Tsuyoshi Wakuda

Generation of 23.4 T using two Bi-2212 insert coils

Development of a 1 GHz superconducting NMR magnet is in progress at the Tsukuba Magnet Laboratory (TML) of the National Research Institute for Metals (NRIM). This magnet will contain a BSCCO inner coil, which should generate a central field of 23.5 T in a backup field of 21.1 T. In order to accomplish this targeted field, we fabricated two Bi-2212 double-pancake coils (Coil A and Coil B). They were installed in the high-field superconducting magnet system at the TML/NRIM. Their performance was measured in a backup field of 18 T. Coil A was made of 20 double-pancakes wound with Ag sheathed Bi-2212 tape conductors. Ag-Mg tape was co-wound for mechanical support. Its winding was 147 mm in outer diameter and 220 mm in height. It generated a central field of 21.4 T in a clear bore of 61 mm. Coil B was located inside Coil A. Its 6 double-pancakes were wound with Bi-2212 tape conductors reinforced with Ag-Mg-Ni alloy sheath. The outer diameter and height of the winding were 48 mm and 63 mm, respectively. Coil B generated the highest field of 23.4 T in a backup field of 21.4 T. This study confirmed that the present performance of the Bi-2212 coils had already satisfied the required conditions for the inner coil of the 1 GHz NMR magnet from the viewpoint of high-field generation.

Development of Superconducting Split Magnets for NMR Spectrometer

We have developed a high-resolution NMR spectrometer with a superconducting split magnet and a cross-shaped bore. Because of the spatial limitations of the magnets structural design, the split magnet requires many pairs of coils to generate a high magnetic field. For this reason, it is difficult to compare the high stability and homogeneity of its magnetic field to conventional NMR magnets. Therefore, we have developed advanced superconducting magnet technologies and successfully fabricated two types of self-shielded superconducting split-magnet systems: a 7 T (300 MHz) and a 14 T (600 MHz) magnet system. The 300 MHz system consists of 12 NbTi coils with 15 joints and a stored energy of 0.7 MJ; it achieved a field drift of about 0.1 Hz/h in persistent current mode operation. The 600 MHz magnet system consists of 12 Nb3Sn coils and 8 NbTi coils with 53 joints and a stored energy of 10 MJ. Its field drift was less than 1 Hz/h.

Development and fundamental study on a superconducting induction/synchronous motor incorporated with MgB2 cage windings

In this paper, a fundamental study of the rotating characteristics of a induction/synchronous motor by use of superconducting MgB2 cage windings is carried out based on analysis and experiment. Current transport properties of the produced monofilamentary MgB2 wires are firstly characterized, and then utilized for the determination of the current carrying capacity of the rotor bars. Then, the motor model is designed and fabricated with the aid of conventional (copper) stator windings. We successfully observe the synchronous rotation of the fabricated motor at a rotation speed range from 300 to 1800 rpm. We can also realize an almost constant torque versus speed curve, and this characteristic is explained from the steep take-off of the electric field versus the current density curve, based on the nonlinear electrical equivalent circuit. These results are promising for the practical applications of a high efficiency motor for a liquid hydrogen circulation pump.

A new symmetrical arrangement of tape-shaped multifilaments for Bi-2212/Ag round-shaped wire

A new Bi-2212/Ag round-shaped wire with tape-shaped multifilaments has been successfully developed. The wire includes 126-960 tape-shaped filaments with triple rotation symmetry, having a good crystal alignment in each filament. We refer to the new wire as ROSATwire, (ROtation-Symmetric Arranged Tape-in-tube wire). Since the ROSATwire structure yields complete symmetrical arrangement of the tape-shaped filaments, it eliminates the need for a rolling machine, but allows us to use a drawing or extrusion machine. We found that the present wire fabrication process markedly improves not only productivity and lowers cost, but also enhances the transport J/sub c/ of the Bi-21212/Ag wire. The I/sub c/ and J/sub c/ reached >340 A and 1000 A/mm/sup 2/ at 28 T and 4 K.

