Search

We have been developing high-strength Nb3Sn strand cables to construct a high-field superconducting outsert for a 45 T hybrid magnet with a 25 T water-cooled resistive magnet. Evidence for cold work strengthening of repeated bending treatment (prebending effect) on Nb3Sn strands internally reinforced with CuNb stabilizer, which exhibits significant enhancement of the critical current density, has been found in the superconducting magnet fabrication process using a react-and-wind method. The strand cables were designed by controlling the constituent number of CuNb/Nb3Sn strands with the prebending effect and stainless steel strands, which are expected to have a stress limit 580 MPa at 0.4% strain. In order to design a compact superconducting outsert, high-strength strand cables are adopted in a magnetic field region below 14 T to maintain relatively large engineering current densities (Je). In a higher-field region above 14 T, YBa2Cu3O7 (Y123) coated conductors are employed for an insert coil. Using combination of Y123, Nb3Sn and NbTi superconductors, a 20 T superconducting outsert with a room temperature bore of 400 mm consisting of three layers made of Y123, two layers of CuNb/Nb3Sn and two layers of NbTi was designed. The coil parameters are 440 mm inner diameter, 1080 mm outer diameter and 1138 mm coil height. A very compact 20 T superconducting outsert with a stored magnetic energy 72 MJ at an operation current 903 A can be developed for a 45 T hybrid magnet.

The High Field Laboratory for Superconducting Materials (HFLSM) and the Tsukuba Magnet Laboratory (TML) conducted in collaboration a case study on development of a 50 T-class hybrid magnet. To construct a high magnetic field magnet with compact and energy-saving design as well as with easy operation and maintenance, one has to develop high-strength NbSn strand cables, with maximized superconducting characteristics and which can withstand a large electromagnetic force over 500 MPa. For this purpose, the HFLSM has proposed and investigated the effect of repeated bending treatment (pre bending) on NbSn strands internally reinforced with CuNb stabilizer leading to significant enhancement of the critical current density. In this report we present our results on application of the pre bending effect to the development of high-strength strand cables. The designed prebent-strand cables are composed of three CuNb/Nb3Sn strands (3 times Phi =1.73 mm) and four stainless steel strands ( 4 times Phi =1.73 mm). High-strength CuNb/Nb3Sn strand cables have shown a stress limit of 552 MPa at 0.4% strain, and a critical current of Ic =1000 A at 18.5 T and 2.0 K. For such high-strength strand cables, a 20 T superconducting magnet with a room temperature bore (Phi =400 mm) consisting of five layers made of CuNb/NbSn and two layers of NbTi was designed. The coil parameters are: inner diameter Phi = 440 mm, outer diameter Phi = 1332 mm, coil height 1321 mm, inductance 350 H and magnetic stored energy 144 MJ at 908 A of the operation current. Winding of the coil was experimentally successfully simulated using dummy 3 + 4 strands cable composed of three Cu strands and 4 stainless steel strands with a similar design to the 3 + 4 strands superconducting cable presented above. The 20 T superconducting coil will be used as a 20 T outsert for a 25 T water-cooled resistive insert to obtain a 45 T hybrid magnet.

The long term reliability of the SMES model coil for the 100 kWh SMES pilot plant was investigated in a 1000 cycles cyclic test in the test facility of the Lawrence Livermore National Laboratory (LLNL) in 1998, in collaboration between Japan and the USA. The coil parameters such as coil inductance, magnetic field and AC loss were measured before and after the cyclic test. They did not change. The coil was successfully charged up to 40.53 kA in the mechanical performance test, which gave mechanically the same conditions as in the SMES pilot plant at rated current. After all performance tests, the coil was checked visually and electrically, and no degradation was observed. Mechanical reliability of the SMES model coil was verified in the performance tests. The obtained technical results encourage recommendation of the construction of the pilot plant.

