Kohei Higashikawa

System Coordination of 2 GJ Class YBCO SMES for Power System Control

YBCO superconducting wire has a relatively low decrease in power distribution at high temperatures and under a high magnetic field. A high-intensity substrate is used for the wire, so the wire has high machine characteristics. Therefore, it is expected that this wire can be used for large-scale high magnetic field coils. Here, coordination between the SMES system for 100 MVA/2 GJ class load fluctuation compensating was conducted using IBAD/CVD-YBCO wire. The SMES system includes a toroidal type YBCO coil consisting of 180 compact, high magnetic field multi-unit coils, a large coil cooling system that uses the conduction cooling method, which does not use a refrigerant medium, and a multi-cell power converter that achieves multi-unit coil connection with relatively low current and low voltage. Studies were conducted for each individual device and for the whole system. Based on the study plan in this paper, it has become possible to develop and coordinate each device of the 100 MVA/2 GJ class power system load fluctuation compensation SMES system using YBCO wire, which up until now had seem impossible as an actual system.

Conceptual Design of HTS Coil for SMES Using YBCO Coated Conductor

High Tc superconducting (HTS) toroidal coil using YBCO coated conductor is designed for 70 MJ class superconducting magnetic energy storage (SMES) for power system control. Its configuration and shape are optimized by means of genetic algorithm (GA) to minimize the required length of the conductor. The optimization is performed for two kinds of constrains, i.e., maximum electric field or flux flow loss of the coil, which are calculated by means of finite element method (FEM). The FEM analysis considers quantitative current density (J)-electric field (E) expressions based on percolation transition model. It is shown that the great transport performance against magnetic field of YBCO coated conductor can realize a very compact SMES coil compared with an existing Nb-Tis one.

Highly effective and isotropic pinning in epitaxial Fe(Se,Te) thin films grown on CaF2 substrates

We report on the isotropic pinning obtained in epitaxial Fe(Se,Te) thin films grown on CaF2(001) substrate. High critical current density values – larger than 1 MA/cm2 in self field and liquid helium – are reached together with a very weak dependence on the magnetic field and a complete isotropy. Analysis through transmission electron microscopy evidences the presence of defects looking like lattice disorder at a very small scale, between 5 and 20 nm, which are thought to be responsible for such isotropic behavior in contrast to what was observed on SrTiO3, where defects parallel to the c-axis enhance pinning in that direction.

Performance Improvement of YBCO Coil for High-Field HTS-SMES Based on Homogenized Distribution of Magnetically-Mechanically Influenced Critical Current

Generally speaking for a HTS coil, perpendicular magnetic field to conductors broad surface should be suppressed as small as possible in relation to the magnetic anisotropy. This is a reason why toroidal coil with relatively many elementary coils is expected for HTS-SMES. On the other hand, from the point of view of the homogenization of critical current distribution in the coil, perpendicular field and parallel field should be balanced corresponding to the ratio of the magnetic anisotropy. This means that a certain level of the perpendicular field is effective to reduce local heat generation in the coil. Furthermore, this concept is especially reasonable for a high-field coil with usual winding method (flat-wise winding) because the perpendicular field does not induce hoop stress which decreases the critical current. In this paper, we show these findings through an optimal design of a MOCVD-YBCO toroidal coil for 2 GJ class SMES taking account of magnetically and mechanically influenced J - E characteristics.

Enhancement of In-Field Current Transport Properties in GdBCO Coated Conductors by

We have investigated the in-field current transport property in BaHfO₃ doped GdBa₂Cu₃O_{7 - δ} coated conductors in a wide range of temperatures and magnetic fields. Significant improvement of in-field critical current <i>I</i>_c was observed, e.g., <i>I</i>_c@77 K, 3 T = 93 A/cm-w, <i>I</i>_c@20&nbsp;K, 17&nbsp;T = 700 A/cm-w, which is a comparable value to that of Nb₃Sn wire at 4.2 K. Enhancement of the irreversibility field was also observed. These results suggest that BaHfO₃ is one of the most promising materials as effective artificial pinning centers and leads to the enhancement of in-field <i>I</i>_c. Furthermore, we have also shown that our analytical expression of electric field versus current density characteristics based on the percolation transition model [1-3] agrees well with the experimental results over a wide range of magnetic fields and temperatures. This analytical expression is useful for the design of superconducting devices because this allows us to predict the current carrying capability of coated conductors not only <i>J</i>_c but also <i>n</i>-value at arbitrary operating conditions of temperature and magnetic field.

