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This paper describes a superfluid 4He (He II) production system with a G-M cryocooler developed to supply continuous cooling capacity for cryogenic devices working below 2 K, such as the sealed-cell lambda-point devices and the photon detectors. Unlike the existing cooling method that injecting and pumping liquid 4He (He I) to cool the devices, high purity 4He gas stored in gas cylinder is firstly liquefied by a G-M cryocooler in the condenser, and then cooled down under 2.17 K at the He II pot through a Joule-Thomson valve along with a dry vacuum pump. This design allows a continuous supplement of He II below 2.17 K by using 4He at ambient temperature, which avoids the temperature oscillation to the devices being cooled when refilling He I. Due to the removing of cryogen injection, no additional cryogenic apparatus and professional operation are required. In this system, the He II pot can reach a temperature below 1.8 K for continuous operation.

A Nb3Sn wire which was manufactured for the ITER toroidal field coil conductor by a bronze route process was prepared for this study to investigate the effect of neutron irradiation on the critical current in a high magnetic field. The critical current of the virgin wire was measured in liquid helium with a 28 T hybrid superconducting magnet at the High Field Laboratory for Superconducting Materials in Tohoku University. It was also measured in vacuum with a heat conduction type variable temperature insert (VTI) at the International Research Center for Nuclear Materials Science at Tohoku University. The wire was irradiated at below 100 °C by fission neutrons at up to 4.9 × 1022 neutrons m−2 (>0.1 MeV) at BR2 in Belgium, and the critical current after the irradiation was evaluated with a VTI in the range of 8–15.5 T. The difference of the critical current measured with two facilities was discussed, focussing on Joule heating of the sample holder which was made of pure copper, and the neutron irradiation effect on the critical current was investigated in the range of up to 15.5 T. The results show that the critical current measured in vacuum becomes lower than that in liquid helium because of the temperature rise of the sample holder where the sample was soldered, the critical current was increased by the neutron irradiation, and the current ratio (IC/IC0) was almost constant of 1.75 in the range of 8–15.5 T at around 4 K.

To perform post irradiation tests of superconducting strands, a 15.5 T superconducting magnet and a variable temperature insert (VTI) were installed at a radiation control area in Oarai center, Institute for Materials Research, Tohoku University. Both of the 15.5 T magnet and the VTI are conductively cooled with GM refrigeration. The commissioning test of the system is still ongoing because unexpected troubles occurred during the commissioning. Also, a SQUID system with the maximum field of 7 T has been installed at another radiation control area to investigate the magnetic property of uranium and its isotopes. These devices are very useful to study the electro-magnetic properties of the neutron irradiated superconducting strands. This paper will introduce the new superconducting test facility, and some data recently obtained will be presented together with the data of magnetization evaluated with the SQUID, and the discussion on the irradiation effect on superconducting properties will be performed.

For developing the fatigue life evaluation method using small specimen, the effect of specimen size and shape on the fatigue life of the reduced activation ferritic/martensitic steels (F82H-IEA, F82H-BA07 and JLF-1) was investigated by the fatigue test at room temperature in air using round-bar and hourglass specimens with various specimen sizes (test section diameter: 0.85–10 mm). The round-bar specimen showed no specimen size and no specimen shape effects on the fatigue life, whereas the hourglass specimen showed no specimen size effect and obvious specimen shape effect on it. The shorter fatigue life of the hourglass specimen observed under low strain ranges could be attributed to the shorter micro-crack initiation life induced by the stress concentration dependent on the specimen shape. On the basis of this study, the small round-bar specimen was an acceptable candidate for evaluating the fatigue life using small specimen.

We measured mechanical loss in an AC magnetic field for a short Bi-2223 superconducting tape. Experimental results indicated that mechanical loss decreased as tension applied to the sample tape increased. Furthermore, when a sample holder with a negative coefficient of thermal expansion was used, mechanical loss reduced. The origin of mechanical loss was confirmed to be the vibration of the sample tape. The expansion holder was the most effective means to reduce the vibration.

Some technical issues still remain in the manufacturing process of the ITER Toroidal Field (TF) coil. Since the impregnation and curing of its insulation system is one of the issues, the authors performed trial impregnation using Cyanate Ester (CE) resin, which is a new candidate among resins because of its excellent resistance to radiation. The authors also studied the feasibility of curing the Winding Pack (WP). A previous study by the authors showed that a void, which may degrade insulation performance, may be generated as a result of an excessively brief impregnation, which was determined by pot life, namely the time required for viscosity to reach 100 mPas. Since the pot life of the improved CE resin was measured to be 200 h, the insulation system is allowed to be impregnated more slowly. Therefore, in an impregnation test, a 0.6 m sample was impregnated more slowly using CE resin than in previous studies, resulting in no voids in the insulation layers. In addition, calculations by the authors indicate that the required curing temperature tolerance of ±5 °C is difficult to achieve through the conventional method of using external heaters because of the heavy mass of the WP. However, it is shown that this demanding tolerance margin is achievable using the conductor as an additional heater. These results suggest that it is time for full qualification testing of the impregnation and curing process in a dummy double-pancake trial.