The stray field of SMES restricts its site location, although SMES has an attractive potential for power management and quality control. The stray field outside a solenoid is analyzed by a series of Legendre polynomials and the result is applied to the stray fields of various SMES coil configurations. As long as the summation of magnetic moments from all coils is zero, the term of a stray field decreasing as r/sub p//sup -3/ can be cancelled out. The higher order of the stray field can vanish if the coil arrangement is optimized. In this paper, the authors consider a single solenoid as a reference, active shield coils, axially displaced coils and multipole coils to reduce the stray field. The multipole coil configuration has high potential to drop the stray field, since the stray field behaves like r/sub p//sup (3+n/2)/, where n is the coil number.

Superconducting conductors and magnets used in the fields of power transmission, generators, MRI, MAGLEV etc. experience AC magnetic fields causing heat generation in the magnets' structure and AC losses in the superconductors. Therefore, AC magnetic shields with wide frequency range in high magnetic fields are desirable to reduce heat generation within the cryogenic region and increase system efficiency. This paper describes the performance of a multilayer superconducting sheet as an AC magnetic field shield, eddy current analysis of MAGLEV coil and eddy current loss measurement of an R&D cryostat with a superconducting magnet. Eddy current loss was reduced to about one fourth of the loss level without a superconducting shield.

Stability margin of the cable-in-conduit conductor is greatly influenced by current transfer performance among strands. When we estimate the stability margin analytically, we must assume the electrical circuit model among strands, but it is difficult to know it for real-scale cables, because the cable has many parallel circuits with a lot of strands and twisted stages. The measurement of the frequency characteristics of impedance among strands shows that the circuit can be regarded as two-wire model. By this measurement we can judge whether the circuit behaves like a distributed constant circuit or a lumped element circuit within the intended frequency band. When the circuit is a lumped element circuit, we can also get the interval distance between each contact point and its resistance. This paper also shows that stable conductors are often represented with a lumped element circuit that has well-contacted points with a short interval.

The transverse resistance among strands of multi-stage twisted cables is one of the most important parameters to decide the stability and the AC losses. This value is varied with the cable compression affected by the void fraction or the Lorentz forces under charging. Furthermore it is also influenced by cyclic number of transversal compression. This paper describes the basic mechanical property of the chrome plated NbTi cable for the transverse compression, and the relation of the transverse resistance and the internal loads of the cable by measuring the impedance and the rigidity for various internal loads. It shows that the transverse resistance, which is much influenced with the performance of conductor, is always changed corresponding to the compressive load and dose not indicate the same resistance after release from the compression because of its elastic hysteresis.

Dry-wound superconducting solenoids offer good manufacturability of large bore magnets but occasionally suffer from premature quenches triggered by small heat generated within the magnets. The principal source of heat is thought to be conductor motion induced frictional heating and/or filler material fracture released energy in windings. This paper describes major specification, stress analysis and training performance and their relation to a series of magnets that were constructed and tested to establish stabilizing methods of a large bore dry-wound superconducting solenoid. The results suggested that conductor motion took place even though radial stress in the magnet was still compressive and a conductor was expected in its original position. >

The stability margin of CIC multi-strand superconducting cables under partial current distribution is affected by electrical resistance among the strands. This means that the stability is governed by the current sharing process among the strands. The achievement of high stability against partial current distribution will be realized by rapid current transfer to the other strands rather than by thermal diffusion of Joule heating to helium. A simplified electrical circuit model, that is a distributed constant circuit for two strands, simulating the current sharing process between strands, is proposed to estimate the stability. The results of the stability analysis, clarify the limiting condition to maintain stability. The circuit constants governing the sharing process can be investigated from the frequency properties of a characteristic impedance measured with CICC short samples. These results are confirmed with experimental results of stability tests under partial current distribution. The proposed estimation method is viable. >

