高尾 智明

In order to obtain a high-intensity neutron beam for neutron scattering experiments, strong sextupole magnets with large apertures are useful as a focusing element. We have started the development of a Nb3Sn sextupole magnet for this purpose. The design field strength of the sextupole is 12780 T/m(2) and the aperture of the. coil is 46 mm in diameter. As a first step of the development, three model coils have be en made in order to learn about Nb3Sn technology and to evaluate the fabrication techniques., Single-pole tests of these coils have been performed to assess the conductor stability and training behavior of the Nb3Sn coils. This paper describes the design of the magnet, the fabrication of the model coils and the results of single-pole tests.

We have fabricated cable-in-conduit conductors (CICCs) with two kinds of nonmetallic conduit materials. Difference of frictional coefficients on surfaces of the materials vary by factors of two or three, and void ratios of the conductors are from 14% to 42%. We wound these CICCs on same-shaped bobbins, and measured the AC losses of the CICC coils. In spite of the fact that the coil shape, the superconducting wire, and background magnetic fields were the same, the measured losses were not, and depended on conduit materials and void ratios. According to the experimental results, in order to decrease the losses, a conduit material with high frictional coefficient is desirable. It is also shown that the losses decrease with the decrease of the void ratios in the CICCs.

When alternating current superconducting coils having cable-in-conduit conductors are fabricated, non-metallic conduit materials are necessary to avoid eddy current losses in the conduits. Using two kinds of non-metallic tapes made of glass fiber reinforced plastics (GFRPs) and Dyneema fiber reinforced plastics (DFRPs), we made the conductors with the GFRP conduits and DFRP conduits, and experimentally studied the total losses of the conductors. The measured results showed that the total loss of the conductor with the GFRP conduit was smaller than that with the DFRP conduit at the same value of the void ratio, the transport current, the external magnetic field, and so on. When the void ratios become high, the superconducting bundle cable can easily move in the conduit. Hence the measured total losses increased with the void ratios of the conductors. (C) 2003 Published by Elsevier B.V.

Local temperature rise due to insufficient cooling is one of the important problems in conduction-cooled high-temperature superconducting (HTS) coils. Assuming two types of plastics having high thermal conductivity as spacers in HTS coils, we measured temperature irises of a conduction-cooled HTS tape under various contact conditions between the plastics and the HTS tape. The cooling performance from the Zylon fiber reinforced plastic to the HTS tape was not so good, since the fibers in the plastic were oriented in one direction. The Dyneema fiber reinforced plastic effectively cooled the HTS tape, because the fibers in the plastic were two dimensions. The possibility to use the plastics with high thermal conductivity as the spacers in the conduction-cooled HTS coils was demonstrated.

We have measured contact resistance between superconducting strands that was a parameter of coupling losses in cable-in-conduit conductors. Assuming some kinds of twist pitches of sub-cables in the multi-stranded cables, we measured the contact resistance under the condition of the intersecting strands. Since not only the resistance but also mechanical properties of the strands (applied force to the strands and deformation of the strands due to the force) were measured, the contact area could be precisely estimated, and then the surface resistance was also evaluated. The experimental results showed that the surface resistance hardly depended on the twist pitches of the sub-cables at both liquid nitrogen and liquid helium temperatures. The results become fundamental data to estimate the coupling losses caused by current loops including more than one sub-cable with long-time constants of current decay.

We have studied a measuring method of a critical current (Ic) for NbTi superconducting wires having a Cu matrix and Cu/CuNi matrices. To prevent the wires from prematurely quenching before reaching their Ic, we tried grease impregnations to sample coils. When the NbTi/Cu wire was used for the coil, silicon grease impregnation was enough to fix the wire and to measure the Ic of the wire. In case that we used the NbTi/Cu/CuNi wire when measuring the Ic, restraint of wire motion by the silicon grease impregnation was insufficient and the sample coil easily quenched before reaching the Ic. Accordingly, the Apiezon® (type: M) grease was used for an impregnation material, and we found the Apiezon grease was effective in fixing the NbTi/Cu/CuNi wire.

Numerical studies on characteristics of electromagnetic behaviors of co-axial multi-layer superconducting cable conductors with superconducting magnetic shields were performed. The inter-layer current distribution and AC loss dependencies on the twist pitches and the diameter of the superconducting layers of the cables were investigated. The numerical results showed that the magnetic field produced by the shielding currents flowing in the superconducting magnetic shield strongly affected the current distribution and the AC losses of the cable. Based on the model, the optimized cable structures for uniform current distribution and to reduce the AC losses were studied. Examples of the optimized structures obtained by our model were presented and compared with those obtained by the conventional electrical circuit model. The differences between the results obtained by our model and those obtained by the circuit model were discussed.