The advantages of MgB2 compound for a future fusion reactor application are not only high Tc (39 K) and low cost but also low activation material. MgB2 compound is non Nb-based superconductor and its decay time of induced radioactivity may be about 10 years and significantly shorter than Nb-based superconductors. In this paper, we fabricated the Mg2Cu doped MgB2 wires using several metal sheath materials to improve Jc property. The selection of metal sheath materials is also important to apply for fusion reactor. Pure Fe, Ta and Cu have faster decay time of induced radioactivity compared the other metals. Tc values of all Mg2Cu doped samples were higher than those of non-doping samples, and Tc values were improved with increasing amount of Cu doping. And the maximum core-Jc value was estimated to be 100 A/mm2 over at 4.2 K under 8 T for 3 at% Cu doping sample. The comparisons of superconducting property between different metal sheath materials are discussed.

The mission of the National Centralized Tokamak (NCT) is to establish high-beta steady-state plasma operation for DEMO and to contribute to ITER. Plasma performance with high-beta conditions is discussed. It is found that near break-even class plasma ( Q DT eq ≥ 0.7 ) with high beta ( β N ≥ 3.5) is achievable with 25 MW of NB power at I P = 4.0 MA and an aspect ratio of 2.6. High beta with steady-state full current drive is possible with 13 MW of NB power at I P = 1.5–1.7 MA. In the engineering design activity of NCT, the strength of the support structure and strength of material for the conductor is confirmed for the superconducting coils, and the successful progress of R&D of boron-loaded resin for neutron shielding is discussed.

In order to improve superconducting property of Bi-2223 superconductor bulk, 5 mm in width, 22 mm in length and ca. 2 mm in thickness, silver (Ag) wires of 0.4 mm in diameter were composed in the bulk. The composite was made by stacking alternately Ag wires and the calcined powder. The pressed sample was sintered at 840/spl deg/C for 50 hours in air. After treatment with a cold isostatic press (CIP) as an intermediate pressing, the sample was sintered again. With 6 times of CIP processing, the maximum critical current density (J/sub c/) at 77.3 K of the sample composed with twenty-four Ag wires was obtained to be 950 A/cm/sup 2/. SEM and EPMA observation results showed that highly c-axis oriented and densely structured Bi-2223 plate-like grains could be formed around the interfacial region between the superconducting oxide and the metal Ag.

Weld specimens of the reference low activation vanadium alloy, NIFS-HEAT-2, were irradiated up to the neutron fluence of 4.5 × 10 23 nm −2 (0.08 dpa) at 563 K in JMTR. The weld metal showed larger irradiation hardening than that of the base metal, and the weld metal showed an embrittlement at 77 K. However, irradiation hardening at the weld metal was effectively reduced by post weld heat treatment at 873 and 1223 K. Mechanisms of irradiation hardening, and behavior of C, N and O impurities in the weld metal was discussed, based on the characterization of radiation defects.

Helical coils of the Large Helical Device are pool-cooled superconducting magnets. The operating current is restricted below about 90% of the design current because a normal-zone has propagated dynamically at several times at almost the same current. In order to estimate the effect of lowering temperatures on the cryogenic stability, an R&D coil was made of the same conductor. The cryogenic stability of the R&D coil was examined in saturated and subcooled helium. A normal-zone was initiated by a heater inserted between the conductor and the layer to layer spacer. The propagation was detected by voltage taps. In saturated helium of 4.4 K and 0.12 MPa, the minimum current to begin propagation is 10.7 to 10.8 kA. It becomes higher at the lower temperature, and it exceeds 11.7 kA in subcooled helium of 3.5 K as a temperature inside the R&D coil.

The effect of the laser welding condition on properties of the weldment, such as bending, tensile and charpy impact properties were investigated in a V-4Cr-4Ti alloy (NIFS-HEAT-2). The microstructu...

A fusion magnet system using superconducting magnets must have a cryogenic support structure to sustain huge electro-magnetic force. Since the structure is constructed by welding and it supports static and dynamic forces, the monitoring system for soundness evaluation of the support structure has been desired. In the Large Helical Device project, strain gages and wires were developed and applied to the strain monitoring system. The gages were attached on the weld line to investigate the change in strain due to crack initiation on the weld root. All gages are still active and the monitoring system is working on well. No significant change in strain has been observed and it results in revealing the soundness of the support structure of the Large Helical Device.

