Kazuya Ohmatsu

Finite element analysis of AC loss in non-twisted Bi-2223 tape carrying AC transport current and/or exposed to DC or AC external magnetic field

Abstract AC losses in Bi-2223 superconducting tapes carrying AC transport current and/or exposed to DC or AC magnetic field are calculated with a numerical model based on the finite element method. Superconducting property is given by the E – J characteristic represented by a power law using equivalent conductivity. First, transport loss and magnetization loss are calculated numerically and compared with measured values. The calculated losses almost agree with the measured losses. Frequency dependencies of calculated and measured transport losses are compared with each other. Next, the influence of DC external magnetic field on the transport loss is studied. DC external magnetic field reduces n that is an exponent in the power law connecting resistivity and current density. The numerically calculated transport loss increases with increasing DC magnetic field. Finally, the total loss of superconducting tape carrying AC transport current in AC magnetic field is calculated. In the perpendicular magnetic field, the calculated total loss is lager than the sum of the transport loss and the magnetization loss, while they almost agree with each other in the parallel magnetic field.

Development of in-plane aligned YBCO tapes fabricated by inclined substrate deposition

Abstract High temperature superconducting thin film tapes, which consist of a flexible metal substrate, buffer layer, and Y 1 Ba 2 Cu 3 O x (YBCO) layer have been developed for power application. Ni-alloy tape was used as the substrate. Both the YBCO layer and the buffer layer were formed by pulsed laser deposition (PLD) because of an advantageous of high deposition rate. Inclined substrate deposition (ISD) was developed to introduce in-plane alignment for the buffer layer on the non-textured metal substrate. YBCO films epitaxially deposited on the ISD buffer layer by normal PLD had high J c values over 10 5 A/cm 2 at 77 K. For verification of high rate productivity of the PLD/ISD method, we demonstrated high rate deposition of the yttria stabilized zirconia buffer layer and YBCO superconducting layer. We also performed continuous deposition of over 10 m YBCO thin film tapes with a reel-to-reel substrate transfer system combined with 200 W high power laser equipment.

Status of Bi-2223 tapes performance and development

We are developing Bi-2223 superconducting tapes on a wide viewpoint from fundamental superconducting property to application usage. In basic characterization of superconductivity, the behavior of critical current density (J/sub c/) in terms of both temperature and magnetic field was studied in detail and a new scaling law which can estimate the value of J/sub c/ in multiple conditions is defined. There were significant improvements in the long length fabricating process. The 1 km order length tape having J/sub c/ at 77 K over 20 kA/cm/sup 2/ can be produced reproducibly. The distribution of J/sub c/ through the whole length is very uniform. These results in both basic characterization and production progress made it possible and easy to design a high field magnet. Then, we have successfully developed a magnet using a refrigerator.

HTS large scale application using BSCCO conductor

The basic property of high-Tc superconducting cables (HTS cables) using Bi-2223-based Ag-sheathed multifilamentary wire (Ag-sheathed wire) have been investigated for the realization of large-scale and compact cables, these being replaceable with conventional cables in existing ducts or tunnels. The AC performance of multi-layer HTS conductors, and three-phase HTS cables with coaxial superconducting magnetic shielding structure was evaluated. The characteristics of the HTS conductor and cable models of long length was investigated on a 50 m scale.

Development of Bi-2223 magnetic separation system

A prototype of a cryocooler cooled Bi-2223 magnet system for magnetic separation was constructed. The magnet system has 200 mm room temperature bore and generates fields higher than 1T in 11-liter room temperature space. The magnet was tested at various temperatures and excitation speeds. The magnet was cyclically excited up to 1.7 T at the speed of 1.7 T/min with various periodic times. The temperature gradually increased with the number of excitations, however, the temperature saturated at 28-38 K for the periodic times longer than 9 min, and stable operation of the magnet was obtained. A demonstration of magnetic separation of aquatic slurry containing fine /spl alpha/-hematite paramagnetic particle was performed by using ferromagnetic stainless steel fibers as a magnetic filter. Almost 100% of hematite particles were successfully separated from the slurry.

