Akihisa Miyazoe

Magnetic Field Induced by Screening Current in Short and Straight Coated Conductors

This paper presents experimental results and analyses of spatial and temporal variations of a screening-current-induced magnetic fields (SCFs) and remnant trapped fields for a short coated conductor (CC) and CCs superimposed each other. The variations are evaluated at temperatures of 4.2 K, 20 K and 30 K. Based on a simple model for SCFs and remnant trapped fields for a short and straight CC, the variations for a CC are formulated. The experimental results of SCFs for superimposed CCs elucidated that a SCF for each CC interacts with other SCFs.

Strain Analysis of

Ic-strain characteristic of YBCO coated conductor was measured using a Walters spring (WASP). In this technique, additional bending and thermal strains induced to the YBCO layer should be considered. In order to produce different initial bending strain to the YBCO layer, the conductor was wound around the springs with different diameters and in the different bending directions. The clear evidence was obtained that -strain curves using a WASP strongly depend on the initial bending strain state. However, when -strain curves were normalized by a maximum value, all of the curves even including the data measured by uniaxial strain method for a short sample fall on to the same curve. Strain analysis based on the rule of mixture probes that the shift along the strain axis for each -strain curve can be explained by the bending and thermal strain during soldering. This result suggests that combined strain of bending and tensile strains can be regarded as simple summation in the present YBCO coated conductor.

I_{c}(\varepsilon)

This paper presents magnetic-field distributions generated by screening currents flowing in short and straight multiple coated conductors exposed to a high magnetic field. The purpose of this study is to understand interactions between the magnetic field and the screening currents flowing in the coated conductors. In this paper, we suggest a method for calculating the magnetic-field distributions from current distributions. The method takes into account of the mutual screening current in the coated conductors. The magnetic-field distributions estimated using the method are in agreement with the experimental results. In addition, the current distributions inducing the magnetic-field distributions are described.

Characteristic of YBCO Coated Conductor Measured by a Walters Spring

Magnetic field trapped in a superconducting bulk magnet has non-negligible inhomogeneity resulting from nonuniform superconducting properties. Counteracting the inhomogeneity is one of the challenging problems in applications of bulk magnets to NMR and MRI. In this study, we fabricated a bulk magnet with concentric slits for trapping a field with reduced circumferential inhomogeneity. A GdBaCuO superconductor, 60 mm in outer diameter, was sliced with a thickness of 2 mm. Three equally-spaced concentric slits were formed using a sandblasting technique. The magnets were cooled with liquid nitrogen in an external field of 600 mT. The z-component of trapped field was measured along circular trajectories. The magnet with slits exhibited smaller inhomogeneities of fields compared with an equal-sized one without slit. On a trajectory 30 mm in diameter, the field inhomogeneities of magnets with and without slits were 7% and 11%, respectively. On a 13-mm trajectory, the slits resulted in the reduction of inhomogeneity from 0.9% to 0.3%.

Magnetic Field Distribution Generated by Screening Current Flowing in Coated Conductor Arranged Edge-By-Edge and/or Face-To-Back

In a measurement of the critical current of a rare-earth cuprate (RE-Ba-Cu-O) coated conductor in liquid helium, we found that the coated conductor delaminated under a magnetic field with angles of 10° and 20° relative to the tape surfaces. To explore causes of the delamination, strains in the direction of the tape widths were observed under an increase in the background magnetic field using two kinds of coated conductors: one had a copper stabilizer surrounding the superconducting layer, while the other had a copper stabilizer that was laminated on the tape surface. The former conductor did not delaminate, but the latter conductor delaminated by applying only a background magnetic field in liquid helium. For the above reason, we concluded that some mechanical support was required to prevent the latter coated conductor from delaminating. In fact, the delamination was preventable by a mechanical support born down to the tape surfaces. As a result, we succeeded to measure the critical current of the coated conductor in liquid helium under a background magnetic field. These results suggest that mechanical support for joints in REBCO coils needs careful consideration.

Concentric Slitting of a Ring-Shaped Bulk Superconductor for a Reduction in Circumferential Inhomogeneity of the Trapped Magnetic Field

This paper presents current distributions consisting of transport and screening currents flowing in a YBCO coated conductor (CC) under flux densities of several tesla in liquid helium, where magnetic flux should be at center along a width of the CC. Distributions of magnetic flux densities generated by the currents in the CC were measured. In order to demonstrate the current distributions, a one dimensional inverse problem was solved using Tikhonov regularization. Sheet current density distributions near one edge with external field, where magnetic flux existed all over the width, were constant even if transport current increased. The constant current density corresponded to a critical current at the field divided by the width. Magnetic flux density distributions were asymmetric to the width of a CC under external magnetic flux density of several tesla. On the other hand, hysteresis of the magnetic flux densities above the CC shrank symmetrically to the width with increases of transport current.

The Critical Current of REBCO Coated Conductors and Experimental Observation of Delamination Due to the Screening Current at 4.2 K

Fabrication of MgB2-based superconducting magnets has been attempted by a new approach using film coated on symmetric tubes. Superconducting MgB2 films have been prepared on iron substrates by electroplating in molten electrolytes. The critical current (Ic) of the MgB2 electroplating films at 4.2 K and at self-field was 15 A on the basis of 1 μV/cm of Ic criterion. A model calculation has shown that MgB2-based superconducting magnets based on MgB2 electroplating films have the potential to generate magnetic fields over 0.5 T.

Current Distribution in Coated Conductor Under External Magnetic Field Up to 5 T

We previously fabricated superconducting magnet component coils that were asymmetrically arranged along the coil axis. In this study, we performed magnetic resonance imaging (MRI) using this magnet. The magnet consisted of seven coils wound up with NbTi wires, generating a homogeneous field zone at 29 mm off-center along the longitudinal axis. A gradient coil, a radio frequency coil, and other hardware were installed for MRI measurements. MRI of water in a sample tube and of a vegetable was performed using a spin-echo method under a static field of 0.77 T. If this concept of magnet design were extended to develop a larger MRI system for the human brain, a subject would have a wide field of vision and could move the hands during imaging.