We have studied a newly designed BPSCCO-2223 bulk composed of mono-cored BPSCCO-2223/Ag sheath filaments. The composite structure design of the combination of BPSCCO-2223 and mono-cored filaments was newly developed with the aim of protection when bulk material is used as a current feeder for large-scale applications. The composites were made by alternately stacking BPSCCO-2223 oxide-cored Ag sheath filaments and the oxide powder by the powder-in-tube (PIT) method, and then the prepared samples were sintered at 840 °C for 50 h. Then, a cold isosatic pressing (CIP) process was applied and they were re-sintered at 840 °C for 50 h. As a result, the maximum transport current (Ic) value of the composite bulk, which is composed of 24 mono-cored sheath filaments of 0.4 mm in diameter, was estimated to be about 240 A at 4.2 K and 0 T. This Ic value was about three times higher than that of a conventional bulk, and the value of the PIT filaments composite bulk was also higher than that of the Ag wires composite bulk. This is why good c-axis oriented and densely structured BPSCCO-2223 plate-like grains were formed on both the outer and inner interface between the oxide and Ag in the PIT filaments. Furthermore, we confirmed that transport current was flowed into the PIT filaments composite bulk after forcing a fracture by the bending test. We guessed that the PIT filaments could act as a bypass for the fracture of the bulk. We thought that a new design of the composite bulk in this study was interesting in terms of safety precautions for large-scale applications.

To improve superconductivity and mechanical properties of Bi-2223 sintered bulk, silver (Ag) wires of 0.4 mm in diameter were added to the bulk. The sample was sintered at 840/spl deg/C for 50 hours in air. After treatment with cold isostatic pressing (CIP), the sample was re-sintered. When twenty-four Ag wires were composed, the critical current density (J/sub c/) at 4.2 K and self-field increased from 1,200 A/cm/sup 2/ to 2,000 A/cm/sup 2/ with the CIP process. SEM observation results showed that high alignment and dense structures of Bi-2223 plate-like grains were formed around the interfacial region between the superconducting oxide and the metal Ag. The mechanical properties were measured by a three point bending test. The maximum bending stress of 60 MPa was obtained in the bulk without Ag wire, which was fractured separately. With the CIP process the maximum bending stress increased to 90 MPa for the composite. The composite did not fracture but only fine cracks were induced after the maximum bending stress.

The bead-on-plate laser welding for V-4Cr–4Ti alloy was performed using YAG laser beams. Full penetration for the 4-mm thick sheet V–4Cr–4Ti alloy was obtained in the various welding conditions. Impact absorption energy in the weld metal was similar to that of the base metal. The width of the weld metal and the heat affected zone (HAZ) increased with increasing the input energy per unit bead length. Hardening in the weld metal and the HAZ occurred. Hardening in the HAZ was attributed to the increase in the impurity level in the matrix induced by precipitate resolution. The optimum welding condition was discussed in terms of controlling the width of the weld metal and the HAZ.

In order to improve superconducting property of Bi-2223 superconductor bulk, Ag wires were composed in the bulk. The composite was made by stacking alternately Ag wires and the calcined powder. The sample was sintered at 840 °C for 50 h in air. After treatment with a cold isostatic press as an intermediate pressing, the sample was sintered again. The maximum J c at 4.2 K and self-field of the sample composed with 24 Ag wires of 0.4 mm in diameter was obtained to be 2000 A/cm 2 . Scanning electron microscope and electron probe microanalyzer observation results showed that highly c -axis oriented and densely structured Bi-2223 plate-like grains could be formed around the interfacial region between the superconducting oxide and the metal Ag.

In the National Institute for Fusion Science (NIFS), the following three research items are being advanced as application of high temperature superconductor (HTS) to an experimental fusion plasma system, through collaboration with universities and laboratories. The first is an application of HTS in the large current feedthrough to a superconducting coil cooled with superfluid helium in order to reconcile low heat load and stable large current transport capability. A 20 kA current feedthrough from 4.4 K to 1.8 K has been being developed using YBCO bulk superconductors manufactured by the QMG process. The second is the development of a 30 kA HTS current lead. The Bi2212 bulk material fabricated by the diffusion process is considered as a candidate material for HTS current leads. The third is the development of the superconducting magnetic-levitation coil for Mini-RT wound with Bi2223 tape conductor. A magnetic-levitation coil operated in persistent-current mode has to levitate for 8 hours without a feed of refrigerant. Present status of these HTS application researches in NIFS is reported.

