Yuzo Fukuda

Theoretical investigation on the detection ratio of the magnetization in superconducting wires by a saddle-shaped pick-up coil

We theoretically investigated the adequate geometrical configuration of a saddle-shaped pick-up coil for the accurate measurement of the ac losses in oxide superconducting thin wires with large anisotropy. Defining the ratio of the apparent signal due to the magnetic moment in sample wires, which is detected by a saddle-shaped pick-up coil, to the actual one as a detection ratio, we calculated the dependences of the detection ratio on the height, width and length of the straight and opening parts of a saddle-shaped pick-up coil. We found out that it is possible to obtain the actual magnetization without compensation by supposing that the effective length of sample wires is equal to that of the straight part of a saddle-shaped pick-up coil and by considering that the signal which is detected by the opening part compensates for the shortage of the signal which is detected by the straight part. Finally, the recommendable dimensions of a saddle-shaped pick-up coil for the measurement of ac losses in currently developed oxide superconducting wires were concretely shown.

Field angle dependence of AC losses in stacked Bi-2223 Ag-sheathed tapes

In case of applying oxide superconducting tapes to power devices, they are usually stacked face-to-face to form a parallel conductor for the enlargement of current capacity. The accurate estimation of ac losses in the stacked tapes is very important for the basic design of the superconducting devices. In this study, we discuss the angular dependence of the ac losses in the stacked Bi-2223 Ag-sheathed tapes exposed to an external ac magnetic field. First we measure the ac losses in the nontwisted Bi-2223 tapes using a saddle-shaped pickup coil. The experimental results show that the ac losses observed in the transverse magnetic field, whose direction has any angle with respect to the wide face of the tapes, can be well reproduced by a simple summation of the parallel- and perpendicular-field losses. Next we compare the measured ac losses with theoretical ones, which were predicted from the critical current characteristics of a short sample observed in dc magnetic fields.

AC loss properties of a 4 kJ conduction-cooled pulse coil wound with a Bi2223 6-strand parallel conductor for SMES

We designed and fabricated a 4 kJ conduction-cooled superconducting pulse coil with a 6-strand interlayer-transposed parallel conductor composed of Bi2223 multifilamentary tapes. We adopted the helium gas forced-flow cooling system where the helium gas flowed inside a copper pipe soldered with the flanges of brass. We succeeded in the continuous pulse operation with an amplitude of 500 A-1.6 T at a sweep rate of 140 A/s at 30 K as designed. Even in the ac operation with an ac loss of 120 W, the difference in temperature inside the winding was only 5 K and it was possible to hold the coil temperature around 30 K. In this paper, we report the design and the test results of the coil system from the aspect of ac loss and thermal properties.

Frequency dependence of AC magnetization in stacked Bi-2223 Ag-sheathed tapes

Abstract The frequency dependence of AC magnetization in stacked Bi-2223 Ag-sheathed tapes is experimentally and numerically evaluated in an external magnetic field perpendicular to their wide faces. The sample consists of 10 pieces of non-twisted Bi-2223 multifilamentary tapes, which are vertically stacked. The magnetization in the external AC field, whose frequency is varied from 1 to 60 Hz, is measured with a saddle-shaped pickup coil. The measured AC magnetization is compared not only with the theoretical curves based on the critical state model but also with the numerically calculated losses which are derived using the voltage–current characteristics represented by the n -value model, where the local electric field is proportional to the power n of local current density. Furthermore, it is shown that the eddy-current losses in a silver matrix cannot be ignored in the perpendicular-field losses at a commercial frequency.

Magnetic field angular dependence of AC losses in Bi-2223 superconducting wires

Abstract We investigated the magnetic field angular dependence of the AC loss in stacked Bi-2223 silver-sheathed superconducting multifilamentary tapes. The form of sample stacked tapes corresponds to parallel conductors, which were adopted to a 22 kV–1000 kVA liquid-nitrogen-cooled superconducting transformer and a 4 kJ conduction-cooled pulse coil. We measured the AC losses in stacked Bi-2223 tapes by using a saddle-shaped pickup coil and rotating the sample stack around its central axis. On the basis of the observed results, we discussed the influence of stacking and the field angular dependence of the AC losses in stacked superconducting tape. In addition we confirmed that the AC losses in stacked superconducting tapes can be estimated for any field angle as the simple sum of the respective AC losses due to the parallel magnetic field component and the perpendicular one.

