Vol.19 No.12
<P>We measured the field profiles near the surface of a coated conductor (CC) under various applied fields by using the scanning Hall probe method. The field, applied in the normal direction, was increased from zero to 171.5 Oe and then decreased to −58.8 Oe. We could not analyse our data completely by the direct use of Brandt’s calculation but by a modification with unusual field dependences of the introduced parameters. Since Brandt’s original calculation was based on homogeneous films, it was not suitable for CCs with coarse granular structures. The modified calculations with appropriate parameters are related to the coarse granular structures. Those parameters, <I>D</I>, <I>J</I><SUB>c</SUB>, and <I>R</I>, represent the three characteristics of the flux penetration network: the average distance of flux penetrations, the density of critical sheet currents, and the range of meandering of the flux penetration front, respectively. The external field dependences of these parameters were different from those of the classical critical state model. </P>
Pushing Coated Conductor Critical Currents Beyond 1 kA per cm Width: Stacks of YBCO Layers
Yehyun Jung,Sheehan, C J,Coulter, J Y,Matias, V,Dojun Youm IEEE 2011 IEEE transactions on applied superconductivity Vol.21 No.3
<P>For a number of superconducting power applications, a high value of the engineering critical current density (<I>Je</I>) for the wire is crucial. The superconducting layer in the coated conductor is typically a small portion of the overall cross-section, so increasing the superconductor fraction will directly result in an increase of <I>Je</I> . However, as the thickness of the superconductor is increased, <I>Jc</I> eventually drops. We describe a way to increase <I>Je</I> by making a stack of superconducting layers using sequential Ion-Beam Assisted Deposition (IBAD)/Superconductor deposition. Reactive Co-Evaporation by Cyclic Deposition and Reaction (RCE-CDR) is used for superconductor. An IBAD-textured layer resets the crystalline structure after each superconducting layer and we use IBAD-MgO for this purpose. However, IBAD-MgO texturing requires an extremely smooth starting surface (about 1 nm root mean square roughness), whereas the YBCO layer is typically 10-100 times rougher. We employ the Solution Deposition Planarization (SDP) process to planarize the rough surface of YBCO. The SDP layer is insulating and it provides for an easy way to separate the superconducting layers electrically. We discuss unique features of the stacking structure that allow for high <I>I</I><SUB>c</SUB>, low ac-losses in applied fields, as well as high <I>Je</I> .</P>