Hidetoshi Hashizume

Numerical analysis of electromagnetic phenomena in superconductors

Governing equations for describing electromagnetic phenomena in superconductors were derived by using the Helmholtz theorem. The Meissner effect, which is common to type-I and type-II superconductors, is treated by introducing the magnetizing vector M. The current flow due to flux invasion into the superconductor, which is a particular phenomenon in the type-II superconductor, is evaluated using the current vector potential together with constitutive equations expressing critical state models. Numerical results based on a two-dimensional finite-element code showed excellent agreement with analytical solutions. >

Numerical analysis of a.c. losses in superconductors

Abstract A two-dimensional computational code to evaluate a.c. losses in superconductors has been developed using the current vector potential method (T-method), where the vector potential T is defined by ▿ × T = J . The current distributions in both superconductors and normal conductors are calculated while changing the conductivity of the superconductor so that the current density never exceeds the critical current density. The hysteresis and coupling losses evaluated by the numerical code agree with analytical solutions, as far as these are available. In order to verify the validity of the code, an experiment to measure the hysteresis and coupling losses was carried out using Nb-Ti/Cu filaments. The total loss evaluated from the numerical code agrees with that from the experiment. The numerical analysis, however, indicates that the hysteresis loss under quasi-steady magnetic field is less than the loss in the superconductor under transient field.

Innovative liquid breeder blanket design activities in Japan

Abstract In order to clarify key engineering issues and to enhance key R&D activities for D-T fusion blankets, many design activities on innovative liquid blanket systems are on going as collaboration studies in Japan. Recently an improved long-life Flibe blanket has been proposed, and the self-cooled Li/V blanket design has started. For Flibe systems, much progress has been made on tritium permeation barrier, energy conversion system, free surface designs, and thermofluid loop experiments. For Li/V systems, evaluation studies have proceeded on Be-free nuclear properties and allowable crack fraction on multilayered MHD insulation coatings.

Design and development of high-temperature superconducting magnet system with joint-winding for the helical fusion reactor

An innovative winding method is developed by connecting high-temperature superconducting (HTS) conductors to enable efficient construction of a magnet system for the helical fusion reactor FFHR-d1. A large-current capacity HTS conductor, referred to as STARS, is being developed by the incorporation of several innovative ideas, such as the simple stacking of state-of-the-art yttrium barium copper oxide tapes embedded in a copper jacket, surrounded by electrical insulation inside a conductor, and an outer stainless-steel jacket cooled by helium gas. A prototype conductor sample was fabricated and reached a current of 100 kA at a bias magnetic field of 5.3 T with the temperature at 20 K. At 4.2 K, the maximum current reached was 120 kA, and a current of 100 kA was successfully sustained for 1 h. A low-resistance bridge-type mechanical lap joint was developed and a joint resistance of 2 nΩ was experimentally confirmed for the conductor sample.

Proposal of mechanically jointed superconducting magnet using high critical temperature superconductors

Abstract For future design of fusion reactors, development of the remountable superconducting magnet can become one of the important technical issues, which will bring huge benefit to reduce both costs of reactor construction and maintenance. In the conventional design, however, there is a fatal problem of heat generation occurring at the joints, which causes quenching of the magnet. On the other hand HTc superconductors, which has large specific heat, are now being used for magnets. In the previous research [International Journal of Applied Electromagnetics and Mechanics, IOS Press, in press. Joint Conference of the 12th International Toki Conference on Plasma Physics and Controlled Nuclear Fusion and The Third General Scientific Assembly of Asia Plasma Fusion Association, Toki. 2001, in press], the butt jointing method of HTc S.C. tapes had been proposed and its contact performance had been reported to be relatively excellent. In this research, therefore, the current distributions in the tape are calculated by numerical analysis and then the heat generations in the jointing region are estimated to evaluate the difference of contact performance in some jointing cases. The results show that the contact performance does not dependent on relative location of each S.C. filaments.