The authors describe the electromagnetic results and the quench phenomena of the DPC-TJ large cable-in-conduit Nb/sub 3/Sn coil experiment which took place in the summer of 1991. The DPC-TJ coil was developed by Toshiba and JAERI in collaboration to demonstrate the realization of (NbTi)/sub 3/Sn superconducting coils for fusion with high average current density of 40 A/mm/sup 2/. The coil was charged up to its rated current of 24 kA (40 A/mm/sup 2/) at 7.6 T without quench in the DPC test facility. Thereafter, the electromagnetic performance was tested by measuring I/sub c/ and T/sub cs/, and the critical current at 12 T was estimated to be 41 kA. The quench phenomena of the DPC-TJ coil were also tested by measuring normal-zone propagation velocity to get the protection design. In these experiments, very slow and very fast propagation were observed. Many other electromagnetic results were also obtained, which are necessary for the design of fusion machines such as International Thermonuclear Experimental Reactor. >

The disturbance energy of a forced flow cooled superconducting coil (FCC) is discussed. The authors built a FCC and carried out stability and quench tests. The test coil had a 36-m-long cooling path with a 0.37-mm hydraulic diameter. It was cooled by supercritical helium at 4.1 K. The stability margin of the FCC was evaluated by using RF heating. In addition, the coil quench test was performed. The disturbance energy at quench is estimated by the stability margin characteristics. The disturbance energy obtained agrees well with that of a simple analytical model. >

The DPC-TJ test coil was developed in the collaboration between the Japan Atomic Energy Research Institute (JAERI) and Toshiba for the Demo Poloidal Coil (DPC) project at JAERI to obtain key technologies required for the construction of superconducting coils of a fusion experimental reactor. The DPC-TJ is a 1-m-bore, 40-A/mm/sup 2/ forced-flow cooled superconducting coil wound with a double-walled 24-kA, 12-T (NbTi)/sub 3/Sn cable-in-conduit conductor. The DPC-TJ was tested to investigate stability performance at operating conditions between 15 kA, 2.3 T and 30 kA, 4.5 T. Another test was carried out with a 1/18 reduced-scale model of the DPC-TJ conductor to investigate stability margins at the designed field of 12 T. Results indicated the limiting current of the DPC-TJ was around 25 kA. The DPC-TJ coil was stably charged up to 30 kA without any normal transitions because only a small mechanical disturbance could occur in the newly developed type of conductor. >

The stability and quench characteristics of a NbTi forced-cooled superconducting coil at the subcooled condition were investigated. The test coil was wound in a one-layer solenoid from a cable-in-conduit-type conductor of 1.5 kA at 7 T, whose total length was 34 m. It was installed in a background coil and tested between the subcooled condition below 4.5 K and the normal condition above 4.5 K. Stability, quench current, normal propagation velocity, and pressure rise during a quench of the subcooled coil were measured. The stability margin increased and the pressure rise during a quench decreased as the inlet coolant temperature was lowered. >

The electronic band structure of a ferromagnetic chromium spinel CuCr2S4 is calculated self-consistently by using the discrete variational X alpha method. The main features of the band structure are: relatively narrow valence bands constructed mainly from the S 3p orbitals lie in the lower energy region; very narrow bands having mainly d character range over the intermediate energy region and a few d bands with only majority spin cross the Fermi level, which corresponds to the p-type metallic conduction observed; true conduction bands composed of Cu and Cr 4s, 4p orbitals lie in the higher energy region. The valency of each ion obtained from the self-consistent calculation is Cu1.3+(Cr1.55+)2(S1.11-)4 and the atomic spin-polarisation is 3.2, -0.32 and -0.27 for Cr, Cu and S, respectively.

The phase diagram of the isopleth section of the Ti-7 at. pct Mo-Al system has been improved and expanded to include alloys up to 25 at. pct aluminum. The mechanical and thermal stability of alloys aged in the two-phase region, β +Ti3Al, was correlated to the microstructure. X-ray rocking-curve studies of the polycrystalline specimens showed that after 2 pct deformation of a Ti-7 Mo-16 Al alloy theβ matrix became preferentially plastically deformed, while the Ti3Al particles functioned as hard particles undergoing little lattice distortions.