Sangjun Oh

A Variable Temperature Walters Spiral Probe for the Critical Current Measurement of Superconducting Strands

We have developed a probe for the critical current measurements of low temperature superconducting strands at various field, temperature and strain. A 30 cm-long strand sample is soldered on a Walters spiral made of beryllium copper alloy and compressive or tensile axial strain can be applied up to 0.7%. Temperature control ability was tested using a MgB2 wire up to 30 K. Three cernox sensors are attached on a spiral adjacent to the strand and the temperature of the spiral is controlled within plusmn50 mK of target temperature during the critical current measurement up to 80 A. Extensive critical current measurements have been carried out for an internal-tin processed Nb3Sn strand. It was found that the measured field, temperature and strain dependence of the critical current for the internal-tin Nb3Sn strand is in agreement with the recent scaling law based on strong coupling theory of superconductivity.

Effects of Cr Diffusion on

Nb3Sn strands which will be used for the ITER TF coils should be chrome plated thickness of 1.8 mum to reduce AC loss and improve the thermal stability of cable. During heat treatment, the RRR value can be affected by Cr diffusion. In this work, the effect of Cr diffusion was systematically studied, using the KSTAR and ITER candidate strands. For the KSTAR strand, the Cu/non-Cu ratio was systematically varied from 1.53 to 0.69 by chemical etching whereas the samples were heat treated by the same schedule. For the ITER candidate strand, on the other hand, the Cu/non-Cu ratio was fixed to the specification value of 1.0, and the variation of the RRR value was studied with respect to the heat treatment schedule, especially the retention time of the 650degC plateau was varied from 100 to 200 hours. It was also compared with the Cr diffusion distance obtained from EPMA (electron probe microanalysis). We found that the RRR value of the ITER candidate strand becomes lower than the ITER requirement of 100, if the heat treatment at 650degC is longer than 200 hours. The RRR can be reached up to 161 when the duration of 650degC plateau is reduced to 100 hours, while satisfying all other ITER requirements including the critical current density.

RRR

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 ...

Values of Cr-Plated Nb

Ni tapes were textured by taking advantage of their secondary recrystallization. The 18 cm long tapes obtained had textures of single crystalline quality with their [001] axes tilted with respect to the surface and their [010] axes parallel to the rolling direction. YBCO/CeO2/YSZ/CeO2 films grown on the Ni tape had the same crystalline orientations. Magnetic field dependent I-V relations were measured on a 5 cm section of the tape. Jc defined by 1 mV criterion was 1.5×105 A cm-2 at 77 K under zero field and was reduced by ~50% under the applied magnetic field of 5 T.

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Abstract Detailed fabrication conditions for the biepitaxial growth of six types of multilayers are reported. Each of them comprises various layers, such as Y1Ba2Cu3O7−x, CeO2, MgO, and SrTiO3 layers, on the MgO or the SrTiO3 substrate. We found that the proper biepitaxial growth was possible for only three types of multilayers. Our experimental results reveal that the MgO seed layer plays an important role in epitaxial growth with a desired crystalline orientation, which must be due to a particular property of the MgO seed layer that is absent in the bulk MgO.

Sn Strands Fabricated by Internal-Tin Process

We have developed a new type of magnetocardiograph (MCG) system employing 6 high-TC SQUID magnetometers. We have fabricated high performance YBCO SQUID devices for the system. The lightweight MCG system, requiring liquid nitrogen as a coolant, consists of 6-channel SQUID sensors and an adjustable patient bed with sliding motion. The MCG system showed stable operation in a room with moderate magnetic shielding. In normal operation with a moving subject, we could obtain 36-point and 48-point MCG mappings with high enough signal qualities for clinical trials.

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

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.

Fabrication of YBCO coated conductors using nickel tapes textured in single crystalline qualities

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.

The effect of a MgO seed layer on the biepitaxial growth of Y1Ba2Cu3O7−x overlayers in various multilayered thin films

We have developed an apparatus to investigate the effect of transverse stress under axial strain on the critical current of superconducting strands. An internal tin Nb3Sn wire is soldered on a C-shape ring (called Pacman) made of 2% beryllium doped copper alloy and axial strain from -0.7 to 0.7% can be applied. The actual strain on a sample was estimated from a comparison with the critical current measurement results using a Walter spiral probe. Reversible transverse stress effects on the critical current were studied in this work and the transverse pressure was applied up to 40 MPa under ±0.5% axial strain. The critical current was initially increased about 3% as transverse load was applied and then decreased almost linearly under compressive axial strain. Similar behavior is observed under tensile axial strain but the critical current decreases rather sharply at higher load. A unified description based on 3 dimensional deviatoric strain was difficult for the sample studied in this work.

Development of a 6-Channel High-

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.