Atsushi Ishiyama

Normal transition and propagation Characteristics of YBCO tape

YBCO tape is expected to be used in future HTS applications, because it has better J/sub c/ characteristic in high temperatures and in high applied magnetic fields. It is important to consider the stability in superconducting applications. To establish the stability criterion for coated conductors, the characteristics of the transition to the normal state and normal-zone propagation in YBCO sample tapes were examined experimentally and numerically. The YBCO sample tapes were produced by IBAD/PLD method. Measurements were performed as a function of both heat input and operating current for operating temperatures of 55 K, 60 K, and 65 K in a background magnetic field of 5 T. The experimental results were compared with a simulation using a newly developed computer program based on the finite element method (FEM). In the computer program, the I-V characteristics of the YBCO sample tapes were taken into account. The computer program could simulate the normal transition and propagation phenomena and its validity was confirmed. The influence of the thickness of a Ag-stabilizer on the transient thermal behavior of YBCO tapes was examined experimentally and numerically and the results were presented here.

A Criterion for Determining Stabilizer Thickness of YBCO Coated Conductors Based on Coil Protection

The purpose of this study is to propose a criterion for determining stabilizer thickness of YBCO coated conductors. The permissible disturbance energy density in high-temperature superconductors (HTSs) is two or three orders of magnitude greater than that in low-temperature superconductors (LTSs). On the other hand, to enhance the reliability and safety of HTS coils, an establishment of a protection scheme assuming a quench caused by a failure in power supply, cooling system or similar problem is also required for HTS coil systems. Quenching produces excessive temperature or voltage in the coil winding; overheating may cause the conductor to meltdown, or high voltage may punch a hole in the winding. To avoid these types of damages in YBCO coils, a suitable stabilizer thickness must be determined for the sophisticated structure of YBCO coated conductors, which are composed of highly textured thin-film on a flat flexible substrate. Therefore, we investigate the appropriate structure for YBCO coated conductors from the perspective of energy dumping (i.e., quench protection). Based on the protection scheme that the stored magnetic energy in the coils is dumped by an external dump resistance prepared as a quench protection, we derive a function that examines the heat balance of a quenching YBCO coil in an adiabatic condition and propose a criterion using the function to determine an appropriate stabilizer thickness for YBCO coated conductors.

An optimal design method for superconducting magnets using HTS tape

This paper describes an optimal design method for superconducting magnets wound with high temperature superconducting (HTS) tapes (e.g. Bi2223/Ag tapes). The properties of HTS tapes have been advancing and HTS magnets have been constructed and demonstrated. However, the HTS tapes have thermally different characteristics compared with the low temperature superconducting (LTS) wires. Therefore it is necessary to consider these characteristics of HTS tapes at the magnet design stage. We propose a new configuration of the superconducting magnets using Bi2223/Ag tapes. The proposed configuration is optimized by using simulated annealing (SA), which is one of the optimization algorithms, under a lot of constraints such as B-I characteristic, central magnetic field, field homogeneity and so on. The details of the optimization method and an example of its application to a 12-Tesla superconducting magnet using Bi2223/Ag tape are shown.

Analyses of Transient Behaviors of No-Insulation REBCO Pancake Coils During Sudden Discharging and Overcurrent

Stability margin of a high-temperature superconducting (HTS) coil is two or three orders of magnitude greater than that of a low-temperature superconducting coil. In recent years, many papers have reported test results of turn-to-turn no-insulation (NI) HTS coils having extremely enhanced thermal stability, such that burnout never occurs in an NI coil, even at an operating current exceeding 2.5 times the critical current. Thus, The main goal of this paper is to clarify transient electromagnetic and thermal behaviors and mechanism of the high thermal stability in an NI REBCO coil. A partial element equivalent circuit (PEEC) model is proposed for the numerical simulation of an NI REBCO coil, which considers a local electrical contact resistance between turns, an I-V characteristic of an REBCO tape, and local self and mutual inductances of the NI REBCO coil. Using the PEEC model, we investigate the influence of the turn-to-turn contact resistance on the transient behavior of the NI REBCO coil during sudden discharging. We also perform thermal conduction analyses with the PEEC model to clarify the transient behavior of an NI REBCO coil during an overcurrent operation.

