Keiji Fukushima

Persistent-mode operation of a 920 MHz high-resolution NMR magnet

Development of a high-field NMR magnet has been underway at the Tsukuba Magnet Laboratory of the National Institute for Materials Science. The magnet succeeded in a persistent-mode operation at 21.17 T in December 1999. A 283-day long-term operation was carried out from October 2000 to August 2001. It included a persistent operation at 21.6 T (920 MHz) for 108 days. This was the highest field that the superconducting magnets have ever achieved in a persistent operation. Field decay was less than 2 Hz/h. Field homogeneity after correcting with superconducting shim coils were less than 0.1 ppm in a sample volume. These results confirmed that this magnet had been successfully developed as a high-resolution NMR magnet.

Transport properties of long-lengths of RHQT processed Nb/sub 3/Al conductor

A rapid heating and quenching (RHQ) operation along a long-length of jellyroll Nb/Al precursor over 300 m has been established. The J/sub c/ varies sensitively with the joule-heating current (I/sub RHQ/) that determines the temperature from which a precursor wire is quenched, excluding the plateau region where the bcc supersaturated-solid solution (Nb(Al)/sub ss/) only forms. The RHQ operation using such a plateau condition can suppress a scatter of J/sub c/ from point to tail of a long-length of wire. The standard deviation of J/sub c/ (4.2 K) at 21 T along a long-length of wire was about 5%, which was less than the variation of J/sub c/ with I/sub RHQ/ in the plateau region (/spl plusmn/7%). The mechanical deformation process needed when incorporating the Cu stabilizer also plays an important role in ensuring uniform heat treatment of the coil that is wound with a long-length of wire and, hence, large heat capacity. Although a temperature ramp-up rate might differ from place to place in the coil, the deformation processing makes such a ramp-up rate insensitive to these critical parameters. The coils using 300.5 m and 254 m lengths of Cu-clad RHQT JR Nb/sub 3/Al conductor generated additional 2.3 T and 3.2 T in bores of 90.2 mm/spl phi/ and 64.6 mm/spl phi/, respectively, indicating that a long-length of RHQT JR Nb/sub 3/Al conductor can be trustworthy for practical coil applications.

Bi-2211/Ag Superconducting Insert Magnet for High Magnetic Field Generation over 22 T

On the title of this paper (page L623 and contents): Bi-2211/Ag is a misprinted symbol. It should be corrected to Bi-2212/Ag.

Fabrication and transport properties of Bi-2212/Ag prototype magnets for 1 GHz-NMR magnet system

Over 2 km-long ROSAT wires with round-shaped cross-section have been successfully fabricated to a prototype magnet having the same dimensions as the actual insert magnet of a 1 GHz-NMR spectrometer. 400 m-long ROSAT wires with rectangular shaped cross-section have also been manufactured without any degradation of the current carrying capacity through wire processing. Another prototype magnet has been fabricated with the rectangular ROSAT wire. For each magnet, the present study demonstrates the feasibility of processing both 1 m-long Bi-2212/Ag leads and ROSAT wire-NbTi wire joints at each end of the leads to the magnet construction.

Transport properties of Bi-2212/Ag multifilamentary composite tape in high magnetic fields

Silver sheathed Bi-2212 multifilamentary tapes with 19-55 cores were successfully developed through the continuous pressing followed by a partial melting process. The multifilamentary tapes showed J c s of 1800 A/mm 2 (B ∥ tape face) and 1100 A/mm 2 (Bltape) at 4.2 K and 23 T, showing a practical range of current carrying capacities. The maximum J c , 4900 A/mm 2 (I c = 550 A), was observed for a 55-filament tape at 4.2 K and 0 T. A single pancake coil using 3.5 m long tape generated a maximum magnetic field of 1.9 T, showing a possibility for realizing over 5T-class coils.

