Hiroshi Okamoto

Anisotropic current transport properties and their scaling in multifilamentry Bi-2223 Ag-sheathed tapes

Using a statistic E-J model describing percolative current transport properties, we analyzed pinning strength, its distribution and angular dependence of Bi-2223 Ag sheathed tapes. It has been shown that the anisotropy can be attributed to angular dependence of the depinning transition field, and a scaling holds with respect to the pinning force density. The angular dependence of the transition field can be described precisely by an extended Tinkham 2D model taking into account misalignment angle of the filaments. We derive an analytical expression describing the magnetic field and angular dependence of J/sub c/ and n value of the tape. It allows us to predict the angular dependence of the transport properties using only limited data set, i.e., (1) magnetic field dependence in perpendicular field and (2) anisotropic ratio of the transition fields between perpendicular and parallel configuration.

Optimal design of a Bi-2223/Ag coil for superconducting magnetic energy storage at different operating temperatures

We carried out optimal design of a Bi-2223/Ag toroidal coil for a superconducting magnetic energy storage system. The objective was to minimize the total length of Bi-2223/Ag tape at fixed conditions of operating temperature, stored energy and upper limit of the loss. The energy loss was calculated by means of the 3D finite element method (FEM) with the use of J?E expressions, which can quantitatively estimate the experimental data obtained from the tape in wide ranges of temperature (20?77?K), external magnetic field (0.02?3?T) and its applied direction (arbitrary). On the other hand, the optimal solution was obtained by a genetic algorithm (GA), which was a particularly effective optimization method when the objective function had a number of local minimum points. We performed such optimal design under various constraint conditions, and found that the optimal configuration of the coil drastically changed as its operating temperature varied.

Thermal Conductivity of YBCO Coated Conductors Reinforced by Metal Tape

We have measured the thermal conductivity, κ(T), of YBCO coated conductors (YCCs) reinforced by Cu or CuNi tape. κ(T) of YCC reinforced by Cu tape with 100-300 μm in thickness, YCC-Cu, was 250-400 W m-1K-1 at 77 K, which depended on the thickness of Cu tape and was roughly one order of magnitude larger than that of the bare YCC. On the other hand, κ(T) of YCC reinforced by CuNi tape of 300 μm thickness, YCC-CuNi, was comparable to that of the bare YCC except for the low temperatures. The contribution of each reinforcing material to the thermal transport was estimated by analyzing the measured κ(T) using an equivalent heat current circuit. The applied heat flows mainly through the Cu tape for the YCC-Cu tapes and through both CuNi tape and stabilizing Ag layer for the YCC-CuNi tapes, respectively. Phonons cannot be ignored as thermal carriers in YCC-CuNi tapes.

Noncontact Characterization of In-Plane Distribution of Critical Current Desity in Multifilamentary Coated Conductor

Using a scanning Hall-probe microscopy, we have developed a noncontact characterization method for multifilamentary coated conductors. In-plane distributions of sheet current density in a multifilamentary coated conductor were visualized at different conditions of external magnetic field. From these results, we could extract local critical current and equivalent width of each filament as a function of longitudinal position. These items will be indispensable for quality control of multifilamentary coated conductors and for the precise estimation of their AC losses. This means that this characterization method will be a key technology for the establishment of multifilamentary coated conductors which promise electric power applications with low AC losses.

Development of REBCO Superconducting Transformers With a Current Limiting Function—Fabrication and Tests of 6.9 kV-400 kVA Transformers

We are developing RE 1 Ba 2 Cu 3 O 7-δ (REBCO, RE: Rare Earth, Y, Gd, etc.) superconducting transformers with a current limiting function. The target is a 3φ-66 kV/6.9 kV-20 MVA one for a distribution power grid. We designed and built two pieces of 1φ-6.9 kV/2.3 kV-400 kVA superconducting transformers with YBCO superconducting tapes. One was wound with a YBCO tape with a copper stabilizing layer with a thickness of 300 μm. The other was 50 μm in thickness of copper. Making sudden short-circuit tests with a 200 MVA short circuit generator, we investigated the response of the YBCO superconducting windings against the fault excess current over the critical current. Repeated sudden short-circuit tests revealed the quite different response between them. In the case of fully stabilized with copper 300 μm thick, short circuit current flowed without decay. On the other hand, in the case of copper 50 μm thick, short circuit current was reduced around three times as much as the rated current. Using the numerical simulation program, which we made up on the basis of the results of sudden short-circuit tests of a 10 kVA test transformer, we quantitatively investigated the transition phenomenon of REBCO superconducting windings to normal state through flux-flow state due to fault excess current. As a result, theoretical simulation explained experiment quantitatively and clarified how to control the short circuit current.

Investigation of Three-Dimensional Current Distribution at Silver Diffusion Joint of RE-123 Coated Conductors Based on Magnetic Microscopy Combined With Finite Element Method

Combining scanning Hall-probe microscopy (SHPM) with finite element method (FEM), we have investigated three-dimensional current distribution at a silver diffusion joint of RE-123 coated conductor (CC). This research aims at the understanding of electromagnetic behaviors in a jointed CC and at the establishment of an analysis model for it. Two-dimensional distribution of sheet current density in a jointed sample was visualized by a SHPM measurement with a spatial resolution of a few hundred micrometers. Then, it was found that such a distribution included information about current transfer between the jointed pieces. Furthermore, the experimental results were successfully reconstructed by a three-dimensional FEM analysis. We believe that these will be very important findings for the establishment of jointing processes on CCs and for the designs of practical applications which need such processes.

Thermal conductivity of YBCO coated conductors fabricated by IBAD–PLD method

We have measured the temperature dependence of the thermal conductivity κ(T) of YBCO coated conductor tapes with various thicknesses of stabilizing Ag layer and analyzed the measured κ(T) using the equivalent heat current circuit. κ(T) of the tape increased with increasing thickness of Ag layer. In the analysis, the dominant thermal carriers were found to be the electrons in the Ag layer in the entire temperature range. The thermal conductivity of the Ag layer, κAg(T), was estimated to decrease with increasing thickness of Ag layer, which suggested that impurities and/or crystal defects were introduced during the long time deposition of Ag.

Analysis of Discharging Characteristics in a Bi-2223/Ag Coil for SMES With Consideration of Cooling Capacity of a Cryocooler

We investigated discharging characteristics of a Bi-2223/Ag coil by means of thermo-electromagnetic coupled analysis. The objective was to evaluate the thermal behavior of conduction-cooled high temperature superconducting (HTS) coils when they are applied to superconducting magnetic energy storage (SMES) systems for voltage dip compensation. The electromagnetic analysis adopted finite element method (FEM) considering the J-E expressions, which can quantitatively estimate the experimental data obtained from the tape in wide ranges of temperature (20-77 K), external magnetic field (0.02-3 T) and its applied direction (arbitrary). The thermal analysis also employed FEM, and was performed under new boundary condition where heat flow rate into a cryocooler does not exceed its cooling capacity. It was shown that transient temperature rise occurred in the coil during discharging, and such temperature rise was underestimated for generally applied boundary condition, i.e., fixed value of temperature