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An isotope effect of negative ion motion in plasma near a plasma grid (PG), which is a plasma-beam boundary grid, has been investigated in a negative hydrogen-ion source with the surface production process of the negative ion on the PG surface. Negative deuterium-ion (D-) density was higher than negative hydrogen-ion (H-) density in the condition without extraction-grid bias (V egb) and approached the H- density in the condition with V egb. Thus, the D- density responded stronger than the H- density to the V egb. The density of the negative hydrogen-ion isotopes and the density response to the V egb have been organized into respective identical trends by momenta of isotopes emitting from the PG. The Larmor motion can be a dominant mechanism of the negative ion transport from the PG to the plasma.

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.

Stability margin of the Cable-in-Conduit Conductor used under the pulse field is greatly influenced by inter-strand coupling losses and current unbalance with coupling current among strands. The large transverse resistance trades sufficient current transfer among strands for low AC losses. The large stability margin is brought by sufficient current transfer, low AC losses and uniform current distribution. Then we should decide the reasonable value of transverse resistance corresponding to the pulse condition. In this paper, the stability is estimated by the simplified distributed constant circuits simulating unbalanced current, that is transient coupling current, and transfer current among strands. In the non-insulated strands, the inductive voltage occurred during charging in a twisted area of half pitch must be enough lower than normal voltage of local quench at the limiting current. This paper also shows that the short twisted pitch and long cables about 1000 m give not only low AC losses but also uniform current distribution during charging for insulated strands.

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 stability and limiting current of a NbTi forced-cooled superconducting coil have been investigated experimentally and analytically at the subcooled condition below 4.5 K. The test coil was wound in a one layer solenoid from a cable-in-conduit type conductor of critical current of 5.1 kA at 4.2 K and 6.5 T, whose total length was 10 m. It was installed in the bore of a back-up superconducting magnet that provided field 8 T at the test coil. The stability margin of the coil as a function of normalized current (transport current/critical current) were measured. The results are compared with the formula proposed by Dresner and reported to predictions of stability margin and limiting current at subcooled condition below 4.5 K. >

Low AC loss and high current density NbTi/Cu/CuNi double stage cabling superconductor for quick response superconducting generator (SCG) has been developed. Its characteristics were measured by short sample and several coil models. The critical current of 14 kA at 6 T and the AC loss of 10 kw/m/sup 3/ were achieved, and coil models were excited under high current change rate operation without the quench. The developed superconductor will be able to be applied to a 70 MW class quick response SCG. >

A forced-cooled superconducting coil has been studied for the next step fusion machines because of many merits. The coil can be cooled by supercritical helium below 4.5K, which improves the stability and other superconducting characteristics. To study the stability of the coil at lower temperature, a test coil wound in a one-layer solenoid with a CIC conductor was tested below 4.5K. It was proved that the stability at lower temperature would be able to predict from the relationship between the critical current below 4.5K and the stability characteristics at 4.5K. The limiting current at lower temperature is in good agreement with the Lottin's experimental results.

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. >

A force-cooled superconducting coil has been studied for the Large Helical Device (LHD), which will be a main experimental apparatus of the Japanese National Institute for Fusion Science (NIFS). To achieve the main requirements for the LHD, a large-current, high-current-density NbTi cable-in-conduit-type conductor of 10 kA and 47 A/mm/sup 2/ at 8 T was developed. The test coil was wound in the form of a one-layer solenoid and tested at an operating current of 10 kA in a superposed field of about 7 T, at a hydraulic condition of 1.0 MPa, 4.5 K, and 10 g/s supercritical helium. The stability, normal propagation velocity, and pressure rise during a quench for the high-current-density case were also studied.

A D-shaped 12 Tesla superconducting coil with forced cooling Nb 3 Sn superconductor is developed. Cryogenic stability and conduit stress analyses, result of winding test of copper dummy coil, add cooing system for the test coil are presented.

A trial construction is described of a six-phase cylindrical VR type linear pulse motor which has been developed as being applicable to the linear actuator for the assist artificial heart. The VR (variable reluctance) type motor has a smaller thrust/input ratio than the PM (permanent magnet) type motor, but is superior in low noise and high-speed operation. The trial construction has made use of this advantage. According to the static thrust characteristic of the linear pulse motor, increase in the number of exciting phases to six is effective to suppressing the thrust regulation, resulting in stable operation. As to the kinetic characteritics, the capability of the motor to be driven at low frequencies leads to such a great reduction in iron loss as to make possible the continuous operation with a kinetic thrust of 160 [N].