Optimization of a Mechanical Bridge Joint Structure in a Stacked HTS Conductor

The mechanical bridge joint (bridge-type lap joint) of a stacked high-temperature superconducting conductor has been investigated for a “remountable” or a segment-fabricated high-temperature superconducting magnet. In a previous study, joint resistivities were evaluated experimentally for the bridge joints of single-layer and double-layer stacked GdBCO coated conductors. However, the joint resistivity increased with an increase in the number of layers due to nonuniform contact pressure distribution caused by a gap or misalignment in the joint region. In this study, therefore, we were aiming at the reduction of joint resistance by achieving more uniform contact pressure distribution. In addition, we investigated the effects of temperature while applying pressure to the joint and positioning the joint structure to investigate its application in an actual large-sized magnet. First, we inserted an indium film between the joint surfaces to make contact pressure uniform. Experimental results showed that joint resistivity with the indium film did not depend on the number of layers. In addition, applying force at room temperature was more effective in decreasing joint resistivity than that at 77 K. Finally, we examined the effect of the joint structure with screw bolt tightening. The result showed that the structure of the convex plate has better joint performance than others.

Transverse Stress Effects on Critical Current and Joint Resistance in Mechanical Lap Joint of a Stacked HTS Conductor

This paper discussed feasibility of bridge-type and multi-layer mechanical lap joint of a stacked HTS conductor. In the multi-layer mechanical lap joint, a decrease in critical current induced by transverse compressive joint force is one of the important factor to decide joint resistance. In addition, misalignment of joint section could be a problem because respective HTS tapes consisting the stacked HTS conductor could have slightly different thickness. In this study, therefore, transverse stress effect on critical current of stacked BSCCO and GdBCO conductors was experimentally investigated at first. Then, bridge-type mechanical lap joints of single layer and 2-layer HTS conductors were tested to confirm their performance and to consider the misalignment of joint section. Experimental results showed that critical current of the BSCCO conductor decreased at lower stress when the number of layers increased whereas that of the GdBCO conductor was less influenced by the stress. Joint resistance in the bridge-type mechanical lap joint was comparable to those in normal mechanical lap joint and soldered lap joint in the case of the single layer. Joint resistances in the 2-layer lap joint were two or three times larger than those in the single layer lap joint due to the misalignment. To solve this problem, compliant metal layer should be inserted into joint section.

Flow Visualization and Heat Transfer Characteristics for Sphere-Packed Pipes

Particle image velocimetry visualization to identify the complex flow structures in a sphere-packed pipe is carried out by using a matched refractive-index method with a sodium iodide solution as the working fluid. The following three flows were confirmed as representative flow structures in the pipe: a meandrous bypass flow with a high-flow velocity due to the wall effect, two pairs of unstable twin vortices accompanied by a strong impinging flow to the pipe wall, and a spouting flow from the central area of the pipe. In an experiment on heat transfer using water as the working fluid, the wall-temperature distribution is measured with thermocouples and infrared thermography, which makes clear a relation between the flow structures and the local heat transfer performance. Though an area with a high wall temperature is formed by the flow stagnation located at a contact point between the sphere and the heating wall, the colliding effect of the high velocity and of the meandrous bypass flow with the spheres significantly affects the heat transport from the stagnation areas. On the other hand, the heat transfer performance is quite high in a large gap area between the upstream and downstream spheres because of the influence of the strong impinging flow and the vortices that are both induced by the meandrous bypass flow.

Design study of National Centralized Tokamak facility for the demonstration of steady state high-β plasma operation

Design studies are shown on the National Centralized Tokamak facility, formerly called JT-60SC. The machine design is carried out to investigate the capability for flexibility in aspect ratio and shape controllability for the demonstration of the high-β steady state operation with nation-wide collaboration, in parallel with ITER towards DEMO. Two designs are proposed and assessed with respect to the physics requirements such as confinement, stability, current drive, divertor and energetic particle confinement. The operation range in the aspect ratio and the plasma shape is widely enhanced consistent with sufficient divertor pumping. Evaluations of the plasma performance towards the determination of the machine design are presented.

Numerical and experimental analysis of eddy current testing for a tube with cracks

A novel method for calculating the impedance signal in eddy current testing (ECT) for a tube with surface cracks is developed using the two-dimensional current vector potential method. Three-dimensional current flow around a surface crack is approximated by introducing quasi-conductivity in the crack region. A final matrix relating the current vector potentials is obtained by decreasing the width of the crack to zero, and then the elements in the crack region are not necessary any longer. Numerical results show good agreement with the experimental ones, which indicates the validity of the proposed method and the possibility of designing a more efficient probe of ECT using the code. >