High field superconducting solenoid magnets sometimes quench by wire motion induced by electro-magnetic force. High strength polyethylene fiber reinforced plastic (DFRP) has negative thermal expansion coefficient and low frictional coefficient. DFRP pipes made by filament winding method could be constructed so as to expand in the circumferential direction when cooled to low temperature with an appropriate selection of winding angle and shape of the pipes. In the case of a superconducting coil wound on a DFRP bobbin, it is expected that wire motions in high field are decreased by expansion of the coil bobbin. In this work, the sample coils wound on DFRP bobbin and stainless steel (SUS) bobbin were prepared. The sample coil voltages were measured during increasing current in back ground external field. The coil using SUS bobbin showed many sharp peaks in tap voltage induced by quick wire motions. In contrast, those using DFRP bobbin showed no peaks. These results suggest that the quick wire motions were constrained by DFRP bobbin that had negative thermal expansion coefficient and low frictional coefficient.

We experimentally studied frictional coefficients on surfaces of Dyneema^[○!R] fiber reinforced plastics that were paid attention as new structural materials of superconduction coils. The frictional coefficients on a surface cut by a diamond cutter were measured; the Dyneema fibers were exposed on the surface. We also measured the coefficients on a grind surface of the plastic on which the Dyneema fibers were not exposed. We measured the coefficients on the two kinds of the surfaces at three temperatures (room, liquid nitrogen, liquid helium), and discussed the expermental data.

Hybrid composite pipes reinforced with high-strength polyethylene fiber (DF) and alumina fiber (AlF) were prepared to develop the coil bobbin for stable superconducting coils. The bobbin in which the circumferential thermal strain expands with cooling and in which the circumferential Young's modulus is large would be effective for stable coils. The unidirectional hybrid composite (ADFRP) showed 0 thermal expansion coefficient when the ratio between DF and AIF volume was 5/5 in fiber direction, and its Young's modulus was larger than that of DF reinforced plastic (DFRP) both in parallel and perpendicular to fiber direction. The circumferential and longitudinal Young's moduli of ADFRP pipe were larger than those of DFRP pipe. The average value of inner and outer circumferential thermal strains with cooling down showed 0 with a filament winding (FW) angle of 90deg when the ratio between DF and AIF volume (D/A) was 5/5. When D/A equals 5/5, the calculated thermal strain with cooling of the pipe showed good agreement with the average of observed inner and outer thermal strains. The circumferential thermal strain showed an expansion with a FW angle of 50-90deg, and an absolute value was smaller than those of DFRP. The inner and outer circumferential thermal strains were different. The difference decreased with an increase of the ratio inner diameter/thickness, and the differences were smaller than those of DFRP with a decrease of the degree of anisotropy of thermal expansion coefficients in UD-FRP. The experimental data were obtained to make it possible to devise a coil bobbin with negative thermal expansion coefficient by ADFRP.

Transient heat transfer of liquid state helium in centrifugal acceleration fields up to 1300G have been investigated for studying the stability characteristics of the rotor field windings of a superconducting generator. In the experiment, temperature resistive platinum-0.5% cobalt (Pt-Co)thin wire was used as the temperature sensor and heater to measure the heat transfer characteristics. The thermal mass of the Pt-Co wire was small enough to observe the temperature fluctuation due to bubbling in the boiling heat transfer. In this paper, influence of the centrifugal acceleration on the transient heat transfer and its mechanism are discussed based on the experimental results. It was observed that, for step heating with heat flux higher than maximum heat flux at steady state nucleate boiling,quasi-nucleate boiling appeared also in centrifugal acceleration field at the beginning of the heating, and that duration of the quasi-nucleate boiling was not much dependent on the acceleration field for relative high heat flux. This phenomenon was explained by bubble formation process in the boiling heat transfer. In this study, a high speed video camera was used to observe the relation of the bubble formation process and the temperature fluctuation of the Pt-Co wire in the normal gravity.

We are developing technologies of a superconducting linear induction motor (SLIM) for steel making processes. We have developed and tested a prototype SLIM. This paper presents configuration and test results of the SLIM.

We are developing a 4 pole SO Hz 30 kVA class fully superconducting generator to investigate the characteristics of superconducting armature windings subject to a rotating magnetic field. Compared with other fully superconducting generators that have been developed and tested, our machine is designed to obtain a higher armature current. It is generally observed in coils wound from AC superconducting cable that the AC quench current of the coil is much lower than its DC quench current. Countermeasures against these AC current degradation phenomena were developed and applied to the AC cable and armature winding of our generaor. In the paper, results of the electrical tests of the generator are presented and the validity of the countermeasures is discussed.

We are developing a 4 poles 30 kVA class fully superconducting generator to investigate the characteristics of superconducting armature winding subject to the rotating magnetic field produced by the superconducting rotor and behaviour of a superconducting generator connected to an electric power utility grid. A static test of the armature winding have been performed by applying 50 Hz AC current. AC quench currents of the armature windings have reached to 200 Arms after several quenches which was well over the rated current. A static test of the field windings have been also performed to verify its rated performance. In the paper, detailed configurations and electrical test results of the generator are shown.

The frequency dependence of the quench current of the epoxy-impregnated windings of a superconducting linear induction motor (SLIM) has been measured at f = 20 - 300 Hz; the quench current decreases with increasing frequency. The a.c. losses of SLIM windings have been measured and the temperature rise of the windings due to the a.c. losses studied by a numerical analysis using the finite element method. Reasons for the decrease in quench current with increasing frequency are discussed in relation to the a.c. losses.