To improve the superconducting properties of the high-T/sub c/ superconductors, we prepared the (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ (BPSCCO-2223) bulk composite with embedded Ag wires. The composites were hand-made by stacking alternately Ag wires into the oxide powder in the thickness direction, and then the prepared samples sintered at 840/spl deg/C for 50 hours. SEM and EPMA observation showed that good c-axis oriented and densely structured BPSCCO-2223 plate-like grains could be formed around the interfacial region between BPSCCO-2223 oxide and metal Ag wire. We measured critical current density (J/sub c/) performance at 77.3 K and self-field and/or 4.2 K under several magnetic fields. As the J/sub c/ values of the BPSCCO-2223/Ag wires composite bulks were higher than that of BPSCCO-2223 bulk without Ag wires, J/sub c/ values were improved with increasing the number of Ag wires at several measurement conditions. The maximum J/sub c/ was obtained about 230 A/cm/sup 2/ at 77.3 K and self-field, 800 A/cm/sup 2/ at 4.2 K and self-field when forty-two Ag wires were composed. These J/sub c/ values corresponded to about twice that of BPSCCO-2223 oxide bulk without Ag wire. The improvement of J/sub c/ value was brought about by forming good c-axis oriented and densely structured BPSCCO-2223 plate-like grains.

The National Institute for Fusion Science (NIFS), in collaboration with universities and laboratories in Japan, the Forschungszentrum Karlsruhe (FZK) and the Max-Planck Institut fur Plasma Physik (IPP) in Germany, is planning to develop high temperature superconducting (HTS) current feeders for large-scale superconducting coils. Two programs are being progressed: one is a current feedthrough for superfluid helium (He II) cooled superconducting coils; the other is current leads for experimental fusion system. The paper describes the present status of the two programs, including joint experiments of a 20 kA Bi-2223 current lead developed by FZK and development of a prototype YBCO bulk current feedthrough for He II cooled superconducting coils. In addition, the test results of Bi-2212 tubes fabricated by diffusion process, applicable to HTS current leads, are described Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/.

Characterization of a weld joint of a Fe–Cr–W ferritic steel (JLF-1) has been carried out in comparison with heat-treated specimens. The heat-treatment was carried out to simulate heating history effects of the base metal (BM), the heat-affected zone (HAZ) and the weld metal (WM) of the joint. Change in X-ray diffraction patterns and hardness of the weld joint and the heat-treated samples are compared and discussed. The results of X-ray diffractometry and the hardness measurements suggest that phase transformation should occur around the heat-treatment temperature of 820–830°C, and that the transformation does not necessarily cause hardening. Although the hardness of the HAZ changes with the distance from fusion line, the internal strain and the residual stress do not change significantly throughout the HAZ. The single heat-treatment test seems insufficient to correlate directly to the HAZ of the weld joint, because repeated heating with different maximum temperatures and different cooling rates would have been applied to the HAZ.

The initial experiments on the Large Helical Device (LHD) have extended confinement studies on currentless plasmas to a large scale (R = 3.9 m, a = 0.6 m). Heating by NBI of 3 MW produced plasmas with a fusion triple product of 8 × 1018m-3keVs at a magnetic field strength of 1.5 T. An electron temperature of 1.5 keV and an ion temperature of 1.1 keV were achieved simultaneously at a line averaged electron density of 1.5 × 1019 m-3. The maximum stored energy reached 0.22 MJ with neither unexpected confinement deterioration nor visible MHD instabilities, which corresponds to β = 0.7%. Energy confinement times reached a maximum of 0.17 s. A favourable dependence of energy confinement time on density remains in the present power density (~40 kW/m3) and electron density (3 × 1019 m-3) regimes, unlike the L mode in tokamaks. Although power degradation and significant density dependence are similar to the conditions on existing medium sized helical devices, the absolute value is enhanced by up to about 50% from the International Stellarator Scaling 95. Temperatures of both electrons and ions as high as 200 eV were observed at the outermost flux surface, which indicates a qualitative jump in performance compared with that of helical devices to date. Spontaneously generated toroidal currents indicate agreement with the physical picture of neoclassical bootstrap currents. Change of magnetic configuration due to the finite β effect was well described by 3-D MHD equilibrium analysis. A density pump-out phenomenon was observed in hydrogen discharges, which was mitigated in helium discharges with high recycling.

Fe–Cr–W ferritic steels are candidate low activation materials for fusion reactor structural components. Under a surveillance test program of the Japanese low activation Fe–9Cr–2WVTa steel (JLF-1), JLF-1-HEAT2 was made by Japanese universities. The present paper reports the results of microstructural observation and hardness testing of JLF-1-HEAT2 and its weld joint. The relation of microstructure with local hardness and tensile properties at various positions on the weld joint was investigated, and the correlation qualitatively interpreted in terms of the martensitic lath width.

