SangMoo Lee

Relation between the critical current and the n value of ReBCO thin films: A scaling law for flux pinning of ReBCO thin films

Detailed field and angle dependencies of the critical current and the n value for a SmBCO coated conductor have been measured. It was found that the field dependence of the n value can be fitted by an empirical power law with three parameters including the irreversibility field. We also found that there is a correlation between the critical current and the n value which can be described by the Kramer model including thermal activation. The model fits the field dependence of the empirical critical current data at various angles and temperatures with three fitting parameters, the pinning force maximum, the g factor, and the upper critical field. The upper critical field found from fitting was higher than the irreversibility field, and the angular dependence of the upper critical field is in agreement with the Tinkham model. The pinning force maxima do not show a correlation with the upper critical or the irreversibility fields, which is attributed to the difference in the pinning mechanism with a variation ...

Scanning Hall probe measurements of field distributions of a coated conductor under applied fields

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 Brandts calculation but by a modification with unusual field dependences of the introduced parameters. Since Brandts 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, D, Jc, and R, 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.

The effects of a magnetic field dependent critical current on the hysteresis loss of a coated conductor

We calculated the current distribution in a coated conductor (CC) from magnetic induction profiles measured using the scanning Hall probe method. An external magnetic field was applied in the normal direction with respect to the tape surface and was decreased stepwise from Hpeak to −Hpeak. From the hysteretic behavior of the current distributions, the energy loss, QM, was calculated for various values of Hpeak. Comparison of the present results with Brandts theoretical calculations revealed that his formula is still valid if one substitutes the magnetic field dependent critical current density into his equation. The theoretical results obtained were consistent with the experimental QM values obtained.

Comparative Study on the Profiles of Intra-Granular Critical Current Density in SmBCO and YBCO Coated Conductors

We measured the magnetic hysteresis loops (M-H loop) of SmBCO coated conductor (CC) and YBCO-CC by using SQUID-magnetometer. These samples were fabricated by RABiTS method and IBAD method. We found the peak shifts of their M-B loops were qualitatively different. To understand this, we measured the magnetic field profiles, H(x), of SmBCO-CC and YBCO-CC by using scanning Hall probe method and calculated their current profiles, J(x). We found that the contrasting difference of M-B loops of the two types of CC was partly due to the inhomogeneous distributions of field and current, which depend on the sample shapes.

A simple model for the estimation of hysteresis energy loss in a coated conductor under the simultaneous application of a magnetic field and current

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 Ib(Ha) to determine the major parameters used in this model. The value of Ib(Ha) was based on the scanning Hall probe measurements (SHP) on a Sm1Ba2Cu3O7 − δ coated conductor. During SHP measurement, a magnetic field (Ha) and current (Ia) were applied simultaneously and were varied along 11 different load lines. From the values of SHP measurements, the current density profiles, J(x, Ha, Ia), were calculated using a numerical inversion method. We define the quantity Ib = ∫ − ww|J(x, Ha, Ia)| dx and we calculated Ib at many points (Ha, Ia) in every load line. We found that when Ia is less than Ib and the flux trap regions are absent, the values of Ib for all points (Ha, Ia) constitute a single line Ib(Ha), which can be easily extrapolated to a high field. This line provided a major parameter for our model.

Estimation of hysteresis loss in Sm1Ba2Cu3O7-δ coated conductor via scanning Hall probe measurements with simultaneously applied field and current

The hysteresis loss in a Sm1Ba2Cu3O7−δ coated conductor was estimated from magnetic field profiles measured by the scanning Hall probe method. Current, Ia, and magnetic field, Ba, were applied simultaneously; Ba was applied in the normal direction with respect to the tape surface. Ba and Ia were varied from Bpeak to −Bpeak and from Ipeak to −Ipeak, respectively, with the ratio α = Ia/Ba fixed during the variation. Three values of α were taken for the three load lines. The values of Bpeak/Ipeak 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, Q, were then calculated for various values of Ipeak(= αBpeak) in each load line. The results were compared with theoretical calculations based on Brandts model. When Bpeak was about 300 mT, the estimated values of Q were several times smaller than the theoretical values of Q with the self-field Ic0. The low value of Q in this case is due to the field dependent Ic and the saturation effect of the current profiles, which results in significant reduction of the induced magnetic moment, M.

Lorentz-force dependence of the critical current for SmBCO coated conductor

Angular dependence of the critical current in a varying Lorentz-force configuration for a SmBCO coated conductor has been studied. Near the transition temperature, the angular dependence of the critical current in a varying Lorentz-force configuration was quite similar to the result of a constant Lorentz-force measurement. As the temperature is lowered and as the field is aligned along the ab-plane, the critical current measured in a varying Lorentz-force configuration becomes larger than the constant Lorentz-force measurement. We found that the field dependence of the critical current and n-value can be described by the same pinning model, the Kramer model including thermal activation, reported for constant Lorentz-force measurements of various ReBCO (Re, rare earth, Sm, or Y) thin films [S. Oh et al., J. Appl. Phys. 102, 043904 (2007)]. As a possible reason, the Lorentz force acting on segments of twisted vortex lines due to collective pinning or by thermal excitations is discussed. It is further argued that the difference in the critical current for the varying and constant Lorentz-force measurements can be understood as mainly due to a variation in the pinning force, if we interpret the Lorentz force on twisted vortex lines as a scalar product between the applied field and current.Angular dependence of the critical current in a varying Lorentz-force configuration for a SmBCO coated conductor has been studied. Near the transition temperature, the angular dependence of the critical current in a varying Lorentz-force configuration was quite similar to the result of a constant Lorentz-force measurement. As the temperature is lowered and as the field is aligned along the ab-plane, the critical current measured in a varying Lorentz-force configuration becomes larger than the constant Lorentz-force measurement. We found that the field dependence of the critical current and n-value can be described by the same pinning model, the Kramer model including thermal activation, reported for constant Lorentz-force measurements of various ReBCO (Re, rare earth, Sm, or Y) thin films [S. Oh et al., J. Appl. Phys. 102, 043904 (2007)]. As a possible reason, the Lorentz force acting on segments of twisted vortex lines due to collective pinning or by thermal excitations is discussed. It is further argued...