Design of an Axial Flux Inductor Type Synchronous Motor With the Liquid Nitrogen Cooled Field and Armature HTS Windings

A liquid nitrogen cooled axial flux type high temperature superconducting (HTS) synchronous motor was designed. This motor is an eight-pole inductor type synchronous motor with six armature windings. The field windings and armature windings are made of the BSCCO HTS wires. We called it inductor type flux control superconducting (IFCS) motor, and its output is 400 kW at the rated speed of 250 rpm. Until now, there were few armature windings using HTS wires, because of AC losses generation when the wires are used in the alternating magnetic field. In this motor, we used iron cores to decrease AC loss. Also because of the help of iron cores, it is possible to make use of armature winding at liquid nitrogen temperature. Furthermore, inductors were employed in the rotor. As a result, the cryostats that contain field HTS windings and armature HTS windings can be fixed; also we do not need the excitation devices such as the brushes and slip ring and the cooling devices such as the rotary joint to offer the path for coolant to flow.

Measurement and FEM analysis of magnetization loss in HTS tapes

Magnetization loss of Bi-2223/Ag tapes was measured with several types of pick-up coils to study the influence of pick-up coil configuration and sample length on the measured losses. Magnetic flux distributions in tapes were calculated numerically by the finite element method to obtain their AC losses. The numerical and experimental losses were compared with each other to clarify the nature of the measured AC losses. In the parallel magnetic field, the height of the pick up coil must equal or be larger than the tape height for the accurate loss measurement. In the perpendicular magnetic field, the size pick up must be much larger than the width for accurate loss measurement. If the length of the sample tape is short, the measured loss contains coupling loss component at commercial frequencies, while the filaments are almost decoupled in DC measurement by a SQUID magnetometer.

Development of High-Tc Superconducting Magnet Using Ag-Sheathed Bi2223 Tapes

We have succeeded in developing two types of the refrigerator cooled High-Tc superconducting magnets; 4-Tesla and 7-Tesla types. They have the same inner diameter of 80 mm and outer diameter of 292 mm, but the coil height was increased to 200 mm of 7-Tesla type from 68 mm of 4-Tesla type. The 4-Tesla type successfiilly produced the central magnetic field of 4.0 Tesla over 30 minutes and 3.5 Tesla for 48 hours. High ramp rate excitation up to 0.35 Tesla per second was achieved, which was a hundred times faster than that of the refrigerator-cooled magnets using the metallic superconductors. The 7-Tesla type magnet stably generated 7.1 Tesla over 24 hours and was excited at a rate of 2 Tesla per minute.

Development of RE123 coated conductor by ISD method

Abstract High rate deposition of HoBa2Cu3O7−δ (HoBCO) layers using the pulsed laser deposition method were experimented for long high temperature superconducting (HTS) coated conductor. Laser conditions and oxygen ambient during HoBCO deposition were precisely inspected. This revealed that the deposition rate of HoBCO reached to 5 μm/min. Even at such high deposition rate, the Jc of HoBCO films remained over 105 A/cm2 on the biaxial buffered substrate fabricated by the inclined substrate deposition (ISD) method. Moreover continuous tape production equipments were installed to make a long and flexible coated conductor by ISD method. These equipments will enable us to fabricate of over 100 m long coated conductor. 55 m long coated conductor were fabricated by using these equipments as our first trial of long length fabrication. Evaluation of Jc distributions of 55 m coated conductor revealed that all parts of 1 m in the 55 m conductor had the Jc over 104 A/cm2.

Development of 10 kA class Nb/sub 3/Al superconducting cable by jelly-roll process (for fusion magnets)

A multifilamentary Cu/Nb/sub 3/Al composite strand was developed with a noncopper critical current density of more than 400 A/mm/sup 2/ at 12 T, by a jelly-roll process. A 10-kA cable-in-conduit conductor was fabricated to demonstrate its applicability to fusion magnets. The cable had 324 strands, and a circular CuNi seamless pipe was used as the conduit. The strand diameter was 0.88 mm and the filament diameter was 28 mu m. The heat treatment was performed at 820 degrees C for 2 h.