Fabrication and Characterization of Superconducting Probe Coil for High-Sensitivity NMR Spectroscopy

A solenoidal-shaped superconducting probe coil for taking NMR measurements was fabricated using Bi2Sr2CaCu2Ox (Bi-2212), and then its quality factor was evaluated. The Bi-2212 probe coil was fabricated by dip-coating process. The quality factor values were Q = 5960-8820 (4.2 K, 0 T) in a frequency range of 400-600 MHz. These values were 2.3-2.5 times higher than those for a normal metal probe coil under the same measurement set up. In addition, the Bi-2212 probe coil maintained its superiority over a normal metal one even in a magnetic field up to 7 T. The measurement sensitivity of NMR depends on the quality factor of the probe coil. Therefore, the present experimental results obtained using the superconducting Bi-2212 probe coil are promising for taking high-sensitivity NMR measurements.

Standardization of the pickup coil method for AC loss measurement of three-component superconducting wires

Abstract In order to extend the scope of the proposed standard of the pickup coil method for AC loss measurement in IEC/TC90-WG9, a round robin test (RRT) of AC loss measurement for three-component (Cu/Cu–Ni/Nb–Ti) superconducting wires has been carried out. Three kinds of three-component superconducting wires were supplied and two laboratories participated in the RRT. The frequency dependence of total AC loss was measured in an alternating transverse magnetic field with the amplitude of 0.5 and 1 T at 4.2 K in the frequency range between 0.01 and 20 Hz. The hysteresis loss per cycle was obtained as an extrapolated value of the total AC loss per cycle at f =0 Hz. The coupling loss was also obtained by subtracting the hysteresis loss from the total AC loss. From the RRT between the two laboratories, the coefficients of variation of hysteresis loss and coupling time constant were both within 2%. Consequently, it was found that the procedure of AC loss measurement for the three-component wires was in principle the same as that for the Cu/Nb–Ti composite ones by extending the upper point of the measuring frequency range from 1 Hz up to about 10 Hz.

AC loss properties of Bi-2212 Rutherford-type cables

We investigated the AC loss properties of a Bi-2212 Rutherford-type cable with large current capacity for future SMES. The cable is composedof 20 strands with a diameter of 0.8 mm and reinforced by Ni based alloy, which is located in the center of the cable. The width and thickness of cable are 9.05 and 2.25 mm respectively. The strand has 61×7 filaments and the average diameter of the 61 filament bundle is about 0.2 mm. The critical current of the cable is about 2.9 kA at 4.2 K and 0 T. We measured the AC losses in the strand and the cable at 4.2 K by pick-up coil method. The AC loss in the strand was scarcely dependent on frequency since it was not twisted. On the other hand, the AC losses in the cable had remarkable frequency dependence. It may be due to the coupling current between strands. We quantitatively discuss the AC loss of the cable in comparison with theoretical evaluation.

Ac Losses in Bi2212 ROSATwires Exposed to an Ac Magnetic Field

We experimentally studied the ac loss properties of Bi2212 ROSATwires(Rotation-symmetric arranged tape-in-tube wire) with 19x6x3 thin filaments and pure silver matrix. Ac losses and I c -B characteristics of the twisted and non-twisted straight sample wires with a diameter of 1.95mm were measured in subcooled LN2 at 66K. The ac loss was reduced by twisting. The effective filament diameter of the twisted wire was equal to the average diameter of the 19x6-filament bundle. Decoupling between the filament bundles was confirmed. The observed coupling current loss roughly agreed with the theoretical prediction on the supposition of an ideal multifilamentary wire notwithstanding the complicated arrangement of filaments with anisotropy of J C .