A large superconducting magnet system for a fusion device requires large support structures which must have enough stiffness and toughness in cryogenic service. In the large helical device (LHD), an austenitic stainless steel SUS 316 with a thickness of up to 100 mm has been used for a supporting shell, shell arms and helical coil covers. Since these structures are assembled by welding, fracture toughness of the thick-section weld joint of SUS 316 has been investigated at cryogenic temperature using a large bend bar with a cross-section 87.5 × 175 mm. In three tests, unstable fracture (large pop-ins) occurred at maximum load. The toughness at the first large pop-in showed a higher resistance to crack initiation than that obtained in a conventional compact tension specimen, however, the crack propagated rapidly and no stable tearing was observed. When a linear elastic failure concept is employed to estimate the critical section thickness, it is confirmed that the LHD cryogenic structures have enough safety margin to resist linear elastic breaks.

This paper describes a short history of material selection for the cryogenic support structures for the Large Helical Device (LHD) which has superconducting coils. Since the support structures are cooled down to 4.4K together with the coils, SUS316 was chosen because of its stable austenitic phase, sufficient mechanical properties at cryogenic temperature and good weldability. Also, outlines of the design and fabrication processes of the support structures are summarized. On the design of the support structures, a deformation analysis was carried out to maintain the proper magnetic field during operation. Afterwards, a stress analysis was performed. During machining and assembling, tolerance was noticed to keep coil positions accurate. Special welding grooves and fabrication processes were considered and achieved successfully. Finally, a cryogenic supporting post which sustains the cryogenic structures and superconducting coils is presented. CFRP was used in this specially developed supporting post to reduce the heat conduction from ambient 300K structures.

The largest superconducting fusion machine, Large Helical Device (LHD), is now under construction in Japan and will begin operation in 1997. Design and construction of related R&D programs are now being carried out. The major radius of this machine is 3.9 m and the magnetic field on the plasma center is 3 T. The NbTi superconducting conductors are used in both helical coils and poloidal coils to produce this field. This will be upgraded in the second phase a using superfluid coil cooling technique. A negative ion source is being successfully developed for the NBI heating of LHD. This paper describes the present status and progress in its experimental planning and theoretical analysis on LHD, and the design and construction of LHD torus, heating, and diagnostics equipments.

We have been developing a flexible superconducting bus-line as a unit electrical feeder between large-scale superconducting coils and their power supplies away from the coils. The designed superconducting bus-line consists of a pair of +/- aluminum stabilized NbTi/Cu compacted strand cables and a coaxial four-channel transfer line. A full-scale model of the SC bus-line (20 m long) has been constructed and tested successfully up to 40 kA without a quench under the short-circuit condition. Stability tests were also done by inducing a forced quench with heaters. A minimum propagation current larger than 32.5 kA was confirmed. Thus, the bus-line was cryogenically stabilized at the rated current of 30 kA. We have examined the test results and evaluated the stability and safety margins of this bus-line. The design criteria for a superconducting bus-line are also shown for large-scale superconducting coils with operating current as a parameter. >

To investigate neutron irradiation effects on superconducting properties, fast neutron irradiation tests of Nb3Sn and Nb3Al strands were carried out using 14 MeV neutron source and fission reactors. Neutron fluence of 2.65×1021 n/m2 was achieved at 14 Mev neutron facility and it was clarified that the critical current in the lower magnetic field increased significantly whereas it did not change in the higher magnetic field, and that the critical temperature decreased by 0.2 K and the critical magnetic field did not change. The fission neutron irradiated samples were investigated by SQUID and clear change in magnetization was measured. The 1.0 × 1022 n/m2 irradiation at JRR-3 increased the magnetization hysteresis curve, which means improvement of the critical current, and the over 1024 n/m2 irradiation degraded and made the strands no superconductivity. In case of Nb3Al strand, the proper heat treatment recovered the property remarkably

On the development of the superconducting helical coil, it is important to examine the mechanical behavior of the coil and the evaluation of the coil rigidity at liquid helium temperature. The helical coils in the Large Helical Device are subjected to high electromagnetic force of about 10MN/m. This paper deals with the mechanical behavior and the compressive rigidity of the superconducting coil pack under compressive load at liquid helium temperature. Finite element method is used for the numerical analysis of the composite structure of the coil pack. The numerical results of displacements and strains are obtained and compared with the experimental results. The validity of numerical techniques for evaluating the rigidity of the coil pack is also examined.

In order to improve the critical current density ( J c ) of Bi-2223 sintered bulk, silver (Ag) wires of 0.2 and 0.4 mm in diameter were composed into the bulk. The composite was prepared by stacking alternately the calcined powder and Ag wires, and then sintered at 840 °C for 50 h. The results of the scanning electron microscope observation showed that good c -axis oriented and dense structure of Bi-2223 plate-like grains could be formed around the interfacial region between Bi-2223 oxide and the Ag wires. The J c values of the composites at 77.3 and 4.2 K were higher than that of the sample without Ag wires. The J c values improved with increasing the pieces of Ag wires corresponding to the diameter of the Ag wire. The improvement of J c values was dependent on the contact area between Bi-2223 oxide and Ag wires.