An optimal configuration design method for HTS-SMES coils

Recently, the properties of high temperature superconducting tapes have been in advance and high temperature superconducting magnets have been constructed and demonstrated. However, the high temperature superconducting tapes have different thermal characteristics compared with low temperature superconducting wires. Therefore, it is necessary to consider these characteristics of high temperature superconducting tapes at the magnet design stage. We proposed an optimal design method for superconducting coils wound with Bi2223/Ag tapes. In this paper, the configuration of 72 MJ SMES coils wound with Bi2223/Ag tapes are optimized.

3D Electromagnetic Analysis of Levitating Transporter Using Bulk Superconductor

We have constructed and demonstrated levitating transporter models composed of the superconductor bulks (HTS bulks) and permanent magnets. The dynamic characteristics of lift force, lateral force and magnetic stiffness are very important for the design of superconducting devices such as a levitation system. The electromagnetic phenomenon should be clarified through the accurate analyses of the magnetic field and shielding current distributions within HTS bulks. In this study, we developed a new simulation program based on the three-dimensional hybrid finite element and boundary element (3D hybrid FE-BE) method, which combines with the fast multipole method (FMM). By using the FMM, it is possible to solve the large-scale problem with a reduction of the required memory size and improvement of the accuracy. This simulation program can consider the anisotropic and nonlinear E-J characteristics of HTS bulks. The results of experiment and numerical simulation are in good agreement

An optimization method for design of SMES coils using YBCO tape

Recently, the properties of high temperature superconducting (HTS) tapes have been in advance and HTS magnets have been constructed and demonstrated. However, the HTS tapes have thermal different characteristics compared with the low temperature superconducting (LTS) wires. Moreover, Bi2223/Ag tapes and YBCO coated conductors have the different properties about the dependence on the degree of external magnetic field. Therefore it is necessary to consider these characteristics of HTS tapes at magnet design stage. We proposed an optimal design method for superconducting coils using Bi2223/Ag tapes. And the proposed configuration is optimized by using the simulated annealing (SA), which is one of the optimization algorithms, under a lot of constraints such as B-I characteristic, storage energy, and so on. The details of the optimization method and an example of its application to SMES coils (72 MJ) using YBCO coated conductors are shown.

Characteristics of lift and restoring force in HTS bulk-Application to two-dimensional maglev transporter

One of the advantages of magnetic levitation using a high-temperature superconducting (HTS) bulk is that stable levitation can be achieved without any control systems. The authors have been investigating the electromagnetic behaviors of an HTS bulk to realize a two-dimensional magnetic levitating transporter without any fixed guides. The characteristics of lift and stability are key parameters to design and optimize such a device. They measured the lift and the restoring force of a YBCO bulk, displaced by a distance in lateral direction, for various field-cooling conditions and permanent-magnet arrangements. Both lift and restoring force are closely related to the air gap in the field-cooling process, distance between the permanent magnets, number of permanent magnets, and permanent magnet arrangement, that is, external magnetic field distribution. The most suitable arrangement of permanent magnets depends on the required levitation height and the weight of the levitating part. It can be considered that the size of the levitating part and geometry are also very important to determine the optimal arrangement in the maglev device.

Magnetic characteristics of a high-T/sub c/ superconducting cylinder for magnetic shielding type superconducting fault current limiter

The superconducting fault current limiter (SCFCL) is expected to be the first application of high-T/sub C/ superconductors (HTSs) in power systems. To develop a magnetic shielding type superconducting fault current limiter, we have carried out some fundamental experiments concerning the magnetic shielding characteristics of an HTS bulk cylinder. In this paper, the experimental results of magnetic flux penetration into a Bi(2223) cylinder are shown. AC magnetic field is applied to the sample by a primary winding (copper coil) excited by AC triangular- and sinusoidal-waveform currents in the frequency range of 0.1 Hz to 100 Hz. We also developed a finite element method (FEM) computer program for evaluation of the dynamic electromagnetic behavior of the HTS cylinder in a time-varying external magnetic field. The results of computer simulations considering the voltage-current (E-J) characteristic are compared with experiments.

Magnetic shielding for MRI superconducting magnets

An optical design is presented of a highly homogeneous superconducting coil system with magnetic shielding for magnetic resonance imaging (MRI). The optimal design method is a combination of the hybrid finite-element and boundary-element method for analysis of an axially symmetric nonlinear open-boundary magnetic field problem and a mathematical programming method for solving the corresponding optimization problem. The multiobjective goal programming method and the nonlinear least-squares method are adopted. The optimal design results of 1.5- and 4.7-T-magnet systems with different types of magnetic shielding for whole-body imaging are compared, and the advantages of a combination of active and yoke shields are shown.