High thermal conductive epoxy resins with controlled high-order structure [electrical insulation applications]

The thermal conductivities of resins can be improved by introducing a high-order structure having microscopic anisotropy while maintaining their macroscopic isotropy. We studied four kinds of diepoxy monomers with a biphenyl group or two phenyl benzoate groups as mesogens, and cured them thermally with an aromatic diamine curing agent. Their thermal conductivities were up to 5/spl times/ higher than those of conventional epoxy resins, because the molecular groups, mesogens, form highly ordered crystal-like structures which suppress phonon scattering. We confirmed the existence of crystal-like structures in the epoxy resins directly using TEM observation. We also observed mezoscopic structures in the resins using an AFM. The results suggest a novel method to improve the thermal conductivities by controlling the high-order structures. Furthermore, the laminates were prepared with the high thermal conductive epoxy resin containing a biphenyl group and ceramic fillers as a part of a feasibility study. Thermal conductivities more than 10/spl times/ higher than ordinary epoxy resin laminates were obtained for test pieces of the laminates.

Persistent mode operation of Bi2Sr2CaCu2Ox/Ag stacked double pancake coils with superconducting joints

A persistent current mode magnet was successfully fabricated using Bi2Sr2CaCu2Ox /Ag tapes. The magnet system consisted of four Bi2Sr2CaCu2Ox /Ag double pancake coils as a magnet and a noninductively wound Bi2Sr2CaCu2Ox /Ag–Au single pancake coil as a persistent current switch. The pancakes were stacked and jointed using a butt-jointing method to form a superconducting closed loop. Persistent current mode operation of the magnet was initiated in liquid helium, trapping a magnetic field of 1 T which corresponds to a loop current of 134 A. The voltage-current ( V–I) characteristic of the closed loop was estimated from the decay of the trapped field. An index number in the V–I curve, n, above 30 and a loop resistance of 2×10-10 Ω were observed for the loop current above 30 A after 116 hours.

Persistent-mode operation of a cryocooler-cooled HTS magnet

Persistent-mode operations of a cryocooler-cooled HTS magnet were carried out. The closed circuit of the magnet consisted of a Bi-2212 solenoid coil and a NbTi/CuNi persistent-current switch (PCS). Thermal conductance between the PCS and the cooling stage was designed not to heat up the coil. The temperature increase was maintained within 0.1 K during the normal state of the PCS. The transition times of the PCS between a superconducting and a normal state were sufficiently short for practical use. The magnet started a persistent-mode operation from an initial magnetic field of 2.1 T. Decay behaviors during persistent-mode operations were successfully observed over 90 hours. Except for the initial period, field decay was limited by resistance of the joint. A method to compensate the field decay was performed by installing a NbTi cylinder in the coil bore.

Persistent Mode Operation of Bi-2212/Ag Solenoid Magnets

The recent progress in our development of wind and react type Bi-2212/Ag solenoidal magnets is presented. Recently, we succeeded in fabricating solenoid magnets with 70mm clear bore, a 130mm outer diameter and a height of 100–600 mm, using Bi-2212/Ag ROSATwire (rotation-symmetric arranged tape-in-tube wire). The transport properties of the magnets were tested in self field conditions at 4.2K. The details of the transport property of the solenoid magnet will be discussed. Furthermore, we succeeded in fabricating a closed circuit with a PCS made by a NbTi/CuNi conductor. The closed circuits were successfully trapped a field of around 1T with a current of over 200A for over 50h at 4.2K.

Performance tests of Bi-2212 insert magnets fabricated by Ag sheath method and dip-coating method

We have fabricated small magnets using Bi-2212/Ag tapes prepared by the combination of a powder-in-tube method or a dip-coating method and a wind-and-react method. The magnets were inserted in a 21 T superconducting magnet and tested under various bias fields at 4.2 K. All of the magnets could sustain large electromagnetic force under high magnetic fields, and no damage was introduced during the test. Critical current density of a magnet consisting of 4 double pancake coils was 9.5/spl times/10/sup 4/ A/cm/sup 2/ in 21 T which corresponded to the central generated field of 1.76 T. Thus the total field of 22.76 T was attained in a full superconducting state. No degradation of superconducting properties was produced by a thermal cycle and moisture in air.