The module coil TOKI-MC is a twisted solenoid coil wound with an aluminum stabilized superconductor developed as an R&D program for the Large Helical Device (LHD). The TOKI-MC can simulate the conductor and winding structure cooled by pool boiling helium, the twisted winding and the large electromagnetic force of the helical coils for LHD. The TOKI-MC was designed as a cryostable coil at an operating current of 20 kA, but the coil quenched around 17 kA in excitation tests. The cause of quenches was thought to be the result of wire movement. Stability tests were also carried out and the measured recovery current was less than the designed value. The degradation of recovery current was due to the excess magnetoresistivity of the copper clad aluminum stabilizer. The stability of TOKI-MC was evaluated and compared with the data of short sample tests. >

For development of superconducting conductors and coils, the National Institute for Fusion Science prepared a new superconducting test facility which is a combination of an electro-mechanical testing device and a large cryogenic cooling system at the new Cryogenics and Superconductivity Building at the Toki site. The cryogenic cooling system has a capacity of 250 liters of liquid helium per hour or 500 W at 4.5 K, and can also supply supercritical helium up to 50 g/s. A dc power supply generates 75 kA current at 21 V. A split-type superconducting coil generates a magnetic field of 9 T of 100 mm in width. The mechanical testing machine has a capacity of 10 MN at liquid-helium temperature. The following R & D experiments for the Large Helical Device have been done using the facility: critical current and stability measurement of helical coil conductors of which the nominal current is 21 kA at 7 T, an elasticity measurement of a helical coil, and stability measurement of a cable-in-conduit conductor.

A construction standard for a large superconducting magnet for a fusion device has been established by Japan Society of Mechanical Engineers in Japan. This paper describes the outline of the part of “Material” and interprets some special features provided in the standard. The standard was composed based on Japan Industrial Standard. The materials used for the superconducting magnet structure was limited and divided into two groups. The first group is for the materials of which cryogenic strength are not limited and will be used under the design with the strength at room temperature. The other group composes of the materials of which cryogenic strength are provided. The former group includes forged austenitic stainless steel, Inconel 718, Ni and Ni alloy, Cu and Cu alloy and welding wires for these materials. The latter group holds cryogenic structural materials, such as JJ1 and 316LN steels. The design strength of these materials is given in a quadratic function from room temperature to cryogenic temperature when the material is used in an as-solution heat treated condition. The standard also equips the process to register the new materials. One shall be able to prepare the data set necessary to make registration, and submit to the JSME Code Committee to register the materials with the standard.Copyright © 2009 by ASME

The Japan Society of Mechanical Engineers (JSME) has set up and published the construction standard for superconducting magnet structures to be used in nuclear fusion facilities. The present target of the standard is tokamak-type fusion energy facilities, especially the International Thermonuclear Experimental Reactor called ITER for short. The standard contains rules for structural materials including cryogenic materials, structural design considering magnetic forces, manufacture including welding and installation, nondestructive testing, pressure proof tests and leak tests of toroidal field magnet structures. The standard covers requirements for structural integrity, deformation control, and leak tightness of all the components of the superconducting magnets and their supports except for superconducting strands and electrical insulators. The standard does not cover deterioration which may occur in service as a result of corrosion, radiation effects, or instability of material. The standard consists of seven articles and twelve mandatory and non-mandatory appendices to the articles; i.e., (1) Scope, roles and responsibilities, (2) Materials, (3) Structural design, (4) Fabrication and installation, (5) Non-destructive examination, (6), Pressure and leak testing, and (7) Terms used in general requirement. The present paper describes the general view of the standard. The detailed descriptions of the standard are described by the other papers in this session CS-21, being divided into four categories; i.e., (1) quality assurance, (2) materials, (3) structural design and replacement, and (4) welding, bonding and examination.

Bead-on-plate welds were produced on the 4 mm-thick V-4Cr-4Ti alloy (NIFS-HEAT-2), using a 2.0 kW YAG laser. The post-weld heat treatments (PWHT) were carried out in various conditions. Microstructures, Vickers hardness and Charpy impact properties were obtained for the weld metal after the PWHT. After PWHT for one hour, the hardness increased and after the peak declined with temperature. At 873 K, the hardness increased and after the peak declined with the time of PWHT. Microstructural observation showed that high density of fine precipitates formed homogeneously when the hardness increased, but the precipitate distribution changed into heterogeneous forming islands of developed precipitate aggregates, coincident with decrease in hardness and recovery of impact properties. Optical microscope observations suggested that a cellular structure of precipitate aggregate region was formed by PWHT. Microchemical analysis showed that Ti was enriched in the precipitate aggregate region. Therefore the areal oscillation of Ti concentration with cellular structures formed by melting and resolidification during the welding resulted in the heterogeneous precipitation by the following PWHT. The precipitation in the Ti-rich area purified the matrix of the weld metal and induced the recovery of hardening and impact property degradation. Optimum PWHT conditions were discussed according to the present results.

