Kazunori Jikihara

11 T liquid helium-free superconducting magnet

Abstract An 11 T liquid helium-free superconducting magnet designed at 6 K in vacuum using high temperature superconducting current leads was developed. The NbTi Nb 3 Sn coil was conductively cooled down from room temperature to 4.1 K in 40 h by two 4 K GM-cryocoolers. In a performance test, the coil temperature rose to 6.8 K for the inner Nb3Sn coil and 5.9 K for the outer NbTi coil, while sweeping the field at 5 A min−1. A central field of 10.7 T in a 52 mm room temperature bore was generated at an operating current of 149 A. Holding the field at 10.5 T was achieved continuously for 24 h at a constant coil temperature of 4.8 K.

Preparation of Highly Oriented Microstructure in the (Bi, Pb)–Sr–Ca–Cu–O Sintered Oxide Superconductor

A highly oriented microstructure in the high-Tc (Bi, Pb)-Sr-Ca-Cu-O bulk samples has been obtained by a modified powder method. Disc-shaped samples consisting of layered grains with the c-axis of the crystal normal to the disc surface were prepared by applying an intermediate pressing between the sintering heat treatments. Enhancement in formation of the high-Tc phase and a significant increase in the critical current density have been reported.

Superconducting current leads of Bi-based oxide

The application of Bi-based oxide bulk superconductors to current leads for various types of superconducting devices was investigated. The bulk was manufactured in the form of a cylindrical tube (inner diameter: 10 to 30 mm; wall thickness: 1.2 to 1.4 mm; length: 200 mm) and had the composition (Bi+Pb):Sr:Ca:Cu=2:2:2:3. Values greater than 1000 A/cm/sup 2/ were obtained for critical current density at 77 K under self-magnetic field. The maximum critical current was 1600 A. Heat leakage was measured between 77 K and 4.2 K while a DC current was passed through the conductor. The heat leakage of a bulk lead pair was 0.16 W, less than one-tenth of that of conventional gas-cooled Cu leads.<<ETX>>

Properties of Pb-Doped Bi–Sr–Ca–Cu–O Superconductors Prepared by the Intermediate Pressing Process

Effects of an intermediate press process on the microstructure and superconducting properties of the Pb-doped Bi-Sr-Ca-Cu-O system have been studied. Samples were made by sintering uniaxially pressed pellets after the conventional pellet-sintering process. Highly textured microstructure with strong (001) reflections was confirmed by X-ray diffraction and SEM. A short sintering time was sufficient to form only the high-Tc phase. By this intermediate pressing and sintering process, Jc was much increased and the magnetic field dependence of Jc was also improved. The highest Jc achieved was 3560 A/cm2 at 0 T and 77.3 K.

Properties of Ag-Sheathed Bi-Pb-Sr-Ca-Cu-O Superconducting Tapes Prepared by the Intermediate Pressing Process

The influence of an intermediate pressing process on the superconducting properties of Ag-sheathed Bi-Pb-Sr-Ca-Cu-O superconducting tapes has been investigated. The critical current density (Jc) was improved by the effect of the uniaxial pressing and by a sintering process after the initial sintering. The maximum transport Jc at 77.3 K under zero magnetic field was indicated as 2.36×104 A/cm2, and that at 4.2 K under zero magnetic field was indicated as 1.5×105 A/cm2. The magnetic field dependence on the Jc at 4.2 K was summarized as follows: 5×104 A/cm2 (I⊥H, C-axis⊥H, 10 T) and 3×104 A/cm2 (I⊥H, C-axis//H, 10 T).

Improvements in the Current Carrying Capacity in High-Tc BiSrCaCuO Superconductors

Remarkable improvements in the critical current density of the BiSrCaCuO sintered specimens have been achieved by a modified combination of heat treatment and pressing. The disk-shaped specimens were pressed uniaxially at room temperature after an initial sintering treatment and heated again at the same sintering temperature. This intermediate pressing method has produced a strongly oriented microstructure with a remarkably large critical current density. Correlation between microstructure and the critical current density, Jc, as well as anisotropy of the Jc with respect to the magnetic field direction has been described.

Cryocooler Cooled Superconducting Magnets and Their Applications

Various types of cryocooler cooled superconducting magnets have been constructed and already used for some applications. An 11 T-52 mm room temperature bore magnet is used for a high-field heat treatment equipment, a 6 T-220 mm room temperature bore magnet is used for a new experiment on the electrochemical reaction in high fields, and a 5 T-50 mm bore with 10 mm gap split type magnet has been combined with an X-ray diffraction apparatus.

Critical current measurement unit utilizing Bi-based oxide superconducting current leads and cryocoolers

A measurement unit has been developed to investigate the dependence on temperature and magnetic field of superconducting cable critical current without the use liquid helium or liquid nitrogen. A test specimen, which is tested using the four probe method, is cooled by a Gifford-McMahon (GM) type cryocooler to a temperature of 20 K to 90 K in a vacuum vessel. Transporting direct current, up to 500 A, is supplied to the specimen through the Bi-based oxide superconducting current leads. The advantages of utilizing oxide superconducting current leads are that the leads have low thermal conductivity, and also that the leads create no Joule heating effect, so the heat input to the sample is minimized. The external magnetic field of up to 3 T, which is applied to the specimen, is generated by a superconducting magnet which also uses Bi-based oxide superconducting current leads and is also cooled by a GM cryocooler. Detail design of the unit, the results of operating test and an example of the measurement result on the Bi-based oxide bulk specimen are presented in the paper.<<ETX>>

Construction of the cryogen-free 23 T hybrid magnet

In order to settle problems requiring a large amount of liquid helium and limiting the operation time for a wide bore superconducting magnet of a hybrid magnet, a cryogen-free 23 T hybrid magnet is being constructed at the High Field Laboratory for Superconducting Materials for the first time. An outer compact superconducting magnet is wound with highly strengthened CuNb/Nb/sub 3/Sn multifilamentary wires and is refrigerated conductively by GM-cryocoolers. The maximum stress value of 210 MPa was designed for the CuNb/Nb/sub 3/Sn coil. The cryogen-free superconducting magnet will be operated using dual power supplies independently, and has potential to generate central fields of 4.59 T at 198 A for the outer section NbTi coil and 3.41 T at 145 A for the inner section CuNb/Nb/sub 3/Sn coil. When the cryogen-free 7.5 T superconducting magnet with a 360 mm room temperature bore is combined with an inner 15.5 T water-cooled resistive magnet, a cryogen-free hybrid magnet will achieve 23.0 T in a 52 mm room temperature experimental bore.

A cryocooler cooled 5 T superconducting magnet with a horizontal and vertical room temperature bore

We designed and constructed a cryocooler cooled 5 T superconducting magnet with a horizontal room temperature bore of 50 mm and a vertical room temperature bore of 90 mm without liquid helium. The magnet, which is directly cooled by 4 K Gifford-McMahon cryocooler in vacuum, consists of NbTi coil, Bi(2223) bulk current leads and cryostat. The coil with an inner diameter of 130 mm, an outer diameter of 301 mm, a height of 66 mm and a gap of 80 mm is made using NbTi wires and Cu-plated SUS bobbin. Bi(2223) bulk current leads are thin-walled sintered cylindrical tubes. The outer diameter, height and weight of the magnet are 510 mm, 730 mm and 260 kg, respectively. The magnet is cooled down to 3.8 K in approximately 62 hours. A continuous operation at 5 T, which is generated by an operating current of 122 A, has been performed.