AC losses in monofilamentary MgB2 round wire carrying alternating transport currents

AC losses in a monofilamentary MgB2 round wire with niobium and copper metal sheaths and carrying alternating transport currents are evaluated at several temperatures and frequencies. First, the transport current losses are observed electrically using a lock-in amplifier. Experimental results show that the AC losses decrease with an increase in the temperature if the amplitude of the transport current normalized by the corresponding critical current is maintained constant. On the other hand, the AC losses increase slightly with the frequency. Next, the AC losses are calculated numerically by a finite difference method. The numerical results for the superconductor filament show a good agreement with the results of the conventional theoretical expression formulated using the Bean model over a wide range of current amplitudes. It is also found that the AC losses in the niobium sheath are negligible whereas those in the copper sheath are comparable with those in the superconductor. On the basis of the numerical calculations, an expression is analytically derived for estimating the eddy current loss occurring in a metal sheath. The derived expression well reproduces the AC loss properties of both the copper and niobium sheaths.

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.

Development of Stator Windings for Fully Superconducting Motor With

Stator windings for a superconducting motor with MgB2 wires are fabricated and tested. The stator windings are composed of 12 element coils in the form of a racetrack, and located in iron core slots to generate a rotating magnetic field of three phases and four poles. Preliminary estimations by means of numerical calculations with a finite element method indicate that the magnetic-flux distribution in a gap between the stator and rotor cores contains relatively significant components of fifth and seventh harmonics compared with a fundamental component. An MgB2 rotor prepared in the previous work can be rotated at a synchronous speed successfully by energizing the fabricated stator windings with a pulse width modulation inverter and compensating for the higher harmonics with a torque boost voltage.

\hbox{MgB}_{2}

Abstract Bi-2212/Ag conductors show practical transport properties in high magnetic field regions above 20 T and 4.2 K, where it is considered difficult to use metallic superconductors. In this paper, the recent progress of our development of Bi-2212/Ag high field insert magnets is presented. Bi-2212/Ag stacked double pancake coils with a 49–150 mm outer diameter, 15–65 mm inner diameter and 50–220 mm height have been fabricated and tested in a superconducting backup magnet system up to 18 T at the Tsukuba Magnet Laboratory of National Research Institute for Metals (NRIM). The Bi-2212 insert magnets successfully generated 5.42 T with a backup field of 18 T, i.e. the total magnetic field of the superoconducting magnet system reached 23.42 T, which is the highest magnetic field ever achieved by a superconducting magnet.

Wires

A highly sensitive NMR spectrometer with a superconducting split magnet and a solenoid-type probe coil is being developed. One of the most challenging tasks is to achieve high homogeneity of magnetic field in a measuring volume with this configuration, because inevitable structural asymmetry and limited production accuracy of the magnet cause larger inhomogeneity of magnetic field compared with a conventional solenoid-type NMR spectrometer. Numerical estimation revealed that field correction capability of conventional shim coils, i.e., superconducting solenoid and saddle coils placed outside main coils, is insufficient for the split NMR magnet. Therefore, we have designed and constructed superconducting shim coils with sufficiently high capability by adopting the following three novel ideas: (1) superconducting shim coils are placed not only outside main coils but also inside them in the vicinity of a measuring volume; (2) superconducting shim coils with ldquoperiodically wavyrdquo shape are utilized to correct non-axial inhomogeneous magnetic field; (3) the current of each superconducting shim coil is independently controlled to correct plural modes of magnetic field simultaneously.

Bi-2212/Ag high-field magnets

We have been developing superconducting induction /synchronous motor incorporated with MgB2 cage windings. Target application of the motor is the liquid hydrogen circulation pump. In this paper, the performance of the fabricated motor was estimated based on nonlinear electrical equivalent circuit. It was shown that the experimental results of the motor characteristics such as torque vs. slip curve can be reproduced based upon the theoretical estimation. From this result, we can design the MgB2 pump motor by means of commonly utilized method of the electrical circuit.