Bi‐2223 sintered bulk samples with addition of AgMg wires or Ag‐plated Ni wires have been prepared to improve both the superconducting and the mechanical properties, simultaneously. When 24 AgMg wires of 0.4 mm in diameter were added into Bi‐2223 bulk composite sample, the critical current density (Jc) at 77 K increased from 2 A/mm2 to 7 A/mm2. The mechanical properties have been estimated by a three‐point bending test. The maximum bending stress of 70 – 90 MPa at the rupture was obtained for Bi‐2223/metal wires composite bulk, regardless of the number of the wires. After the maximum bending stress, Bi‐2223 bulk without metal wires fractured separately, while the composite did not fracture but fine cracks were induced only. The Jc values of the composites just after the fracture were about half of the Jc value in the composites without bending. It has been observed that the superconducting current flows even after cracking, in Bi‐2223 bulk with metal wires. This is due to the fact that metal wires suppres...

Contact resistance on surfaces of strands is one of important parameters for characterizing stability of cable-in-conduit conductors. Redistribution of strand currents after occurring a local-normal segment in a strand and AC losses due to inter-strand coupling currents strongly depend on the contact resistance between strands in the multi-stranded conductors. Applying compressive force to a cross segment of the stacked strands, we have measured the contact resistance between the strands at three temperatures. We have prepared two kinds of strands for this experiment. The experimental results showed that the contact resistance strongly decreased with increasing of the contact force to the strands. Furthermore, we quantitatively estimated surface resistance based on the contact resistance and the deformation of the strands. When the strands were made of copper, the range of the estimated surface resistance was between 2×10−10 and 8×10−10 Ωm2 at the liquid helium temperature. In case that we used the superc...

(Bi,Pb)2Sr2Ca2Cu3Ox (Bi-2223) superconductor is suitable to be applied under low field and near liquid nitrogen temperature. However, Jc value of Bi-2223 bulk material is much smaller than that of the wire and tape conductors. We prepared Bi-2223/Ag wires composite bulk to improve simultaneously both the superconducting and the mechanical properties. When twenty-four Ag wires of 0.4 mm in diameter were composed, the Jc value of the composite bulk was estimated to 1200 A/cm2 at 4.2 K and 0 T. This Jc value was about three times higher than that of the oxide bulk without Ag wire. The high alignment and dense structure of Bi-2223 phase was observed on the surface of Ag wires. It is mentioned that this structure brought about improvement of the Jc property. The mechanical property was estimated by a three point bending test. The bending stress was 60 MPa in Bi-2223 bulk without Ag wire, and 20 MPa in the composite with twenty-four Ag wires. The Bi-2223 bulk without Ag wire fractured separately after the maxim...

A superconducting (SC) current-feeder system is used as the cuirait transmission lines fon the LHD. It consists of nine SC bus lines with total length of 497 m and nine pairs of gas-cooled current leads. The cooling process is automatically switched from forced-flow of two-phase helium to pool boiling, when faults happen. Liquid helium consumption of the innermost tube of the SC bus line was negligible, because the liquid helium in the second inner channel absorbed the heat load of 0.34– 0.42 W/m. Mass flow rates of the current leads could be controlled in constant values, whenever the pressure in the recovery line reached up from 0.106 MPa to 0.133 MPa. The allowable current carrying time on the fault protection mode is expected to be longer than 30 minutes of the design value. We successfully demonstrated that the SC current-feeder system with high current capacities was reliable and safe, and was useful for the SC experimental fusion device.

Helical coils of LHD are large scale pool-cooled superconducting coils. In order to produce a fine magnetic surface, the accuracy of the coil position is very important. The required tolerance for manufacturing is ± 2 mm. Against large electromagnetic forces, the conductors were designed to be packed into thick cases which are supported by an outer shell structure. In order to suppress the maximum tensile stress of the conductors below the yield stress, the average compressive Youngs’ modulus of the coils should be high. We developed insulator with a high compressive modulus, and we have managed the average gaps between layers within the aimed value while winding. After winding, the top cover of the coil case should be installed without leaving the residual gaps on the coils and without deforming the accurate shape. In order to reduce the assembling gap, we have machined adjusting spacers that were inserted between the coils and the top covers. After that, we tried to diminish the assembling gaps by utilizing the shrinkage by welding.