\hbox{BaHfO}_{3}

We have succeeded in characterizing local critical currents and electromagnetically equivalent widths of slit coated conductors (CCs) as a function of their longitudinal position based on reel-to-reel scanning Hall-probe microscopy (RTR-SHPM). The aim is to make a quality control of slit CCs intended for electric power application, such as electric power cable in particular, for ac losses reduction. Long pieces of slit CCs, i.e., collectively 40-m-long pieces of 2-mm-wide slit CCs, were transferred in their longitudinal direction, and continuously magnetized in a liquid nitrogen bath; the magnetic field distributions of the samples were then measured by scanning a Hall sensor. From the magnetic field distributions, we could evaluate local critical currents and the equivalent widths of the slit CCs as a function of their longitudinal position. The difference between the equivalent width and the geometrical width of a CC corresponds to the damage due to the slitting process. This means that we can discuss such damage precisely together with its statistics along the longitudinal direction. This kind of diagnostics will be a very powerful technique for establishing slitting processes for producing narrow CCs for overcoming the issue of ac losses for electric power application comprising CCs.

Doping

Y_xGd_{(1 - x)}Ba₂Cu₃ O_y coated conductors (CCs) with artificial pinning centers (APCs) were fabricated by the trifluoroacetates-metal organic deposition method using a batch heat-treatment process. This process has been industrially applied to fabricate long superconducting tapes that have high critical current (<i>I</i>_c). From the viewpoint of applications, the characteristics in magnetic fields become further important, and one solution has been shown previously as introduction of APCs such as BaZrO₃. We applied the technique to our CCs fabrication process. The heat-treatment conditions, specifically total gas pressure, oxygen concentration, and crystallization temperature, were optimized for the batch heat-treatment process. As a consequence, fabricated CC showed extremely high characteristic <i>Jc</i> value of 3.5 MA/cm² in self field, and 0.3 MA/cm² at 3 T in liquid N₂. Moreover, by cross-sectional transmission electron microscopy observation, it was confirmed that the APC was distributed uniformly. Furthermore, the anisotropy of the pinning center is also examined based on the evaluation result of the angular dependence of <i>Jc</i> in 3T.

Nondestructive Diagnostics of Narrow Coated Conductors for Electric Power Applications

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.

Development of REBCO Coated Conductors by TFA-MOD Method With High Characteristic in Magnetic Field

Sumitomo Electric has been developing the silver-sheathed Bi2223 multi-filamentary wires since the discovery of Bi-based superconductors. DI-BSCCO is the high performance wires produced using the controlled-overpressure (CT-OP) sintering technique. The present commercial DI-BSCCO can provide the uniform high critical current, Ic, up to 180 A with length over 2000 m, and recently 200 A were succeeded to be obtained by the same kind of 1000 m length wires, resulting from the improvement and control of the microstructure in Bi2223 multi-filaments. The short trial wires of several meters have exhibited the highest Ic over 240 A at 77K in self-field (corresponding to 580 A per 1 cm-width). Besides, the optimization of carrier density after CT-OP led to further enhancement of Ic, reached 250 A. All the derivative products also have uniform critical current properties over entire length even after lamination with the reinforcing metals. The performances of DI-BSCCO can meet the growing needs for various application of high temperature superconductor like high in-field applications, such as magnets and motors. The recent progress in transport properties of commercial DI-BSCCO and R&D short trial wires is shown.

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

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.