Hirofumi Fukai

Numerical simulation of trapped magnetic field for bulk superconductor

Abstract We performed numerical simulation of the trapped magnetic field by varying the thickness and the side length of the bulk superconductor, based on the sand-pile model and Biot–Savart law. It was found that the trapped magnetic field density at the center of the sample saturated when the sample thickness exceeds a certain value. Such thickness dependence is explained in terms of the geometrical effect or de-magnetizing effect.

Jc-B properties of large RE-Ba-Cu-O disks

We estimated Jc-B characteristics based on the sand-pile model from the magnetic field distribution on large disk samples with dimensions of 10×10×1 mm3. Such thin disks show a large demagnetizing effect. Thus simple estimates of Jc from the field distribution failed to determine correct values, and a large discrepancy was observed with Jc values determined from magnetization loops of small specimens. Jc-B characteristics obtained from the sand-pile model were in good agreement with those obtained from small specimens.

The effect of geometry on the trapped magnetic field in bulk superconductors

We have studied the effect of geometry on the trapped magnetic field in bulk Y–Ba–Cu–O superconductors by varying the thickness. The trapped field first increased with increasing thickness of the bulk superconductor, and then saturated when the thickness reached a certain value. We have also performed simulation studies based on the sand-pile model and the Biot–Savart law, which were in fairly good agreement with the experimental results.

The effect of inhomogeneous flux penetration into bulk superconductor by pulsed field magnetization

Pulsed field magnetization (PFM) is a practical method for magnetizing bulk superconductors, but the amount of trapped magnetic flux is usually smaller than that magnetized by the field cooling (FC) method due to the heat generation by the flux motion during pulsed field application. In this study, we magnetized a Y–Ba–Cu–O bulk superconductor grown from a seed which was placed at deviating positions from the centre of the bulk to investigate the effect of inhomogeneous penetration of the flux into the bulk superconductor during the PFM. The maximum trapped field by the PFM was almost the same value as by the FC method. This suggests that the inhomogeneity of the bulk superconductor affects the trapped field on the PFM. The magnetic flux can penetrate into the centre of the bulk even if the applied field is small, which contributes to reducing the extra heat generation due to the flux motion.

Numerical simulation of the trapped magnetic field for large bulk superconductors

Abstract In the present study, we modified the simulation method for calculating the trapped field distribution of large bulk Nd–Ba–Cu–O superconductors by taking account of B dependence of J c . First we modeled the J c – B property of Nd–Ba–Cu–O with the secondary peak effect by assuming that the sample has two different J c components: one for normal B dependence and the other with the peak effect. Based on the J c – B results thus determined, we could obtain the trapped field distribution of large grain Nd–Ba–Cu–O, which was in good agreement with the empirical data.

Comparative study of pulsed field magnetization for Y–Ba–Cu–O and Sm–Ba–Cu–O

Abstract We have performed a comparative study of the magnetization behavior with applying pulse fields for Y–Ba–Cu–O and Sm–Ba–Cu–O, which exhibit the different field dependence of critical current densities (Jc’s). Time variation of the field was monitored with the Hall probe sensor which is attached to the sample surface by varying the maximum field values from 0 to 2 T. In a low field region, the trapped magnetic field of Sm–Ba–Cu–O was slightly smaller than that of Y–Ba–Cu–O, however, it exceeded the value of Y–Ba–Cu–O in a high field region. The results reflect the Jc–B characteristic of Sm–Ba–Cu–O, in which the Jc values show the secondary peak effect and high field Jc values are higher than those of Y–Ba–Cu–O.

Improvement of thermal stability with alloy impregnation in Gd-Ba-Cu-O superconductors for pulsed field magnetization

We present a post-fabrication treatment that improves thermal conductivity of bulk Gd-Ba-Cu-O magnets. A small hole about 1 mm in diameter was artificially drilled into the centre of bulk Gd-Ba-Cu-O 25 mm in diameter and 18 mm in thickness. An aluminium wire was inserted into the hole, and then the sample was subjected to the impregnation by using Bi- Su-Cd alloy. A pulsed-field magnetization was performed for Gd-Ba-Cu-O bulk with and without the alloy impregnation treatment. The temperature of samples was monitored with thermocouples and local magnetic field density was measured with a Hall sensor at several positions. The maximum temperature rise was depressed by 4 K and the trapped field was increased by 25 % at 44 K. The result shows that the alloy impregnation is very effective in enhancing the thermal conductivity and thereby improving the field trapping ability.

Pulsed field magnetization of large-grain Gd–Ba–Cu–O bulk superconductor

Abstract We measured the field distribution and the variation of temperature in a large-grain Gd–Ba–Cu–O bulk superconductor during pulse field magnetization. When the applied field was below 3.1 T, the temperature rise due to flux motion was small because the magnetic flux penetrated only the surface thin layer. The trapped field increased with increasing field, although the sample temperature was raised and the trapped field reached 1.6 T for the applied pulse field of 3.1 T. However, when the applied field exceeded 3.1 T, the trapped field suddenly decreased due to an excessive temperature rise in the sample. Thus there is an optimum peak field for the pulse field magnetization process.

Penetration Dynamics of RE-Ba-Cu-O Superconductors on Pulsed Field Magnetization

It is known that melt processed RE-Ba-Cu-O (RE: rare earth elements) superconductors can trap magnetic field over 1T at liquid nitrogen temperature (77K). The trapped field is dependent on the critical current density (J c ). J c -B characteristic of oxygen-controlled-melt-growth (OCMG) processed Sm-Ba-Cu-O superconductors are different from that of Y-Ba-Cu-O. Therefore, we investigated the J c effect on pulsed field magnetization at various maximum applied field values of 0 – 2T for Sm-Ba-Cu-O grown by the OCMG method.

Pulsed Field Magnetization of Sm-Ba-Cu-O

We have investigated how the critical current densities (J c ’s) affect the magnetization behavior with pulse fields by varying the maximum values from 0 to 2 T in Y-Ba-Cu-O and Sm-Ba-Cu-O superconductors. In a low field region, the trapped magnetic field of Sm-Ba-Cu-O was slightly smaller than that of Y-Ba-Cu-O, however, it exceeded the value of Y-Ba-Cu-O in a high field region. The results reflect the J c -B characteristic of Sm-Ba-Cu-O, in which the J c values show the secondary peak effect and high field J c values are higher than those of Y-Ba-Cu-O.