The fatigue crack growth rate of an austenitic stainless steel, SUS 316, used for support structures of the Large Helical Device was investigated in a helium gas environment at 7 K. The fatigue crack was propagated in the rolling direction of the base plate of 75 mm thick. Also, a multi-pass welded joint was prepared by TIG and MAG welding procedures simulating practical joints performed in construction of the Large Helical Device, and the fatigue crack propagation behavior started from a natural crack at a weld root was investigated. The fatigue crack growth rate in the base metal showed almost the same property as that reported before. Nevertheless, the fatigue crack propagated in the weld metal with a slower rate, about one fourth or one fifth of the rate in the base metal. A compressive welding residual stress around the weld root and a roughness-induced crack closure would be considered to reduce the fatigue crack growth rate. The comparison of these data with those of other cryogenic structural materials, Type 316LN, Incoloy 908, CSUS-JN1, was also performed.

The Large Helical Device is a fusion experimental machine with all superconducting coils. Electromagnetic force generated by the coils is supported by the torus shaped “supporting structure” of 100 mm thick 316 stainless steel. This structure consists of 20 fan shaped parts that were welded together on site. The coils needed to be positioned within 2 mm accuracy in the absolute coordinates so that the supporting structure had to be assembled within 5 mm accuracy prior to the coil setting. After completion of the welding, the overall accuracy of < 5 mm was achieved.

To characterize fracture toughness of thin plates or local positions in weld joints, a new fracture toughness test method is proposed. A small round bar with a circumferential notch is tensile-tested, and a load-displacement curve is recorded. J-integral is calculated continuously from a load start point to obtain a blunting line empirically. The critical J is determined at the fracture point on the blunting line. Some specimens show remarkable necking, and others fracture at or near the maximum load. Since necking generates a large plastic flow and cup & cone fracture, a flow diagram to evaluate the critical fracture toughness is assessed by adding some limitations. A comparison of results by this new method with ASTM E813-89 is performed, and both methods reveal nearly equivalent toughness value.

High-Tc superconductor (HTS) is expected to be used for various fields applications. For example, it will be applied to large capacity current leads in a field of a certain energy system. The HTS is a ceramic and naturally mechanically brittle. Therefore, fracture by an electromagnetic force will become an essential problem. There is a fiber-reinforced-method to improve mechanical properties of structural materials. This method will be applicable to improve the mechanical property of HTS. We mixed fibers Al2O3, ZrO2 · Y2O3, ZrO2, SiC with BPSCCO calcined powder. The mixed powder was sintered and fabricated into bulks. The fiber added BPSCCO bulks were investigated by a scanning electron microscopy and a X-ray diffraction method.

Two kinds of partially welded austenitic stainless steel joints were prepared using SUS 316, 75 mm thick. One joint was fabricated using tungsten inert gas welding and metal arc gas welding, and the other was electron beam welded. Compact tension specimens for fracture toughness tests were machined out from these welded plates in the thickness direction. The fracture toughness tests of these specimens with natural cracks were carried out in 0, 8, and 14 T fields at 4 K.

To investigate the cold thermal-fatigue characteristics of an austenitic stainless steel (SUS304), a thermal-cycling test was conducted between ambient and cryogenic temperatures. Test specimens were tubes through which flowed liquid helium or nitrogen for cooling and nitrogen gas for warming; the specimen ends were under thermal-deformation restraint. The center of the pipe contained a thin, narrow test section, 20 mm long, to concentrate the thermal deformation. Large hysteresis loops were obtained in the test section. After 757 cycles, a fatigue crack propagated through the wall of the test section, and nitrogen gas leaked into the vacuum chamber. Cold thermal cracks were observed at a corner of the specimen test section; striations were observed on the fracture surfaces.

Remarkable progress in the physical parameters of net-current free plasmas has been made in the Large Helical Device (LHD) since the last Fusion Energy Conference in Chengdu, 2006 (Motojima et al 2007 Nucl. Fusion 47 S668). The beta value reached 5% and a high-beta state beyond 4.5% from the diamagnetic measurement has been maintained for longer than 100 times the energy confinement time. The density and temperature regimes have also been extended. The central density has exceeded 1 × 1021 m−3 due to the formation of an internal diffusion barrier. The ion temperature has reached 5.2 keV at the density of 1.6 × 1019 m−3, which is associated with the suppression of ion heat conduction loss. Although these parameters have been obtained in separated discharges, each fusion-reactor relevant parameter has elucidated the potential of net-current free heliotron plasmas. Diversified studies in recent LHD experiments are reviewed in this paper.

An overview is given of the National Centralized Tokamak (NCT) programme as a research programme for advanced tokamak research to succeed JT-60U. The mission of NCT is to establish high beta steady-state operation for DEMO and to contribute to ITER. The machine flexibility is pursued in aspect ratio and shape controllability for the demonstration of the high-β steady-state, feedback control of resistive wall modes, wide current and pressure profile control capability and also very long pulse steady-state operation. Existing JT-60 infrastructure such as the heating and current drive system, power supplies and cooling systems will be best utilized for this modification.